Age-related macular degeneration – classification

Description

Age-related macular degeneration (AMD) is the most common cause of irreversible vision loss in people over 50 years of age in Western countries. Loss of central vision in AMD occurs following the deposition of material in Bruch’s membrane (drusen), beneath the retinal pigment epithelium (RPE). Vision is not affected unless there is an associated loss of pigment and atrophy of the RPE, degeneration of the choriocapillaris and photoreceptor atrophy. Ultimately a geographic atrophy may develop. Less commonly, choroidal neovascularization (CNV) may develop and grow through defects in Bruch’s membrane into the sub-RPE or subretinal spaces. CNV carries the likelihood of leakage, leading to RPE detachment, subretinal hemorrhage or lipid, and ultimately disciform scarring and central blindness. A vascular basis for AMD has been suggested in several studies, with associations documented between AMD and systemic conditions such as hypertension, cerebrovascular disease, atherosclerosis and serum cholesterol level. That AMD may reflect systemic conditions as well as local processes is suggested by the Age-Related Eye Disease Study (AREDS), which found higher mortality rates in patients with age-related maculopathy (ARM). Underlying mechanisms proposed for these associations have included effects on the choroidal circulation and lipid deposition within Bruch’s membrane.

Symptoms

In the early stages of AMD, patients may note slight distortion (metamorphopsia) or blurring of their vision.

Signs and classification

The hallmark of AMD is the presence of drusen – subretinal pale yellow deposits 10 located mainly in the posterior pole area.

Small hard drusen are usually considered innocuous and a normal part of ageing. Their size is comparable to the width of the smaller retinal blood vessels. The AREDS research group used the following classification for AMD:

(1). Normal: minimal or no drusen
(2). Mild: multiple small hard drusen (<63 |im), non-extensive intermediate drusen (63-124 j^m) or pigmentary changes. If vision is unaffected, this form of the condition may be termed age-related maculopathy (ARM).
(3). Moderate: extreme intermediate drusen, any large drusen (125 ^im) or non-central geographic atrophy without advanced AMD. The intermediate and large drusen are often termed soft drusen.
(4). Advanced: choroidal neovascularization, other exudative maculopathy, or geographic atrophic maculopathy in one eye but not in the other. These forms of AMD carry a high risk of sudden, severe, central vision loss.
(5). The most severe form of AMD is bilateral advanced AMD.

Only 10-15 per cent of affected patients progress to exudative AMD; however, this form of the condition is responsible for up to 90 per cent of the associated severe vision loss. In more than 40 per cent of patients with exudative AMD, the condition becomes bilateral within 5 years.

Prevalence

Age-related macular degeneration is the leading cause of visual loss in the developed world. The Blue Mountains Eye Study showed the prevalence of AMD in an Australian population to be about 2 per cent, with a significant increase in prevalence in those over 65 years of age. There is a similar incidence in males and females.

Significance

Age-related macular degeneration can lead to profound central visual loss.

Differential diagnosis

Genetic macular diseases such as Sorsby dystrophy and Stargardt disease.

Consider also Intraretinal processes such as intraretinal lipid (exudates), retinal emboli and cotton-wool spots (infarcts).

Management

See management under AMD – dry and AMD – exudative