How does macular degeneration affect vision?
There are two main ways that macular degeneration can affect vision: the loss of retinal cells and the development of abnormal blood vessels.
The inside of the eye is lined by three layers of tissue and each has a critical role in normal vision.
The innermost layer (the layer first struck by the light that enters the eye) is known as the retina and consists of a complex network of nervous tissue. Some of the cells in this layer (the photoreceptors) convert light into an electrical signal, which is then amplified and processed by other cells before being sent to the brain via the optic nerve.
The central part of the retina (the macula) has a number of special structural features that allow images focused on it to be seen with very high resolution. The middle layer is a one-cell-thick sheet known as the retinal pigment epithelium or RPE. The RPE provides metabolic support for the photoreceptor cells and also removes old bits of cellular debris from the tips of the photoreceptor cells as they renew themselves.
The layer farthest from the incoming light is a rich network of blood vessels known as the choroid. These vessels supply oxygen and nutrients to the retinal pigment epithelium and photoreceptor cells and carry away waste products.
In macular degeneration, clumps of yellowish material gradually accumulate within and beneath the retinal pigment epithelium. These deposits are visible to a doctor who looks inside the eye. The clumps appear as small yellow spots known as drusen (singular: druse). With the passage of time, patches of retinal pigment epithelial cells may die, resulting in bare spots known as geographic atrophy. When the support functions of the RPE are lost, the photoreceptor cells overlying the areas of geographic atrophy cannot function and the vision from this patch of retina is lost. If these patches become large and involve the very center of the macula (the fovea), the individual’s visual acuity can fall to the point that they are considered legally blind. This atrophic phase of macular degeneration is sometimes referred to as “dry” macular degeneration and is the most common mechanism of vision loss in affected individuals.
In approximately 10 percent of patients with macular degeneration, the injury of the retinal pigment epithelium described above stimulates new choroidal blood vessels to grow up into the RPE and retina--seemingly in an attempt to heal the defects in these layers. This reparative response is very similar to those that occur elsewhere in the body in response to injury, such as scar formation in response to a cut on the skin. Unfortunately, the retina is such a complex and highly ordered tissue that the ingrowth of these new blood vessels causes more visual loss than the original degenerative process does. In fact, although only 10 percent of patients develop new blood vessels, this complication is responsible for the majority of the legal blindness associated with macular degeneration. The vascular phase of macular degeneration is sometimes called “wet” macular degeneration.
What are the symptoms of macular degeneration?
The most common symptom of macular degeneration is decreased visual acuity, that is, decreased ability to see fine detail. Individuals with macular degeneration can experience small gaps in their vision, which they recognize as the need for larger print in order to be able to discern the letters. Sometimes, the disturbance of the structure of the retinal pigment epithelium causes the surface of the retina to be irregular and this results in distortion of the viewed image. This may be viewed by some as bending or curving of straight lines. When a growth of abnormal blood vessels occurs, such bending and waviness can become quite pronounced. Visual distortions of this type are sometimes more easily seen when viewing a high contrast grid. This is the basis for the test known as an Amsler Grid.
What causes macular degeneration?
The term macular degeneration refers to a group of different diseases that will almost certainly prove to have several different causes.
Physicians have wondered about the causes of macular degeneration for more than a century. In the late 1800s, when doctors first began looking into eyes with ophthalmoscopes, they believed that the yellow spots (drusen) they observed represented some type of infection, or at least inflammation, of the choroid. Even today, there is some evidence to suggest that the body’s immune system plays a role in the development of some forms of macular degeneration, especially the development of neovascularization.
Another group of possible causes are environmental factors. That is, with any late-onset, degenerative process, it is tempting to hypothesize that the degeneration has resulted from an exposure to a bad agent, or lack of exposure to a good agent, sometime during the course of the patient’s life. Scientists have searched for evidence of such factors for decades. The factors studied in this way include various nutritional factors (e.g. zinc, B-vitamins, antioxidant substances), light exposure, drugs (e.g. caffeine, nicotine, oral contraceptives, etc.), and toxins (e.g. plasticizers). Although some of these factors appear to have a demonstrable effect on prevalence or course of macular degeneration (green leafy vegetables and some specific nutritional supplements are good, cigarettes are bad), none has emerged as a likely major cause of macular degeneration.
Another important group of likely causes of age related macular degeneration are mildly abnormal genes. It has been recognized for more than a century that some forms of macular degeneration run in families, and during the past 30 years, an increasing amount of evidence has been gathered that suggests a significant fraction of macular degeneration has a hereditary basis. This has important implications for understanding macular degeneration at the molecular level, as well as for designing improved treatments for the disease.
When a disease like macular degeneration is caused by a single gene, a number of family members may be similarly affected. Such families can be studied by modern molecular genetic methods in ways that allow the causative gene to be identified. In the past 10 years, the chromosomal locations of several genes that cause macular-degeneration-like conditions have been identified, and six of these (ABCA4, VMD2, RDS, ELOVL4, TIMP3, and EFEMP1 have actually been identified. Unfortunately, none of these six genes causes a significant fraction of typical late-onset macular degeneration, but the disease mechanisms are similar enough to the latter condition that scientists can already begin to develop animal models of macular degeneration based on these genes to use in developing new treatments. The genetic approach is particularly appealing because if a genetic predisposition to macular degeneration can be identified, it raises the possibility that individuals can be tested for the predisposition early in life and given some sort of treatment that will delay or prevent the onset of the macular disease. Such treatment has the potential to be safer, simpler, cheaper (and hence more globally available) than some of the other experimental treatments currently under development.