Age Related Macular Degeneration
As part of NAVH's Educational Series we are pleased to provide this excerpt from AGE RELATED MACULAR DEGENERATION. Updated by Michael J. Cooney, M.D., Jason S. Slakter, M.D., and Richard F. Spaide, M.D.
INTRODUCTION: ANATOMY OF THE RETINA
The retina is a tissue in the back of the eye. It is an extremely complex structure which contains millions of vision cells.
The macula, an area of approximately 1/4 inch in diameter, is the central part of the retina and is responsible for central or straight ahead vision. The term macular degeneration, therefore, implies a disease in which a degenerative or aging process affects macula.
The macula contains a very small central area called the FOVE. It is this part of the macula which is responsible for very sharp vision. Thus, if only a part of the macula is diseased but the fovea is intact, vision can still be good.
When rays of light enter the eye, they first pass through transparent structures - the cornea, the lens and the VITREOUS. The cornea and the lens focus light rays onto the part of the macula called the fovea.
The fovea is different from the rest of the retina, since it contains a very high concentration of cells called CONES. Because of this, the fovea is the only area of the retina capable of 20/20 vision. Since the cones must have bright light to function normally, the fovea works best in daylight illumination.
The rest of the retina, including the macula area outside the fovea, has a lesser concentration of cones and also has another type of receptor called the ROD. The rod functions better in dim illumination and is not capable of sharp central vision.
Behind the retina is another tissue called the choroid, which is a very vascular tissue. Between the retina and the choroid is a space called the subretinal space. Abnormal blood vessels from the choroid grow into the subretinal space in age-related macular degeneration. This growth of blood vessels leads to bleeding and scarring which is responsible for the vision loss in age-related macular degeneration.
EPIDEMIOLOGY OF AGE-RELATED MACULAR DEGENERATION
Age-related macular degeneration (AMD) is the leading cause of severe, irreversible vision loss in the Western world. It is predominantly due to the age-related process, although certain patients have a genetic pre-disposition.
Rarer forms of macular degeneration may occur from other disorders. For example, in younger patients, infection, myopia, and unknown factors may cause abnormal blood vessels to grow behind the retina. Although the process is similar to that seen in age-related macular degeneration, in younger patients this disease has different causes and prognoses.
FORMS OF AGE-RELATED MACULAR DEGENERATION
There are two basic types of age-related macular degeneration -- dry and wet.
In the dry type of the disease, aging yellow spots called DRUSEN are present with or without atrophy of the macular region. In this dry form of the disease, the visual acuity is usually not drastically affected. In the wet form, abnormal blood vessels (the blood vessels "leak" -- hence the term wet) grow behind the retina in the subretinal space and leak blood and fluid. With time, these blood vessels proliferate and grow into a scar. Once the scar has formed, there is no treatment except for low vision rehabilitation, as the scar is the end result of this process.
Although the dry form of the disease is the most common, most irreversible vision loss is due to the wet form. It has been stated that approximately 10-20% of patients have the wet form which is responsible for approximately 90% of severe vision loss from AMD.
CONES - Light-sensitive retinal receptor cell that provides sharp visual acuity and color dis-crimination. The majority are found in the macular area.
FOVEA (FOH-vee-uh) - Central pit in the macula that produces sharpest vision; contains a high concentration of cones and no retinal blood vessels.
RODS - Light-sensitive retinal receptor cells, specialized to work at low light levels (night vision). A normal retina contains 150 million rods. The majority are found in the periphery (side vision).
VITREOUS (VIT-ree-us) - Transparent, colorless, gelatinous mass; fills rear two-thirds of interior of the eyeball, between the lens and the retina.
THIS IS AN EXCERPT!
NAVH's members can order the complete article on Age Related Macular Degeneration by David R. Guyer, M.D. by sending $1.50 to NAVH. The cost for nonmembers is $2.50.
Copyright ©1997-2004 by NAVH. All Rights Reserved. Updated January 2005.