高齢患者におけるクレアチンキナーゼ正常値の特発性炎症性筋炎：診断上の課題

Myositis is an inflammatory muscle disorder characterised by progressive weakness, and in elderly patients, diagnosis can be challenging due to symptom overlap with age-related conditions and misleading inflammatory markers. We describe an elderly male with chronic lower limb weakness initially attributed to infection based on elevated inflammatory markers, with persistently normal creatine kinase (CK) levels delaying recognition. Subsequent investigations revealed positive Mi-2 antibodies, an MRI demonstrating bilateral muscle oedema with increased signal intensity, and muscle biopsy findings of HLA class I upregulation with focal complement deposition, consistent with idiopathic inflammatory myopathy. The patient responded well to corticosteroids and mycophenolate mofetil, achieving significant functional recovery. This case underscores the diagnostic difficulties of idiopathic inflammatory myopathy in the elderly, particularly when CK is normal, where initial misattribution to infection led to diagnostic delay and significant functional decline, highlighting the importance of comprehensive evaluation to avoid misdiagnosis and enable timely treatment.