Case of the Week
Section Editors: Matylda Machnowska1 and Anvita Pauranik2
1University of Toronto, Toronto, Ontario, Canada
2BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada
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June 17, 2021
Giant Thrombosed Posterior Cerebral Artery Aneurysm
- Background:
- Giant intracranial aneurysms (GIAs, measuring >25 mm) are uncommon (around 5% of the total aneurysms), but they are potentially fatal, with a mortality rate of up to 11%.
- There is a female predominance (M:F = 1:~3), and patients commonly become symptomatic between 40 and 70 years of age. The incidence of GIAs in the posterior circulation is 34%. (The incidence of aneurysms in the posterior circulation is 15% of all aneurysms.)
- GIA can be classified into 3 categories: saccular aneurysm growing from small berry aneurysms, fusiform aneurysm secondary to atherosclerosis or connective tissue disorder, and serpentine aneurysm probably growing when the main blood flow is diverted from the aneurysmal wall.
- The basilar tip and P1 segment of the PCA account for about 70% of GIAs in the posterior circulation, whereas the cavernous portion of the internal carotid artery or ophthalmic artery is a common site in the anterior circulation.
- Clinical Presentation:
- Mass effect, subarachnoid hemorrhage, and ischemic symptoms are common presentations, while neurologic symptoms may vary depending on the location and size of the aneurysm.
- The ophthalmic and cavernous segments of the internal carotid artery frequently develop giant aneurysms due to surrounding hemodynamic pressures, resulting in ophthalmoplegia and facial pain.
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PCA aneurysm can compress the adjacent temporal lobe or optic tract, resulting in seizures, memory loss, and incongruous hemianopsia.
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The 5-year cumulative rupture risk is about 40% for the anterior circulation, while the risk is about 50% for the posterior circulation.
- Key Diagnostic Features:
- CT: Slightly hyperdense, well-defined extra-axial mass with or without a peripheral calcification
- MRI: An “onion-skin” appearance on T1WI (due to intramural hemorrhage) is seen in partially thrombosed aneurysms. Flow voids are absent in totally thrombosed aneurysms, compared with partially thrombosed aneurysms, in which a flow void is usually present. Thrombosed lumen is poorly visualized on MRA and DSA.
- Differential Diagnoses:
- Cavernoma: Peripheral hemosiderin rim and central hyperintensity on T2WI may appear similar to that of aneurysms. Associated developmental venous anomaly can be seen.
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Ganglioglioma: Solid and cystic mass with variable enhancement, typically found in the temporal lobe, with calcification seen in 30%–50%
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Dysembryoplastic neuroepithelial tumor (DNT): Usually seen as a hyperintense cortical lesion with a bubbly appearance on T2WI; a well-defined hyperintense rim on FLAIR images is both sensitive and specific.
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Pilocytic astrocytoma: Well-defined solid and cystic tumor typically arising from the cerebellar hemisphere; involvement of the optic pathway is the second common presentation, especially in patients with neurofibromatosis type I. The solid portion shows intense-to-heterogeneous enhancement.
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Thrombosed aneurysms of the PCA can be considered when an apparent long-term epilepsy-associated tumor (LEAT) does not appear to be clearly intra-axial and/or has a greater degree of intrinsic cystic-appearing component with peripheral rim enhancement. On CT, an underlying thrombosed aneurysm should be suspected when there is a focal hyperdense mass or hematoma in typical sites along the expected course of vessels, such as in the cavernous sinus (ICA), prepontine cistern (basilar artery), or along the medial temporal lobe (for PCA).
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Treatment:
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Endovascular treatment (parent vessel occlusion or selective coil occlusion of the remaining lumen) or open surgery
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