Health-related quality of life in facial palsy: translation and validation of the Dutch version Facial Disability Index

Purpose Patient-reported outcome measures are essential in the evaluation of facial palsy. Aim of this study was to translate and validate the Facial Disability Index (FDI) for use in the Netherlands. Methods The FDI was translated into Dutch according to a forward-backward method. Construct validity was assessed by formulating 22 hypotheses regarding associations of FDI scores with the Facial Clinimetric Evaluation scale, the Synkinesis Assessment Questionnaire, the Short Form-12 and the Sunnybrook Facial Grading System. Validity was considered adequate if at least 75% (i.e. 17 out of 22) of the hypotheses were confirmed. Additionally, confirmatory factor analysis was performed. Cronbach's alpha was calculated as a measure of internal consistency. Participants were asked to fill out the FDI a second time after 2 weeks to analyse test-retest reliability. Lastly, smallest detectable change was calculated. Results In total, 19 hypotheses (86.4%) were confirmed. Confirmatory factor analysis showed acceptable fit for the two factor structure of the original FDI (root mean square error of approximation = 0.064, standardized root mean square residual = 0.081, comparative fit index = 0.925, Chi-square = 50.22 with 34 degrees of freedom). Internal consistency for the FDI physical function scale was good (alpha > 0.720). Internal consistency for the FDI social/well-being scale was slightly less (alpha > 0.574). Test-retest reliability for both scales was good (intraclass correlation coefficients > 0.786). Smallest detectable change at the level of the individual was 17.6 points for the physical function and 17.7 points for the social/well-being function, and at group level 1.9 points for both scales. Conclusion The Dutch version FDI shows good psychometric properties. The relatively large values for individual smallest detectable change may limit clinical use. The translation and widespread use of the FDI in multiple languages can help to compare treatment results internationally.