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

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.

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.

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.