Segmental arterial mediolysis with subarachnoid hemorrhage following chronic internal carotid artery dissection: illustrative case.

backgroundSegmental arterial mediolysis (SAM) is a rare, noninflammatory vascular disease primarily affecting small to medium arteries. Although neurological presentations like subarachnoid hemorrhage (SAH) and ischemic stroke have been reported, awareness of SAM in neurology remains limited. The authors present a unique case of SAM with SAH following chronic internal carotid artery (ICA) dissection and explore the clinical course and potential ethnic implications of the disease. OBSERVATIONS: A 56-year-old Japanese man with a history of left ICA dissection presented with sudden-onset headache and loss of consciousness. Imaging revealed a diffuse SAH caused by a ruptured vertebral artery aneurysm. Despite initial conservative treatment and subsequent coil embolization, the patient's condition deteriorated, and he died on day 10. Postmortem examination revealed multiple arterial dissections and medial degeneration, confirming SAM. LESSONS: Physicians specializing in vascular diseases should pay attention to the possibility and management of SAM in patients with multiple arterial lesions, including neurological fields. SAM involving craniocervical vessels presents a high risk of morbidity and mortality. Early recognition and appropriate management are crucial, particularly in neurovascular cases. https://thejns.org/doi/10.3171/CASE24810.