Longitudinal Trajectory of Adaptive Behavior in Individuals with Fragile X Syndrome

Background: Fragile X syndrome (FraX) is the leading inherited cause of developmental disability. Adaptive behavior, the term used to indicate an individual’s ability to independently function in their environment changes over the course of an individual’s life. Individuals with FraX have strengths in daily living skills with weaknesses in socialization and communication skills, similar to individuals with autism (ASD).  Some reports suggest that there may be changes in adaptive behavior over time, with different studies showing gains and others declines.  

Objectives:  The purpose of this study was to examine, prospectively and longitudinally, the adaptive behavior of children with FraX. Specifically, we examined the mean level and rates of development for the Adaptive Behavior Composite score and the domain scores as measured by the Vineland Adaptive Behavior Scales (Vineland). In this large scale study, we investigated the rate of adaptive skill acquisition as well as the adaptive behavior profiles in individuals ages 6 through 16 years.

Methods:  The participants were 265 individuals with FraX (186 males, 79 females) and 225 typically developing individuals (124 males, 101 females). All individuals with FraX were diagnosed with full mutation FraX using DNA analyses. The Vineland was used to assess adaptive behavior. Depending on the Vineland domain, 252-265 had one assessment, 186-187 had 2 assessments, 46 had 3 assessments and 6 had 4 assessments. The mean interval between assessments was 1.94 years.

Results:  Common growth model was used to estimate trajectories of Vineland domains. The Adaptive Behavior Composite (ABC) score did not change for control males or females or FraX females. Frax males change rapidly from 6-11 years (p < .001) but not from 11 – 16 years (p>.05). This is similar to results on the socialization domain, though the FraX males continue to show declines through 16 years (p=.003). In the communication domain, both control and FraX males decline from 6-11 (p=.006 and p<.001 respectively). From 11-16 years, all but FraX females decline significantly. With regard to daily living skills, no changes are seen with regard to control individuals however FraX females increased significantly (p=.04) and FraX males decreased significantly (p<.001) from 6-11 years. Comparing typical trajectories to FraX trajectories, for males the gap in adaptive behavior widens with age.

Conclusions:  In this original, large scale, longitudinal study, we were able to elucidate trajectories of adaptive behavior across a wide age range in boys and girls with FraX. Boys with FraX show the largest declines in adaptive behavior with socialization skills declining the least, albeit continually through 16 years. Communication skills decline the most overall. Interestingly, this profile is different from individuals with idiopathic autism who show the largest declines in socialization and communication skills over time. It is important to be aware of developmental periods where skills are particularly likely to diminish in FraX so that those working with affected individuals can attempt to preserve behavioral sets that are most vulnerable.  Understanding developmental trajectories in FraX will also be of value in understanding and interpreting the effects of new treatments for this disorder.