The conditions that overlap with autism generally fall into one of four groups: classic medical problems, such as epilepsy, gastrointestinal issues or sleep disorders; developmental diagnoses, such as intellectual disability or language delay; mental-health conditions, such as attention deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder or depression; and genetic conditions, including fragile X syndrome and tuberous sclerosis complex.

How common are these conditions among people with autism?

It depends on the condition, and estimates vary widely. For instance, between 11 and 84 percent of autistic children also have anxiety2. Similarly, serious sleep problems may affect anywhere between 44 and 86 percent of children on the spectrum3. Differences in diagnostic criteria and other study variables may explain these wide margins. And the age, sex, race and intelligence quotient of the person being evaluated can all influence whether and when they are diagnosed. For instance, autistic black children are more likely than autistic white children to be diagnosed with intellectual disability4. And if a child doesn’t speak, mood disorders may be difficult to detect. Certain conditions, such as anxiety may also look different in people with autism than they do in other people, adding another layer of complexity.

 ADHD is a common co-occurring condition with ASD and as such DSM-5 no longer prohibits ADHD to be diagnosed with ASD. Diagnosis of co-occurring ADHD can be challenging because symptoms of inattention, executive functioning problems, and social cognitive deficits are common in both conditions. It is important to distinguish features of inattention and impulsivity that may be inherent in ASD, such as distractibility related to a special interest, sensory seeking behaviors, or processing problems, from those that warrant an additional diagnosis of ADHD in children with ASD. The hyperactive-impulsive sub type of ADHD can also manifest in people with ASD and is best assessed in the context of the youth’s developmental age, expected activity level, and environmental demands.

• Anxiety disorders: Using findings from their meta-analysis, vanSteensel and colleagues5 estimated that 40% of youth with ASD have a comorbid anxiety disorder. Risk factors for developing anxiety that are prevalent in ASD include social skill deficits, sensory sensitivity, cognitive rigidity, heightened physiological arousal, and difficulties regulating stress.

• Specific phobias: Specific phobia tends to have an onset in childhood. In most cases the phenomenology of specific phobia in ASD tends to be similar to typically developing youth. However, people with developmental disabilities may also develop fears to unusual objects or situations, such as elevators, vacuum cleaners, etc.