A coexisting pseudoaneurysm hidden behind a saccular distal middle cerebral artery aneurysm in long-standing postirradiation vasculopathy: illustrative case.

BACKGROUND: Delayed cerebrovascular complications after childhood cranial irradiation may include progressive vasculopathy and aneurysmal disease. However, attributing a ruptured aneurysm directly to prior irradiation can be difficult in patients with complex preexisting cerebrovascular pathology. The authors describe a diagnostic pitfall in a patient with long-standing postirradiation vasculopathy who presented with hemorrhagic distal middle cerebral artery (MCA) aneurysmal lesions. OBSERVATIONS: A 51-year-old man who had undergone whole-brain irradiation for pineal germinoma at 8 years of age presented with intracerebral and subarachnoid hemorrhage. He had a long history of progressive cerebrovascular disease, including recurrent ischemic strokes, prior intracerebral hemorrhage, intracranial arterial stenosis, and a previously treated internal carotid artery aneurysm. MR angiography and initial digital subtraction angiography demonstrated a newly detected saccular distal MCA aneurysm, which was treated with coil embolization. Hematoma enlargement occurred the following day. Repeat angiography showed complete obliteration of the treated saccular aneurysm but revealed a coexisting pseudoaneurysm in the same vascular territory. Parent artery occlusion achieved definitive hemostasis without additional clinically apparent neurological deterioration. LESSONS: In patients with long-standing postirradiation vasculopathy, a conspicuous saccular aneurysm may obscure a coexisting pseudoaneurysm. Repeat angiography should be considered when hemorrhage progresses despite apparent aneurysm obliteration. https://thejns.org/doi/10.3171/CASE26561.