ADHD and autism spectrum disorder (ASD) are neurodevelopmental disorders that share an early onset, delays and deviances in the development of brain structure/function, cognitive impairments, a male preponderance, and strong genetic influences on individual differences and liability.
ADHD is the most common comorbidity in children with ASD with comorbidity rates in the 40-70% range.
A community-based study of 164 children with ADHD found that 25% of children had parent-reported ASD symptoms in the clinical range and 40% were in the at-risk range, with higher ASD symptoms being positively correlated with ADHD symptoms.
Other data indicate that hyperactive/impulsive symptoms, not inattentive symptoms, may be more closely associated with ASD behaviors.
It is very interesting to know that in a large study of 1496 children with ASD, 20% received a diagnosis of ADHD BEFORE ASD.
Symptoms of ASD and ADHD frequently co-occur in the same individuals and cluster within families.
The co-occurrence of ADHD diagnoses in young children at high risk for ASD and/or with ASD vary widely from less than 5% up to 40%. Co-occurrence rates were higher in children aged 3-6 years than in younger children (about 2.1 years of age), higher for inattention problems than for hyperactivity-impulsivity (but note that ratings of hyperactivity-impulsivity, particularly the verbal items, might be less reliable at a young age), higher in clinical compared to population samples, higher in children with lower IQs than higher IQs and higher in studies that have used less stringent criteria for ADHD.
The amount and severity of ADHD problems in children with ASD seem further related to lower IQ.
ADHD and ASD are both associated with adaptive behavioral deficits.
The co-occurrence of ASD and ADHD increases with age, the severity of symptoms, and lower IQ.
ASD and ADHD share difficulties with control and shifting, but partly opposite behaviors seem to be involved.
Visser, J. C., Rommelse, N. N. J., Greven, C. U., & Buitelaar, J. K. (2016). Autism spectrum disorder and attention-deficit/hyperactivity disorder in early childhood: A review of unique and shared characteristics and developmental antecedents. Neuroscience & Biobehavioral Reviews, 65, 229–263.
Antshel, K. M., Zhang-James, Y., Wagner, K. E., Ledesma, A., & Faraone, S. V. (2016). An update on the comorbidity of ADHD and ASD: a focus on clinical management. Expert Review of Neurotherapeutics, 16(3), 279–293.
Miodovnik, A., et al., Timing of the Diagnosis of Attention-Deficit/Hyperactivity Disorder and Autism Spectrum Disorder. Pediatrics, 2015. 136(4): p. e830-7.