Angiographic doughnut-shaped aneurysm of intracranial internal carotid artery

Morphologic characteristics of angiographic doughnut-shaped aneurysms of the intracranial internal carotid artery (ICA) have been rarely reported in the literature and were discussed only radiologically. In this case, surgical clipping was performed, and the specimen from the aneurysm was examined to correlate the pathologic findings and morphologic characteristics. A 59-year-old male presented with diffuse subarachnoid hemorrhage secondary to a ruptured vascular lesion originating from the intracranial portion of the right ICA. Digital subtraction angiography showed a 1.5-cm doughnut-shaped aneurysm at the junction of the ICA and the posterior communicating artery. The aneurysm was successfully clipped in a neurosurgical hybrid operating room. The pathologic findings of the content inside the aneurysm revealed only fibrinoid exudates, materials from thrombosis. This is the first report to prove that the mechanism of a doughnut-shaped aneurysm formation is central thrombosis inside the aneurysm, possibly related to its complex hemodynamic flow.