Early skeletal muscle manifestations in polyarteritis nodosa and ANCA-associated vasculitis

Skeletal muscle involvement is common in patients with small- and medium-sized vasculitis, particularly polyarteritis nodosa (PAN) and antineutrophil cytoplasmic antibody-associated vasculitis (AAV). Despite being not included in the standard classification criteria for PAN and AAV, skeletal muscle involvement is an important clinical indicator, particularly when vasculitic myopathy is the only pathological evidence in the absence of other organ involvement. Herein, we comprehensively reviewed and compared the clinical features of 71 and 135 patients with PAN and AAV, respectively, with skeletal muscle involvement at the time of disease onset. Most patients with PAN and AAV exhibited skeletal muscle involvement, often characterized by myalgia and occasional muscular weakness, predominantly in the lower extremities. Myalgia and weakness were observed more frequently in the distal lower extremities in patients with PAN than in those with AAV. In contrast, skeletal muscle involvement tended to exhibit a more dispersed distribution across all four extremities in those with AAV. Muscle magnetic resonance imaging T2-weighted and short-tau inversion recovery sequences can effectively identify hyperintense areas attributed to hypervascularity of affected muscle tissues and serve as a sensitive and useful modality for visually determining the suitable biopsy site. >90% of patients with PAN and AAV demonstrated perivascular inflammation in their affected muscle tissues, whereas fibrinoid necrosis of the vessel walls was reported in two-thirds of patients. Serum creatine kinase (CK) levels were within the normal range in approximately 80% of patients presenting with skeletal muscle involvement in PAN and AAV. Furthermore, muscle fiber damage was milder in patients with skeletal muscle involvement in PAN and AAV than those with idiopathic inflammatory myositis. Meanwhile, serum CK levels were elevated in 65-85% of patients with PAN and AAV who had myofiber necrosis and degeneration in the affected muscles. Most patients with PAN and AAV showed improvement in skeletal muscle involvement following glucocorticoids (GCs) administration; however, relapse was observed in some patients during the tapering of GCs. In summary, skeletal muscle involvement is a potential indicator for establishing PAN and AAV diagnoses during the early phases of the disease.