Mycotic aneurysm presenting in a patient with a ventriculoatrial shunt with positive CSF cultures for Cutibacterium acnes: illustrative case.

BACKGROUND: Mycotic aneurysms (MAs) typically present with intracranial hemorrhage but are a rare cause of subdural hematoma (SDH). Although infective endocarditis is the most common etiology, MAs may also result from bacteremia associated with intravenous drug use, meningitis, or poor dentition. Patients with ventriculoatrial shunts (VASs) are at increased risk for shunt-related bacteremia compared to those with ventriculoperitoneal shunts, potentially predisposing them to MA formation. OBSERVATIONS: This is the case of a 61-year-old female with a fusiform distal left middle cerebral artery MA presenting as an acute-on-chronic SDH without endocarditis, but with positive blood cultures and CSF cultures for Cutibacterium acnes from a long-term VAS. LESSONS: MAs can rarely present as SDHs and should be considered in the differential diagnosis for patients with a long-standing VAS due to the risk of subacute bloodstream infections. https//thejns.org/doi/10.3171/CASE25625.