Longitudinal Trajectories of the Behavioral and Health Phenotype of Adolescents and Adults with Fragile X Syndrome

Background: Individuals with fragile X syndrome (FXS) display behavior problems, functional limitations, and health problems across the lifespan (Smith et al., 2016; Raspa et al., 2010). However, much of what we know about the phenotype comes from cross-sectional studies investigating children and adolescents.

Objectives: To use longitudinal data to examine ten-year trajectories of behavior (daily living skills and behavior problems) and health (body mass index [BMI] and number of health conditions) in adolescents and adults with FXS, accounting for sex and autism spectrum disorder (ASD) symptoms.

Methods: Participants (n = 134) were drawn from a longitudinal study of families of adolescents and adults with FXS (Mailick et al., 2014). At study entry, individuals with FXS met three criteria: 1) were 12 years of age or older, 2) had received a diagnosis of FXS, and 3) lived with parents or had at least weekly contact. If there was more than one co-residing child, mothers reported on the child she viewed as most severely affected. Individuals with FXS were mostly males (85.1%), in their 20s on average (M = 20.19, SD = 6.92, range 12–48 years), and most lived with mothers (90.3%). About one-quarter (24.2%) of the sample had a co-occurring diagnosis of ASD. Mothers reported on sons’ or daughters’ daily living skills, behavior problems, height and weight, and number of health conditions, as well as lifetime ASD symptoms. The present sample was restricted to those with intellectual disability.

Results: Hierarchical linear modeling was used to describe change in the behavioral and health phenotype of adolescents and adults over ten years and to examine effects of between-person differences in sex and ASD symptomatology on trajectories. We evaluated separate growth curve models for four outcomes: daily living skills, behavior problems, BMI, and health conditions. Sex and lifetime ASD symptoms at Time 1 were entered as predictors of the intercept and the linear and quadratic slopes. Results indicated increased independence in daily living skills and decreases in behavior problems improved over time. However, BMI increased over time, indicating a greater proportion of individuals meeting the cutoffs for overweight or obesity across the course of the study. The number of health conditions also increased over time. Adolescents and adults who started with fewer ASD symptoms were more independent in daily living skills and had fewer behavior problems.

Conclusions: This is the first study to examine longitudinal trajectories of behavioral and health phenotypes for adolescents and adults with FXS over the course of a decade. Trajectories for daily living skills and behavior problems indicated improved over time. Individuals with elevated levels of ASD symptoms were at risk for lower levels of independence in daily living and more behavior problems at the initial time point than were those with lower levels of ASD symptoms, suggesting possible targets for intervention.