Age-related macular degeneration (AMD) is a disease characterized by progressive degenerative abnormalities in the macula of the eye, a small area in the central portion of the retina. AMD is characteristically a disease occurring in patients older than 50 years of age and has long been recognized as the leading cause of severe and irreversible loss of central vision in adults over the age of 50 in the U.S. and other developed countries around the world [1, 2].
AMD is exceptionally common, currently affecting about 8 million Americans and an additional 8 million Europeans. . It is estimated that approximately 6% of individuals 65-74 years of age, and 20% of those older than 75 years of age are affected with AMD in the U.S. . Because of the increasing life expectancy in developed and developing countries, the elderly sector of the general population is expected to increase at the greatest rate in the coming decades. Using U.S. Census Bureau projections, the number of Americans over 65 years of age will more than double to 80 million by the middle of this century . In the absence of adequate prevention or treatment measures, the number of cases of AMD with visual loss will grow dramatically.
In addition to advanced age, there are environmental and genetic risk factors for AMD including ocular pigmentation, dietary factors, a positive family history for AMD, high blood pressure, and smoking . Recent work has also identified inflammatory and more specific genetic contributions to the disease .
Age-related macular degeneration is classified into one of two general subgroups: the non-neovascular (non-exudative) form of the disease or 'dry' AMD and the neovascular (exudative) form of the disease or 'wet' AMD.
Non-neovascular or dry AMD is the most common form of the disease, accounting for about 90% of all cases. There is currently no approved therapy for dry AMD.
In dry AMD, there is a breakdown or thinning of the retinal pigment epithelial cells (RPE) in the macula. Dry AMD is characterized by the presence of drusen (dots of yellow crystalline deposits that develop within the macula) and thinning of the retina, particularly in the macular region. Slow degeneration of the light-sensitive photoreceptor cells in the macula leads to gradually blurring central vision in the affected eye. The deterioration of vision is usually gradual over a period of years but is considered irreversible and commonly causes mild to moderate visual loss, but may result in more profound visual decline. Additionally, dry AMD can progress to the wet form of the disease.
Neovascular or wet AMD, although less prevalent, more commonly causes sudden, often substantial, loss of central vision and is responsible for the majority of cases of severe loss of visual acuity in this disease . Wet AMD is preceded by dry AMD. It has been estimated that there are approximately 200,000 new cases of wet AMD each year in the U.S. . This type of AMD results when abnormal blood vessels proliferate under and/or within the retina. These blood vessels leak blood and fluid into and under the retina, which results in vision loss. The natural history of wet AMD is that of scarring with progressive destruction of the central retina and loss of vision.
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