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Correction
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The International Study of Unruptured Intracranial Aneurysms Investigators,
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Another concern of ours is the relation between subarachnoid hemorrhage and the size of the aneurysm. Although Wiebers et al.3 have written about the absence of subarachnoid hemorrhage in patients with aneurysms that are less than 10 mm in diameter, many large series have shown that the mean diameter of aneurysms in patients who present with subarachnoid hemorrhage is less than 10 mm, as noted by Dr. Caplan in his editorial.4 Therefore, we conclude that there is little assurance that an aneurysm that is less than 10 mm will not bleed.
We believe that the conclusions of this article should be modified to reflect a higher risk of subarachnoid hemorrhage at certain sites, since this will affect the manner in which treatment options are presented to patients.
Alejandro Berenstein, M.D.
Eugene S. Flamm, M.D.
Mark J. Kupersmith, M.D.
Beth Israel Medical Center
New York, NY 10128
References
We were approached to participate in this study and include any patients with unruptured intracranial aneurysms who were being followed. Although we had several such patients, we declined the invitation because of our concern that the cohort would not be representative of the overall population of patients with unruptured aneurysms. For instance, the vast majority of patients with unruptured aneurysms who are referred to our institution have already been surgically treated, leaving only a small minority with aneurysms with a low risk on the basis of the natural history, and these aneurysms were, for example, heavily calcified, partly intracavernous, tiny and laterally located, or in elderly patients with other medical problems. Unfortunately, aneurysms such as these also often pose a greater surgical risk, further complicating the issue of their inclusion in a study.
We think that the data reported confirm our concern about selection bias. The cohort consisted of 1449 patients at 53 centers who were given a diagnosis over a period of 21 years (1970 to 1991). All the centers are regional referral institutions that would be expected, on the basis of the volume at our institution, to see 60 to 80 patients with newly diagnosed unruptured aneurysms annually. Assuming this to be the case, outcome data would have been reported for only 1.3 patients per year (2 percent of all patients seen). Even if one assumes the volume in the referral centers to be only 10 percent of this value, the cohort would represent only 20 percent of the patients in these centers, which is still too small a percentage to be representative of the population at large.
Thus, although we applaud any plans for a true population-based assessment and some type of randomized trial addressing this problem and hope that this article will trigger enough controversy to see that goal accomplished, we recall the initial impressions that carotid artery disease was benign and not in need of treatment, which were reversed by definitive studies showing that intervention was appropriate for patients who were properly identified. The same could apply to unruptured aneurysms, which the study shows still have a case fatality rate of 66 percent when they are left to bleed.
For those of us treating this disease, the identification of appropriate surgical candidates, combined with more cost-effective screening, probably offers the greatest hope, since ongoing efforts to prevent early rebleeding, cure vasospasm, reverse severe brain damage, and refine microsurgical and endovascular techniques are having little effect on overall morbidity due to this disease.
E. Sander Connolly, Jr., M.D.
J.P. Mohr, M.D.
Robert A. Solomon, M.D.
Columbia University
New York, NY 10032
The rates of surgical morbidity and mortality were higher in this study than in other series.4 There have been microneurosurgical advances since 1970, when the retrospective component began. It would not be accurate to use high complication rates when one is analyzing the risks and benefits of the obliteration of aneurysms. We are sure that the authors accept the seriousness of these lesions and the potential for poor outcomes (a mortality rate of up to 50 percent after rupture). In addition, in 42 of the 205 patients who died during the 7.5 years of follow-up, death was caused by intracranial hemorrhage. Was this due to the aneurysm? Were autopsy data available?
Patients in whom the aneurysm was manipulated within 30 days after diagnosis were excluded. This cohort most likely represents younger patients who have aneurysms with worrisome features that make the decision to operate straightforward. If these assumptions are true, the rate of surgical complications would be lower in this group. The number of such patients and the reasons for treatment are not reported.
The locations of the aneurysms in this study differ from those in previous studies and suggest that there may have been a location-specific bias.5 There were 256 cavernous aneurysms. The likelihood of rupture is low if the lesion is completely intracavernous. These lesions account for 18 percent of aneurysms in the retrospective study, but for only 6 percent of cases of subarachnoid hemorrhage.
To advise patients with unruptured aneurysms properly, three factors require consideration: the size and location of the aneurysm and the patient's age. The study does not provide information regarding the rates of surgical complications as a function of the location or the size of the aneurysm or the patient's age. Unfortunately, all patients are combined into one group that reflects various microneurosurgical techniques. This approach is very misleading. We hope that this report will prompt further investigation into this important clinical issue.
Philip E. Stieg, Ph.D., M.D.
Robert Friedlander, M.D.
Brigham and Women's Hospital
Boston, MA 02115
References
Allan Brett, M.D.
University of South Carolina School of Medicine
Columbia, SC 29203
To the Editor: Approximately half the patients with cavernous aneurysms also had noncavernous unruptured aneurysms. Although intracavernous aneurysms can cause subarachnoid hemorrhage, most are protected from the subarachnoid space. Other internal carotid aneurysms are not protected. The annual rates of rupture are only slightly increased by the exclusion of patients with cavernous aneurysms (Table 1); patients in group 2 who had small (<10 mm) unruptured aneurysms remain approximately 10 times as likely to have a subsequent rupture as patients with small aneurysms in group 1.
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The concern about microneurosurgical advances that have occurred since 1970 and the exclusion of patients who underwent surgery in the first 30 days after diagnosis is not relevant, since operative morbidity and mortality were assessed only in the patients in the prospective group, beginning in late 1991. The base-line characteristics of the surgically treated patients and those who were not so treated were virtually identical, with minor differences in the mean age and aneurysmal size only among patients in group 1, underscoring the lack of consensus about the selection of patients with unruptured intracranial aneurysms as candidates for surgery.
Comparison of our retrospective results with those of a 30-year study (1965 to 1995) in Rochester, Minnesota, of a population-based sample of patients with intracranial aneurysms1 (and unpublished data) yielded strikingly similar demographic characteristics, aneurysmal characteristics, and associated medical conditions, suggesting that our group (representing approximately 40 percent of patients with unruptured aneurysms who were seen at participating centers) may be representative of the general population. A randomized trial would involve a much greater prospective selection bias than our study because many patients would refuse to participate.
Data on the natural history of unruptured intracranial aneurysms confirm the conclusion that judgments about the probability of the rupture of such aneurysms cannot be extrapolated from data on patients with ruptured aneurysms. It appears that most aneurysms that are going to rupture do so when they form or soon afterward and that the critical size in terms of rupture is smaller for aneurysms that rupture early.
In our cohort, the patient's age significantly predicted operative morbidity and mortality. The influences of the size and location of aneurysms in this study are not yet clear because of insufficient numbers of patients; this is one of the central reasons that the study has been extended to involve a total of 5500 patients.
David O. Wiebers, M.D.
David G. Piepgras, M.D.
John Huston III, M.D.
Mayo Clinic
Rochester, MN 55905
for the International Study of Unruptured Intracranial Aneurysms Investigators
References
Louis R. Caplan, M.D.
Beth Israel Deaconess Medical Center
Boston, MA 02215
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