Masticatory Muscle Myositis Presenting as a Neoplasm in a Young Woman: A Case Report and Review of Literature

Masticatory muscle myositis is the most common inflammatory myopathy among canines and is associated with an autoimmune process; however, the cause is unknown. A 28-year-old young woman was notices with left facial swelling and pain, particularly during mouth opening. No obvious trigger factors, including trauma, were found. Computed tomography (CT) scans of the head and neck revealed a heterogenous mass in the left masseter muscle. Due to persistent discomfort, tumor excision was performed. The histopathologic findings of the mass included several degenerative muscle fibers with enlargement and atrophy accompanied by lymphocytic infiltrates. Differential diagnosis including main myopathies was considered, but our case did not fulfill any criteria of these myopathies. Therefore, we diagnosed the patient with masticatory muscle myositis, which is rare in humans. Further investigation is necessary for understanding the disease.