Age-related macular degeneration (AMD) is the leading cause of severe central visual acuity loss in 1 or both eyes in people over 50 years of age in the United States. It is estimated that 15 million North Americans currently have AMD. Prevalence of the disease is roughly 85%–90% nonexudative (dry) AMD and 10%–15% neovascular (wet) AMD. As a greater percentage of Americans live well beyond 60 years of age, more patients will become visually impaired from AMD than from glaucoma and diabetic retinopathy combined. Although the term macular degeneration may be used to describe any degenerative abnormality in the macula, the words age-related should probably be added only for eyes that demonstrate the nonneovascular or neovascular abnormalities described in the following discussion.
Normal aging results in a spectrum of changes in the macula, many clinically undetected, that affect the outer retina, RPE, Bruch’s membrane, and choriocapillaris:
Photoreceptors are reduced in density and distribution.
Ultrastructural aging changes occur in the pigment epithelium, including loss of melanin granules, formation of lipofuscin granules, and accumulation of residual bodies.
Basal laminar deposits accumulate; these consist of granular lipid-rich material and widely spaced collagen fibers collecting between the basal lamina (plasma membrane) of the RPE cell and the inner aspect of the basement membrane of the RPE (Fig 4-3).
Progressive involutional changes occur in the choriocapillaris.
All of these changes represent aging but may not be part of AMD. Abnormalities associated with AMD that are not necessarily part of normal aging may be classified as nonneovascular or neovascular.
Population-based studies have shown that most patients with AMD have only nonneovascular abnormalities, such as drusen, focal hyperpigmentation, or geographic atrophy (RPE degeneration). These patients are usually asymptomatic or may have a mild decrease in vision and/or metamorphopsia. The advanced atrophic form may have central or pericentral scotomata. Severe visual loss from AMD usually occurs in individuals with neovascular abnormalities or subfoveal geographic atrophy of the RPE.
The risk of AMD increases with age. In the Framingham Eye Study, 6.4% of patients aged 65–74 years old and 19.7% of patients older than 75 years had signs of AMD. The Eye Disease Research Prevalence Group reported that, in the US population, the overall prevalence of drusen 125 µm or larger in 1 eye is 6.12% and the prevalence of late AMD is 1.47%, with the majority of these patients having CNV (1.02%). Similar to the findings of the Framingham Eye Study, the group found that patients over 80 years old had a 6-fold higher prevalence than patients aged 60–64 years. Unfortunately, the impact of AMD will continue to increase as the population ages. It is estimated that the number of patients with AMD will increase by 60% by the year 2020.
Other risk factors for AMD include positive family history, cigarette smoking, hyperopia, light iris color, hypertension, hypercholesterolemia, female gender, and cardiovascular disease.