Patients With Small, Asymptomatic, Unruptured Intracranial Aneurysms and No History of Subarachnoid Hemorrhage Should Be Treated Conservatively
It is foolhardy to remove from consideration of treatment any aneurysm solely because its maximal size is <10 mm or 7 mm. Annually, ≥15 000 American patients have subarachnoid hemorrhage (SAH) from aneurysms with a maximum diameter <7 mm and consequently experience irreparable morbidity and severe mortality. The majority of their aneurysms were, of course, unruptured, single, asymptomatic, and even smaller at some point before rupture.
If the rupture rates for <10-mm aneurysms in International Study of Unruptured Intracranial Aneurysm (ISUIA) I1 or <7 mm in ISUIA II2 are used with the known number of ruptured aneurysms of these sizes admitted each year to calculate the prevalence of such small aneurysms in American adults, there is an unbelievably high result: in the range of 7% to 14%. Previous best estimates of prevalence on the basis of retrospective and prospective, radiological and pathological data have been ≈2% for all sizes of aneurysms and for those <10 mm, less than 1%. In autopsy studies, unruptured aneurysms have not outnumbered ruptured ones; the reverse is the case.3 Between ISUIA I1 and ISUIA II,2 the rupture rate has gone from <0.05% per year for <10 mm (no previous SAH), to ≈0.1% per year for <7 mm (no previous SAH), or more than doubled despite the fact that the size has moved in the direction of a lower rupture rate and follow-up for cumulative rates fell from 7.5 to 5 years. Also of note is the fact that this rate can now be contrasted with a morbidity and mortality of ≈5% for anterior circulation aneurysms ≤12 mm in patients <50 years, whereas in ISUIA I,1 only the global rate of 15.7% at 1 year for patients without previous SAH was given for all sizes and sites of aneurysms and all ages of patients.
The 5-year rupture rate of 0% for <7 mm aneurysms of the internal carotid, anterior communicating, or middle cerebral artery sites was compared with 2.5% for posterior communicating/posterior circulation aneurysms. The removal of posterior communicating aneurysms from the anterior circulation had no physiological justification. The remarkable difference in tendency to rupture between single and multiple small aneurysms suggested in ISUIA I1 (1 of 424 versus 17 of 641 at <10 mm) was probably not replicated. In ISUIA II,2 for single aneurysms, 2 ruptures in 535 at risk occurred in <7 mm, but there were 5 in the 7- to 9-mm range. In the multiple aneurysm cases, 7 ruptures in 439 at risk occurred at <7 mm but only 1 between 7 and 9 mm (the denominators are not given for the 7- to 9-mm sizes in the article). However, as in the first report, larger single aneurysms had a higher rupture rate than larger multiple aneurysms. Importantly, if there is not a highly significantly greater tendency for small multiple aneurysms to rupture, one cannot dismiss as irrelevant the longest follow-up data on unruptured aneurysms (almost entirely multiple-aneurysm cases), which found that after 20 years, 18% of 2- to 6-mm aneurysms had ruptured, as had 53% of 7- to 9-mm aneurysms.3 Also, in another large study, there was no difference in the sizes at rupture between single and multiple aneurysms.3
Possible unintended selection bias may be inferred, for instance, from the following. Anterior communicating/anterior cerebral aneurysm cases made up only 10.3% of the 1692 untreated ones in ISUIA II.2 The 5-year rupture rates for <7-mm aneurysms was 0% for some anterior circulation aneurysms including this site. This suggests that these are uniquely safe lesions. However, in ruptured aneurysm clinical series, these aneurysms make up 35% to 50% of cases. They make up an even higher percentage of the smaller ruptured aneurysms in these series, and they rupture at the smallest average size. In pathologic series, the ratio of ruptured to unruptured aneurysms is high at this location, and the proportion of unruptured aneurysms at this site is lowest in elderly people. Aneurysms at this site are the most common exceptions to the rule that in multiple aneurysm cases, it is the larger and more proximal aneurysms that rupture.3
ISUIA II2 was a prospective study, but it was not randomized. Not only were cases selected for no treatment, but there was a large crossover during the study from no treatment to treatment, which resulted in the censoring of such cases. The majority of them were followed for a relatively short time, so only 35 patients were assessed at 6 years after entry in the “without previous SAH” group of aneurysms <7 mm. The conclusions do not constitute class I evidence, and of course it is only 1 study. What cannot be known with certainty is whether the patients that were never treated were truly representative of the universe of unruptured aneurysm cases.
The risk of surgery is taken “up front,” whereas the risk of rupture does not diminish with time. A 40-year-old now has an anticipated life expectancy of >37 years; even the 60-year-old has >20 years.3 A policy based on repeated observation to watch for growth to a dangerous size has limitations because some aneurysms rupture without growing, and growth rates are unpredictable.3 The decision to recommend treatment for small, asymptomatic aneurysms should depend on the patient’s psychological makeup, their ability to understand relative risks in the absence of definitive data, their wishes, life expectancy, comorbidities, aneurysm number, site and morphology, arterial anatomy and anomalies, observed changes in the aneurysm between repeat visualizations, family history, underlying vascular diseases, smoking history, and the experience of the treating physician and hospital (and the size but not just the size).
- Received June 8, 2004.
- Accepted July 26, 2004.