Look Here, Not There! Vasculitic Myopathy and a Tale of Masquerades

Background Myalgias and muscle weakness carry a wide differential of etiologies. Rheumatologists will often need to consider diagnoses such as polymyalgia rheumatica (PMR), statin-induced myopathy, and idiopathic inflammatory myopathies (IIM). There are, however, uncommon causes that can also lead to myalgias and weakness. Myopathy of the lower extremities is uncommonly due to small vessel vasculitis involving skeletal muscle in ANCA associated vasculitis (AAV). More rarely, it could be the sole presenting feature of AAV.[1-3] We describe a case of muscle weakness that was a diagnostic dilemma with investigations supportive of multiple diagnoses, eventually definitively diagnosed on muscle biopsy.

Case Report Our patient is a 76-year-old male, presenting with lower limb myalgias, weakness, and elevated inflammatory markers. He was initially treated by his other physicians as PMR with moderate dose prednisone when we met. Despite therapy with moderate dose prednisone, he did not experience significant improvement, and his symptoms were largely proximal lower extremity, with no upper extremity symptoms. He had no skin rash, or other connective tissue disease symptoms. His lower extremity symptoms were not due to inflammatory arthritis in the lower extremity joints. He had no respiratory or renal disease on history or current symptoms. He did endorse a 1-year history of chronic nasal congestion, altered taste, reduced appetite, fatigue and night sweats. He was initially on a statin which was discontinued, given a concern for possible statin induced myopathy. Imaging for malignancy was negative. Auto-antibody testing was performed, which eventually identified positive anti-SRP and positive anti-MPO ANCA, as well as elevated rheumatoid factor. While his CK had been found to be within normal limits, serum aldolase levels were found to be elevated. EMG-NCS performed by neuromuscular neurology supported a myopathic process and sensorimotor axonal polyneuropathy likely secondary to his diabetes. Ultimately muscle biopsy confirmed presence of a subacute to chronic destructive vasculitic process involving arterioles and small arteries. With these findings, the unifying diagnosis of ANCA-related small vessel vasculitis involving mainly the lower extremity skeletal muscles was made.

Conclusion Small vessel vasculitis involving mainly skeletal muscle is an uncommon presentation of ANCA associated vasculitis. The low titer positivity of multiple antibodies supporting entities which can cause similar symptoms contributed to this patient being a diagnostic dilemma. This case serves as an example of the value of consideration for small vessel vasculitis of skeletal muscle as a cause of weakness, and the importance of tissue biopsy to help confirm diagnoses.

References [1.] Conticini E. Autoimmun Rev 2022;21:103029. [2.] Ruffer N. J Neurol 2025;272:496. [3.] Shimojima Y. Autoimmun Rev 2024;23:103602.