Aspirin and AMD: Separating correlation from causation

in Contributors/Dr. Mark Siegel, MD FAAO by

By Mark S. Siegel

A patient of mine recently asked me if he should stop his aspirin since he read an article about a study that showed that aspirin use may cause Age-related Macular Degeneration (AMD). (Never mind that this patient doesn’t have AMD, but does have a significant past medical history of coronary artery disease, hypertension and hyperlipidemia.) The media has picked up this story and there have been a number of misleading headlines about aspirin and AMD. Some have been quite alarmist. While a recent study did reveal an increased risk of late AMD in people who took aspirin regularly, there is a lot more to the story. And, as they say, the devil is in the details.

The article that was recently published only observed a large group of people who were already part of an ongoing demographic study. In this case, it was the population of Beaver Dam, Wisconsin, where the health of the residents has been monitored for decades.

A review of 5,000 people in that study showed that aspirin users had a 1.4 percent increased risk of getting wet AMD as opposed to a 0.6 percent risk for those who did not take aspirin regularly. Only 10% to 15% of people with AMD develop wet AMD. When the risk statistics are applied to the population as a whole, the actual risk is minimal. It’s actually a very small increased risk.

There has never been a randomized clinical trial on the use of aspirin among people with macular degeneration. That would involve taking two groups of people and giving aspirin to one group and not to the other and recording their eye health over a long period of time. Such a study would give us a lot more useful information than an observational study.

We can say that aspirin use is associated with an increased risk of AMD, because both are happening at the same time. But we cannot say that aspirin use is a cause of the increased risk, for a number of reasons. One is that only a controlled clinical trial might give enough data to make such a statement and we don’t have that data now. Also, the age at which people start taking daily aspirin for heart health is about when early signs of macular degeneration can be seen. Heart disease and poor circulation are known risk factors for AMD. So, is it the aspirin or the heart condition that contributed to the development of wet AMD?

Dr. Barbara Klein, from the University of Wisconsin, is the lead author in the study. She noted that it requires more study and that the link between aspirin and AMD risk is not clearly understood. In the meantime, “One should not alter aspirin use based on these findings. It’s better to have blurry vision but still be here to complain about it than to die of a heart attack.”

Make sure your internist and cardiologist know you have macular degeneration — and that your ophthalmologist knows you take aspirin for your heart. As with most things, the risk needs to be balanced against the benefits.