Trismus has been associated with significant morbidity post–radiation therapy, with significant health implications, including reduced nutrition due to impaired mastication, difficulty in speaking, and compromised oral hygiene. Limitations in jaw opening have been reported in 6% to 86% of patients who received radiation to the temporomandibular joint and/or masseter/pterygoid muscles, with frequency and severity that are somewhat unpredictable.
The loss of function and range of mandibular motion from radiation therapy appears to be related to fibrosis in and damage to the muscles of mastication. Studies have demonstrated that an abnormal proliferation of fibroblasts is an important initial event in these reactions. Additionally, there may be scar tissue from radiation therapy or surgery, nerve damage, or a combination of these factors. Regardless of the immediate cause, mandibular hypomobility will ultimately result in degeneration of both muscle and temporomandibular joint.
Radiation therapy involving the temporomandibular joint, the pterygoid muscles, or the masseter muscle is most likely to result in trismus. Tumors related to this type of radiation can appear in the following locations:
- Oral cavity.
- Base of tongue.
- Salivary gland.
- Maxilla or mandible.
The prevalence of trismus increases with increasing doses of radiation, and levels in excess of 60 Gy are more likely to cause trismus. Patients who have been previously irradiated and who are being treated for a recurrence appear to be at higher risk of trismus than those who are receiving their first treatment.[32,33] This suggests that the effects of radiation are cumulative, even over many years. Radiation-induced trismus may begin toward the end of radiation therapy or at any time during the subsequent 24 months. Limitations in opening the mouth often increase slowly over several weeks or months. The condition may worsen over time or remain the same, or the symptoms may reduce over time, even in the absence of treatment.
Limited mouth opening frequently results in reduced nutritional status. These patients may experience significant weight loss and nutritional deficits. It is generally accepted that weight loss of more than 10% of initial body weight is considered significant. This is of particular importance at a time when the patient is recovering from surgery, chemotherapy, and/or radiation therapy. Additionally, it lowers the ability for social eating and thereby increases the risk of social isolation and decrease in quality of life in patients with HNC.