In my last blog about ARMD (age related macular degeneration) I went over the bare basics of what macular degeneration is. There’s a lot more to macular degeneration than what I discussed and many people are confused about what exactly is going on. In this blog I want to go over one form of macular degeneration: dry macular degeneration.
Dry ARMD is the most common form. It makes up about 90% of all ARMD. The good news though is that it only accounts for about 10% of serious vision loss. That’s not to say that people with dry ARMD don’t suffer a loss of central vision and have trouble seeing, it just means that they are still able to function reasonably well. But what IS dry ARMD. What does it mean? For that we have to dive into a little bit of anatomy and physiology (I promise it's not too hard).
You might remember from science class that our retina has a bunch of different layers. The important ones for ARMD are the photoreceptors (better known as the rods and cones) that detect light and the retinal pigmented epithelial layer that keeps food and oxygen going where it should (these layers were not named to be short… that’s why we call that one the RPE). So what goes wrong in someone with ARMD? It all starts with little yellow flecks called drusen.
Drusen are thought to be clumps of waste product from the cells of the retina that haven’t been cleared out by your blood vessels. Some more recent research suggests they come from ongoing inflammation. Regardless of where they come from they mean trouble. Drusen on their own do not mean that someone has ARMD. In fact the great majority of people with drusen don’t have ARMD at all! Drusen are a huge risk factor for developing the disease though and shouldn’t be ignored.
Like many things in the eye the exact way that drusen cause damage to the RPE isn’t fully understood but the most commonly accepted reason is that drusen decrease the amount of food and oxygen that can get to the RPE. Unsurprisingly this isn’t good for the RPE and it dies. Since the RPE’s job is to feed the photoreceptors in the eye when the RPE goes the photoreceptors in the same area die off too. This degeneration of the RPE and photoreceptors is what we call dry macular degeneration or if you want to be really fancy and impress your friends the official term for this is geographic atrophy.
In some cases though, these drusen do not lead to degeneration of the RPE and photoreceptors. In some cases they cause wet macular degeneration. We'll learn more about wet ARMD in another blog.