Brain Computed Tomography Angiographic Scans as the Sole Diagnostic Examination for Excluding Aneurysms in Patients with Perimesencephalic Subarachnoid Hemorrhage (2007)

To the Editor: Kershenovich et al. (5) reported on a series of 30 patients with a perimesencephalic subarachnoid hemorrhage (SAH) pattern and a negative computed tomographic angiography (CTA). In these patients, subsequent conventional cerebral angiography also failed to demonstrate an aneurysm. The dangerous scenario of a missed cerebral aneurysm in a patient with aneurysmal SAH could occur if CTA were falsely negative and no correlative digital subtraction angiography (DSA) were performed. The authors attempt to simulate a “CTA-only” algorithm of SAH evaluation with their retrospective case series. In a random sample of 30 cases, no false-negative CTA exams were observed—thus, this potential “dangerous scenario” never would have arisen were a CTA-only algorithm applied. On the basis of these data, the authors conclude that “ … CTA alone is a good and conclusive tool to rule out aneurysms in patients … with classic perimesencephalic SAH … and thus can replace DSA and its corresponding risks.” The conclusion that the authors have drawn seems logical, but they are not justified on the basis of their data set. Occasionally, investigators are required to estimate the chance of an event occurring on the basis of a random sample in which the event has not occurred. Methods for estimating the probability of the occurrence of an event on the basis of a zero occurrence in a given sample have been reported by Basu et al. (2) and Louis (6). The classic, one-sided 95% confidence interval for the Kershenovich et al. data set (n = 30) yields an upper limit of 9.5%, the chance of the “dangerous scenario” of a false-negative CTA arising (6). If a Bayesian estimate is applied (2), the upper confidence bound (95%) is 6.2%. Kershenovich et al. would have to observe an additional 170 consecutive patients with “classic perimesencephalic SAH” (increasing the overall sample size to 200) with negative CTAs and DSAs to reduce the upper 95% confidence bounds to 1.5% and 0.95% using the classic and Bayesian estimates, respectively; this rate is concordant with the upper limits of the estimated risk of significant neurological morbidity resulting from cerebral angiography (4). It was previously reported that in patients with “perimesencephalic hemorrhage,” there is a 4 to 9% chance of an aneurysm being the source of the SAH (1,7,8). Because of the relatively low prevalence of cerebral aneurysms in this patient population, one could argue that the perimesencephalic SAH pattern alone would preclude the necessity of a catheter angiogram, as did Ruigrok et al. (9). However, the present series of 30 patients with true-negative CTAs adds little to support a CTA-only algorithm and by no means justifies a change in the accepted diagnostic evaluation of these patients. With modern CTA, we certainly agree that the vast majority of saccular aneurysms in this category are detected prospectively, even to the extent that a definitive treatment plan (endovascular or surgical) can be entirely formulated from the noninvasive imaging data. The questions then are: What types of aneurysms are not detected easily with CTA? and Can anything be done about them? Dissecting and blood blister aneurysms of the vertebral basilar system are capable of producing a perimesencephalic pattern of SAH. These aneurysms often lack a distinct saccular component and occur in atypical locations, thereby decreasing their conspicuity on CTA, even with application of the most-advanced multiplanar reformations and three-dimensional postprocessing techniques. These aneurysms are also among the most dangerous cerebrovascular lesions, with a malignant natural history characterized by frequent and early rebleeding (10). Despite their aggressive natural history, the treatment of these lesions is often technically straightforward and can be achieved safely either through a deconstruction of the parent vessel or, when a constructive strategy is required, via Neuroform stent (Boston Scientific/Target, Fremont, CA) reconstruction (3). At our institution, we presently perform CTA as a first-line test on almost all patients with SAH. In those patients with negative CTA exams, we proceed with thorough catheter-based angiography, largely to exclude one of these dangerous but imminently treatable lesions. Raymond Turner Nancy Obuchowski Vivek Gonugunta Michael Kelly Henry H. Woo David Fiorella Cleveland, Ohio