Myalgia as initial presentation of microscopic polyangiitis: diagnostic utility of kidney biopsy.

Microscopic polyangiitis (MPA), a form of ANCA-associated vasculitis (AAV), can present a diagnostic challenge when it manifests with atypical symptoms. We report a case of MPA where the predominant clinical feature was myalgia with normal creatine kinase levels, underscoring the importance of a comprehensive diagnostic approach. A 60-year-old male presented with bilateral lower leg myalgia and gait disturbance. Physical examination revealed muscle tenderness and weakness. Magnetic resonance imaging (MRI) with fat-suppressed T2-weighted imaging showed heterogeneous high signal intensity in the lower limb muscles, suggestive of myositis. However, laboratory investigations found normal serum creatine kinase levels, and all tested myositis-specific autoantibodies were negative. A muscle biopsy yielded inconclusive results. In contrast, serology was positive for myeloperoxidase-antineutrophil cytoplasmic antibody (MPO-ANCA). Despite preserved kidney function and unremarkable urinalysis, a kidney biopsy was performed, which revealed fibrinoid necrosis in small arteries. This led to a definitive diagnosis of MPA. This case highlights that MPA should be considered a key differential diagnosis in patients presenting with myalgia, particularly in the presence of a positive MPO-ANCA serology. While muscle manifestations occur in ~20% of AAV cases, they are not included in the current classification criteria. Our findings underscore that kidney biopsy remains a critical diagnostic tool for establishing a definitive diagnosis of MPA, even when renal symptoms are minimal, facilitating timely and appropriate immunosuppressive therapy.