Atlas of pathophysiology, 2 Edition

Part II - Disorders

Sensory Disorders

Macular degeneration

Macular degeneration—atrophy or degeneration of the macular disk—is the most common cause of legal blindness in adults. Commonly affecting both eyes, it accounts for about 12% of blindness in the United States and for about 17% of new blindness. It's one cause of severe, irreversible, and unpreventable loss of central vision in elderly patients.

Age Alert

Two types of age-related macular degeneration occur:

·   dry, or atrophic—characterized by atrophic pigment epithelial changes; most commonly causes mild, gradual visual loss

·   wet, or exudative—characterized by subretinal formation of new blood vessels (neovascularization) that cause leakage, hemorrhage, and fibrovascular scar formation; causes severe, rapid vision loss.

Causes

Primary cause unknown

Possible contributing factors

·   Aging

·   Inflammation

·   Trauma

·   Infection

·   Poor nutrition

Pathophysiology

Age-related macular degeneration results from hardening and obstruction of retinal arteries, which probably reflect normal degenerative changes. The formation of new blood vessels in the macular area obscures central vision. Underlying pathologic changes occur primarily in the retinal pigment epithelium, Bruch's membrane, and choriocapillaries in the macular region.

The dry form develops as yellow extracellular deposits, or drusen, accumulate beneath the pigment epithelium of the retina; they may be prominent in the macula. Drusen are common in elderly patients. Over time, drusen grow and become more numerous. Visual loss occurs as the retinal pigment epithelium detaches and becomes atrophic.

Exudative macular degeneration develops as new blood vessels in the choroid project through abnormalities in Bruch's membrane and invade the potential space underneath the retinal pigment epithelium. As these vessels leak, fluid in the retinal pigment epithelium is increased, causing blurry vision.

Signs and symptoms

·   Changes in central vision due to neovascularization, such as a blank spot (scotoma) in the center of a page when reading

·   Distorted appearance of straight lines caused by relocation of retinal receptors

Diagnostic test results

·   Indirect ophthalmoscopy shows gross macular changes, opacities, hemorrhage, neovascularization, retinal pallor, or retinal detachment.

·   I.V. fluorescein angiography sequential photographs show leaking vessels as fluorescein dye flows into the tissues from the subretinal neovascular net.

·   Amsler's grid test reveals central visual field loss.

Treatment

·   Exudative form (subretinal neovascularization): laser photocoagulation

·   Atrophic form: currently no cure

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RETINAL CHANGES IN MACULAR DEGENERATION

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