Weighted frequency of triadic communication growth rate predicts endpoint atypicality within a sample of infant siblings of children with ASD

Background: Later-born siblings of children with ASD (Sibs-ASD) are at elevated risk for autism and related developmental disorders. However, it is currently unclear how to predict which Sibs-ASD will have compromised developmental outcomes at an age when diagnoses tend to be stable (i.e., after 30 months).

Objectives:   We predicted that Sibs-ASD with faster growth rate for triadic communication between the ages of 15 and 34 months would have more optimal diagnostic outcomes than those with a slower growth rate.  

Methods:  Forty-four Sibs-ASD (initial CA = 15.1 mos; SD = 3 mos) were tested using the Screening Tool for Autism in Two-year-old (STAT) at 3 to 5 periods with approximately 4 – 8-month inter-test intervals. Frequency of triadic communication (i.e., communication about an object to a message recipient) was coded from the STAT. A weighted frequency score was also derived by multiplying each communication act by the behavior used to communicate (nonverbal = 1, single word = 2, multiword = 3). Child diagnoses were determined by psychologists at the last data point (i.e., M = 33.7, SD = 4.1 mos) on the basis of standard cognitive and diagnostic measures. The “atypical” subgroup (n = 15) were those with diagnoses of language impairment, developmental delay, autism, PDD/NOS, or the broader autism phenotype (i.e., clinical level of concern regarding social communication, no other diagnosis, and scores above the cutoff on the ADOS social domain).

Results: Sibs-ASD with faster growth in weighted frequency of triadic communication were less likely to have “atypical” diagnoses at the final evaluation (pseudo-R² = .28). Neither the initial value of weighted triadic communication nor any aspect of unweighted triadic communication growth predicted diagnostic subgroup.

Conclusions: If replicated, growth rate of weighted frequency of triadic communication between 15 and 24 months will be a strong predictor of which Sibs-ASD need treatment.