Sensory receptor cells initiate the process of transporting the world to the mind. They send information across increasingly complex and intricate systems that most people take for granted. Some 10,000 taste buds housed in the tongue detect sweet, sour, salty, and bitter flavors. With each inhalation of air, the cilia of the olfactory nerve cells in the membranes lining the upper passages of the nose trigger impulses that carry scents to the brain. Millions of nerve cells lodged in the skin react to pain, heat and cold, pressure, and texture and can feel vibrations, motion, and position. The delicate hair cells of the inner ear signal changes in air pressure in ways that the brain interprets as sound. When light rays activate some of the 126 million receptor cells of the retina at the back of the inside of the eye and those cells send their impulses to the brain, the brain processes the impulses, integrates them with any other sensory information just received, and informs a person of what he or she has seen.
For most people, the sense of sight plays the mediator role for the other senses to help organize and negotiate the environment and put tastes, sounds, aromas, tactile impressions, objects, and people in perspective. This sense of sight and the vision of the world gained from it incorporate electromagnetic, chemical, and electrical energy and require precise muscle coordination to control the movements of the exquisitely sensitive structures of the visual system, of which the eyes are a part.
This chapter presents information about the eyes and their key role in the process of seeing and in the visual system overall. The first section focuses on anatomy and physiology—the structures of the healthy eye and how they function. Following sections address the causes of visual impairments and factors that can damage structures and interrupt function. In addition, the chapter describes the clinical information that eye specialists contribute to the analysis and understanding of visual function and illustrates how clinical information obtained from eye specialists can be interpreted by teachers of students with visual impairments and used for educational and instructional purposes.
An examination of the anatomy, that is, the structures of the eye, provides a better understanding of how the structures of the eye can function alone and in relation to each other to contribute to the efficiency of the visual system (see Figure 3.1). A basic understanding of structure and function of the eye lays the foundation for studying the growth and development of children with visual impairments in later chapters. This examination of the anatomy of the eye starts with the point that light rays first encounter on their route into the eye and works back to their destination in the occipital lobe of the brain, where sensory data are given meaning.
Each eyeball lies in a pear-shaped bony orbital cavity (also called the orbit or eye socket), the front of which can be closed off by the upper and lower eyelids. The eyelids play a vital role in protecting the entire visual system. They can shut to prevent light, dirt, and wind from entering the eye and open to allow light rays to pass through and travel to the receptor cells inside the back of the healthy eyeball. The eyelids contain additional glands that secrete oils and substances to help lubricate the front of the eye and prevent the evaporation of tears. Opening and closing the eyelids aids the flow of tears across the eyeball. The eyebrows, eyelashes, and eyelids, together with the bony tissue of the orbit, provide a cushion against bumps and strikes and a shield against dirt, perspiration, and bright lights.
The orbital cavity is made up of seven bony, triangle-shaped plates that form a pear shape or pyramid with the apex positioned posterior to (toward the rear of) the face and the wider open end anterior (toward the front), so when the eyelids are open, light rays can enter the eye (Wright, 1997). The eyeball fills about one-fifth of the orbit. The nose separates the two orbital cavities. The nasal or medial plates of the two orbits are approximately parallel to each other, and the two lateral walls of the orbits are about 90 degrees from each other.
The fat and connective tissues that surround each eyeball inside the orbit provide more protection for the eyeball, also called the globe, as well as for the optic nerve that exits from the back of the eyeball and the six extrinsic muscles (also called the extraocular muscles) that are attached to the eyeball and to the walls of the orbit. The extrinsic muscles of each eye work together to move the eyeball in the directions of gaze. The four rectus muscles are primarily responsible for turning the eye toward the nose or toward the temple and up or down. The two oblique muscles act to turn and rotate the eye up and out or down and in, depending on the position from which the eye is moving. Table 3.1 lists the primary and secondary actions for which each muscle is responsible.
The six muscles of one eye are yoked with the six extrinsic muscles of the other eye so both eyes can move together to focus on a distant visual target. Table 3.2 illustrates how the six pairs of muscles are coordinated in normally functioning eyes to move the eyes together in the cardinal directions of gaze. Which muscles contract and relax to direct the eyes in the desired direction to the desired target depends on their current position and status. For example, for looking away from a page of text to an object to the right while holding one's head in the straight ahead position, the primary muscle action would come from the right lateral rectus muscle and the left medial rectus muscle, as shown in Figure 3.2. To allow these muscles to move the eyes to the right evenly and smoothly, the right medial rectus and the left lateral rectus must relax. The muscles also work together so they can converge to focus on a visual target at close range.
The extrinsic muscles are innervated by cranial nerves in the central nervous system. Clear binocular vision (the ability of the two eyes to focus on one object and the ability of the brain to fuse the two images into one single image) depends on the ability of the eyes to move together in a finely and smoothly coordinated fashion so that the messages that reach the brain from the eyes are clean and sharp. When an individual has a condition, such as cerebral palsy, that affects the central nervous system, problems are often found with eye muscle coordination.
Besides the eyeball, muscles, protective fat, and connective tissue, the orbit contains blood vessels, nerves, and the lacrimal gland. The lacrimal gland is situated in the forward upper outer portion of the orbit, as shown in Figure 3.3. It secretes tears that flow down over the surface of the globe into the fold below the margin of the lower eyelid and finally drain out through the lacrimal sac that empties into the nose and nasopharynx. Ordinarily, one is not aware of the flow of tears, but if the supply is insufficient to lubricate the front parts of the eye, the eye can feel gritty and irritated or burning. The oversupply or overproduction of tears can result from allergens, environmental irritants, foreign bodies, upper respiratory infections, bright lights, or emotional upset, to name a few of the causes (Vaughan, Asbury, & Riordan-Eva, 1999).
The conjunctiva, a transparent mucous membrane, covers the posterior surface of the eyelids and the white front portion of the eyeball. Some of the specialized cells in the conjunctiva contribute mucous to the tear layer that lubricates the front of the eyeball, especially the cornea. In addition, other cells in the conjunctiva fight off germs and attack foreign matter and microorganisms that enter when the eyelids are open. One of the most common eye disease in Western countries is conjunctivitis, an inflammation of this thin protective covering.
The eyeball is a slightly elongated and flattened sphere, approximately 24 mm front to back, 23 mm vertically, and 23.5 mm on the horizontal axis (Wright, 1997). One can think of the eyeball as having three layers: the outer protective layer; the middle vascular layer; and the inner nerve layer, where light rays should come to a point of focus. Information from the nerve layer is transmitted as electrical impulses to the occipital (rear) lobe of the brain, where it is interpreted and processed for storage and retrieval.
The outer layer consists of the tough, fibrous white part of the eye called the sclera and the transparent, avascular cornea. Although the cornea is only about 1 mm thick at the periphery in the mature eye and about 0.5–0.6 mm thick at the center, it has five clearly defined layers of cells, membranes, and fibers (see Figure 3.4). The cornea must remain avascular (without blood supply) and in a state of relative dehydration to retain its transparency. Injury or infection can upset the delicate balance and introduce germs that can lead to corneal scarring. As was previously mentioned, the tears help wash away irritants and ward off germs that enter the front of the orbital cavity through the open lids and can cause inflammation and infection. The cornea gets oxygen from the environment and from the aqueous humor, a fluid that circulates inside the eye in the anterior chamber behind the cornea.
The middle layer of the eyeball, referred to as the uveal tract, consists of the choroid, iris, and ciliary body. The choroid is rich in blood supply and lies between the sclera and the inner nerve layer. The function of the choroid is to provide nutrients to the retinal nerve layer.
The iris is a "musculovascular diaphragm with a central opening" (Wright, 1997, p. 28), that is, a ring with an inner hole that can vary in diameter to let more or less light into the back of the eye. The dilator muscle contracts to increase the size of the pupil while the sphincter muscle contracts to draw the edges of the iris together to decrease the size of the pupil. These muscles are antagonistic; they must work together, one relaxing and one contracting, to control the pupil. The iris vascular system provides nutrition for the front of the eye. The color of the iris depends on the amount of pigmentation in the front portion of the iris.
The ciliary body serves two major functions carried out by the ciliary process and the ciliary muscle. The ciliary process secretes a liquid, called the aqueous humor, that circulates through the pupil from the posterior chamber into the anterior chamber right behind the cornea. The aqueous flows out of the eye through the trabecular meshwork into the canal of Schlemm. The aqueous provides nutrition as well as some oxygen for the cornea. The intraocular pressure depends upon the production; outflow; and, to a lesser extent, the absorption of the aqueous (Wright, 1997). If the aqueous builds up in the anterior and posterior chambers, the pressure within the eye can rise to dangerous levels associated with the various types of glaucoma. Until recently, increased intraocular pressure beyond the range considered to be normal was considered the hallmark of glaucoma. Recent research has suggested, however, that although increased pressure in the eye is a common and important risk factor for glaucoma, the significant feature of glaucoma of all types is a specific pattern of optic nerve damage (Wright, 1997). The glaucomas account for approximately 10 percent of blindness in the United States today.
The ciliary muscle helps control the thickness of the lens of the eye by contracting and relaxing the suspensory fibers that hold the lens in place and that regulate the tension on the lens. The lens lies behind the iris and pupil, the hole through which light rays pass to enter the lens and continue to the back of the eye. Changes in the tension of these fibers or ligaments, called zonules of Zinn, allow the transparent lens to vary its refractive power (power to bend light rays and to accommodate to preserve clear focus for near as well as distant objects). The lens is not really a part of the middle layer of the eye, but its work is affected by the action of the suspensory ligaments that enable the lens to alter its curvature. The lens is the only refractive medium or light-bending structure in the eye that can adjust its curvature and thus its refractive power. Most of the refraction of light that enters the eye is accomplished one by the cornea. The lens, however, is responsible for the fine-tuning of light rays so they can come to a point of focus on the inner retinal layer. This ability to adjust the curvature is essential for accommodation to near and distant objects. As an individual grows older, the elasticity of the lens decreases as the fiber like cells continue to develop and compress within the lens capsule. The ability of the lens to alter its curvature also decreases. Reading glasses are usually necessary around age 45, when the loss of the accommodative power of the lens is significantly great enough to make it difficult to see detail at close range. This loss of accommodation that is due to the natural aging process is called presbyopia.
For a variety of reasons, the lens may lose its transparency, which results in the formation of a cataract. A cataract, an opacity, or clouding, of part of or the entire lens, prevents light from traveling to the retinal layer. A cataract cannot be "cured," but the cloudy lens can be removed when a person experiences blurred vision. Once the natural lens of the eye has been removed, the optical system of the eye is out of balance. Without some compensation for the loss of the refractive power of the natural lens, light rays cannot come to a point of focus on the retina. Until recently, contact lenses were typically worn to make up for the absence of the natural lens, but today most adults and some children over age 2 are candidates for intraocular lens implants. At the time of surgery to remove the opaque lens, an artificial lens, often made of silicone, is positioned where the natural lens had been. After surgery with implants, some individuals have good vision, but most need spectacles for normal or near-normal vision.
The aging process accounts for the majority of cataracts. Since it often brings with it other changes in the structures of the eye, especially in the cells of the retina, not all people have good vision after cataract surgery. In addition to aging, certain metabolic disorders, injuries, inflammations, reactions to medications, and some systemic diseases may lead to the formation of cataracts. Some children are born with cataracts, and their early treatment, even in the first month of life, is critical to the development of good, functional vision.
The inner layer of the eye is the nerve layer called the retina. The retina is made up of nine distinct cell layers and approximately 126 million rod and cone cells (Wright, 1997). Its thinnest point, the macula, is also the point of clearest vision. The macula is located on the temporal side of the optic disk near where the many ganglion cells of the retina exit the back of the eye as the optic nerve. The fovea centralis, the central portion of the macula, contains only tightly packed cone cells that are sensitive to color, form, low spatial frequency, contrast sensitivity, and fine detail. Some cone cells are dispersed among the 120 million rods that are dominant in the peripheral regions of the retina. Rods are sensitive to motion and the presence of light and thus are essential for night vision or vision in areas of reduced light. A healthy retinal layer is critical to the efficient transmission of impulses back to the brain. In a premature infant, the retinal layer may not be completely developed and may be vulnerable to the oxygen that frequently must be administered to maintain life. In a full-term infant, the macula and fovea are typically not fully developed until several months after birth (Vaughan et al., 1999; Wright, 1997).
Degenerative diseases of the retina that damage the cone cells in the macular region can cause losses in central vision, a common occurrence in older adults who, simply because of age, are at risk for age-related macular degeneration. Color vision becomes vulnerable if the cone cells are compromised for any reason. Other conditions, like retinitis pigmentosa, can affect the rod cells and lead to decreased night vision.
Other structures of the internal eye are the anterior and posterior chambers and the vitreous cavity. The anterior chamber lies behind the posterior surface of the cornea and the anterior surface of the iris. The posterior chamber lies behind the iris and pupil and in front of the anterior surface of the lens. Both chambers are filled with aqueous humor, the clear watery liquid secreted by the ciliary process already described.
The vitreous cavity, a large chamber, is filled with a transparent, avascular, physiological gel. The vitreous is 99 percent water and makes up about two-thirds of the volume of the eyeball and three-fourths of the weight. The vitreous gel may become stained by blood if there are hemorrhages in the back of the eye, as may occur in some types of diabetic retinopathy, which is discussed later in this chapter.
Of the retina's nine layers, the internal limiting membrane is the layer that is adjacent to the vitreous. The retinal pigment epithelium, the outer layer, rests on the inner layer of the choroid, which provides nutrition for the retina. The rods and cones lie under the retinal pigment epithelium. In the course of time and use, discs in the outer portion of the rods and cones are removed and replaced by new ones. The new ones at the bottom of the stack move up in the rods and cones as the old discs are pushed out to where they can be digested by the retinal pigment epithelium in a process called phagocytosis, an efficient waste management system.
When the rods and cones of the retina are stimulated by light rays, they send their messages to the brain via the 1 million fibers in back of the optic nerve. The optic nerve is the second cranial nerve (CN 2) and, like other central nerves, cannot regenerate or be repaired if it is damaged. The fibers of the optic nerve divide after leaving each eyeball and orbital cavity. Fibers that carry impulses from the nasal retina of the left eye decussate (separate) and cross over at the optic chiasm, the place where they join fibers from the temporal retina of the right eye. Together, they form the right optic tract and travel to the lateral geniculate nucleus, where they synapse with cells that continue and fan out as radiations that relay information to the visual areas of the occipital cortex of the occipital lobe of the brain. Right nasal retinal fibers cross at the optic chiasm to join left temporal retinal fibers to form the left optic tract, which carries information to the lateral geniculate nucleus and from there along the optic radiations to the left side of the occipital cortex (see Figure 3.5). Information is then transferred to other areas of the cortex for interpretation and processing (Newell, 1996; Wright, 1997). Information from each eye in the normal visual system arrives on each side of the brain.
Damage to various parts of the optic pathways can sometimes be localized by determining what portions of an individual's visual field are restricted. For example, if damage occurred to the right optic nerve at a point in front of the optic chiasm, vision in the right eye would be affected. If damage occurred at the optic chiasm, impulses from the nasal portions of each retina would be interrupted, resulting in losses in the left and right temporal fields. When the eyes are directed straight ahead to a distant visual target, light rays entering the eyes from the left strike the nasal retina in the left eye and the temporal retina in the right eye and light rays entering from the right strike the nasal retina in the right eye and the temporal retina of the left eye. Therefore, if impulses from the left and right nasal retinas are not transmitted past the damaged point, the result is the loss of both temporal fields in both the left and right eyes. Figure 3.5 suggests how injuries at specific sites along the optic nerves and pathways would affect the receipt of stimuli from regions of the visual field.
In some instances, an individual is said to be cortically blind or to have cortical visual impairment. Cortical visual impairment can result from lesions that occur along the visual pathway from the lateral geniculate nucleus back to the visual cortex. Problems along the optic nerve between the retina and the lateral geniculate body are grouped with conditions affecting the eyeball, all of which are considered ocular visual impairments (Jan & Groenveld, 1993). The visual behavioral characteristics of those with ocular impairments and children with cortical impairments are typically different, unless the impairments involve sites on both sides of the lateral geniculate body. The eyes and eye movements of children with cortical visual impairments appear normal, as do the children's pupillary reactions to light. But these children often have variable visual function even within a few minutes, a short visual attention span, attraction to light and light gazing, and limited field of vision, among other characteristics (Jan & Groenveld, 1993).
Many children with cortical visual impairments have other neurological problems as well. Although sophisticated electrophysiological and other diagnostic tests can aid the diagnosis of cortical visual impairment, careful observations of general and visual behaviors can provide valuable information for both diagnosis and instruction.
In the normal and healthy eye, rays of light travel through the transparent cornea, the major refractive surface of the eye; through the aqueous to the lens, where fine adjustments are made; on through the vitreous; and, finally, to the retina. Refractive errors and muscle imbalance can, however, introduce difficulties that lead to degradation in the image that is transmitted to the brain from the retina via the optic nerve fibers.
Many individuals, children as well as adults, have refractive errors that result in reduced visual acuity because the light rays do not come to a point of focus on the retina (see Figure 3.6). In myopia (nearsightedness), the rays come to a point of focus in front of the retina because the eyeball is longer than normal on the horizontal axis. The converging, or bending, power of the cornea and the fine-tuning power of the lens are too strong to make the light rays reach the retina to produce a clear image. In hyperopia (farsightedness), the point of focus is slightly (and hypothetically) behind the retina. In this case, the converging power of the cornea and the fine adjustments of the lens are not strong enough to bend the light rays to meet at the focal point (point of clear focus) on the retina. In astigmatism, some rays may converge on the retina while others may converge in front or behind the retina because the surface of the cornea is uneven or oblong, rather than spherical. In the case of myopia, near vision is better than distance vision. In hyperopia, distance vision is typically better than near vision. In astigmatism, vision can be blurry at all points of focus.
Most refractive errors can be corrected with eyeglasses or contact lenses. Concave spherical (divergent or minus) lenses are used to correct for myopia, and convex spherical (convergent or plus) lenses are used to correct for hyperopia. For astigmatism, cylindrical lenses are prescribed that have different refractive powers along specific meridians to provide what the eye requires in convergent or divergent power to bring light rays to a point of focus on the retina (see Figure 3.6).
To understand the meridians on a lens, one can think of a ball that has a circle drawn around it to represent the vertical axis and another circle to represent the horizontal axis. More circles could be drawn around the ball that pass through the midpoint, where the horizontal and vertical axes cross. These additional circles represent the meridians of a lens, although the lens is biconvex—not round like a ball, but oval like a football. In many cases, the refracting power of all meridians of the lens of the eye is essentially the same, and a spherical corrective lens can be used to correct for myopia or hyperopia because a spherical lens has the same refracting power on all meridians. In cases of astigmatism, however, along one or several adjacent meridians, the bending power of the lenses is prescribed to add bending power for the weaker meridians and to compensate for the stronger meridians so rays of light come to focus on the retina, not in front of or behind it.
In many people, visual acuity for near or distant points remains below the normal range even with the best correction. For some of them, generally those over age 18, recently refined surgical procedures to reshape the cornea have dramatically improved vision so that eyeglasses may not even be necessary or if they are, the refractive power of the spectacle lenses is much less than it was before surgery. The procedures include radial keratotomy for myopia, to reduce the curvature of the cornea and thus reduce the refractive power; astigmatic keratotomy, to reduce astigmatism by flattening areas of the cornea to even out the refractive power or the cornea; and eximer laser correction/ablation, to reshape the cornea stroma (Laser Vision Correction, n.d.).
Children with refractive errors who are served in programs for students with visual impairments are those whose visual acuity still falls below the normal range in the better eye after the best correction and adversely affects their educational progress. Some of these children may have other ocular conditions that also interfere with functional vision.
Another problem in eye structure that can lead to possible difficulties with visual function is muscle imbalance. The paired or yoked muscles work together to produce conjugate eye movements in the six cardinal directions of gaze. If these muscles are not innervated equally, that is, if the strength of the muscles is unequal, or if any muscle is paralyzed, then the eyes may not appear straight and may not achieve good, clear, binocular vision.
Strabismus is the condition in which an eye deviates from either the horizontal or vertical axis. It occurs in approximately 2 percent of children (Vaughan et al., 1999) and is sometimes first noticed in infants just several months old. If there is a tendency for misalignment, the turn is referred to as a -phoria. If the misalignment is constant, the term used is -tropia. The turn may be in toward the nose (eso), toward the temple (exo), or up (hyper) or down (hypo) in relation to the horizontal axis. A person whose right eye tends to turn in, for example, when he or she is tired or has done a lot of close eyework, is said to have a right esophoria. If the turn was constant, it would be called a right esotropia.
If a child has strabismus, there is a danger of diplopia (double vision) in which the child sees two images that may be partially superimposed or totally separated because the image from one eye does not originate from a point on the retina that corresponds to the point on the retina in the other eye. As was discussed earlier, the information from both eyes is carried to both sides of the occipital lobe. Because the two eyes are directed at a visual target from a slightly different position, two slightly different images, one from each eye, arrive in the occipital lobe. The association areas of the lobe fuse the two images into one three-dimensional stereoscopic image in ways that are not clearly understood. If the two images cannot be fused into one clear image, the resulting double vision can be confusing and eventually intolerable. At some point along the visual pathways or possibly in the brain, the bothersome, usually weaker, image is suppressed and ignored. This suppression can lead to amblyopia in the suppressed eye, that is, poor vision from lack of use, rather than from an organic disease. If amblyopia is caught early, at least by age 5 or 6, and treatment of the underlying cause is successful, then vision may improve to within the normal range (Amblyopia, 1997).
Options for the treatment of strabismus include the correction of any refractive errors with spectacle or contact lenses; prism lenses to redirect the line of sight; patching or occlusion of the good eye for carefully prescribed periods each day to stimulate use of the weaker eye and equalize the visual acuities; orthoptic eye exercises in selected cases to stimulate the fovea; medication, again to force use of the weaker eye; and surgery. The major goals in the treatment of strabismus are clear vision (acuity), cosmetically straight eyes, and binocular vision (fusion). In many cases, only the first two goals are achieved.
Although refractive errors and muscle imbalance contribute to the population of children and adults who have impaired vision, these conditions are not considered major causes of visual impairment. Most refractive errors can be corrected with lenses and, in some cases, with surgery. Muscle imbalance can be treated successfully in most cases if it is caught early, ideally before a child is one year old. But if these conditions are not diagnosed and treated while a child is still young, or at least before age 5 or 6, then the effects on functional vision and sometimes on social-emotional development may be serious.
Many conditions contribute to the impairment of eye structures and tissue. Whether the impairment actually leads to limitations in visual function, however, depends on such factors as the site and severity of the tissue damage and the age of the individual at the time the problem occurs. Some conditions originate during the prenatal period, others stem from events that occur during or shortly after birth, and still others develop or occur later as a result of disease or trauma. Conditions that are caused by diseases or accidents after birth are called adventitious.
Some eye conditions are inherited; that is, they are passed on to the affected individuals by a parent or, in some cases, both parents. At times, the inherited condition may appear to skip a generation. A genetic study can help determine a family's pattern of transmission and how the condition might have been passed from one generation to the next and to children within a generation.
There are many ways eye conditions can be inherited. An autosomal dominant pattern means that a condition or disease can be passed to an offspring if one parent transmits the gene responsible for the condition at the moment of conception, as is the case with some types of glaucoma and night blindness. An autosomal recessive pattern means that a recessive gene for the condition must come from each parent, as is the case with some metabolic diseases and at least one type of albinism. In sex-linked patterns, the female carrier, the mother, contributes the affected sex chromosome and the son manifests the condition. Examples of sex-linked diseases are some forms of albinism and color blindness. When inherited conditions are manifest, or expressed, they may appear as mild, moderate, or severe. The variable severity of expression of a disease from one generation to another can complicate attempts to determine penetrance, that is, whether the inherited condition in a particular individual is mild, whether it is skipping a generation, or whether it is genetically present but not manifest. Geneticists who try to determine patterns of transmission of diseases look at expression in previous generations of a family and in members of the same generation of a family and then examine the pedigrees of the individuals of concern in the family (Vaughan et al., 1999).
Some hereditary conditions do not become manifest or obvious until adolescence or even adulthood, while others may be congenital (present at birth). Not all hereditary conditions are congenital, and not all congenital conditions are hereditary. Some hereditary and other later developing conditions, such as retinitis pigmentosa and Usher syndrome, may be diagnosed before their clinical appearance with the aid of sophisticated diagnostic procedures.
This section describes the more common causes of ocular visual impairment; briefly reviews other notable causes, such as cortical visual impairment; and highlights several systemic conditions that can have significant ocular manifestations. In subsequent discussion, diagnostic procedures are reviewed to enhance teachers' understanding of clinical eye reports that are typically required for students who receive special education services because of their visual impairments.
The major causes of visual impairments can be described according to their site, type, and etiology. Site refers to the location within the orbit or the eyeball; type indicates the diagnosis, like glaucoma; and etiology refers to the underlying cause, such as an infection.
There are no accurate data on the number of individuals with impaired vision or no vision, let alone on the causes of the visual conditions, because in the United States there is no national registry of individuals who are blind or who have significant low vision (Kirchner & Schmeidler, 1997). The American Printing House for the Blind (APH) collects annual figures to show the number of school-age children and youths who receive educational services because their visual impairments interfere with their learning, but etiology and site are not reported. No national data are available for the number of infants, toddlers, or preschoolers with visual impairments, although efforts are underway in some states with federally funded Project PRISM to establish a multistate registry (Ferrell, with Shaw & Dietz, 1998). For adults over age 40, more accurate and recent estimates are found in Vision Problems in the U.S. (1994), published by Prevent Blindness America (PBA), formerly the National Society to Prevent Blindness. These PBA estimates are based on 1990 population data taken from the summary tapes of the U.S. Bureau of the Census. The best estimates of the prevalence of clinical low vision and blindness are based on data drawn from data pools collected in 1970 from a 16-state region that included approximately 31 percent of the population of the United States in that year. Although there are many questions regarding the accuracy and representativeness of this information, these data are still the best available for overall estimates of prevalence and causes across all ages.
What can be determined from all these data sources with some degree of certainty is that the most common causes of blindness in the United States for people of all ages are macular degeneration, cataracts, glaucoma, and diabetic retinopathy (Vision Problems, 1994). In infants and toddlers, including those born prematurely, retinopathy of prematurity (ROP), optic nerve atrophy, and cortical visual impairment account for an increasing number of children with impaired vision.
Macular degeneration is most frequently age related; the more years one lives beyond age 50, the greater the probability that macular degeneration will develop. Because the macular region of the retina and the fovea within the macula are responsible for carrying information about detail and color to the brain for interpretation, problems with blood supply or the removal of old cell tissue can interfere with the work of the cone cells that make up the macular area and degrade the fine resolution a healthy macula can provide. Whether macular degeneration occurs in childhood or is age related, there is no cure for it. In some cases, however, photocoagulation (the use of a high-energy light source to destroy tissue) may stop or delay further neovascularization, capillary seepage, and degeneration. In the case of macular degeneration, photocoagulation with a laser can destroy the fragile and abnormal blood vessels and capillaries that form and seep or leak blood.
As was explained earlier, a cataract is a lens that has become opaque, so that light rays can no longer pass through it into the vitreous and on to the cells of the retinal layer. Congenital cataracts are found in approximately 1 in 250 live births (Wright, 1997) and may be linked to maternal rubella during the first trimester of pregnancy, to an autosomal-dominant or other inheritance pattern, or to an infection like syphilis or cytomegalovirus. Some cataracts occur as a result of a systemic disease, but for many, the cause cannot be identified.
As with cataracts in young children, many attempts to identify the causes of blindness and low vision among the general population or among the school-age population end with "undetermined" or "not specified" or "not clear at this time." The glaucomas, except for those secondary to disease or trauma, fall under this category because the reason for the increase in pressure within the eye is not clearly understood. As was noted earlier, the significant feature of all types of glaucoma is a specific pattern of optic nerve damage, usually coupled with increased pressure in the eye. The eye care specialist can determine the status of the optic disc by using an ophthalmoscope to examine the back of the eye, as long as the cornea, lens, and vitreous are clear. A slit lamp and high-power handheld lens provide an even better magnified stereoscopic view of the optic disc and surrounding retina (Wright, 1997). But the underlying reason, the etiology, for the pattern of damage to the disc and the increase in intraocular pressure is usually not evident.
Infants with low birth weight (under 1500 grams, or 3.5 pounds), typically as a result of premature birth, are at risk of developing retinopathy of prematurity (ROP). For many such infants, oxygen is necessary to sustain life. About half the premature infants born at 26 weeks gestation and weighing only 700 grams now survive (Wright, 1997). But the oxygen given to sustain life, even when blood gasses and oxygen uptake are carefully monitored, may stimulate the growth of fragile and abnormal blood vessels in the underdeveloped retina. These tiny vessels may grow in the wrong direction into the vitreous or form a ridge instead of branching out and sometimes break down and cause hemorrhages. Skilled ophthalmologists can determine the stages of the development of ROP in a careful examination. If Stage 3+, indicating severe and critical disease, is determined, the treatment of choice is laser therapy or cryotherapy (use of extreme cold to freeze tissue). Without treatment, the tiny abnormal vessels that grow may exert enough pull on the retina to lead to retinal detachment. In some babies, ROP resolves with no apparent decrease in visual function as the babies grow and develop. In other babies, ROP is just one of a number of conditions related to prematurity and low birth weight; vision may or may not be compromised, as can be the case with other areas of motor and cognitive development. With the increase in the number of extremely premature infants of low birth weight who survive, the incidence of ROP is growing (Wright, 1997). Children with ROP seem to have a higher-than-normal incidence of myopia, astigmatism, and strabismus (Wright, 1997).
Optic nerve hypoplasia and optic nerve atrophy, in which the optic nerve either does not develop normally or develops but then degenerates, also contribute to the number of school-age children with visual impairments. Trauma, a tumor causing pressure against the optic nerve, and decreased blood supply may lead to the atrophy. In some cases, if the cause is eliminated, vision may improve. In other cases, the condition is associated with other congenital abnormalities (Vaughan et al., 1999).
Eye injuries account for many known instances of visual impairment among school-age youngsters, but are not considered a major cause of visual impairment because often only one eye is involved. Such injuries are a leading cause of preventable impaired vision, at least monocular impaired vision. Burns, flash burns, contusions, foreign bodies (such as pieces of glass or metal) embedded in the eye, motor vehicle accidents, sharp toys or pencils, sport ball injuries to the orbit when goggles are not worn, exposure to sunlamps, and other items and events are associated with eye accidents that may, but fortunately do not always, lead to blindness in the injured eye. Teachers, parents, and other responsible caregivers can help prevent instances of vision loss by being alert to these potentially hazardous objects and situations when youngsters are under their supervision.
Infectious diseases still account for cases of impaired vision in some children. Rubella, also called German measles, can cause damage to the eyes, ears, heart, and central nervous system of a developing baby if contracted by the mother during her first trimester of pregnancy. These systems and structures are especially vulnerable to infection during the early weeks after conception, and the rubella virus attacks the cells of the developing structures when they are most at risk during those first few months. The mother may not even notice any symptoms, but the virus apparently can interfere with the transcription of genetic information in the cells of the developing structures of her baby. The result can be hearing loss, cardiac malformation, cataracts, and mental retardation in the child. A vaccine to immunize individuals against the disease is available. Although it is of no value to those who have already had rubella or who have been affected by it, this vaccine can decrease the number of persons who may contract the disease and transmit it to any pregnant woman who has not been inoculated against it.
Other infectious diseases besides rubella that may result in damage to the eyes, either before or after birth, include toxoplasmosis, tuberculosis, and trachoma. Trachoma, a leading cause of preventable blindness in Africa, Asia, and the Middle East, is an infection of the conjunctive and cornea that can lead to corneal scarring if not treated but is preventable with the use of antibiotics. Infectious sexually transmitted diseases, such as AIDS and HIV, can lead to eye infections and inflammations with a significant loss of vision.
Tumors may cause damage to eye structures and may even necessitate enucleation (removal of the affected eye). Retinoblastoma, a life-threatening malignant tumor that occurs bilaterally in approximately 20 percent of the cases, usually appears before a child reaches his or her third birthday. At present, the treatment generally is enucleation (Wright, 1997). In selected cases when the tumor is small, radiation, chemotherapy, laser photocoagulation, and cryosurgery may be options (Wright, 1997).
Nystagmus is a condition that is frequently found in children and adults whose vision was impaired at birth or during the first two or three years of life. It is an involuntary, rhythmical oscillating movement of one or both eyes from side to side, up and down, in a rotary pattern, or in some combination. The movement can be pendular and regular or jerky with comparatively slow movement in one direction and a rapid return. Nystagmus may accompany other ocular conditions, usually those that have existed for an extended period; it is not characteristic of corticalvisual impairment or post-optic chiasm disorders such as tumors or cerebrovascular disease (Wright, 1997). Treatment is directed at the primary condition if the nystagmus accompanies another condition. Some children and adults may turn or tilt their heads in an effort to decrease the speed, amplitude, or duration of the eye movements. Under certain circumstances, nystagmus can be elicited in individuals with normal vision, for example, by looking as far to the side as possible for a period or by watching a rotating drum marked with alternating dark and light bands or looking at railroad cars moving along the tracks. Since nystagmus can be stimulated in the normal eye with intact ocular motor and visual pathways, some visual tests make use of rotating drums with bands of various thicknesses and intensity in an effort to elicit nystagmus as evidence of some degree of visual function.
As was mentioned earlier, some children have cortical visual impairment, meaning that their decreased visual function is the result of a lesion located somewhere along the visual pathways between the lateral geniculate body and the occipital cortex in the brain. Many of these children (although not all of them) have additional disabilities, including cognitive delays, epilepsy, and cerebral palsy. The incidence of such children seems to be increasing, according to data being collected by Ferrell and her colleagues (Ferrell et al., 1998). Students with cortical visual impairments do not appear to have eye problems, but they demonstrate difficulties with depth perception, the visual detection of objects, estimation of distance, and spontaneous visual learning (Groenveld, Jan, & Leader, 1990).
Many general systemic diseases that affect the vascular, neurological, and metabolic systems can put the eyes at risk. Diabetes is a prime example of a metabolic disorder that can result in retinopathy with changes in the retinal blood vessels, hemorrhages, and proliferation of blood vessels. The occurrence of diabetic retinopathy seems to be more closely related to the duration and control, rather than to the severity, of diabetes. About 50 percent of those with diabetes for 7 years show some signs of retinopathy, and approximately 90 percent show signs after 20 years. Damage to the retina can occur as a result of tiny leaks from the retinal capillaries that weaken those capillaries or the growth of new but abnormal capillaries in an attempt to bypass areas of poor perfusion (blood flow). Children who develop early-onset diabetes may not experience eye difficulties until years after they leave school, but regular eye examinations need to be a vital part of their ongoing health care.
Another systemic condition that is frequently accompanied by difficulties in ocular movement is cerebral palsy, a disorder of voluntary movement and posture caused by damage to the brain before, during, or soon after birth (Rosen, 1998). Considered a multidimensional disorder because of the variety of daily functions that can be affected by the associated problems, such as seizures, impaired communication, and mental retardation, cerebral palsy also carries with it a high incidence of sensory impairments. Many youngsters have difficulty tracking or visually fixing on objects, and may have strabismus or their eyes may show nystagmus.
Retinitis pigmentosa (RP), an inheritable condition leading to progressive deterioration of the retina with decreased night vision and restricted fields of vision, can occur in just the eyes or may be associated with systemic conditions (Understanding Retinitis Pigmentosa, 1996; Wright, 1997). The rods are usually affected first. The loss of night vision typically begins in adolescence, and eventually the cones may be affected. The severity of the visual loss is apparently related to the type of inheritance pattern, with the sex-linked pattern leading to more severe loss (Newell, 1996; Wright, 1997). An individual with Usher syndrome has congenital neurosensory deafness and then later manifests RP. About 90 percent of individuals with Lawrence-Moon-Biedl-Bardet syndrome, which includes such characteristics as mental retardation and obesity, have RP. Although there is no cure for RP, some researchers have suggested that vitamin A supplements and avoidance of exposure to bright sunlight may delay the deterioration. Specialized physiological testing may detect signs of RP in the retina before a person is aware of any decreased function, but since there is no cure and no certainty of the eventual degree of vision loss, some professionals have been reluctant to suggest testing when nothing can be done to improve the condition. Others have noted the value of genetic counseling, along with the use of appropriate visual aids to enhance remaining vision.
Multiple sclerosis, a chronic neurological disease that appears usually between age 15 and 50, is sometimes difficult to diagnose unless magnetic resonance imaging reveals the typical indicators in the brain. It is a demyelinating disease of the central nervous system, but its course is unpredictable, and there are remissions and relapses. If the optic nerve is involved, vision may decrease and then improve, just as motor weaknesses, ataxia, tremors, and other physical symptoms come and go (Vaughan et al., 1999).
Along with multiple sclerosis, disorders of the thyroid gland, certain vitamin deficiencies, and other systemic diseases can lead to severe eye problems and vision loss. Although the total number of children affected may be relatively small, some cases do occur and are of extreme significance to the individuals who are affected.
Parents and teachers may be the first to suspect that something is wrong with a child's eyes. They may notice the child having difficulty accomplishing visual tasks at home or school. Difficulties with vision might also be identified through screening procedures that are commonly conducted in preschool and kindergarten. These screenings provide a gross judgment about how the child sees. If the child's performance on the screening task, such as naming objects at a set distance and size or matching shapes at a set distance and size, does not meet the criteria for "passing" the screening, the child is referred for a clinical eye examination. The real value of such screenings, usually conducted by trained volunteers, lies in the follow-up of those children who are referred for professional eye examinations. If there is no follow-up to ensure children are examined by eye specialists, then the screenings serve little purpose. Screenings can be thought of as sorting children into two groups: those whose visual acuity is probably within the normal range and those who need further evaluation to determine their visual status.
Various eye specialists—ophthalmologists, optometrists, orthoptists, and opticians—are trained in the use of diagnostic procedures to determine specific causes or rule out problems with children's eyes. An ophthalmologist is a physician who concentrates on the diagnosis and comprehensive treatment of defects and diseases of the eye, prescribes lenses, performs surgery, and uses drugs and other forms of medical treatment. An optometrist is a trained and licensed eye specialist who examines the eyes to detect signs of disease, measures refractive errors and muscle disturbances, and prescribes and fits lenses. In many states, optometrists are permitted to use certain medications to provide a better view into the eye, just as ophthalmologists do. Some optometrists and ophthalmologists specialize in the evaluation of patients for the possible use of low vision devices (for further information on low vision devices, see Chapter 14 of Foundations of Education: Instructional Strategies for Teaching Children and Youths with Visual Impairments). In some low vision clinics, both of these eye specialists are on the staff, as is an orthoptist, who is trained to give eye exercises in cases of muscle imbalance, amblyopia, diplopia, and suppression of foveal stimulation, and who works in conjunction with these eye specialists. An optician is a technician trained to design, fit, and dispense lenses according to optometrists' or ophthalmologists' prescriptions, fit contact and spectacle lenses, and adjust spectacle frames to the wearers. Some opticians may also make low vision aids available (Common Eye Problems, 1997).
Anyone who is referred to an eye specialist generally receives a clinical examination that includes a history, a discussion of why the person has come for an examination, and the physical examination itself. Sometimes special procedures are necessary to determine the integrity of the eye structures.
Eye specialists use a wide variety of diagnostic procedures and tests to assess the integrity, health, and function of the eyes and the optic pathways that lead to the back of the brain. The following are among the more common procedures, but an eye specialist decides which of these or other more sophisticated procedures are called for in a given situation.
The eye specialist notes any symptoms the person exhibits—pain, double vision, tearing, dryness, blind spots, halos around lights, floaters, photophobia (sensitivity to light), poor night vision, blurriness, difficulty reading, and so forth. In combination with a careful and detailed history, this is a rich source of information and helps the eye specialist determine the type and extent of further testing.
The eye specialist pays attention to the size, shape, position, and color of the eyes; the presence of discharge or inflammation; and whether the pupils are of equal size.
Typically, visual acuity is checked with the Snellen Letter or E Chart,the latter preferred for children and others who cannot accurately identify letters. In some cases, special equipment with symbol cards showing objects or shapes other than letters may be used. Acuities are taken for both distance and near vision.
The normal field of vision covers approximately 150 degrees on the nasal to temporal axis and approximately 120 degrees on the superior to inferior axis (Wright, 1997). (See Sidebar 3.1 for a further explanation of visual field.) A number of different confrontation techniques using grids and screens, as well as computerized software that presents points of light at predetermined spots and intensities and mechanical devices that allow points of light to be presented at selected locations, can be used to determine defects in the central field and the peripheral field of vision in each eye (Wright, 1997). The eye specialist selects the one most appropriate for the specific situation.
Use of a slit lamp with a high-power magnification and illumination provides the eye specialist with a more detailed view of the eyelids and the inside of the eyeball.
Intraocular pressure may be measured by several techniques. Tonometers require the insertion of a local anesthetic solution in the eye. Most glaucoma screening programs use a technique that does not involve touching the eye directly. This type of screening procedure is satisfactory for initial screening purposes, but indentation and applanation tonometry are considered more precise for clinical purposes. In applanation tonometry, as with the Goldmann tonometer, a variable amount of force is applied directly to the anesthetized cornea to determine how much force is needed to produce a predetermined amount of indentation. In indentation tonometry, such as with the Schoitz tonometer, weights of various amounts are applied to the corneal surface and the amount of indentation is measured. The purpose of all these techniques is to determine the pressure within the eyes, a critical element in the diagnosis of glaucoma.
The ophthalmoscope provides a good view of the retina and the internal structures of the eye. The ophthalmologist routinely instills a drug that dilates the pupil but does not affect accommodation, so as to obtain visual access to the peripheral retina and regions of the fundus.
Particularly in suspected cases of glaucoma, gonioscopy is done to examine the angle of the anterior chamber by direct visualization. A local anesthetic, special lighting, a special microscope, and a special lens, called a goniolens, are required for the examination.
A dye, frequently fluorescein, may be instilled in the eye to reveal corneal abrasions and irregularities, to help locate foreign bodies in the eye, or to determine the drainage of tears.
Tests for color perception require a person to identify patterns, often numbers, made up of colored dots on a background of dots of a different color. The colors are selected so patterns in the frequencies of these colors are not discernible to persons with defects in color perception. Red-green confusion is the most common problem in both males (8 percent) and females (0.4 percent), whereas blue-yellow confusion is rare in both sexes (Newell, 1996; True Colors, 1996). Problems with color vision are almost always sex linked and transmitted by mothers to their sons.
To detect blind spots or scotomas in the central visual field, an eye specialist may ask a person to focus on a dot on a grid chart and then indicate any area of distortion on the grid or absence of a grid pattern. Blind spots in the field can indicate where on the retina or in other structures of the eye there may be damage, as with macular degeneration.
Once an individual has had a clinical examination and has been fitted with the best possible corrective lenses, his or her functional vision may still not be efficient for some tasks the person would like to do. In these cases, a low vision evaluation may result in the identification of optical aids, magnifiers, closed-circuit television (CCVT), or other adaptations that can enhance visual performance. Sidebar 3.2 summarizes information on three types of eye examinations in addition to screening: clinical, functional, and low vision.
In most programs for school-age youngsters with visual impairments, an eye examination report completed by an eye specialist, either an ophthalmologist or an optometrist, is required annually for each student who is in or referred to the program. Many school districts, intermediate units, counties, and administrative units have designed forms for this annual report or have adopted or modified the form suggested by PBA (Prevent Blindness America). All annual reports for a student should be retained in the student's permanent record folder.
The information in an annual eye report is clinical and may not appear to be directly useful in determining instructional objectives and teaching strategies. Nevertheless, much of the information is valuable to teachers of students with visual impairments. Therefore, teachers need to be able to interpret the information in these reports to determine what may be relevant for instructional purposes and the signs that may indicate a significant deterioration or improvement in a child's eye condition. The following sections describe the various types of information that are reported on most annual report forms, which are illustrated in Figure 3.8.
A record of the child's name, age, address, and school placement identifies the child for whom the report is requested. Although in some cases, a parent takes the form to an eye specialist, who completes it at the time of the child's eye examination, in most cases, the report form is mailed to the eye specialist, along with a cover letter that explains why the report is needed. In either case, the identifying information should be filled in before the eye specialist gets the form. If the teacher accompanies the child and parent to the examination, he or she can present the form and request that the information for the other sections be reported at the completion of the examination (for a list of abbreviations that are commonly used in these reports, see Table 3.3).
The information on the eye report should be considered confidential, as is clearly indicated on most such forms, just as are any other records pertaining to the student's achievement, performance, health, behavior, and potential. Teachers, guidance counselors, and administrators who legitimately have access to the student's records must respect this confidentiality. Teachers who serve more than one school district or program need to investigate the procedures for maintaining confidentiality of information for all the programs they serve.
A complete and accurate history of previous health problems, treatments, and habits is considered to be one of the most critical components of any type of physical examination. Certain elements of the health history are particularly important for teachers of students with visual impairments to know about, not only for what they may contribute to a better understanding of the present condition of the student who is being examined but for what they suggest about the student's record of past eye care, the parent's follow-through with care and the possible implications for other family members.
One of the first questions asked when a health history is taken is the probable age of onset of the eye problem. A preschooler who has had little or no functional vision since birth has a considerably different repertoire of experiences (in both quantity and quality) from that of the child who has had normal vision since birth. Children learn much by just watching what others are doing, by visually exploring their surroundings, and by combining their visual examination with tactile investigations of objects they grasp and put in their mouths. Chapter 4 in this volume explains the development of vision and visual perception and its impact on cognitive, psychomotor, and psychosocial development. For the purposes of the present discussion, it is sufficient to point out that a child whose visual impairment and subsequent limitations in functional vision have existed since birth or shortly thereafter generally has learning characteristics and instructional needs that are significantly different from those of a child whose visual impairments and limitations in visual function developed after age 3 or 4 (Lowenfeld, 1981). The instructional needs of both children may call for modifications in the regular school program so these children can receive an appropriate education.
Other aspects of the health history are also important: previous eye problems that have required medical treatment, the age at which these problems occurred, and immediate or extended family members who have the same eye condition as the child or other eye problems that may be hereditary. At some point, the teacher of students with visual impairments needs to discuss with the parents whether other family members have been examined to determine if signs of any hereditary eye problems are evident.
Clinical measurements of vision include distance and near visual acuities with and without correction, a report of what correction has been prescribed for spectacle or contact lenses, a record of the field of vision for each eye, and information about any problem with color vision. Distance visual acuity is perhaps the measurement that many teachers check first, but all the other measurements, especially near vision, are also important for what they can suggest about functional vision that should be explored in the classroom.
In many programs, eligibility for services because of visual impairment is determined by considering the impact of the visual impairment on the student's ability to benefit from the educational program offered to classmates with no visual impairments. If the visual status after treatment and correction indicates the need for specialized educational programming, then the student is deemed eligible for services. The determination of eligibility for services is sometimes still based on the definition of blindness used for legal purposes: a restricted visual field (20 degrees or less in the better eye) or a limited distance visual acuity (20/200 or less in the better eye after correction). On the eye report, distance acuity may be reported with and without correction, although many children who wear corrective lenses are checked only while wearing their lenses. For those for whom no correction with lenses is possible, no acuity is reported in that portion of the report form.
Near vision acuity is extremely important. Acuity is reported with and without correction for near vision. When a child has been fitted with a low vision device, either some type of telebinocular or monocular aid for distance or a magnifying aid for near work, acuity is reported for vision when using that device.
At times, there may be a discrepancy between the acuity demonstrated in the clinical setting, and the acuity demonstrated at school, or at home. Many factors can contribute to this discrepancy, not the least of which is anxiety. Therefore, teachers need to consider various possible reasons for a student's poor performance in one setting but not in others, as in the following example:
A 5-year-old child in kindergarten was referred to a school program because the eye specialist found her visual acuity to be 20/200 OU and was concerned about her school performance. The teacher of students with visual impairments who received the referral note talked with the girl's kindergarten teacher and with the school nurse who knew the family and then spent time observing the girl draw, color, and do her reading readiness activities. It was clear that although the girl could copy shapes and letters accurately, she did not know the names of the shapes or letters. She appeared to have no problem seeing details or color and worked with materials at the normal distance from her eyes.
As the teacher gave the eye specialist the eye report form to complete when he next examined the girl, she mentioned that the girl did not yet know the names of the letters. When the eye report form was returned to the school several weeks later, the girl's visual acuity was reported to be well within the normal range for a 5 year old. During the first examination, the girl's acuity had been checked with the Snellen Letter Chart, and her performance was, of course, poor because she had not yet learned the names of the letters and could not match them to the shapes of the letters. When the girl had been checked with the Snellen E Chart during the next examination several weeks later, her performance improved dramatically.
It is important to note the distance from the visual target at which visual acuities are taken. For example, in the United States, distance visual acuity is generally based on a 20-foot test distance from the visual target. In some cases, when equipment like the Titmus vision screener is used, the 20-foot distance is simulated by adjusting the size of the target picture or letter. Visual acuity within the normal range is reported as 20/20, meaning that the individual identified the letter or shape at a distance of 20 feet. An acuity of 20/200 indicates that the individual identified the letter or shape at a distance of 20 feet that a person with normal vision could identify at a distance of 200 feet. If the individual cannot identify or even see the visual target from the 20-foot mark, then he or she can move closer to the target. Successful identification at a point closer than 20 feet would be recorded as, for example, 7/200, indicating the individual saw at 7 feet what the person with normal vision would see from 200 feet. These notations are not fractions; they indicate distance from the target and size of the target—two different values. Such information is helpful for some purposes, such as the determination of eligibility for services, but it does not reveal much useful information regarding visual efficiency and performance in a nonclinical setting.
Measurements of near visual acuity are generally reported in inches from the target, with 14 inches considered the standard (14/14); in centimeters from the target with 35 centimeters the standard (35/35); or sometimes in the size of type read, such as J4, a number shown on a Jaeger chart, a chart using graded sizes of letters or numbers for testing. Some eye report forms contain conversion tables that can be helpful in deciphering the specialist's notations if they are not in the anticipated figures. But again, these notations suggest where to start in learning media assessments or functional vision assessments; they do not show what the individual may be able to do in a particular instructional setting.
Visual acuity measures taken in a clinical setting that may be strange or anxiety producing, may certainly be different from those taken in the typical work setting at home or in school. They can give a good idea of visual function in that clinical setting. However, they do not necessarily reveal how efficiently a given child will use his or her vision at school or home. Thus, it is imperative for the teacher of students with visual impairments to determine how each student uses whatever vision he or she has at school; under what conditions the student can work best; and what modifications in the task, time limits, materials, setting, and lighting may increase the student's visual function. A summary of that information should be given to the eye specialist for review before the student's examination.
A student's prescription can provide information about the strength or power of the lenses the student is to wear and suggest factors the teacher of students with visual impairments and the general classroom teacher should keep in mind. Although a detailed description of the intricacies of prescription lenses is beyond the scope of this chapter, teachers may find the following basic information helpful.
Prescriptions for corrective lenses are reported in terms of the refractive power, that is, the bending power, of the lenses. The unit of bending power of an optical lens is measured in diopters. For a convex lens, one that converges light rays, as would be necessary to sharpen vision in a hyperopic eye, the shorter the focal distance, the more powerful the lens (see Figure 3.6). A convex, or plus, lens of +10D would have more refractive power than a +2D lens. The same is true for a concave, or minus, lens that diverges light rays, as would be necessary for the myopic eye; the shorter the focal length, the more refraction, in this case diverging, power the lens has. A -10D lens would be more powerful or stronger than a -2D lens.
Corrective lenses can be spherical or cylindrical. A spherical lens has the same refractive power on all axes, while a cylindrical lens has more power or less power along one axis than along another. Spherical lenses are prescribed for simple refractive errors, and cylindrical lenses are used to correct for astigmatism. The eye report form should provide space for recording the power of the spherical lens prescribed for each eye, any cylindrical lens, and the axis or direction in which the cylindrical power is to be set. If an individual has been given a bifocal lens, then that power is usually reported as OU Add +3D (both lenses will have an added power of +3 diopters for near vision, fixed typically for a comfortable reading distance). With advances in lens technology, correction for astigmatism, as well as bifocal adds, can sometimes be fitted in contact lenses.
What, then, is important for the teacher of students with visual impairments to note when examining the prescription portion of a student's eye report? A number of items should stand out.
The more power, generally the poorer the individual's vision is without correction and the more important it is for the lenses to be worn all the time. The exception to this rule is that some nearsighted students may remove their eyeglasses for near vision tasks, since their natural near vision may be sufficient.
This notation could suggest that a student's own lens had been removed. To confirm or reject this supposition, a teacher can check other portions of the report to see if the student is aphakic, which means that the lens has been removed or, in rare cases, did not completely form to begin with. It could also mean that the student is extremely farsighted. In such extreme cases, even distance vision after correction may not be good.
A -6D to -8D or stronger lens probably mean that the wearer is quite nearsighted.
When the difference between the refractive powers of the two eyes is large, as in the case of a student who is extremely farsighted in one eye and nearsighted in the other (anisometropia), the student may tend to favor one eye for some tasks, like reading. The reason is related to the difference in the size of the image received on the retina of each eye; a plus lens that converges light rays tends to increase the size of the image. A difference in size, if noticeable to the individual, may be resolved by favoring one eye for tasks that require near vision.
With astigmatism, it becomes even more important for students not only to keep their eyeglasses on but to keep the eyeglasses properly adjusted on their faces so the power of the corrective lenses matches accurately the power of the eyes on the various meridians. That is, the power of the lenses in the eyeglasses is positioned to provide the power the natural lenses need where they need it.
Ideally, all people who wear eyeglasses should have safety lenses that are shatter resistant and frames that are fire resistant. Some tragic accidents have been reported to Prevent Blindness America (PBA) as a result of lenses shattering, cutting the eyelids, or penetrating the cornea or deeper structures of the eye. Members of the PBA-sponsored Wise Owl Club and Wise Owl Jr. Club give ample testimony to the advantages of wearing safety lenses or safety goggles in laboratories and workshops. Each year, new members whose eyesight was saved in industrial or school laboratory accidents as a result of having worn safety lenses or goggles are added to the clubs. Many sports can also be safer when the participants wear safety goggles.
A great variety of low vision devices can be prescribed to help a student who is visually impaired use vision efficiently to perform certain tasks. Low vision devices include handheld, stand, and head-borne magnifiers for enlarging images; telescopes to see targets at a distance; electronic aids, such as CCTVs, voice output systems, and handheld cameras that enlarge text; nonoptical aids like markers, reading stands, large-print dials, and talking clocks; and filters to reduce glare and improve contrast (Faye, 1999). (For additional information, see Chapter 14, Foundations of Education: Instructional Strategies for Teaching Children and Youths with Visual Impairments.) If a low vision device has been prescribed or it has been recommended that the student should be evaluated at a low vision clinic to determine whether a device may be helpful, then the teacher of students with visual impairments needs to be certain either that the student actually has the device and knows how and when to use it or that the appointment for the low vision evaluation has been made and kept.
Some students have good central visual acuity but limited field or peripheral vision. The field loss may be in one or both eyes and may or may not be in the same location in each field. Students with impaired central vision may have patches or islands of useful vision of various sizes that require that they learn to redirect their gaze to permit rays of light entering the eye to stimulate that portion of their retina that is actually activated by light.
Usually the eye report form contains an illustration to indicate where the fields are restricted and to what degree. The eye specialist shades in the portion of the field that is constricted and the areas of the central field where there are scotomas (blind spots) or macular degeneration has resulted in reduced visual acuity. These field drawings can be useful to explain why students may turn their heads slightly when attempting to see an object or person directly in front of them (see also Figure 3.7).
Several types of tests, as mentioned in the discussion of specialized testing, can determine the extent of any field loss. The purpose of all of the tests is to identify where in the visual field for each eye the individual can see the test objects or light points. The results are generally used to make inferences about what portions of the retina of each eye are sensitive to light simulation. If the field is restricted for some other reason, as may be the case after retinal hemorrhage or optic nerve damage, the test results can indicate from where in the field the light rays can actually pass unimpeded through the various media of the eye to the retina and produce a reaction in the retinal cells.
Students with extensive field losses need to be alert to auditory and other cues that warn of people or objects that are present or approaching from the area in which visual cues are absent. They also must learn to move their heads and eyes to scan their environment in such a way that light rays can reach those portions of the retina that still function (see Figure 3.7).
Contrast sensitivity refers to an individual's ability to detect differences in light and dark parallel lines with predetermined changes in both the thickness and darkness of the lines. When visual acuity is tested with the Snellen Chart letters, lighting in the room where the test occurs is held constant and the visual targets, the letters or the letter E, are dark. What changes is the size of the letters. In contrast sensitivity, the visual targets are usually parallel lines. The light in the room is constant, but the lines on the test chart get thinner and lighter, decreasing the contrast and requiring more visual sensitivity to detect them. In some retinal and optic nerve diseases and with cataracts, the sensitivity to these changes decreases (Newell, 1996; Vaughan et al., 1999; Wright, 1997). Contrast sensitivity tests may yield useful information for functional vision assessments because they are sensitive to differences in the lightness and darkness (intensity) as well as size (frequency) of visual targets.
Visual capacity refers to an individual's visual endurance. It is one thing to read letters on a chart; it is another to do sustained reading for a long period. A person may be able to read regular or enhanced print, as with a CCTV or handheld magnifier, for only a few minutes before tiring. Clinical eye tests typically do not provide information about visual capacity. Visual capacity is mentioned here simply to stress its importance in determining how an individual uses whatever vision he or she has and to highlight that it is different from a measure of either near or distance visual acuity.
Problems discriminating color are much more common in males than in females (Newell, 1996; Vaughan et al., 1999). The complete absence of color vision (achromatopsia) is quite rare, 0.3 per 100,000 in males and lower in females (Jose, 1983), and most people who have difficulty interpreting color have normal visual acuity (Vaughan et al., 1999). Children and adults may confuse colors because their retinal cone receptors lack the pigment necessary or their cones are less sensitive, in general, to light waves and levels of light intensity and do not detect lengths of red-orange, yellow-green, or blue light waves when light waves strike them.
Usually, no serious learning problems are involved as long as a teacher and student are aware that the student has difficulty distinguishing various colors and hues. But beginning readers, for example, whose activities may include "reading" picture stories or drawing their own pictures to illustrate stories they make up or hear, may have difficulty using colors and color words appropriately, which may result in unusual confusion or combinations of colors.
Another area of concern is the recognition of color in traffic lights when a student must cross light-controlled intersections. A student who cannot clearly distinguish red from green can learn which color appears where on the traffic signal and what is correct behavior in response to the red, green, and yellow positions. One second grader who had such a problem and had learned that he was to cross only when the bottom (on the vertical sign) or right (on the horizontal signal) green traffic light was on, asked his teacher what he should do when the middle light switched on. He was advised not to do what his friends did, which was to run. That boy had taught himself to read the names of the colors that were printed on his crayons so he could color his pictures according to the teacher's instructions. He had difficulty, however, when no instructions were given and when the teacher based the questions asked during reading class on color aspects of the pictures, such as "What is the boy in the red shirt doing?"
If a student is suspected of having problems with color perception, the teacher of students with visual impairments should consider them when selecting instructional materials or discussing strategies with the student's general education teacher. Color deficiency can be a serious problem when a person is trying to follow color-coded directions or is seeking employment in occupations that require the best possible perception of all colors. School-age children's problems with color perception usually do not create major difficulties as long as the teacher understands the possible limitations and works with the students to find ways to compensate for them.
Some individuals have only one eye because of an injury or birth defect or have normal vision in one eye but limited vision in the other. A student who has poor or no vision in one eye may need assistance to develop compensatory skills but may not require modifications in the instructional program. For example, a student with a right temporal peripheral field loss may need to learn to be alert for sound cues that people or objects are approaching from that side and to turn his or her head in that direction to pick up visual cues. A student with poor central vision in one eye needs to determine where to hold reading materials so he or she can read with comfort and efficiency with the good eye. A student with useful vision in only one eye will need to be particularly alert for auditory signals from his or her blind side and needs to develop strategies to make up for the absence of depth perception. These strategies may include looking for shadows and intervening objects and using a knowledge of distance perspective to help determine the space between the viewer and other stationary or moving objects. Knowing specific situations (like running on a playground at recess, reaching for objects on a desk or shelf or items in the cafeteria line, or driving behind a pickup truck from which boards or ladders extend) in which it is particularly difficult to judge distance is also important.
Most eye report forms provide space for the eye specialist to report what condition is affecting the eyes at that time and what previous conditions—injuries, diseases, infection, and so forth—may have led to the present condition. For example, a student with degenerative myopia may be extremely nearsighted and at a high risk for retinal detachment or even secondary glaucoma. The degree of degeneration, as determined by a clinical examination of the sclera, optic disk, choroid, retina, and vitreous, may actually have little relationship to the severity of the myopia as measured by the strength of the correction, that is, the dioptric power, needed to bring visual acuity as close to 20/20 as possible. Although functional vision is a prime concern for instructional purposes, the degree of degeneration is a major focus in the clinical examination and in the determination of any restriction on physical activities. The teacher needs to be cognizant of the implications of eye conditions and should examine carefully any information on etiology.
Frequently, the eye specialist indicates if the condition is hereditary and what implications there may be for other family members. If the genetic patterns of transmission are known, the eye specialist may explain what they are. The teacher of students with visual impairments can use this information in discussions with the family of the importance of examinations for other family members who may be at risk because they manifest clinical signs of the condition or because they may be carriers, although not affected. In addition, during adolescence, a student may question whether his or her eye condition can be passed on to offspring and approach the teacher for an answer. Although the teacher is not likely to be trained in genetics or to be an expert on patterns of genetic transmission, frequency of penetrance, degree of expression, or determination of pedigree, he or she needs to know enough to respond to basic questions about dominant, recessive, and sex-linked patterns of transmission and to recognize when additional sources of information or a referral to a genetic counseling clinic may be necessary and helpful. Teachers of students with visual impairments are often called on to explain what certain diagnoses mean; how the eye is affected; or what the educational implications of a particular disease, defect, or hereditary condition are.
If the condition was caused by an injury or poisoning at school that occurred under circumstances that could have been avoided had adequate precautions been taken or supervision provided, school personnel need to do what is possible to prevent a similar event from happening in the future. As was mentioned before, teachers may be able to prevent needless eye injuries and possible loss of vision by anticipating thoughtless actions on the playground; noting where dangerous equipment or supplies are kept; recognizing the improper use or placement of sports, shop, and laboratory equipment; and insisting that protective goggles and safety lenses be used for risky instructional and athletic events.
Many times, the etiology of a student's eye condition is simply not known. Sometimes a condition can be given a name and clearly identified, as in the case of glaucoma or congenital cataracts, but the reason the condition exists in the first place may not be known at the time. Additional research on causes and treatment and more information about the effects of disease, infection, environmental pollution, nutrition, and lifestyle on the human body should lead eventually to the identification of ways to prevent or at least reduce the incidence of visual impairment among both children and adults.
The eye specialist should note for the student's file any information regarding the stability of the student's eye condition. If the condition may deteriorate, the teacher needs to know what signs the student may exhibit in school that could indicate a significant change. For example, a student with Marfan syndrome, an inherited disorder of the connective tissue that can cause subluxated (partially displaced) lenses, among other things, may complain of blurry vision, a possible indication of a dislocated lens. The presence of significant signs calls for immediate contact with the student's family to arrange for an examination by the student's eye specialist. It is better to err on the conservative side than to allow complaints that could indicate serious trouble to go unheeded.
The eye specialist usually reports what treatment has been recommended and when the student's next examination should be scheduled. The teacher should note this information and follow up with the parents to make certain the recommendations are carried out. If medication is to be administered during school hours, the teacher can help obtain the permissions that the school district requires for school personnel to give the prescribed pills or eyedrops. Some medications have side effects that can affect school performance; therefore the time that a student takes them should be scheduled so as not to disrupt school activities, if possible. Teachers should be alerted to any side effects, such as drowsiness or blurred vision, that may affect the student's energy level or quality of work and report these observations to the eye specialist.
Some eye conditions, like degenerative myopia, call for restrictions on certain types of physical activity, primarily activities that may involve hard physical contact with other players, such as football, field hockey, diving, dodgeball, and volleyball. In some cases, wearing polycarbonate spectacle lenses may reduce the danger to acceptable risk levels (but what is acceptable to one person may not be to another).
If limits on physical activity or impact are advisable, then such information needs to be communicated to the student's teachers, especially the physical education teacher. But just because physical contact sports are restricted or ruled out does not mean that the student cannot participate in other types of physical activities and athletic events. This message also needs to be conveyed to the student's teachers as well as to the student.
Eye specialists' recommendations for lighting in the classroom, seating for classroom work, type or mode of reading to be taught, and even school placements may be helpful, but they must be considered in light of the context in which they were determined—the clinical environment, which generally bears little resemblance to the facilities in the student's school. If the teacher of students with visual impairments can send a concise report of the student's current level of visual function in school to the ophthalmologist or optometrist, along with the eye report form, the eye specialist will have additional information that can supplement the clinical information gathered in the clinical setting. Working together, the student and teacher need to determine the most comfortable levels of ambient and task lighting for classwork, the placement of the desk or table and chair for efficient use of functional vision, and the best working distance from a task or visual target to the eyes.
Eye report forms frequently provide space for the eye specialist to report additional information about the student's eye condition or visual functioning. For example, for some individuals who are not able to see the test objects at any distance, notations other than those previously discussed measures that indicate distance from and size of the target (for example, 20/200) may prove informative: HM, to indicate the ability to detect hand movements; light projection, to indicate the ability to detect the direction from which the light comes; light perception, to indicate the ability to tell the difference between light and dark; and no light perception, to indicate no sense of light (Vaughan et al., 1999; Wright, 1997). In addition, as was mentioned earlier, near visual acuity may be reported in inches or centimeters from the target or the size of type, based on numbers on the Jaeger chart. Furthermore, information on restrictions in fields of vision, discussed earlier, needs to be given to and interpreted for students and their parents to confirm where areas of functional field are and to suggest directions of gaze that may collect more stimuli than a direct gaze could, especially if a student's central vision is not clear or sharp.
As should be clear from this discussion, much information of significance for educational and instructional purposes can be gleaned from clinical eye reports by teachers who have basic knowledge about the anatomy and physiology of the eye, conditions that can interfere with how the eye functions, and implications of eye disorders in school-age children and youths.
Over the years, the percentage of youngsters in school programs for students with visual impairments who have used braille as their primary reading medium has decreased. In the late 1950s, approximately 58 percent of the students who were registered with APH used braille as their chief mode of reading; in 1979, about 16 percent; and in 1997, about 8 percent. Many factors have contributed to this seemingly drastic reduction in the percentage of school-age students whose visual acuity is 20/200 or less in the better eye after correction and whose primary reading medium is braille. Among these factors are changes in the characteristics of some of the students being served. First, many students with disabilities were excluded from neighborhood schools in the 1950s in some states. Second, medical advances have led to the survival of more students with a variety of disabilities, including visual impairments. Third, the development of technology has enabled individuals with low vision (that is, those with severe visual impairments that, even with the best correction, interfere with the performance of daily tasks) to use optical and other aids to enhance their functional vision. Fourth, for many students with low vision who may not have had an opportunity to learn braille, braille reading may be more efficient than print reading. It is essential that a student's most effective way of accessing information be determined early and on a regular basis. For this reason, the performance of learning media assessments has become a critical part of the educational programming for students with visual impairments. (Further information on assessments for determining literacy media is presented in Chapter 4 of Foundations of Education: Instructional Strategies for Teaching Children and Youths with Visual Impairments.)
The previous section touched on the relationship of visual acuity and preferred reading media to illustrate that many factors can contribute to choice of reading medium. In this section, the discussion is broadened to explore the relationship between visual impairments and clinical information about the impairments, including visual acuity, and an individual's functional vision. It is important for teachers and other professionals who work with students with visual and sometimes additional impairments to understand the relationship and the fact that a visual impairment and clinical information may remain stable while visual function may fluctuate from day to day or even from task to task.
Hoover (1963), whose work with newly blinded veterans of World War II led to the development of the field of orientation and mobility (O&M), identified the major components of visual function as visual acuity measured for near, midrange, and distant objects; visual versatility, meaning accommodative power for switching from near to far to near; convergence, or the ability to focus both eyes on a single visual target; light-dark adaptation, color vision, and binocular vision; and visual capacity, including field of vision, lighting requirements and preferences, and the length of time an individual can sustain a visual effort. These factors and others discussed in this book play a role in determining the use of vision by an individual who has some vision. (It should be noted that a person who is functionally blind has no usable vision or, at best, can sense light and dark.)
The effects of visual impairment on functional vision and how an individual uses whatever vision he or she has at a particular time or for a particular task can rarely be predicted with much certainty because so many other factors enter into the situation—age of onset of visual impairment, present age, quality of the repertoire of visual experiences, presence of other disabilities, cognitive skills, risk-taking behaviors, and general psychological makeup, to name a few. Other factors in visual function are the specific task and the environment in which the task is to be performed, the amount and quality of available light, the degree of contrast between visual targets and surrounding objects or platforms on which they sit, color, glare, time allotted, the time of day, the individual's degree of interest or engagement in the task, and the time the individual is willing to spend on the task (Blanks by & Langford, 1993; Corn, 1983).
Although little may be altered regarding the visual impairment itself, the professional who understands the relationship of visual impairment to visual function should be prepared to look for environmental adaptations, instructional strategies, learning materials, and conditions for learning that may be modified to enhance a student's opportunities to learn visually as efficiently as possible. The critical factor in determining what can enhance functional vision is the skill of the teacher, counselor, parent, eye care specialist, or O&M specialist in observing the individual's behavior and documenting how the individual is using vision for learning. Careful observation and recording are the first steps in planning appropriate instruction for students who appear to be functionally blind.
Often the teacher of students with visual impairments recommends a low vision evaluation after clinical and functional vision examinations have been completed (see Sidebar 3.1). Among the purposes of such an evaluation are to help students learn to use the vision they have efficiently and to determine whether any low vision devices may further enhance functional vision for specific visual tasks.
This chapter has provided the groundwork—structures of the eye and their function, causes of visual impairments, and clinical information and its interpretation for instructional purposes—for understanding the process of seeing. The following chapters of this text will address the impact of visual impairment on growth and development through early childhood and adolescence and consider the educational implications accompanying this impact.
The field of vision refers to the area one can see without shifting one's gaze. Because of the location of the eyeballs in the orbits and the position of the orbits in the head, the normal field of vision is approximately 150 degrees horizontally and 120 degrees vertically (Wright, 1997). The nasal fields of vision from each eye overlap when the eyes look straight ahead, but not the temporal. Field defects are denoted as nasal, temporal, superior, and inferior, terms that refer to space, not to the retinal site. For example, if a person has right superior field loss in the right eye, some of the retinal cells in the inferior nasal retina of the right eye are not functioning adequately or their message is not relayed accurately to the appropriate occipital lobe of the brain. In other words, one way to check retinal function and integrity of the optic pathways is to check visual fields and infer retinal function and integrity from what a person reports in each quadrant of the field. See Figure 3.7 for an illustration of visual field.
Not really an examination but, rather, a gross sorting of individuals whose visual performance on a visual task meets a predetermined cutoff from those whose visual performance does not. Examples of screening include:
An eye examination done by an eye specialist to determine the health and integrity of the eyes. Typically, it involves a history; physical examination of the eyes, including a check of visual acuity, appearance of the eyes and surrounding structures; and certain tests, such as for glaucoma or macular degeneration. Treatment may or may not be recommended, depending on what is found. Treatment may include refraction to improve near and/or distant acuity, medication, surgery, referral to another specialist, follow-up care at a later date, or further specialized tests. The following are examples of specialized tests:
An assessment carried out by a low vision specialist, teacher of students who are visually impaired, or another individual who is knowledgeable about the development of visual skills and the effects of visual impairment on visual performance. The assessment considers visual performance in a variety of tasks and settings, visual fields, eye movements, and responses to such specific environmental conditions as light and color. The report of a functional vision exam should indicate recommendations for any modifications in instructional procedures, materials, strategies, or conditions for learning that can enhance the use of vision in school, as well as at home and in the community (Corn & Koenig, 1996).
An evaluation to determine if any low vision devices may enhance visual performance for specific tasks the individual has identified. A low vision evaluation should follow any clinical examination and treatment and may include elements of a functional eye examination if one has not been completed. This evaluation explores ways to carry out tasks, such as reading a book or a newspaper, watching television, locating places on a map, or reading labels on bottles or packages.
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