Subarachnoid hemorrhage (SAH) is a potentially fatal but rare event occurring in approximately 10 per 100,000 population each year. Most cases of SAH are believed to be caused by rupture of intracranial aneurysms. In a review of the International Study of Unruptured Intracranial Aneurysms, Kirkpatrick notes that approximately 5 percent of the population will develop an aneurysm, although very few ever become symptomatic.

Physicians now face a dilemma when cerebral aneurysms are identified in asymptomatic patients. In this study of 2,621 patients, the risk of rupture from small aneurysms (less than 10 mm diameter) was estimated to be 0.05 percent per year. The risk increased to 0.5 percent per year if the patient had previously bled from another aneurysm. For aneurysms greater than 10 mm in diameter, the annual risk of rupture was approximately 1 percent. Giant aneurysms (diameter over 25 mm) are rare but have a 6 percent risk of rupture within one year of diagnosis.

The risk of rupture for all aneurysms increases with patient age and location; areas such as the basilar tip are particularly vulnerable to rupture. The study concluded that most intracranial aneurysms are at extremely low risk of rupture, and that characteristics such as size, location, previous SAH and patient age indicated those aneurysms that are at increased risk of SAH.

Conversely, surgical treatment of aneurysms has significant morbidity and mortality. The mortality within one year of surgery was 3.8 percent; 12.0 percent of patients had significant morbidity. Morbidity and mortality increased with patient age. Rates of complications varied greatly between centers, emphasizing the role of clinical experience and the need for increased audit.