AMD: A Leading Cause of Vision Loss
Age-related macular degeneration (AMD) is a chronic disease of the retina that causes loss of central vision. AMD affects the macula, the area of the retina responsible for fine visual detail. AMD is the leading cause of severe loss of central vision (defined as visual acuity of 20/200 or less in the affected eye) in elderly individuals. An estimated 15 million North Americans have AMD.
Risk Factors for AMD
The primary risk factor for AMD is age. The prevalence of disease and the risk for progression increase with advancing age. A family history, female sex, and white race are additional risk factors for AMD. Modifiable risk factors include smoking, obesity, hypertension, hypercholesterolemia, excessive sun exposure, and a diet deficient in fruits and vegetables.
Classification of AMD
AMD is classified into 2 types: nonexudative (dry) and exudative (wet).
Dry AMD is characterized by drusen (small, yellow, submacular lesions) and disruption and atrophy of the retinal pigment epithelium (Figure 1).
Figure 1. Nonexudative (dry) AMD: confluent soft drusen, geographic atrophy, and pigment clumping.
Dry AMD is the more common and less severe form but is often a precursor to wet AMD. Although 80% of patients with AMD have the dry form, the wet form is responsible for 90% of severe loss of vision associated with AMD.
Wet AMD is characterized by choroidal neovascular membranes (Figures 2, 3).
Figure 2. Exudative (wet) AMD.
Figure 3. Optical coherence tomography of a patient with exudative (wet) AMD shows subretinal fluid as well as a detachment of the retinal pigment epithelium.
Progression of AMD leads to a break in the basement membrane of the retina. The defect in the basement membrane allows buds of neovascular tissue from the underlying choroidal vasculature to grow into and under the retina. The vessels are fenestrated and can leak into the intraretinal and subretinal spaces. Accompanying fibroblasts create fibroglial and fibrovascular scars, which disrupt the normal retinal structure and function.
AMD can also be divided by severity of disease into early, intermediate, and advanced AMD. These distinctions are used for research purposes, but they also determine clinical prevention and treatment strategies. Early AMD features small-to-intermediate drusen and minimal to no pigment disturbance. Intermediate AMD exhibits several intermediate drusen or at least 1 large drusen (>125 µm) in 1 or both eyes. Advanced AMD is defined as exudative/neovascular AMD or geographic atrophy involving the fovea.
Symptoms of AMD
Dry macular degenerative changes usually occur slowly over time. The patient may notice a need for brighter light when reading. Other symptoms include difficulty adapting to low light levels, increased blurriness of printed words, decrease in brightness of colors, or a blurred or blind spot in the center of the field of vision.
In contrast, visual changes in wet AMD tend to occur rapidly, resulting in an abrupt decline in central vision. Patients describe the abrupt visual change as visual distortions, such as straight lines appearing wavy or objects appearing larger or smaller than they are. Visual changes may also manifest as a well-defined blind spot in the center of vision.
Medscape Ophthalmology © 2012
Cite this: Kyle N. Klingler, Sophie J. Bakri. Age-Related Macular Degeneration - Medscape - Jun 11, 2012.