Red flags in asymmetric axonal neuropathy revealing MPO-ANCA–associated microscopic polyangiitis: a case report

BACKGROUND: Mononeuritis multiplex is a recognized, though sometimes overlooked, clinical feature of myeloperoxidase antineutrophil cytoplasmic antibody (MPO-ANCA)-associated vasculitis. When renal or pulmonary involvement is initially mild, the condition may mimic common neurological disorders and diagnosis can be delayed. CASE PRESENTATION: A 51-year-old woman presented with a five-month history of progressive bilateral lower-limb pain and edema, followed by three weeks of rapidly progressive asymmetric weakness affecting all four limbs. Nerve conduction studies showed severe multifocal axonal neuropathy with marked side-to-side compound muscle action potential asymmetry, consistent with mononeuritis multiplex. Laboratory evaluation demonstrated anemia, hypoalbuminemia, proteinuria, microscopic hematuria, MPO-ANCA positivity, and a subsequent creatinine rise from 76 to 121 μmol/L, indicating early but clinically evident systemic involvement. Chest computed tomography showed right-sided nodular/parenchymal pulmonary lesions with bilateral chronic/interstitial changes. Treatment with glucocorticoid-based induction therapy together with a documented cyclophosphamide-containing discharge regimen was associated with marked early improvement in pain, strength, inflammatory markers, and renal indices. CONCLUSIONS: Three red flags should prompt early ANCA testing in patients with rapidly progressive asymmetric axonal neuropathy: multifocal asymmetric axonal loss with relatively preserved conduction velocities on nerve conduction studies; early renal abnormalities such as hematuria, proteinuria, or rising creatinine; and concordant systemic findings including hypoalbuminemia, elevated inflammatory markers, or pulmonary imaging changes. Early recognition may allow treatment before axonal injury becomes irreversible.