1536: Non-Aneurysmal Subarachnoid Hemorrhage as a Presentation of Undiagnosed Microscopic Polyangiitis

Introduction: Anti-neutrophil cytoplasmic antibody–associated vasculitides (ANCA-AAV), such as microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis, predominantly affect small vessels and can involve the kidneys, lungs, and peripheral nerves. Central nervous system (CNS) involvement is uncommon; subarachnoid hemorrhage (SAH), particularly due to p-ANCA–associated vasculitis, is exceedingly rare. Description: We report a case of a 52-year-old woman with untreated systemic lupus erythematosus (SLE) who presented with sudden-onset severe headache and vomiting. She was hypertensive and somnolent, without focal neurological deficits. Laboratory workup revealed acute kidney injury (creatinine 3.2 mg/dL) and significant proteinuria (urine protein/creatinine ratio 23.83). Head CT showed SAH (Hunt and Hess grade 2, Fisher grade 4). CT angiography revealed no aneurysms, suggesting a non-aneurysmal etiology. Her mental status subsequently declined, and she developed hydrocephalus requiring placement of an external ventricular drain. She also developed hypoxia and hemoptysis; bronchoscopy confirmed diffuse alveolar hemorrhage. Serology revealed a positive antinuclear antibody, elevated p-ANCA (8.0 antibody index), normal complement levels, and negative anti–double-stranded DNA. Renal biopsy demonstrated pauci-immune crescentic glomerulonephritis with fibrinoid necrosis, confirming MPA. She was treated with high-dose methylprednisolone and plasmapheresis. Despite this, she developed severe ARDS requiring intubation. Chest CT demonstrated bilateral ground-glass opacities. Rituximab and IV Immunoglobulin were initiated, but her condition continued to worsen. Care was transitioned to comfort measures, and she passed away in hospice. Discussion: CNS involvement in AAV occurs in approximately 7–10% of cases, usually as ischemic stroke or intracerebral hemorrhage. Non-aneurysmal SAH is particularly rare in p-ANCA vasculitis. This case mirrors others in the literature describing SAH due to small-vessel fragility. Though SLE broadened the differential diagnosis, biopsy and serology supported a primary diagnosis of AAV. This case underscores the need to consider vasculitis in angiography-negative SAH, especially in patients with systemic manifestations such as renal or pulmonary involvement.