Intracranial aneurysms are a significant cause of morbidity and mortality. Aneurysm rupture is the most common cause of atraumatic subarachnoid hemorrhage. Death or severe disability in seen in 40 to 60% of patients following the initial hemorrhage. When untreated, an additional 25 to 35% of these patients die due to subsequent hemorrhagic events.1,2,3

Berry aneurysms comprise the vast majority (97%) of intracranial aneurysms. They are found in 1% of the general population,2 and have a female to male occurrence ratio of 3:2. The incidence of berry aneurysm increases with advancing age and with associated conditions, including adult polycystic kidney disease, Ehlers-Danlos syndrome, neurofibromatosis, aortic coarctation, and fibromuscular dysplasia.

Historically, the first anatomic description of intracranial aneurysm dates back to 1778, when Biumi reported an intracavernous aneurysm of the internal carotid artery.2 The first radiographic description of intracranial aneurysms is credited to Moniz who, in 1933, published an angiogram demonstrating a large aneurysm of the internal carotid artery at the skull base in a young man affected by syphilis. More recently, the advent of computed tomography4 and magnetic resonance imaging, along with improved angiographic, endovascular, and neurosurgical techniques, have allowed for more rapid and accurate diagnosis and treatment of intracranial aneurysms. The diagnosis is commonly suggested by the presence of subarachnoid hemorrhage on unenhanced computed tomography. In some patients, however, a less common pattern of hemorrhage may be present. Prompt recognition of the unusual patterns of hemorrhage resulting from aneurysm rupture is important for a better chance of a favorable treatment outcome. The following pictorial essay will discuss and illustrate the spectrum of uncommon patterns of hemorrhage associated with ruptured intracranial aneurysms.

Radiologic manifestations

Unenhanced CT examination is undoubtedly the initial exam of choice in the diagnosis of a ruptured intracranial aneurysm, due to its high sensitivity for acute hemorrhage (90 to 95%).1 Extraluminal blood will appear as high attenuation material along the subarachnoid cisterns. The pattern of distribution of subarachnoid blood seen on computed tomography can offer clues as to the likely location of the aneurysm. For instance, hemorrhage found predominantly in the interhemispheric fissures is suggestive of an anterior communicating artery aneurysm; hemorrhage collecting mainly along the sylvian fissure suggests a middle cerebral artery aneurysm. Posterior cerebral artery aneurysms may display significant hemorrhage in the basilar cisterns.

While subarachnoid hemorrhage is essentially a universal finding of aneurysm rupture, other less common patterns of hemorrhage may be present. These include intraparenchymal, intraventricular, and subdural hemorrhage. The pathophysiology of these unusual patterns of hemorrhage has not been established. However, it has been postulated that aneurysms which have ruptured previously may become associated with fibrosis along the surrounding tissues, and this may predispose to later rupture into the parenchyma or the subdural space.

Anterior cerebral, pericallosal, and anterior communicating artery aneurysms (figures 1-3) may be associated with intraparenchymal hemorrhage in the frontal lobe. The hematoma can have intraventricular extension, often involving the frontal horns of the lateral ventricles and, at times, extending to the third ventricle. The hematoma also may dissect along the sulcus of the corpus callosum to the region of the splenium.

Middle cerebral artery and carotid terminus aneurysms (figures 4-6) also can present as intraparenchymal hemorrhage, often affecting the region of the basal ganglia. As this area is a common site for hypertensive hemorrhage, the radiologist must determine that a hemorrhage that is felt to be hypertensive in nature is not contiguous with the sylvian fissure, i.e., with the subarachnoid space. In such cases of hemorrhages with a basal

ganglia and a subarachnoid component, further study with conventional angiography is necessary to exclude an aneurysm.

Rarely, a ruptured middle cerebral artery aneurysm can present as a subdural hematoma (figure 6). Such a finding may be seen in up to 2% of those patients eventually diagnosed with a ruptured intracranial aneurysm;2 most of these involve the middle cerebral artery bifurcation, likely owing to its lateral location. Intraparenchymal hemorrhage secondary to aneurysm rupture can also affect the temporal lobes; such a pattern has been seen in the setting of posterior communicating artery aneurysms that project laterally (figure 7).

Aneurysms affecting the posterior circulation (figures 8-9) can likewise demonstrate unusual patterns of hemorrhage.5 For instance, intraparenchymal cerebellar hemorrhage involving the tonsils has been seen in the setting of aneurysms in the distal posterior inferior cerebellar artery. When such aneurysms are found, they are not infrequently associated with arteriovenous malformations that are being fed from the aneurysmal artery.6 Rupture of a posterior inferior cerebellar artery aneurysm may be associated with isolated subarachnoid hemorrhage at the level of the foramen magnum; such a finding may be fairly subtle on computed tomography (figure 8) due to the beam-hardening artifact. Intraventricular hemorrhage also can be seen in cases of a ruptured posterior circulation aneurysm. Posterior inferior cerebellar artery, superior cerebellar artery, or basilar tip aneurysms can present with hemorrhage extending into the fourth ventricle.7 AR

References

1. Camarata PJ, Latchaw RE, Rufenacht DA, Heros RC: Intracranial aneurysms. Invest Radiol 28(4):378-382, 1993.

2. Heiserman J, Bird R: Cerebral aneurysms. Neuroimaging Clin North Am 4(4):799-822, 1994.

3. Atlas S: Intracranial vascular malformations and aneurysms. Radiol Clin North Am 26(4):830-837, 1988.

4. Schwartz R, Tice H, Hooten S, et al: Evaluation of cerebral aneurysms with helical CT: Correlation with conventional angiography and MR angiography. Radiology 192:717-722, 1994.

5. Kamiya K, Nagai H, Koide K, et al: Peripheral anterior and inferior cerebellar artery aneurysms. Surg Neurol 42(1):46-51, 1994.

6. Mabuchi S, Kamiyama H, Abe H: Distal aneurysms of the superior cerebellar artery and posterior inferior cerebellar artery feeding and associated arteriovenous malformations: Case Report. Neurosurgery 30(2):284-287, 1992.

7. Zingale A, Chiaramonte I, Consoli V, Albanese V: Distal posterior inferior cerebellar artery saccular and giant aneurysms: Report of two new cases and a comprehensive review of the surgically treated cases. J Neurosurg Sci 38(2):93-104, 1994.

Dr. Iaia, Dr. Schwartz, and Dr. Klufas are in the Department of Radiology at Brigham and Women's Hospital and Harvard Medical School in Boston, MA. Dr. Steig is in the Department of Neurosurgery, also at Brigham and Women's Hospital and Harvard Medical School.