Trismus or lockjaw is the reduction in mouth opening. Radiotherapy‐induced trismus occurs when the masticatory muscles are within range of radiation treating head and neck cancer, resulting in muscular fibrosis and trismus. We aim to assess the measurement properties of available validated patient‐reported outcome measures relevant to radiotherapy‐induced trismus in head and neck cancer patients. Systematic electronic searches were performed in MEDLINE and EMBASE databases to identify studies validating radiotherapy‐induced trismus with patient‐reported outcome measures up to January 2018. Two reviewers independently rated the methodological quality of identified validation studies, using the consensus‐based standards for the selection of health status measurement instrument (COSMIN) checklist. The level of evidence for measurement properties was then summarised based upon pre‐defined criteria. Only one of the 280 retrieved studies were deemed fit for inclusion. The Gothenburg Trismus Questionnaire was assessed and found to have a good methodological quality of measurement properties except for reliability, which was poor. There was a moderate level of evidence supporting internal consistency, content validity, structural validity and hypothesis testing. The evidence for test‐retest reliability remains unknown in the population of interest. The Gothenburg Trismus Questionnaire is the only patient‐reported outcome measure in the field of showing some good measurement properties, and it may therefore represent a useful instrument for the assessment of radiotherapy‐induced trismus in head and neck cancer patients. However, further validation studies are needed especially with respect to the reliability, measurement error and the responsiveness of this instrument.
