February is Macular Degeneration Awareness month. Many of you have already heard of this retinal disease and the new treatments to prevent its potentially devastating effects. If you are reading this blog, you or someone you love may have already been affected by Age-Related Macular Degeneration. If you haven’t heard of the condition, you soon will. Often called “ARMD” or simply “AMD”, macular degeneration is the most common cause of vision loss and blindness among Americans age 65 and older. With the aging of the baby boomers and increasing life expectancy AMD is a rapidly growing problem. Current studies indicate that 7% of Caucasian Americans between the ages of 65 and 70 already have early retinal changes that are known to lead to advanced AMD. That number rises to 15% by age 75 and 30% by age 80.
The macula is a term referring the central part of the retina, which coats the back of the eye wall like a delicate wallpaper of nerve tissue and photoreceptors. This tissue is like the “film” in the camera of the eye, if you will, and is responsible for collecting light from the world you see. The light patterns trigger nerve signals to the brain creating vision as you know it. Macular Degeneration is a term that collectively refers to a degenerative breakdown of this central portion of the retina, called the “macula”. This specialized portion of the retina is loaded with photoreceptors and allows for the most detailed and precise portion of our visual sense directly in the center of the visual field. What we call AMD is likely several disease entities with varied combination of gene and environmental contributing factors that result in this similar clinical picture of central vision damage. Recent studies have revealed connections with a number of gene products that may play a crucial role in either a) protecting against the development of AMD or b) increasing the likelihood of developing the disease. We’ll write more about this interesting field of research in an upcoming blog.
Wet and Dry AMD
AMD is usually divided into two major categories of the disease: 1) Dry and 2) Wet. Of course, the back of the eye is all immersed in fluids and blood vessels course through the various sections of the eye in a network of blood vessels. I’ve had patients struggle with understanding what we mean when we refer to macular degeneration as “dry” or “wet”. To better understand this concept, let offer a little review of the micro-anatomy of the eye and a couple of analogies that help make sense of this concept. The retina lines the back of the eye wall and has a very delicate structure, with organized blood vessels running through the inner portion of the retina and providing oxygen and other nutrients to the inner layers of the retina. The retina itself lies on the surface of a sheet of pigmented cells called the “retinal pigment epithelium”, or RPE. The RPE (and Bruch’s membrane) separate the retina proper from the underlying choroid. The choroid is a thick network of larger caliber vessels deep within the eye layers. Nutrients from the choroid (and associated choriocapillaris) provide for the outer retinal layers, but these nutrients must be transported through the RPE. As such, you have the delicate and well-organized retina separated from the thick, high-volume network of vessels beneath it by a thin sheet of RPE and Bruch’s membrane.
With age, environmental factors, and an unlucky combination of genes, the RPE often becomes thinner and breaks down over time. Atrophic (wasted away) areas of the RPE may spread, giving the appearance of a threadbare portion of fabric. Waste products from the retina that are normally managed by the underlying RPE build up and form yellowish deposits in the eye. These round yellow deposits are called “drusen” (or singular, druse), a German word for “geode” or “stone”, in reference to the appearance of tiny glistening pebbles seen beneath the retina. “Dry AMD” refers to some combination of drusen, RPE break-down, and atrophy.
“Wet AMD” occurs when the fluids and blood from the underlying choroid – that was previously separated from the retina by the RPE layer – begin to grow and leak into underneath and into the retina. Wet macular degeneration typically progresses more rapidly because abnormal blood vessels may leak or hemorrhage into the retina in dramatic fashion over short periods of time, resulting in more rapid vision loss.
Sidewalks and Macular Degeneration
When I consider the process of macular degeneration, I can’t help but think back to the sidewalks in beautiful Iowa where I spent six years of my training. Iowa is a gorgeous place with wonderful people and a unique combination of rural countryside and advanced centers of education. It was a great place to train and raise my family. During my ophthalmology residency and fellowship, I enjoyed free moments when I could go running or biking on the many paths that circled Iowa City and spread out into the countryside. I remember early in my residency that I had been studying the research surrounding macular degeneration and learning to first truly understand the physiologic principles behind this concept of the “wet” and “dry” forms of the disease. I was out jogging on the path with my two oldest sons who were riding their bikes on the sidewalk beside me. I noted the beautiful winding concrete path through the tall green stands of corn on one side and the towering oak trees on the other while we enjoyed the smooth uninterrupted path before us.
When first poured, the relatively thin layer of cement poured for a sidewalk creates a smooth surface that is organized and clean. For many years, this very thin layer of cement will separate the smooth top surface of a sidewalk or driveway from the massive network of roots and weeds coursing through the soil beneath it. As many years pass, the weathering effects of time take their toll. Traffic will wear at the surface. The heat of the sun in the summertime alternating with the damaging effects of ice and snow in the wintertime will stress the concrete and may result in cracks. This aging process is analogous to dry macular degeneration. The surface has simply aged due to a combination of a) external effects on the concrete and b) the potential inherent shortfalls in the mixture and quality of the materials and construction used to create the concrete slab in the first place. Similarly, the damaging effects of oxidizing agents in our diet and environment, toxins (like cigarette smoke exposure), and other insults on our retina and RPE will combine with the inherent genetic makeup of each persons’ tissues to influence the degree to which one patient’s retina will age as compared to another.
Picture again the aged sidewalk. The surface has become imperfect and has some cracks in it, but this may be tolerable and have only a minimal effect on the function of the concrete. In and of itself, this process is a slow degeneration. Remember, however, that seeds, grasses, and perhaps the large roots of a nearby oak tree are running randomly beneath the surface. Normally, a sprout would not burst through the intact surface of a sidewalk. But what if that surface barrier had been compromised, as in our example? Now, tiny shoots from the oak roots or miniscule sprouts of weeds have the ability to rise through the crack and spread across the surface. Left unchecked, these abnormal sprouts can grow and multiply until an entire young sapling with surrounding grasses and weeds is rising through the gaps in the concrete. Such it is with wet macular degeneration. Once the RPE and Bruch’s membrane have been broken down by the dry process, the potential exists for abnormal vessels to sprout from the choroid and grow into the sub-retinal space. The resultant percolation of fluids and periodic hemorrhaging into and beneath the retina results in a more rapid process of visual decline. It is this rapid vision loss that is a feared consequence of wet AMD.
This last decade has offered significant advances in the treatment of AMD. Most especially, the advent of treatment modalities that can halt the effects of wet macular degeneration has changed the landscape of this disease forever. Agents that can cause the abnormal blood vessels to regress and speed up the resorption of fluids have given significantly greater hope to patients who suffer from wet AMD. To return to our analogy of weeds sprouting up through the cracks in the driveway, science has given us agents that are effectively weed-killer for wet AMD.