Localized Immune-Complex Small-Vessel Vasculitis Presenting as Unilateral Myositis After Hepatitis A.

Immune-complex small-vessel vasculitis (ICSVV) is a subtype of vasculitis characterized by inflammation and necrosis of small blood vessels caused by deposition of circulating immune complexes in vessel walls, followed by complement activation and recruitment of inflammatory cells. It typically targets postcapillary venules in the skin, but other small vessels in muscles, kidneys, gastrointestinal tract, and other organs can also be affected. Vasculitic myositis is usually bilateral and symmetric, with only a few reports of muscle-limited disease. Hepatitis A virus (HAV) is an uncommon trigger of vasculitis, in contrast to the well-recognized associations of hepatitis B with polyarteritis nodosa and hepatitis C with cryoglobulinemic vasculitis. We report a case of a woman in her early 30s who experienced intense discomfort in her left groin and thigh, accompanied by swelling and fever, two months after a confirmed acute HAV infection. Laboratory studies demonstrated severe systemic inflammation (WBC 30×10³/µL, C-reactive protein (CRP) 348 mg/L, erythrocyte sedimentation rate (ESR) >100 mm/h) with normal creatine kinase (CK). Axial fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) and coronal MRI revealed diffuse left-thigh inflammation without abscess. MRI-guided biopsy of the vastus lateralis demonstrated transmural inflammation and fibrinoid necrosis of intramuscular arteries with mixed CD4+/CD20+ infiltrates and C5b-9 deposition, consistent with ICSVV. The patient received oral prednisone 1 mg/kg daily (tapered over 10 weeks) and methotrexate 15 mg weekly; CRP decreased to 3.4 mg/L within four weeks. Remission was achieved by six months, with only mild residual numbness. This case represents the first report of biopsy-proven unilateral immune-complex vasculitic myositis temporally associated with HAV infection. It emphasizes the importance of considering vasculitic myositis in the differential diagnosis of focal limb swelling with systemic inflammation, even when CK levels are normal. It also highlights the role of MRI-guided biopsy in establishing the diagnosis. Early immunosuppressive therapy can improve outcomes in this rare but treatable condition.