Focal Myositis of the Masseter Muscle Masquerading as Temporomandibular Disorder: Diagnostic Challenges and Management.

Focal myositis (FM) is a rare, localized inflammatory muscle disease that is often incorrectly diagnosed due to nonspecific symptoms, especially in the head and neck region. We present a case of FM involving the masseter muscle in a 76-year-old man, which was initially presumed to be a temporomandibular joint disorder. The patient exhibited trismus and swelling of the right cheek. Magnetic resonance imaging (MRI) showed masseter muscle swelling and high signal intensity on diffusion-weighted image (DWI). Histopathological findings showed FM with moderate inflammatory cell infiltration and muscle atrophy. Empirical treatment with sulbactam sodium and ampicillin sodium was administered, followed by corticosteroid treatment with prednisolone, which significantly improved the symptoms. Persistent trismus remained due to fibrosis, highlighting the importance of early diagnosis and intervention. This case highlights the importance of considering FM in the differential diagnosis of unilateral masticatory muscle swelling and trismus. Imaging and histopathological findings are necessary for accurate diagnosis, and early corticosteroid therapy is essential to prevent sequelae such as fibrosis. A multidisciplinary approach is essential for effective management.