I ANCA Kidding: ANCA-Associated, Pauci-Immune Crescentic Glomerulonephritis with Normal Kidney Function Presenting with Transverse Myelitis

Introduction: Microscopic polyangiitis (MPA) frequently (>80%) manifests with kidney involvement, often presenting with rapidly progressive glomerulonephritis. However, MPA can have a variable presentation with normal kidney function, underscoring the importance of recognizing the extrarenal manifestations of the disease. Case Description: A 63-year-old Black female was admitted with a 2-week history of falls and progressive lower extremity weakness. Vital signs and electrolytes were normal with creatinine at a baseline of 0.8 mg/dL. Spinal MRI revealed longitudinally extensive transverse myelitis extending from the corticomedullary junction to the thoracic cord. A broad workup for inflammatory, neoplastic, and infectious etiologies was performed and revealed positive ANA (1:320), anti-RNP (1.2U), and anti-myeloperoxidase (1.5U) autoantibodies with P-ANCA positivity raising the concern for MPA with CNS involvement. The patient started plasma exchange (x6) and pulse steroids before being tapered to 1 mg/kg. Despite normal creatinine, nephrology was consulted due to hematuria (RBC >100) and pyuria (WBC >500) on urinalysis and 0.4g/g UPCR. Further, CT chest done for trauma/fall evaluation had revealed asymptomatic lung infiltrates which together with ANCA and renal findings raised concern for a pulmonary-renal syndrome. A transbronchial biopsy was performed first. The initial pathology report from our institution suggested capillaritis, but a second, specialty evaluation reported no vasculitis. The patient subsequently underwent a kidney biopsy which showed pauci-immune crescentic glomerulonephritis (5/20 glomeruli with crescents--3 fibrocellular/2 fibrous crescents; negative immunoglobulins and complements on immunofluorescence) with minimal (~5%) interstitial fibrosis/tubular atrophy. These findings were consistent with MPA and a rituximab-based regimen was started. At four months, the patient’s neurologic sequelae have completely resolved while her kidney function has remained at baseline. Discussion: Although MPA commonly involves the peripheral nervous system, severe central nervous system manifestations such as transverse myelitis are uncommon. In this case, early recognition of the underlying pulmonary-renal syndrome was crucial to ensuring adequate therapy and preserving kidney function.