Rett syndrome: establishing a novel outcome measure for walking activity in an era of clinical trials for rare disorders

Abstract

Background: Rett syndrome is a pervasive neurological disorder with impaired gait as one criterion. This study investigated the capacity of three accelerometer-type devices to measure walking activity in Rett syndrome. Methods: Twenty-six participants (mean 18 years, SD 8) wore an Actigraph, ActivPAL and StepWatch Activity Monitor (SAM) during a video-taped session of activities. Agreement was determined between step-counts derived from each accelerometer and observation. Repeatability of SAM-derived step counts was determined using pairs of one-minute epochs during which the same participant was observed to walk with the same cadence. Results: The mean difference (limit of agreement) for the Actigraph, ActivPAL and SAM were −41 (SD 33), −16 (SD 21) and −1 (SD 16) steps/min, respectively. Agreement was influenced by a device/cadence interaction (p < 0.001) with greater under-recording at higher cadences. For SAM data, repeatability of step-count pairs was excellent (intraclass correlation coefficient 0.91, 95% CI 0.79–0.96). The standard error of measurement was 6 steps/min and we would be 95% confident that a change ≥17 steps/min would be greater than within-subject measurement error. Conclusions: The capacity of the SAM to measure physical activity in Rett syndrome allows focus on participation-based activities in clinical practice and clinical trials.Implications for RehabilitationMany girls and women with Rett syndrome are able to walk on their own or with assistance but with altered movement patterns.Validated measures of physical activity, such as step counts, have potential to monitor function during daily life.Compared with other forms of accelerometer-type devices, such as ActiGraph and ActivPAL, the StepWatch Activity Monitor (SAM) measured step counts with good accuracy and repeatability.The capacity of the SAM to measure physical activity in Rett syndrome allows focus on participation-based activities in clinical practice and clinical trials.