MRI findings in patients with severe trismus following radiotherapy for nasopharyngeal carcinoma.

The aim of the study was to document MRI findings in masticator structures in patients with trismus developing after radiotherapy for nasopharyngeal carcinoma (NPC). MRI neck examinations were reviewed in 35 patients with marked trismus, defined as an interincisal gap of 25 mm or less, post-radiotherapy for NPC. Patients with trismus before treatment, infiltration of masticator structures at the time of trismus, or previous surgery involving the masticator structures were excluded. Sixteen patients had no significant abnormality in their masticator structures (46%). Nineteen patients (54%) had abnormalities comprising radiotherapy-induced masticator muscle fibrosis (n = 19), denervation atrophy of the masticator muscles secondary to mandibular nerve damage (n = 1), mandibular ramus signal abnormalities (n = 5), mandibular condyle sclerosis with or without capsular thickening (n = 5), perimasticator fibrosis extending into the masticator space (n = 3) and inflammation secondary to severe sinusitis extending into the masticator space (n = 2). Nine patients (26%) had more than one type of abnormality. Twenty-two patients (63%) had concomitant skull base osteoradionecrosis which extended into the pterygoid bases in 16 patients (45%). The presence of several MRI abnormalities in the masticator structures of patients with trismus after radiotherapy suggests that trismus is multifactorial. This study advances the understanding of mechanisms behind this debilitating side effect of radiotherapy.