On Sale | Home | Contact Us | News | About | Search | View Cart 

Low Vision Facts

Definitions of Terms Related to Blindness and Visual Impairment

Visual impairment is a term used by researchers that refers to a “functional loss of vision” (National Dissemination Center for Children with Disabilities (NICHCY), 2003). This term is generally not used as a clinical reference because of a lack of specificity regarding the intensity of impairment. Visual disabilities occur at varying degrees, dependent upon the level of disability and the cause of the impairment. Prevent Blindness America (Shoemaker, 2002) defines visual impairment as vision of 20/40 or less in the better eye with corrective lenses or a visual field of less than 20 degrees diameter. Various estimates for the total number of people in the U.S. with visual impairments are included in the demographics section of this document.

Low vision is a clinical diagnostic term that refers to vision in the range of 20/70 to 20/160 (American Optometric Association (AOA), 2004a). People with low vision often retain some portion of usable vision and are able to make use of assistive technology devices to perform activities of daily living (National Advisory Eye Council, 1998).

Severe visual impairment is primarily used to describe vision loss in the range of 20/200 to 20/400 (AOA, 2004a). The definition of severe visual impairment varies widely between studies. However, as the level for diagnosis for legal blindness is 20/200 or a visual field of less than 20 degrees, many of the people within this category may be labeled legally blind.

Legal blindness refers to a diagnosis of 20/200 or a visual field of less than 20 degrees. While the term “legally blind” has no medical significance it does mark an entry point for many low vision services and benefits. Therefore, it is an important distinction to note (Shoemaker, 2002).

The AOA (2004a) includes other levels of visual impairment including profound low vision at 20/500 to 20/1,000. Acuity levels of less than 20/1000 generally qualify as near total blindness. Total blindness is considered to be present when there is no light perception (AOA, 2004a; Shoemaker, 2002).

Common Eye Problems

There are many eye diseases and dysfunctions that can lead to visual impairment. The most commonly found disorders are myopia (near sightedness) or hyperopia (far sightedness). Myopia, which results from elongation of the eyeball, causes the image to fall in front of the retina instead of on its surface. Myopia affects approximately one-third of the American population. When an individual is nearsighted they have difficulty seeing objects at a distance, tasks that require near vision are unaffected. Hyperopia is caused by a slightly shortened eyeball that causes images to focus slightly behind the retina. Hyperopia affects vision for close tasks. It is estimated that one-fourth of Americans have hyperopia. Presbyopia, which generally affects people over 40, is believed to be caused by a hardening of the lens within the eye that makes it difficult to focus on objects that are close to the person’s eyes (Lee and Bailey, 2000). Common corrections for myopia, hyperopia, and presbyopia are corrective lenses or glasses. An astounding 150 million Americans spend $15 billion per year on corrective lenses supporting a $30 billion optical industry (Shoemaker, 2002). Many Americans are beginning to turn to surgery to correct both myopia and hyperopia.

Visual Impairment in Adult Populations

Glaucoma is a disease of the eye that is caused by a gradual degeneration of cells in the optic nerve. The loss of these cells leads to a gradual narrowing of the field of vision beginning at the periphery (Shoemaker, 2002).There is no known cause for the most common form of glaucoma, primary open angle glaucoma, but it is commonly believed to be associated with the inability of fluid to properly drain from the eyes causing an increased intraocular pressure (National Eye Institute (NEI), 2003). Primary open angle glaucoma affects more than 2.2 million people, ages 40 and over in America alone (Shoemaker, 2002). Ritch (2000) emphasizes the point that glaucoma does not result from a single eye disease that can be treated by simply relieving intraocular pressure, but is a “final, common pathway of many diseases that affect the eye.” Onset generally occurs later in life and people over 60 are six times more likely to get glaucoma than the younger population (Glaucoma Research Foundation (GRF, 2003). In some cases congenital glaucoma will be found in children as young as two and three. Not only do these children experience more signs and symptoms of eye disease, but these cases directly relate to an inability of fluid to drain from the eye (McLeod, Wisnicki, and Medow, 2000). Glaucoma is the leading cause of blindness among African-Americans, and Hispanic Americans over the age of 60 are also at an increased risk. Common symptoms include elevated inter-ocular pressure, optic disk cupping, and visual field loss (Shoemaker, 2002). Often people will lose vision from primary open angle glaucoma with little warning or noticeable symptoms (GRF, 2003). Major risk factors include advanced age, African or Hispanic descent, heredity, and prolonged smoking or steroid usage (Weih, Nanjan, McCarty, and Taylor, 2001; Liebmann, 2003; GRF, 2003). While there is no way to prevent glaucoma, it can be successfully treated if diagnosed early.

Age-related macular degeneration (MD) is caused by the malfunction of photosensitive cells in the macula which results in a loss of the central field of vision (Macular Degeneration Foundation, 2003). The Royal National Institute of the Blind (2002a) reports that additional symptoms may include a distortion of images especially at the center of the visual field; a darkened area in the center of an image; and diminished color perception. The peripheral vision of people with macular degeneration is unaffected. Although the disease affects nearly 1.7 million Americans over the age of 50, and is the leading cause of blindness in developing countries, no exact cause is known (Shoemaker, 2002; Schwartz, 2000). In rare cases, juvenile MD occurs as a result of mutated genes and is generally an inherited condition (MDF, 2003). Dry MD is the most common form of the disease, totaling approximately 85% to 90% of all cases. It is related to the development of drusen, or small yellow fat deposits, under the macula. These deposits cause the macula to thin and dry out which relates directly to the loss of vision (American Macular Degeneration Foundation (AMDF), 2003). There is no known treatment or cure for dry MD. Wet MD accounts for approximately 10% of all cases of MD in older Americans. It is caused by the growth of new blood vessels that bleed and leak fluid into the macula causing distorted vision and the formation of scar tissue (Shoemaker, 2002; AMDF, 2003). Laser therapy is often used as a treatment in wet MD, but this intervention does not guarantee that vision will be saved.

Optic nerve atrophy (ONA) is caused by tissue damage in the optic nerve resulting in either partial or profound loss of vision (Douglas, 2002). The causes of ONA vary widely. The most common type, ischemic optic neuropathy, most often impacts elderly Americans and is estimated to affect between 6,500 and 29,000 people in the United States. Arteric optic neuropathy, which is caused by poor blood flow to the optic nerve, affects approximately 1,000 people in the United States (Younge, 2001). In adults, ONA can be caused by trauma, toxic substances, radiation, and shock. Disease related causes include multiple sclerosis, brain tumor, or stoke (Douglas, 2002). In children, ONA is commonly caused by anoxia, tumors, hydrocephalus, heredity, and rare degenerative disorders (Blind Babies Foundation, 2002). Optic nerve atrophy reduces central vision acuity resulting in an inability to see detail. It also reduces the field of vision, causing images in the periphery to be lost. Finally, there will be a decreased reaction of the pupil to light sources. As ONA progresses, the pupil will cease to react to light altogether (Douglas, 2002). Once vision is lost through ONA, it cannot be recovered.

Diabetic Retinopathy is a disease of the eye that all people with diabetes should be aware of. It is a visual disorder associated with diabetes that causes retinal blood vessels to leak into the retina causing macular edema. In the advanced stages, called the proliferative stage, new blood vessels grow along the retina and into the vitreous humor (Shoemaker, 2002; National Eye Institute (NEI), 2000). It is estimated that nearly 5.4 million Americans, ages 18 and over currently have diabetic retinopathy. It causes over 8000 cases of new blindness annually, and is the primary cause of blindness for people ages 25 to 74 (Valero and Drouilhet, 2001). Vision loss from diabetic retinopathy generally worsens over time. It will begin with a blurring of the vision and as it develops will cause development of cloudy vision, blind spots, or floaters (Access Media Group, 2002). Careful control of diabetes and regular eye exams can delay the development of the disorder (Eyes on Diabetes, 2004). While diabetic retinopathy will often develop with no pain and minimal symptoms in its early states, it can be treated if it is diagnosed early. Photocoagulation is a treatment option for people with diabetic retinopathy. Photocoagulation is a laser surgery that is used to destroy leaking blood vessels that lead to macular edema (Shoemaker, 2002). In cases when the vitreous humor fills with blood, a virectomy is performed to remove the liquid and replace it with a salt solution (NEI, 2000).

Retinitis Pigmentosa (RP) is a progressive disorder that results from the degeneration of photoreceptor cells (rods and cones) of the retina. As these cells degenerate, gradual vision loss occurs. The disease often first occurs in adolescence and continues to progress as the individual ages often resulting in blindness in young adults. RP is a genetic disorder that is linked to more than 70 different genetic defects (de Beus and Small, 2003). Retinitis pigmentosa affects 50,000 to 100,000 people in the United States, making it a relatively rare disorder (Healthcommunities.com, 2004). In cases where the rod cells are primarily affected, vision loss generally begins as night blindness and progressive vision loss in the periphery results in tunnel vision (Foundation for Fighting Blindness (FFB), 2003). Another form of RP, known as rod-cone dystrophy is associated with loss of central vision and color perception. RP is caused by a group of hereditary disorders that include Usher’s syndrome, Leber’s congenital amaurosis, choroideremia, Laurence-Moon syndrome, and Best syndrome. There is no known cure for RP, although Vitamin A is said to slightly slow progression (FFB, 2003).

Cataracts result from a clouding (opacification) of the normally slightly yellowish lens of the eye (NEI, 2003). The loss of transparency causes light to be diffused as it enters the eye which impacts the clarity of the visual image (Chylack, 2000). The lens slowly develops a greenish and later a brownish tint which impedes the ability of light to pass through the lens (Mayo Foundation, 2002). Symptoms of cataract include blurred vision, light sensitivity, double vision, and an apparent fading or yellowing of colors. Night vision is generally impacted as is the amount of light needed to complete near tasks (American Academy of Ophthalmology (AAO), 2003b). While the most common cataracts are age-related, there are other types of cataracts, including secondary cataracts (resulting from other diseases, such as glaucoma or diabetes); traumatic cataracts (which may develop as a result of injury to the eye); or radiation cataracts (which develop as a result of exposure to radiation) (AAO, 2003b). Congenital cataracts, a very common cause of blindness in the pediatric population, can result in bilateral vision impairment if not treated meticulously (McLoed, Wisnicki, and Medow, 2000). Additional risk factors include prolonged use of corticosteroids, excessive consumption of alcohol, smoking, and excessive exposure to sunlight (Mayo Foundation, 2003). Many people develop cataracts as a result of the normal aging process. In the United States, cataract surgery is an outpatient procedure that replaces the damaged lens of the eye with an intraocular lens (Chylack, 2000). This surgery is performed on approximately one half of a million people each year (Annis, 2000). According to Chylack (2000) cataract surgery is the first line item in Medicare budget as it is a very common surgery for people over the age of 65. Medicare’s administrative bodies are attempting to reduce the amount of reimbursement allowed to doctors and hospitals for performing cataract surgery (Annis, 2000).

Credit for Industry Profile

This information was compiled by the Technology Transfer Rehabilitation Engineering Research Center (T2RERC) as part of the Industry Profile of Vision Impairment. Please visit the T2RERC website at http://cosmos.buffalo.edu/t2rerc/research/industry-profile/IP/vi-buy.htm for additional information or to purchase the complete Industry Profile on Visual Impairment. The T2RERC is funded by the National Institute on Disability and Rehabilitation Research of the Department of Education, under grant number H133E030025.

All links provided on this page are provided by the Rehabilitation Engineering Research Center on Technology Transfer (T2RERC) at the University of Buffalo. We cannot be responsible for Web pages that have moved.

Low Vision Facts - Anatomy and Physiology of the Eye


Figure 1: Diagram of the human eye identifying the various segments described below.
The image shows a cross-section of the human eye. It also identifies principle parts of the eye, such as the sclera, iris, cornea, pupil, lens, conjunctiva, vitreous, choroid, optic nerve, macula, and retina.

 

Illustration by Mark Erickson

The protective outer layer of the eye, sometimes referred to as the “white of the eye” is called the sclera and it maintains the shape of the eye. The front portion of the sclera, called the cornea, is transparent and allows light to enter the eye. The cornea is a powerful refracting surface, providing much of the eye's focusing power (Cassin and Solomon, 1997). Attached to the sclera are six extraocular muscles responsible for movement of the eyes (Bianco, 2002). The choroid is the second layer of the eye and lies between the sclera and the retina. It contains the blood vessels that provide nourishment to the outer layers of the retina (Cassin and Solomon, 1997). The iris is the part of the eye that gives it color. It consists of muscular tissue that responds to surrounding light, making the pupil, or circular opening in the center of the iris, larger or smaller depending on the brightness of the light (Pachler and Rizun, n.d.).

Light entering the pupil falls onto the lens of the eye where it is altered before passing through to the retina. The lens is a transparent, biconvex structure, encased in a thin transparent covering. The function of the lens is to refract and focus incoming light onto the retina for processing (Moorfields Eye Hospital, 2002).

The retina is the innermost layer in the eye. It converts images into electrical impulses that are sent along the optic nerve to the brain where the images are interpreted. The retina can be compared to the film of a camera. It is composed of light sensitive cells known as rods and cones interconnected by a complex mesh of neurons that provide early stage visual processing. Rod cells are primarily in the outer retina, do not discriminate colors, have low spatial resolution, support vision in low light (“night vision”), are sensitive to object movement and provide peripheral vision. Cone cells are densely packed within the central visual field, function best in bright light, process acute images and discriminate colors (Montgomery, 2002).

The macula is located in the back of the eye, in the center of the retina. Within the macula is an area called the fovea centralis. This area contains the highest concentration of cones, produces the sharpest vision, and is used to see details clearly (Moorfields Eye Hospital, 2002).

The inside of the eyeball is divided by the lens into two fluid-filled sections. The larger section at the back of the eye is filled with a colorless gelatinous mass called the vitreous humor. The smaller section in the front contains a clear, water-like material called aqueous humor (Discovery Fund for Eye Research, 1999). A circular canal, called the Canal of Schlemm provides a drainage system for the aqueous humor from the eye into the bloodstream. Blockages in the Canal of Schlemm are believed to be contributing factors in the development of glaucoma (Bianco, 2002).

The conjunctiva is a mucous membrane that begins at the edge of the cornea and lines the inside surface of the eyelids and sclera, which serves to lubricate the eye. Inflammation of this membrane results in conjunctivitis, commonly known as pink eye (Bianco, 2002; Cassin and Solomon, 1997).

References

©2003, Technology Transfer Rehabilitation Engineering Research Center, University at Buffalo

 
 
 
 

Sign up for your own ProStores storefront
 Commerce enabled by ProStores

Copyright © 2006 Next Level Assistive Technology. All Rights Reserved.