ASD in Sibling Pairs Discordant for ADHD

Thursday, May 14, 2015: 11:30 AM-1:30 PM
Imperial Ballroom (Grand America Hotel)
A. M. Reiersen and M. B. McGrath, Psychiatry, Washington University in St. Louis School of Medicine, St. Louis, MO

Symptoms of Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) frequently co-occur in clinical and population-based samples, and there is evidence that this co-occurrence is largely due to overlap of genetic influences that contribute to the two disorders. The diagnosis of both disorders in one person is now allowed according to DSM-5 diagnostic criteria, so it is important to identify appropriate methods for determining which individuals should receive just one versus both diagnoses. Many existing reports of ASD symptoms in ADHD have been based on parent-rated questionnaires, but few studies have used the Autism Diagnostic Observation Schedule (ADOS) to investigate the presence of likely ASD among individuals with ADHD.


To examine evidence for ASD among individuals showing parent-reported lifetime symptoms of combined type ADHD and their “unaffected” siblings. 


The ADOS was administered to individuals classified as having a severe combined form of ADHD (n=96) and their siblings (n=91) with few if any lifetime ADHD symptoms. Subjects were selected from a population-based sample of large sibships (n=169) or recruited through study advertisements (n=18). In each case, ADOS module 3 (N=43) or module 4 (N=144) was administered and scored according to the original ADOS algorithm to determine the ADOS classification. The ADOS-2 module 3 algorithm was then also applied to all ADOS assessments (regardless of module administered) to determine an ADOS-2 (module 3) classification.


Subjects were 8-31 years old, 96% Caucasian (includes 4 Hispanic subjects), 4% African-American, and 59% male. Using the original ADOS algorithm, 17% of ADHD probands and 8% of siblings had a classification of either “autism” or “autism spectrum”.  Using the ADOS-2 module 3 algorithm, 19% of ADHD probands and 12% of siblings had a classification of either “autism” or “autism spectrum”. When ADHD probands qualified for an ADOS-2 diagnosis, they were more likely to be classified as “autism” (n=13) rather than “autism spectrum” (n=5). This was not true of siblings, who were more likely to be classified in the less severe “autism spectrum” category (n=8) than the “autism” category (n=3).


We estimate that approximately 17-19% percent of individuals with lifetime combined type ADHD may be classified as having a co-occurring ASD based on the ADOS or ADOS-2.  Consistent with previous findings of genetic overlap between ADHD and ASD, a substantial proportion (8-12%) of “unaffected” siblings of ADHD probands may also be classified as having ASD by the ADOS or ADOS-2.