Septic embolus resulting in M1 occlusion with separate, multiple distal mycotic aneurysms, complicated by surrounding concurrent cerebral abscess: illustrative case.

backgroundIntracranial mycotic aneurysms are rare, representing only 0.7%-5.4% of all intracranial aneurysms, and typically arise from septic emboli secondary to infective endocarditis. Large-vessel occlusion due to a septic embolus is exceedingly uncommon and carries high morbidity. OBSERVATIONS: A 29-year-old man with methicillin-resistant Staphylococcus aureus endocarditis and intravenous drug use presented with intracerebral and subarachnoid hemorrhage. CT angiography revealed a left parieto-occipital hematoma with midline shift. An emergency craniotomy for hematoma evacuation exposed a ruptured distal middle cerebral artery (MCA) mycotic aneurysm, which was clipped, and pathological analysis confirmed abscess formation within the aneurysm wall. Despite antibiotic therapy, a new distal MCA aneurysm developed within an abscess cavity. Cerebral angiography later demonstrated M1 occlusion requiring mechanical thrombectomy, achieving partial reperfusion consistent with Thrombolysis in Cerebral Infarction grade 2a. The persistent abscess and aneurysm required Onyx embolization and abscess drainage in a hybrid operating room. LESSONS: Sequential mycotic aneurysms with concurrent large-vessel occlusion and abscess formation represent a rare and aggressive manifestation of septic emboli. Successful management depends on coordinated microsurgical clipping, endovascular embolization, thrombectomy, and abscess evacuation. Early recognition of evolving vascular pathology and combined surgical-endovascular approaches improve outcomes in infective mycotic aneurysm-related cerebrovascular disease. https://thejns.org/doi/10.3171/CASE25902.