Eye disorders

Disorders of form and function that affect the eye. Disorders of form affect the way the eye looks or feels; disorders of function affect vision. Normal vision requires proper performance from the entire visual system, from the precorneal tear film to the occipital cortex. See also Eye (vertebrate); Vision.

The eyelids contain many glands that are susceptible to acute or chronic infection. Acute infection produces a hordeolum, or sty, which is a localized nodule up to several millimeters in diameter swelling may develop rapidly. Chronic infection results in a granulomatous nodule, or chalazion. In many cases, surgical removal is indicated.

Conjunctivitis is an inflammation of the conjunctiva, regardless of cause, which may include viral, bacterial, or chlamydial infection; mechanical or chemical trauma; allergy; and so on. The symptoms of conjunctivitis depend on the cause and generally include conjunctival redness (hyperemia), swelling (chemosis), mild to moderate discomfort, and tearing. If the conjunctiva is the only structure involved, vision is usually affected minimally or not at all. Infectious conjunctivitis is often termed pink eye because of the bright red appearance of the conjunctiva. Bacterial infections are usually responsible for pink eye in children. Infectious conjunctivitis in adults is commonly caused by viral agents. Chlamydial infections are responsible for inclusion conjunctivitis and ophthalmia neonatorum in the United States and trachoma in many arid regions of the third world. The initial infection involves the conjunctiva, but ultimately trachoma results in blindness due to a scarred, vascularized, opaque cornea. See also Visual impairment.

If the cornea and conjunctiva are both involved, the condition is referred to as keratoconjunctivitis. Adenovirus infection causes keratoconjunctivitis. If lubrication by the lacrimal glands or accessory lacrimal glands is interrupted by infection, trauma, or autoimmune disease, one of the dry-eye syndromes may ensue. The symptoms include a mild foreign-body sensation, light sensitivity (photophobia), and gritty feeling in the eyes that progresses throughout the day. Paradoxically, increased tearing (epiphora) can be a symptom of dry eyes.

Glaucoma is a serious eye disorder with many subclassifications. It is characterized by intraocular pressure sufficiently high to cause characteristic damage to the optic nerve. See also Glaucoma.

The leading cause of blindness in the world is cataract, an opacity within the crystalline lens. The overwhelming majority of cataracts have no specific cause and are associated with aging. Effective therapy almost always involves surgical removal of the lens. See also Cataract.

Degeneration of the retina produces a painless distortion or loss of vision. The function may be disrupted in many ways. The retina itself may be affected by infectious processes. Cytomegalovirus retinitis, for instance, is a frequent cause of visual loss in individuals with acquired immune deficiency syndrome (AIDS). Primary retinal degeneration may occur in many forms. Retinitis pigmentosa tends to manifest itself early in life and progresses from peripheral to central visual loss; prognosis depends in part on the hereditary pattern of the disease. Involutional macular degeneration is fairly common in the elderly. It affects the central vision first and rarely spreads to the peripheral vision. Between these two extremes is a very large number of progressive retinal dystrophies and degenerations, involving central or peripheral vision or both. Although the ultimate visual loss is variable, it is often severe.

Retinal detachment occurs when the sensory layers of the retina are separated from their supporting foundations. It is classified as rhegmatogenous (caused by a retinal hole) or nonrhegmatogenous. Rhegmatogenous retinal detachment occurs spontaneously or following trauma. Nonrhegmatogenous retinal detachment occurs as a final stage of such pathologic conditions as retinopathy of prematurity or diabetic retinopathy. The symptoms of retinal detachment are a painless and sudden segmental or total visual loss in one eye. Treatment is aimed at reestablishing the connection between the sensory-neural retina and its supporting structures.

Diabetic retinopathy is a common cause of severe retinal disease. Research has developed many methods of interrupting the progression of diabetic retinopathy. The most effective include laser therapy for localized retinal edema, hemorrhage, and neovascularization, and vitrectomy for late-stage diabetic retinopathy with vitreous hemorrhage and retinal traction.

Human vision is dependent on the successful interaction of optical structures in the eye. When these structures malfunction, vision disorders occur. The key to treatment and resolution of these disorders is early detection through regular eye exams and prompt consultation with an ophthalmologist when problems occur.

The best way to describe how vision works is to use the analogy of a camera. The pupil manages the incoming light rays, opening and closing—like a camera shutter—according to the amount of light available. These light rays are progressively refracted and focused by three structures: the cornea, a transparent, convex cover over the iris and pupil in front of the eye; the lens, a spherical body behind the cornea, and the vitreous humor, a gelatinous substance that fills the back of the eyeball. It is important that the rays be in sharp focus when they reach the retina, a sensory membrane that lines the back of the eye and acts like film in a camera. The retina converts the light rays into electrical signals that are sent to the brain by way of the optic nerve. The brain then translates these electrical signals into what we know as sight.

Refractive Errors. The most common vision disorders are refractive errors—specifically nearsightedness, farsightedness, and astigmatism. In each case, the eye does not refract the incoming light properly, so the image is blurred. While they are not diseases, refractive errors affect every age range and comprise the largest treatment effort of ophthalmologists. Refractive errors can be successfully corrected with eyeglasses, contact lenses, and laser refractive surgery.

Cataract. A cataract results when the normally transparent lens of the eye clouds, blurring vision. Most cataracts are age-related, advancing slowly and progressively until functional blindness occurs. Cataract cannot be prevented or cured with medication or optical devices, but it can be successfully treated through a surgical procedure that removes the damaged, natural eye lens and replaces it with a permanent, intraocular lens implant. The procedure has over a 90 percent success rate. After refractive errors, cataract is the most common vision disorder.

Macular Degeneration. Located in the retina, the macula is responsible for central vision. When people have macular degeneration, they can no longer bring the center of the picture they see into focus. The most common type of the disease is agerelated, and there are two forms: "wet" and "dry." Whereas the wet form comprises only about 10 percent of cases, it causes the greatest vision loss, striking quickly and without warning as a result of erupting blood vessels. The dry form is characterized by a slow, progressive loss of vision from the thinning and tearing of the macula. Although both forms are being extensively researched, definitive causes and treatments have not yet been identified. Age-related macular degeneration is the leading cause of blindness in most developed countries.

Glaucoma. Glaucoma is a disease of the optic nerve. If the aqueous humor (the clear fluid that fills the front of the eye) does not drain properly, intraocular pressure builds, damaging the optic nerve and causing blind spots to develop. When the entire nerve is destroyed, blindness results. If glaucoma is detected and treated in the early stages, loss of vision can be averted. However, the disease is chronic and cannot be cured or reversed. Unfortunately, the early stages are symptomless. Once symptoms occur, usually manifested by loss of peripheral or side vision, irreversible vision loss has already taken place. Treatment consists of medication and/or surgery, depending on the type of glaucoma, the patient's medical history, and the stage of the disease. Glaucoma is the leading cause of blindness worldwide and the second-leading cause in developed countries.

Diabetic Retinopathy. Retinopathy is a side effect of diabetes and occurs as a result of fluctuations in the body's blood sugar, a daily problem for diabetics. When blood sugar fluctuates over time, it affects the blood vessels in various parts of the body, including the retina of the eye, where the blood vessels can break and bleed, causing blurred vision. The longer a person has diabetes, the higher the risk of retinopathy; good diabetic control can forestall the disease, however. Signs of retinopathy often occur before symptoms appear. Treatment includes the use of laser photocoagulation to seal leaking blood vessels. Often undetected and untreated, diabetic retinopathy is the leading cause of visual disability among working-age people.

Retinal Degenerations. Retinal degeneration is an umbrella term for a number of hereditary and degenerative disorders that range from mild to profound vision loss and blindness. Retinitis pigmentosa is the most common type of retinal degeneration, affecting one in three thousand people. Its many forms have widely varied symptoms, and onset and progress of the disease can be slow or rapid. In general, symptoms occur in childhood or young adulthood. Patients complain of night blindness followed by loss of visual field. There is no treatment, though researchers are hopeful that genetic therapies may be possible in the future.

Strabismus. Unlike most other vision disorders, strabismus is a physical defect. One or both of the eyes are misaligned and point in different directions. One eye may look ahead while the other eye points up, down, in, or out. Strabismus is more common in children than in adults. In adults it can be a side effect of head trauma or brain disorder. Treatment may involve eyeglasses, an eye patch (in some cases), or surgery on the eye muscles.

(SEE ALSO: Diabetes Mellitus)

Bibliography

O'Toole, M. (1997). Miller-Keane Encyclopedia & Dictionary of Medicine, Nursing, & Allied Health, 6th edition. Philadelphia, PA: W.B. Saunders.

— BARBARA L. PAWLEY