The Great Paradox of the Unruptured Intracranial Aneurysm
We have never detected or treated as many unruptured intracranial aneurysms as we do today. Yet subarachnoid hemorrhage is becoming less frequent and less lethal: its incidence fell from 10.2 to 6.1 per 100,000 between 1980 and 2010, and case fatality dropped from 51% to 35%.
The paradox has another side: most aneurysms never rupture (the average annual risk is below 1%), but most of those that do rupture are small, the ones we usually call «low risk». Moreover, the decline in hemorrhage seems to follow blood pressure and smoking control more closely than preventive treatment.
In this essay I review the evidence on rupture rates, the biases of natural-history studies and a dynamic model of risk. I propose that the decision should not rest on a millimeter threshold but on an individual estimate: the challenge will be less about how to occlude aneurysms, and more about which ones to leave alone.
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