Case Report of an Atypical Presentation of Inclusion Body Myositis Masquerading as Polymyalgia Rheumatica.

Introduction: Idiopathic inflammatory myopathies are a group of rheumatologic disorders presenting with progressive muscle weakness and the presence of inflammatory infiltrates in muscle tissue on histopathology. Inclusion body myositis classically has an insidious onset and slow progression and affects the older population, most commonly men. Muscle weakness is usually asymmetric and involves the distal upper extremity muscle groups.

Case presentation: This case describes a 59-year-old man presenting with worsening symmetrical upper and lower extremity proximal muscle weakness and disabling muscle pain in his shoulders and hips. Further, weakly positive antinuclear antibodies were also observed. The creatinine phosphokinase was also remarkably elevated, uncharacteristic of both inclusion body myositis and polymyalgia rheumatica. He was initially thought to have polymyalgia rheumatica, but given the time frame and the presence of muscle pain, a muscle biopsy was done, which confirmed the inclusion body myositis.

Conclusion: This case underscores the challenges in diagnosing inclusion body myositis due to its slow progression and overlapping features with other conditions, highlighting the importance of recognizing its distinguishing characteristics.