Thursday, May 5, 2011

Warning: Simplification May be Hazardous to your Health

Would that some knowledgeable authority could take responsibility for attaching a label similar to the above title to articles that purport to be news!  Consider the headline attached to an article by James Gallagher, Health Reporter for BBC News:

Sex and coffee 'trigger stroke'

As Cole Petrochko nicely demonstrated in a post to MedPage Today Blogs, it is dangerously easy to take the results of a “study” and run wildly with them in all directions.  Petrochko’s piece involved a recent European study that seems to contradict current beliefs about the risks of high salt intake.  Gallagher’s concerns research in the Netherlands concerned with the risk of stroke.

The good news about Gallagher’s report is that he tries to ground it in the nature of stroke itself.  Stroke arises from bleeding on the brain.  This, in turn, tends to arise when the walls of blood vessels weaken.  Such blood vessels are known as aneurysms.  Bleeding on the brain ensues when a brain aneurysm bursts.  There are a variety of reasons why such a burst could occur, one of which is high blood pressure.  Hence, high blood pressure is regarded as a risk factor for stroke.

The Netherlands study basically sweeps by all the links of this causal chain and chooses, instead, to focus of factors that may “induce a sudden and short increase in blood pressure” (in the words of Dr. Monique Vlak, the lead author in the published accounts of the Netherlands results).  What are the risk factors for that blood pressure change?  That is what the Netherlands study actually addresses.

I am not sure what factors were chosen.  However, Gallagher’s report gives a “top eight” list of factors and the percentage of bursts associated with them
  1. Coffee 10.6%
  2. Vigorous exercise 7.9%
  3. Nose blowing 5.4%
  4. Sex 4.3%
  5. Straining to defecate 3.6%
  6. Drinking cola 3.5%
  7. Being startled 2.7%
  8. Being angry 1.3%
My guess is that these results are consistent with the intuitions many of us have, but do they actually say anything about stroke?

As I see it, we need to go higher up the causal chain, so to speak.  How much do we know about aneurysms?  What are the factors that actually contribute to the weakening of blood vessel walls?  Is that weakening irreversible, or can the strength of a blood vessel be restored like the strength of a muscle?  Consider the hypothesis that vigorous exercise can strengthen blood vessels through a mechanism similar to strengthening muscle.  Wouldn’t it be a misrepresentation to put it on the sort of risk list given in the Netherlands report?

Enquiring minds want to know!

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