Assessment, diagnosis, severity, and considerations for phenotype classification of trismus: a systematic review.

BACKGROUND: Trismus is a prevalent and functionally debilitating consequence of head and neck cancer (HNC) and its treatments, yet variability in assessment practices limits accurate incidence estimation, cross-study comparison, and development of targeted interventions. This systematic review maps current methodologies used to assess, diagnose, and classify trismus across clinical contexts. METHODS: Following Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines, eight databases and gray literature sources were searched (March 2025). Studies were included if they assessed, categorised, or diagnosed trismus in adults. Data were extracted on clinical context, assessment methodology, diagnostic thresholds, severity grading, intent of measurement, and modifications for dentition. Due to heterogeneity, findings were synthesised descriptively using best-evidence methodology. RESULTS: Of 882 records identified, 215 studies (23,554 participants) met eligibility criteria. Maximal interincisal opening (MIO) was the most common diagnostic measure (n = 111), typically with a < 35 mm threshold, though cut-offs and tools varied widely. Oncology toxicity frameworks (Common Terminology Criteria for Adverse Events, n = 44; European Organisation for Research and Treatment of Cancer, n = 29) were frequently applied but lacked alignment with functional outcomes or validated patient-reported outcomes (PROs). Management of edentulous patients was inconsistently reported. CONCLUSION: Assessment of trismus lacks standardisation in diagnostic criteria, measurement tools, and severity classification. Current approaches, dominated by aetiology-agnostic MIO thresholds and oncology toxicity scales, fail to capture the multidimensional nature of trismus or distinguish its diverse phenotypes and burden to HNC survivors. While the 35 mm MIO threshold provides a reproducible diagnostic standard, it is agnostic to aetiology and to the impact on an individual's life and function, limiting its utility as a standalone characterisation tool. These findings highlight the need for a more multi-dimensional approach to trismus assessment that integrates objective measurement, patient-reported outcomes, and clinical evaluation of contributing aetiologies, to support diagnostic precision, treatment evaluation, and patient-centred care in cancer survivorship.