The target area of ARMD, as evidenced by the name, is the macula. This small region of the retina is located on the posterior pole or inside of the eye. It has a very different retinal makeup than the rest of the retina which is primarily composed of nine specific layers and is commonly referred to as sensory retina. All portions of the sensory retina are responsible for sight, but only the macular region gives us the clear straight ahead vision necessary for functioning in everyday life. The other regions of the retina, in comparison, are responsible for our ability to distinguish periphery images. Attached loosely to the sensory layer is the retinal pigment epithelium (RPE) cells. These cells contain a high amount of pigment necessary for light absorption and also sustain the health of the outer retina. The next layer, known as Bruch's membrane, is tightly bound to the RPE and also provides support to the retina. Beneath this layer is the chroroid which contains a network of blood vessels, nerves, immune cells, and fibroblasts. Its vital function is to supply the RPE and outer part of the sensory retina with all of their nutritional needs. Encasing all of these layers is the sclera which maintains the rigidity of the eye.
As mentioned, the macular region contains a very different retinal configuration, especially towards the center or in the foveal region. In essence, it appears as if someone pressed their finger into the retina and left an indentation. Here the retina is maximally thinned and chiefly consists of photoreceptors and their nuclei. The purpose of foveal thinning is to permit greater light absorption by the dense array of photoreceptors creating the characteristic sharp imaging capabilities of the macula. In contrast, the photoreceptors in the rest of the sensory retina absorb light only after it has traveled through several other retinal layers. Another interesting aspect of the fovea is the absence of blood vessels over the photoreceptors. This also contributes to the high degree of visual acuity found in the macular region because the light path to the photoreceptors is less hindered. Comparitively, the vascularization in the rest of the macula is very dense and creates the potential for many vascular related diseases. Quite obviously, the macular region is imperative to good eyesight and it is understandable why eyecare practitioners are concerned with maintaing its health.
The dry or geographic form of macular degeneration is by far the more common. It traditionally affects people over the age of 60 and will be found in 15% of eyes by the age of 80. The earliest warning sign of dry ARMD is the appearance of drusen in the macular region. Drusen formation is not necessarily the cause of dry ARMD, but it is a fairly good indicator that a person might eventually develop some form of ARMD. It is known that drusen is evidence of an unhealthy RPE, the layer of cells between the retina and Bruch's membrane. When properly functioning, the RPE maintains the health of the outer retina by protecting it from choroidal neovascularization, discarding damaged photoreceptors, and performing many other vital functions. As the fitness of the RPE is compromised, it discharges extracelluar materials which are then deposited onto Bruch's membrane. The excreted material result in drusen which can be seen in the eye as yellow-gray nodules located between the RPE and Bruch's membrane. Drusen deposits vary in size and may exist in a variety of forms from soft to calcified. With increased drusen formation the RPE are gradually thinned and begin to lose their functionality. Consequently, the retina which depends on the RPE for its vitality may be affected and vision problems may arise. Signs of drusen do not always mean ARMD has set in but it does indicate compromised RPE cells which could eventually develop into the disease.
.gif)
Other characteristics of dry ARMD include RPE alterations (most likely caused by drusen formation) and depigmentation. Generally RPE changes, like atrophy and clumping, exhibit no real pattern; however, with time these transformations can develop in a geographical manner especially in the macular region. In addition, continued degeneration of the RPE leads to photoreceptor loss and enhances the risk of central vision impairment. Besides the gradual loss of central perception, a change in color vision is another possible symptom of dry ARMD. The photoreceptors, specifically the cones, are responsible for this attribute and as they are degraded the eye loses the ability to adequately distinguish color.
Rarely does Dry ARMD affect visual acuity to the extent that a person becomes legally blind. Though, as mentioned, some loss of central vision and color distinguishing capabilities may occur. Currently, there are no procedures to counteract the onset or effects of Dry ARMD, but with proper education and regular eye examinations patients can learn to maximize the available vision they do have and lead normal lives.
Unlike its relatively benign counterpart, wet ARMD is the leading cause of vision loss in the US for people over 65. It accounts for approximately 10% of all ARMD cases and is sometimes found in conjunction with dry ARMD. As for early warning signs, drusen formations in the macula seem to be the only precursor of wet ARMD's possible commencement. This may sound familiar because drusen is also used as a plausible indicator of dry ARMD. Unique to wet ARMD is a breakdown in Bruch's membrane. This generally occurs near drusen deposits and is possibly due to granular infiltrants in the membrane. Often times one of these jeopardized areas will become the site of choroidal neovascularization (development of new retinal blood vessels), a trademark of wet ARMD. Choroidal neovascularization or CNV originates in the choroid which lies directly beneath Bruch's membrane. Small vessels in this layer called choriocapillaris grow through the break in Bruch's and may leak causing RPE damage or even retinal detachment which then results in severe vision loss. Sometimes, this form of RPE or retinal detachment is confused with a different type of leakage induced separation. In this case, no new vascular growth develops and detachment is caused by fluid seeping through the break in Bruch's membrane. To distinguish between the two, fluorescein angiography is used.
The fluorescein technique causes the developing vessels to fluoresce (glow). Under blue light, these vessels can be observed and the two mechanims of separation can be characterized by the presence or absence of a new vessels.
Besides leaking, the developing vascular network may also hemorrhage producing a sub-RPE or sub retinal hemorrhage. This, like vascular leaking, might produce RPE or retinal disjoinment. Neovascularization may also cause disciform scarring which represents the final stage of CNV. The scarring can be seen in the macula as a large dark-spot with hemorrhaging around the outlying edges. Usually, if CNV has been allowed to progress to this stage severe eyesight damage is evident.
.gif)
The consequences of this form of ARMD include rapid reduction of vision, image distortion, and/or loss of color vision. While some of these symptoms may sound similar to those of dry ARMD, the rapidity and manner of their onset separates the two varieties. Wet ARMD, if not caught early, can lead to severe vision loss whereas the loss of vision is a gradual and not an absolute process with dry ARMD. In addition, procedures such as laser photocoagulation do exist that may stop further progression of wet ARMD. This technique and other measures that can reduce the risk of ARMD can be found in the Treatment and Prevention section.
Back
to Patient Education menu
Back
to Optometry Home Page