Abstract
INTRODUCTION: ANCA-associated vasculitis (AAV) typically presents with renal or pulmonary involvement, while skeletal muscle manifestations are rare. This case highlights a diagnostic dilemma where a predominantly neuromuscular presentation—initially attributed to infection or primary myopathy—ultimately revealed systemic vasculitis on renal biopsy. This case underscores the importance of recognising non-classical presentations of AAV where inflammatory myopathy is the predominant initial manifestation. The diagnostic pathway was complex and highlighted the value of multidisciplinary collaboration, together with careful balance of immunosuppressive agents. CASE DESCRIPTION: An 80-year-old female presented with a two-week history of lethargy, fever, weight loss, dyspnoea, and progressive lower limb weakness. She was initially managed for presumed community-acquired pneumonia following but chest imaging revealed no consolidation, and empirical antibiotics were ineffective. Blood tests showed elevated inflammatory markers and a mildly raised creatine kinase. Neurological evaluation revealed bilateral lower limb weakness and EMG changes consistent with inflammatory myopathy. MRI of the thighs and spine revealed symmetrical muscle oedema. Despite a negative myositis antibody screen, autoimmune testing revealed strong MPO-ANCA positivity. While rheumatology initially favoured an infective cause, she subsequently developed significant proteinuria and rising creatinine levels. Renal biopsy confirmed necrotising glomerulonephritis consistent with ANCA vasculitis. Immunosuppressive treatment was initiated with rituximab, avacopan and a tapering course of corticosteroids. The patient responded well to treatment initially but experienced a relapse upon steroid tapering, with worsening renal function and neuromuscular symptoms. Increasing the steroid dose led to marked clinical improvement. She continues to be followed in renal and rheumatology clinic, with gradual steroid tapering and ongoing monitoring of renal and neurological function. [Image: see text] DISCUSSION: This case presented diagnostic and therapeutic challenges due to its atypical features. The initial clinical picture mimicked infection, and the presence of inflammatory myopathy—uncommon in AAV—diverted attention from vasculitis. Negative myositis serology further complicated the picture. However, persistent symptoms and MRI findings prompted a broader autoimmune work-up, leading to the diagnosis. Steroid initiation provided early benefit, and the addition of rituximab and avacopan was appropriate given the renal involvement and steroid-sparing need. However, the relapse during tapering highlights the complexity of managing steroid-dependent disease in elderly patients and raises questions about optimal tapering speed and maintenance strategies. This case underscores the importance of considering vasculitis in the differential diagnosis of unexplained myopathy, especially when accompanied by systemic symptoms. It also illustrates the value of timely renal biopsy in identifying pathology that can be masked by non-specific presentations. Discussion points include the evolving role of avacopan in AAV management, its impact on relapse rates during tapering, and the potential overlap between inflammatory myopathy and vasculitis. This case reinforces the need for a multidisciplinary approach and highlights the benefit of early rheumatology input when classical features are absent. KEY LEARNING POINTS: • Atypical presentations: AAV can present initially with neuromuscular symptoms, including inflammatory myopathy, before classic renal or pulmonary features emerge. • Diagnostic complexity: Negative myositis serology does not exclude immune-mediated myopathy. Persistent systemic symptoms should prompt consideration of vasculitis, especially in older adults with multisystem involvement. • Multidisciplinary approach: Early involvement of rheumatology, neurology, nephrology, and radiology was essential in reaching a diagnosis and guiding tailored treatment. • Therapeutic considerations: The patient responded well to a combination of corticosteroids, rituximab, and avacopan. However, her relapse during steroid tapering highlights the need for careful balancing of immunosuppression, especially in elderly and frail patients.