Confined Aneurysm Rupture into an Arachnoid Cyst with Focal Intracystic Vasospasm

Introduction: Arachnoid cysts are well-described intracranial lesions. They occasionally present with spontaneous intracystic or subdural hemorrhage, or appear following CNS insults. Rare is their association with a ruptured intracranial aneurysm. This scenario often leads to delayed diagnosis, as intracystic clot can mimic other cerebral pathology. Here, we present an unusual case of intracystic aneurysm rupture; to our knowledge the first to either be treated with endovascular embolization or cause focal intracystic vasospasm. We follow with a review of the literature and a discussion of important points.

Case Report: A man presented 1 week after sudden onset of severe headaches, visual disturbances, and nausea with a 4.5 cm hyperdense lesion in the left Sylvian fissure. MRI demonstrated a 12-mm flow void within the lesion at the left MCA bifurcation, consistent with an aneurysm, which was confirmed angiographically. The larger lesion was consistent with a blood-filled arachnoid cyst and surrounded the majority of the M1 branch. As he presented during the peak vasospasm window, we elected to treat the aneurysm endovascularly, which was successful. His course was later complicated by severe focal vasospasm of the left M1, which required intra-arterial verapamil, balloon angioplasty, and vasopressors. He recovered with minimal residual deficits.

Review of Literature: We identified 11 additional cases of intracystic subarachnoid hemorrhage in the international literature. No prior ruptured intracystic aneurysm was treated with coil embolization. The mean patient age was 40.9 years, and patients were split evenly between the sexes. Of the 12 arachnoid cysts, 10 were located in the middle fossa. There were six MCA bifurcation aneurysms, four PCommA aneurysms and two ICA bifurcation aneurysms. Three patients had multiple aneurysms. Two-thirds of the ruptured aneurysms were left sided. Only this patient progressed to symptomatic vasospasm. The mean delay to diagnosis of acute intracystic rupture was 5.8 days.

Conclusion: Subarachnoid hemorrhage into an arachnoid cyst is exceptionally rare, but represents a significant danger to patients because of its mimicry of other conditions and common delay to diagnosis. Neurosurgeons should be aware of this diagnostic entity and have a high index of suspicion in clinical scenarios suggestive of aneurysm rupture.