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CEREBRAL ANEURYSMS

Unruptured Aneurysms

Ratio estimates of ruptured:unruptured aneurysms range from 5:3-6, so a working estimate is 1:1¹.  In other words, it is estimated that approximately 50% of aneurysms rupture¹.  Risk of rupture of incidentally found unruptured aneurysms depends upon a variety of factors, not the least of which is the size of the aneurysm.  Other factors such as location, presence of multiple aneurysms and bleeding from prior aneurysms also play a role.  Annual risk of rupture of incidentally found unruptured aneurysms has classically been quoted as 1-2% per year, but other studies call these numbers into question.  For example, the International Study of Unruptured Intracranial Aneurysm Investigators (1998; Wiebers, 2003) found that anterior circulation aneurysms smaller than 7 mm had an annual risk of rupture of .05%².  Aneurysms at other locations, aneurysms greater than 10mm and aneurysms in patients who had previous bleeding aneurysms were found to have a risk of rupture of 0.5% a year².  There continues to be considerable controversy over these findings as some critics point out a selection bias favoring patients who surgeons felt were less likely to bleed².   Aneurysms which present with focal neurological deficits such as third nerve palsy, visual loss or brainstem compression have a much higher risk of rupture-about 6% per year².  Keep in mind when you go into practice that aneurysms which present with third nerve palsy represent a neurosurgical emergency and must be dealt with immediately². 

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