Dermatomyositis Case with normal Creatine Kinase level diagnosed by F-18 FDG-PET/CT

Objective: Inflammatory myopathies are typically associated with elevated creatine kinase (CK) levels due to muscle fiber destruction. However, in this case, where CK levels remained within normal limits, the diagnosis was established through radionuclide imaging followed by muscle biopsy. Case: A 52-year-old male presented with complaints of fatigue and persistent fever. Biochemical tests, serological investigations, and whole-body FDG PET/CT were performed. The patient’s CK level was 49 U/L (reference: 39–308 U/L). PET/CT imaging revealed focal FDG uptake in the right cricoarytenoid cartilage (SUVmax 7.3), patchy uptake in both shoulders and all four extremities (SUVmax 5.6), and increased FDG uptake in the frontal and right orbitofrontal subcutaneous tissue (SUVmax 5.6 and 7.8, respectively). Based on the PET/CT findings, dermatomyositis was suspected and subsequently confirmed by histopathological examination of a muscle biopsy. Treatment was initiated with 80 mg/day of prednisolone, which was tapered to 40 mg after 6 days, and methotrexate at 10 mg/week was added. The patient’s fever resolved within two days of corticosteroid initiation, and serum CRP levels normalized rapidly. Conclusion: FDG PET/CT proved to be a critical diagnostic tool in this atypical presentation of dermatomyositis, where standard serological markers, including creatine kinase, remained within normal limits. By revealing characteristic patterns of muscular and cutaneous involvement, it not only supported the clinical suspicion but also guided the optimal biopsy site, thereby enabling a definitive diagnosis. This case underscores the importance of considering FDG PET/CT in the diagnostic algorithm of inflammatory myopathies, particularly when classical laboratory findings are absent. Early imaging-based detection facilitates timely initiation of immunosuppressive therapy, which can significantly improve patient outcomes and prevent unnecessary delays in management.