Wednesday, September 9, 2009

Association versus Causality

I find it fascinating to read reports in the newspapers of new advances in medicine and new insights into medical care.  I try to find the hidden, or not so hidden, mistakes or misinterpretations that often occur.  In a previous blog, I wrote about reports of basic research that are years away from practical use that give the impression that therapies are immediately changing in response to the research.  Today I get to discuss another common error.  Proving an association between two different illnesses or conditions and implying that there is a causal relationship that can impact treatment when one may or may not exist. Let’s get specific:

Depression and Heart Disease

It has been known for many years that people who are depressed are more likely to get heart attacks.  In a study this past week in the Archives of General Psychiatry, it was shown that people who have already had a heart attack, have a doubling of mortality after the heart attack if they have major depression.  This article was the basis of a news article in the Wall Street Journal which asked the right question in its title “Will Depression Treat Heart Disease?”  Based on the results of this study, should the medical profession be more aggressive in treating people for depression after heart attacks?  Should cardiologists and others who treat these patients be trained in screening people after heart attacks for depression and treating them with appropriate medications and therapies?  What about the risks of using those medications on people after heart attacks?  There are risks especially in people who are known to have heart disease.  It almost seems obvious that if people with depression after heart attack have a double risk of dying, that we should be as aggressive as possible in identifying and treating depression in people with heart disease.  However it may not be as obvious as seems.

Does Depression Cause Death in Heart Disease or Does Bad Heart Disease Make You Depressed (or neither) 

That is the major real question here.  We know from the study that I cited that there is an association between heart attacks, depression and death but we don’t know whether one causes the other and which causes which.  Two recent editorials in the Sept 1 issue of the Journal of the American College of Cardiology commented on this.  One article pointed out that there was no evidence that screening for depression actually improved the outcomes of coronary heart disease.  The other, while agreeing with that fact took the position that identifying depression was good in anyone so it should be done.  While we know that depression, heart disease and death are associated, we don’t really know how best to use that knowledge to help people.  We don’t know if the heart disease is causing the depression in which case treating the heart disease more successfully might improve the depression, or if the depression is worsening the heart disease in which case we would want to identify and treat the depression in order to improve the heart disease.  Of course the increased mortality with depression may just be due to the fact that the depressed people are not taking their medications and doing their cardiac rehabilitation.  Maybe all we need to do is improve the way we give medications and encourage people to exercise and not deal directly with the depression at all.

Have I Confused You Yet?

In some ways I hope so.  Not because I want to frustrate all of you but because I do hope to show you that even an easy straightforward association may not imply a straightforward therapeutic direction.  We all want simple answers however even when the facts are simple, the answers they imply may not be.