Developmental Trajectory in ASD and Broader Autism Phenotype in the First Three Years of Life

Background: Autism is a heritable disorder; the genetic liability includes autism and milder developmental differences or impairments.  Understanding developmental trajectory in the first three years of life of children at high risk for ASD will inform early detection efforts and research on neurobiological mechanisms of ASD.

Objectives: To define developmental trajectories associated with the Broader Autism Phenotype (BAP) and ASD from 6 to 36 months in younger siblings of children with autism. 

Methods: A longitudinal, prospective design was used.  Children were separated into groups based on an ‘outcome’ assessment at 36 months of age.  The Non-Broader Autism Phenotype group (Non-BAP; n=140) did not meet outcome criteria for language or social delays or ASD (n=140).  The Broader Autism Phenotype group (BAP, n=20) had an outcome diagnosis of language or social impairment (did not meet criteria for ASD).  The ASD group (n=55) met outcome criteria for autism or Pervasive Developmental Disorder-Not Otherwise Specified.  Dependent variables include standard (T) and raw scores from the Gross Motor, Fine Motor, Visual Reception, Receptive Language, and Expressive Language scales of the Mullen Scales of Early Learning, and algorithm summary scores from the Autism Diagnostic Observation Schedule (ADOS).  Mullen data were available at 6, 14, 28, 24, 30, and 36 months of age.  ADOS data were available at all ages except 6 months.  Analytic methods involved linear growth curve random effects models implemented using the R statistical software package.  All participants gave informed consent for this IRB approved research protocol.

Results: No group differences were found for Mullen scores at 6 months of age.  By 14 months of age, and for each subsequent age, the ASD group scored significantly lower on all Mullen scales than the Non-BAP group (p’s=0.03 to <0.01).  Language difficulties in the BAP group became more evident at age three, as increasingly complex linguistic skills were expected based on chronological age.  Growth curve analyses showed that Mullen raw and standard scores decreased in the ASD group with increasing age (p<0.01, indicating slowing and, in some cases, regression).  The qualitative social and communication abnormalities associated with autism were present in the ASD and BAP groups by 14 months (p<0.01).  Growth curve analysis showed that ADOS scores for the Non-BAP and BAP groups remained stable with increasing age, but ADOS scores for the ASD group increased with age (p<0.01, indicating worsening).

Conclusions: Siblings of children with autism are at increased risk for altered patterns of development.  Our data indicate that developmental slowing begins between 6 and 14 months of age for children with ASD outcomes, and persists through the third birthday.  Social and communication symptoms of ASD, as well as repetitive behaviors and interests, appear by 14 months of age in siblings with 36-month diagnosis of BAP or ASD.  There is a continuum of severity of expression of these symptoms, and they remain stable from 14 through 36 months for siblings with the BAP.  Findings underscore the importance of developmental surveillance for younger siblings of children with autism.