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An Overview of Common Eye Conditions

Editor's Note: Editor's Note: Sara Bennett is a certified rehabilitation teacher of the blind and vision-impaired. This article is compiled from information taken from the websites of the CNIB and RNIB, as well as from the book, "The Human Eye", by Kay Ferrell and Naomi Tuttle, the Hadley School for the Blind, 1996.

Blindness takes many forms. While legal blindness is generally defined as visual acuity of 20/200 or less with best possible correction in the better eye, or a visual field of 20 degrees or less, vision loss varies among people in degree, cause, age of onset, prognosis and treatment. This article attempts to outline some of the most common eye conditions today, starting with the eye's anatomy.

The human eye and vision system are often compared to a camera--with a shutter, lens and film. As with an automatic camera, light enters the eye, an image of what is seen is projected, interpreted and produced.

Light enters the eye through the pupil, which is the black hole in the front of the eye. The coloured part of the eye, known as the iris, acts much as a camera's shutter, opening and closing over the front of the eye, causing the pupil to either contract or dilate. In good light, the iris "closes", causing the pupil to contract, thus allowing less light into the eye. Conversely, in dim light, the iris "opens", causing the pupil to dilate, enabling more light to enter. Overlaying the pupil and iris, on the surface of the eye, is a clear tissue called the cornea. The cornea directs light into the eye. Toward the front of the eye, fluid known as aqueous humor circulates and nourishes the eye.

Once inside, light is further refracted by the clear lens, which is behind the iris in the centre of the eye, and suspended from the eye's rim. The lens directs light onto the retina at the back of the eye by changing shape, a process known as accommodation. Toward the back of the eye, fluid known as vitreous humor fills the eye and helps to provide its shape.

The retina is the innermost layer of the wall at the back of the eye. It is comprised of light-sensitive cells called rods and cones. Rods, primarily located on the outer edge of the retina, are responsible for night vision and peripheral vision. Cones, on the other hand, mostly lie in the centre of the retina (macula), and are responsible for fine, detailed, colour, central vision. Like a camera's film, the retina receives the light or image, and then converts it into electrical impulses. These impulses are transmitted via the optic nerve to the visual cortex in the brain. This part of the brain receives and interprets the image, thus producing "vision".

A structural or functional anomaly with any part of the eye or vision system can cause blindness or vision impairment.

The most common cause of corneal scarring in the world is trachoma, an inflammation of the eyelid's lining, caused by a micro-organism. Usually found in the developing world where poverty, overcrowding and inadequate sanitation exist, this condition is contagious. Vision becomes increasingly blurred as scarring of the normally clear cornea progresses. Improved hygiene, antibiotics and possibly corneal transplant surgery serve as preventive and treatment measures.

Glaucoma is one of the most common eye conditions in North America, and results from the blockage of the circulation of aqueous humor. An abnormal build-up of pressure within the eye damages the optic nerve. It can be congenital or adventitious. Initially, in open-angle glaucoma, peripheral vision is gradually affected, and vision loss is unnoticed. Once vision changes are detected, the damage is irreversible. Central vision is usually the last to be affected. In closed-angle glaucoma, vision lossis sudden. Depending on the type of glaucoma, medication or surgery may be used to relieve the intraocular pressure. The best treatment, however, is prevention--through regular eye exams that monitor eye pressure.

The most common condition affecting the lens is a cataract. This clouding of the eye's lens, once begun, cannot be prevented. It may be congenital--sometimes as part of a genetic syndrome or resulting from an infectious disease of the mother--or it can be acquired--through age, trauma, medications, infections or other eye diseases. In most cases, symptoms progress slowly, with vision becoming blurry, colour perception diminishing, and problems with glare (photophobia) occurring. Surgery is usually performed to remove the cloudy lens and replace it with an artificial implant.

Retinopathy of prematurity is a major cause of new blindness in babies in the developed world. Unknown before the use of high oxygen levels for premature, low birth weight babies, this condition stems from the indiscriminate growth of retinal blood vessels. Vision levels can range from near normal to total blindness depending on the retinal areas affected before diagnosis and treatment. Possible interventions range from observation and monitoring, to cryotherapy and laser treatment to allay the retinopathy's progression. Milder forms may regress spontaneously.

Diabetic retinopathy also involves abnormal growth of retinal blood vessels. Fluctuating blood sugar levels change the structure and function of these vessels. This condition's most common form is non-proliferating, which affects only small vessels, and may not affect vision. The proliferating form, on the other hand, involves the retina and vitreous, and may lead to severe vision loss. In this form, the blood vessels grow and bleed into the vitreous. Laser treatment to seal the blood vessels, a victrectomy to replace the vitreous, or increased control of the diabetes, may be used to minimize vision loss. Regular eye exams are important. Diabetic retinopathy is a leading cause of blindness.

Finally, macular degeneration, estimated to be the most common form of vision loss in Canada, involves the loss of central vision. The "dry" form involves distortion of the macula, and gradual, almost unnoticeable, vision changes. The "wet" form, however, involves abnormal growth and leaking of retinal blood vessels, with quick and often severe vision loss. Problems may occur in reading and driving. Blurring, distortions in colour, haziness and blind spots or scotomas develop. People can usually use their peripheral vision, however, or look around the blind spot. Age and genetics are primary contributing factors in this eye condition. Certain vitamins may help the dry form and again, laser surgery and other therapies may be used to seal leaking blood vessels. Research is underway to develop other possible treatments.

These are just a few common eye conditions. Others are discussed, to a greater or lesser degree, elsewhere in this magazine. Whatever the eye condition or situation, however, people adapt and live healthy and productive lives.

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