Methods: Developmental trajectories of ASD symptoms from ages 6 to 18 months, indexed by the total score on the Autism Observational Scale for Infants (AOSI; higher scores indicate greater symptomatology), were identified by semi-parametric group-based modeling using SAS (PROC TRAJ) in a combined sample of high- and low-risk infants (n = 401 and 160, respectively). We then examined whether trajectory membership was associated with diagnostic outcomes among the 225 high-risk and 91 low-risk infants followed to age 3 years. ASD diagnoses were based on the ADI-R, ADOS and expert clinical judgment using DSM-IV, blind to prior study data.
Results: A 3-group solution provided optimal fit to variation in ASD symptom trajectories. One group, which included all but one of the low-risk infants and 68% of the high-risk infants, had minimal ASD symptoms from ages 6 to 18 months. A second group, who scored higher on the AOSI than the first group but had a similarly flat trajectory, included 24% of the high-risk sample: 6 of 22 diagnosed with autistic disorder (27%), 13 of 30 with other ASDs (43%) and 36 of 173 (21%) who were not diagnosed with ASD (21 with clinical concerns such as language delay, and 15 who were typically developing). One of 91 infants in the low-risk group belonged to this trajectory (the only low-risk child diagnosed with ASD). Thus, membership in this second group (moderate but stable levels of early symptoms) was associated with increased risk of ASD (χ2 = 5.36, p=.021), but was non-specific and had relatively limited positive predictive value. A third group had similar levels of 6-month symptoms as the second group, but a markedly inclining trajectory. This included 12 of 52 children with ASD (10 of 22 or 46% with autistic disorder; and 2 of 30 or 7% with other ASDs), compared to 4 of 173 siblings who were not diagnosed with ASD (2.3%), 3 of whom had other clinical concerns such as language delay at age 3 (χ2 = 65.2, p<.001). Overall, 12 of 16 high-risk infants (75%) in the third group were diagnosed with ASD.
Conclusions: The presence of increasing ASD symptoms between 6 and 18 months was highly predictive of autistic disorder at age 3. However, 27% of high-risk infants later diagnosed with autistic disorder and 43% of those diagnosed with other ASDs had stable moderate-level symptom trajectories to age 18 months shared by some high-risk infants not diagnosed with ASD.