Rett Syndrome Behaviour Questionnaire: Variability of Scores and Related Factors.

Rett syndrome (RTT) is a severe neurodevelopmental disorder affecting predominantly females and associated with variants in the gene. Recent success in clinical trials have resulted in an expanded use of the Rett Syndrome Behaviour Questionnaire (RSBQ) for clinical and research purposes. Implementation of the RSBQ as a global clinical severity scale has raised concerns about its construct validity considering its content, structure, and psychometric features. To further understand RSBQ data, we analyzed RSBQ scores available in the literature with a focus on variability and influencing factors. We identified publications reporting RSBQ total and/or subscale scores and summarized relevant study information, such as type of investigation, administration method, and descriptive data. We then analyzed means and standard deviations, calculating variance-to-mean ratios (VMR), as a measure of variability, when raw score descriptive statistics were available. Where appropriate, we compared means and VMRs by Welch t-tests. Of the 14 publications identified, raw total scores from 5 observational studies and 4 clinical trials (baseline) were available. Raw subscale scores from four of the five observational studies were also available. We found a wide but comparable range of mean total scores for observational studies and clinical trials. However, VMRs were significantly higher in observational studies. Subscale scores showed either high (i.e., General Mood, Breathing Problems) or low (e.g., Hand Behaviours, Body Rocking and Expressionless Face) variability. Available data demonstrated greater variability in pediatric than adult groups and less variability when using interviews or electronic RSBQ administration compared with paper forms. Total score changes over time did not affect variability. Although certain studies offered insight into the relationship between the RSBQ and other measures, overall, data were insufficient for characterizing how RSBQ variability relates to other factors. Our findings on score variability support the need for more comprehensive reporting of RSBQ data, cohort characterization, and methodology; and the deployment of standardized RSBQ administration methods, such as advanced data capture systems. There is potential for use of subscales as outcome measures, subject to further psychometric validation studies, including prospective investigations testing the stability of RSBQ scores and influencing factors. Further examining the relationship between RSBQ scores and other instruments will aid in its interpretation as a clinical outcome measure.