Primary Angiitis of the Central Nervous System Mimicking a Cardioembolism: Posterior Circulation Infarction With Subarachnoid Hemorrhage and Obstructive Hydrocephalus.

Primary angiitis of the central nervous system (PACNS) is a rare inflammatory vasculopathy that frequently mimics more common cerebrovascular disorders. We report a 44-year-old woman who presented with acute posterior circulation stroke symptoms consistent with lateral medullary syndrome. Initial neuroimaging revealed a right cerebellar infarct with minimal non-aneurysmal subarachnoid hemorrhage and multifocal intracranial arterial narrowing. Concomitant atrial fibrillation and a patent foramen ovale initially suggested a cardioembolic source; however, subsequent high-resolution vessel wall magnetic resonance imaging (VW-MRI) demonstrated concentric enhancement of multiple intracranial vessels, supporting an inflammatory vasculopathy. Extensive infectious and autoimmune workups were unremarkable. Although brain biopsy remains the diagnostic gold standard, it was declined by the patient, and the diagnosis was established on clinical and radiological grounds. The clinical course was complicated by obstructive hydrocephalus requiring external ventricular drainage, and initiation of anticoagulation was deferred, given the hemorrhagic component and evolving neurological status. Treatment with high-dose corticosteroids followed by maintenance immunosuppression resulted in clinical and radiological improvement without recurrent ischemic or hemorrhagic events. This case highlights the diagnostic complexity of PACNS presenting with concurrent ischemic and hemorrhagic stroke and underscores the value of vessel wall imaging in distinguishing inflammatory vasculopathy from competing embolic etiologies.