Electrodiagnostic Confirmation of Mononeuritis Multiplex in Evolving Myeloperoxidase-Antineutrophil Cytoplasmic Antibody–Positive Microscopic Polyangiitis: A Case Report

Peripheral neuropathy in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis may present as mononeuritis multiplex, but early recognition can be difficult when focal foot drop is the dominant manifestation.We report the case of a 63-year-old man with fibrotic interstitial lung disease and prior atypical perinuclear ANCA positivity who developed painful, asymmetric lower-extremity sensory symptoms, followed by recurrent right foot drop.Nerve conduction studies and needle electromyography demonstrated an asymmetric, non-length-dependent, multifocal axonal neuropathy involving the right common peroneal distribution, bilateral tibial nerves, selected distal sensory nerves, and bilateral ulnar sensory nerves, consistent with mononeuritis multiplex.Five days later, he was hospitalized for a left lateral medullary infarction; brain magnetic resonance imaging showed diffusion restriction in the left lateral medulla, and noncontrast time-of-flight magnetic resonance angiography favored left posterior inferior cerebellar artery dissection rather than definite angiographic vasculitis.Repeat testing disclosed high-titer myeloperoxidase (MPO)-ANCA, elevated inflammatory markers, and worsening renal function.Sural nerve biopsy was nondiagnostic.The rheumatology team diagnosed MPO-AN-CA-positive microscopic polyangiitis, and glucocorticoids, cyclophosphamide, and later rituximab were administered.This case illustrates that microscopic polyangiitis may evolve over years and may present with predominant peripheral neuropathy before overt systemic disease becomes apparent.Electrodiagnostic testing provided objective confirmation and localization of clinically suspected mononeuritis multiplex, supporting repeated diagnostic reassessment for systemic vasculitis.