Progressive Purpura in Microscopic Polyangiitis: A Case Report.

Purpura is one of the most common cutaneous manifestations of microscopic polyangiitis (MPA), typically presenting as palpable lesions on the lower extremities due to leukocytoclastic vasculitis. Although nonspecific, these findings often appear early and may provide important diagnostic clues before life-threatening organ involvement becomes evident. A 72-year-old woman presented with a two-month history of dry cough and a one-month history of bilateral femoral myalgia. Physical examination revealed fine crackles in the bilateral lower dorsal lung fields and localized purpura on her right lower extremity. Grasping of the limbs elicited tenderness in the bilateral quadriceps femoris, tibialis anterior, and extensor hallucis longus. A plain chest computed tomography scan revealed interstitial opacities in the subpleural regions of both lungs. Laboratory tests showed a hemoglobin level of 9.5 g/dL and a C-reactive protein level of 8.81 mg/dL. Urinalysis revealed proteinuria (0.32 g/gCr) without hematuria. Five days after the initial presentation, palpable purpura was noted on both lower extremities, along with tingling pain radiating from the dorsal feet to the toes. Myeloperoxidase-anti-neutrophil cytoplasmic antibody (MPO-ANCA) was elevated at 16.4 IU/mL. Nerve conduction studies revealed an axonal degeneration pattern in the bilateral lower limbs. A skin biopsy demonstrated leukocytoclastic vasculitis within the dermis, indicating vasculitis. Based on these findings, MPA was diagnosed. Treatment with prednisolone and azathioprine resulted in the rapid resolution of the purpura. This case underscores the importance of recognizing unilateral or progressive purpura as an early diagnostic clue of MPA, even before renal or pulmonary involvement becomes evident.