Gross Motor Function in Individuals With SCN2A-Related Disorders: The Clinical Trial Readiness Study.

Background and objectives: SCN2A-related disorders (RDs) are genetic conditions characterized by severe to profound impairments in multiple domains including gross motor function, which could serve as a nonseizure outcome in precision medicine therapy trials. This study evaluated specific properties of the Vineland Adaptive Behavior Scales-3 (VABS3) and other motor assessments for their fitness for use in trials of SCN2A-RDs.

Methods: Sixty-five families recruited through the FamileSCN2A foundation enrolled their affected children ("participants," 28 female, median age 6.4 years, interquartile range [IQR] 4.1-10.5) in a 1-year, longitudinal study. Assessments were administered at 0 (study entry), 6, and 12 months. Assessments included the VABS3, Adaptive Behavior Assessment System 0-5 years (ABAS), a modified Functional Mobility Scale (FMS), and the Functional Activities Questionnaire-Walking Level (FAQ-WL).

Results: The VABS3 composite score (34 [IQR 26-46]) indicated overall adaptive function >4 SDs below the normative mean. Forty percent of participants aged 2 years or older required wheelchairs for home distances, and 28% could not take any steps. The median standardized scores (SSs) for the VABS3 motor domain (20 [IQR 20-32]) and gross motor subdomain (1 [IQR 1-2]) reflected performance at the floor of the measures. Standardized motor scores discriminated poorly among participants with different levels of mobility (FAQ-WL and FMS) and different markers of diseases severity (presence of epilepsy, history of epileptic spasms, number of seizure medications). Cross-sectionally, SSs declined with increasing age. By contrast, raw scores of the VABS3 and ABAS and growth scale values (GSVs) of the VABS3 had relatively little floor effects. They distinguished well between participants based on FAQ-WL and FMS scores and between those with different disease severity markers. Test-retest and inter-rater reliability for all scores were excellent. No motor score changed significantly over time in the longitudinal analyses.

Discussion: Gross motor function in people with SCN2A-RDs is so severely impaired that it cannot be adequately measured with norm-referenced (standardized) scores. GSVs and alternative scoring assessments used out of their intended age range have superior and promising psychometric features in this severely impaired group, and they should be considered in future precision medicine trials for SCN2A-RDs and other similarly severe, rare disorders.