Masticator Muscle Fibrosis

Epidemiology

Radiation therapy for head and neck cancers may induce fibrosis in the muscles of mastication (masseter, temporalis, medial and lateral pterygoid muscles), resulting in trismus. This complication, which may be seen in 5 to 15% of patients, is most commonly encountered following treatment for malignancies involving the nasopharynx, tonsil, retromolar trigone, and paranasal sinuses. The frequency of masticator muscle fibrosis and trismus is higher if there is also a history of surgical treatment or malignant infiltration of the masticator muscles.

Clinical Findings

Patients with radiation-induced masticator muscle fibrosis typically present with increasing trismus a few years following treatment. Some patients may initially have trismus as a result of tumor involvement of the masticator muscles. Following successful tumor treatment, trismus subsides but may return later because of masticator muscle fibrosis. Clinical evaluation shows varying degrees of atrophy of masseter and temporalis muscles with associated limited excursion of the temporomandibular joint.