Proliferative Fasciitis/Myositis Involving the Facial Muscles Including the Masseter Muscle: A Rare Cause of Trismus.

BACKGROUND Proliferative fasciitis/myositis is a benign disease that can be treated conservatively. However, some patients are mistakenly treated surgically because of a misdiagnosis of the condition as a malignant tumor. CASE REPORT A 50-year-old Japanese man developed swelling in his left cheek 12 days before admission; he developed a fever and trismus 3 days later. He was admitted to our hospital because of worsening of his condition despite treatment with sitafloxacin for 5 days and needle-aspiration drainage. On admission, he had a fever of 38.1°C, swelling in his left cheek spreading to the lower jaw, and several dental caries. Although ceftriaxone and clindamycin were administered for 7 days because an odontogenic infection was suspected, his condition did not improve. T2-weighted magnetic resonance imaging of the facial muscles on Day 5 of hospitalization showed swelling and high-intensity signals in the left masseter, temporalis, and pterygoid muscles. Macroscopic findings from a biopsy of the left temporalis muscle performed on Day 17 showed white and thickened fascia. Histopathological examination revealed fibrous hyperplasia of the fascia, increased fibrous connective tissue between muscle fibers, and infiltration of inflammatory cells, providing not a definite but a compatible diagnosis of proliferative fasciitis/myositis. Beginning on Day 18, the patient's fever lessened with gradual improvement of his facial swelling and trismus. CONCLUSIONS It is imperative to include proliferative fasciitis/myositis as a possible diagnosis when patients present with facial swelling and trismus of unknown cause.