A psychiatric disorder refers to a significant dysfunction in an individual’s cognitions, emotions or behaviours that reflects a disturbance in the psychological, biological or developmental processes underlying mental functioning.

Comorbidity refers to two or more forms of psychopathology within the same person. For some, a second diagnosis requires symptom presentation without assumptions of causality (i.e. disorder one causes disorder two). For others, a second diagnosis requires the dysfunction to be significant enough to require treatment in addition to the primary condition.

There are several difficulties in assessing comorbidity in people with ASD:

• There is diagnostic overshadowing.
• There is often a different symptom presentation in children with ASD than in children without ASD.
• There are no clear systematic methods of identifying and treating comorbid psychiatric disorders across the autism spectrum.
• Many symptoms of psychopathology overlap with symptoms of ASD.
• ASD is very heterogeneous making complete distinguishability of ASD and other disorders difficult.
• The presence of intellectual disability complicates diagnosing other disorders.

Diagnostic overshadowing refers to not recognizing a second disorder because the problems are attributed to the first disorder. Intellectual disability is common in ASD.  There are several benefits of having accurate and early recognition of comorbidity:

• Early treatment has substantial effects on cognitive ability (1), adaptive behaviour (2) and psychopathology (3).
• Knowledge on comorbid disorders can inform treatment planning.
• Knowledge on comorbid disorders can focus researchers on developing specific diagnostic tools (1), treatments (2) and psychopharmacology (3).

Comorbid disorders may cause impairment in learning, which makes the treatment of ASD more problematic. There are few standardized assessment tools to assess psychiatric symptoms among individuals with ASD. Tools developed to assess typically developing children cannot always be used because the validity has not been established for individuals with ASD.

It is not clear whether challenging behaviour is an atypical symptom of a comorbid psychiatric disorder. The severity of either ID or ASD may alter or mask the mental health symptomology presentation. It is imperative to study the relationship between individual and environmental factors that influence development of psychiatric disorders in children an youth with ASD. This may have a significant impact on a person with ASD’s vulnerability or resilience to developing particular disorders. Individual factors may include genetics (1), cognitive profile (2), diagnosis (3), Asperger’s syndrome (4), PDD-NOS (5) and self-management skills for coping with stress and aversive events (6). Environmental factors may include community placement and parenting style.

Problems arising from ADHD appear similar to core symptoms of ASD (e.g. impairments in social interactions; poor communication restricted and stereotyped interests). There are several common comorbid issues between ASD and ADHD:

• Executive functioning deficits.
• Learning disabilities.
• Low processing speed.
• Sleep disorders.
• Early language delay.
• Deficits in attention, motor control and perception.
• Fewer social relationships than same age peers.
• Disruptive behaviour problems.
• Difficulty adapting to change.

It may be irrelevant to diagnose ADHD when people are diagnosed with ASD due to the large overlap. High-functioning ASD has often been misdiagnosed as ADHD. This means that it may be necessary to screen people with ADHD for ASD. However, ADHD does not always occur in people with ASD.

ADHD symptoms are similar regardless of the presence or absence of secondary diagnosis. ASD symptoms are similar regardless of the presence or absence of ADHD symptoms. Children diagnosed with both ADHD and ASD have higher rates of hospitalization (1), psychopharmacological treatment (2) and psychological treatment (3). Children with ADHD and ASD do not report more impairment but have more difficulty in daily life than children with ASD alone. ASD and ADHD may partially originate from similar familial and genetic factors.

Psychopharmacology is the most popular treatment for ADHD in typically developing children. Medication may also be effective for treatment of children with ADHD and ASD although research is not unequivocal. Negative side effects of medication are social withdrawal (1), irritability (2), tics (3), sleep disturbance (4), appetite changes (5), sedation (6), headaches (7), anxiousness (8) and gastrointestinal problems (9).

Obsessive-compulsive disorder affects 2-3% of children and adolescents. A diagnosis requires that a person reports intrusive thoughts, impulses or images that produce anxiety (i.e. obsessions) and repetitive behaviours that are carried out to reduce anxiety (i.e. compulsions). The symptoms must cause marked distress (1), take more than 1 hour a day (2) and significantly interfere with personal relationships or activities (3). Obsessions refer to recurrent, persistent thoughts, impulses or images that are experienced as intrusive and inappropriate and that cause anxiety or distress.

It is difficult to distinguish OCD from other repetitive behaviours in children with ASD due to their limited communication skills. However, the prevalence of OCD among individuals with ASD appears to be higher than typically developing peers. The prevalence is 11% for children with ASD with higher functioning (i.e. IQ > 70) adults being more vulnerable to develop the disorder.

Overlapping behaviours of autism and OCD are excessive involvement in special interests (1), engagement in compulsive rituals (2), stereotyped and repetitive motor mannerisms (3) and insistence on sameness (4). However, individuals with ASD may not experience distress associated with their rigid beliefs and do not perform their rituals to alleviate anxiety. It is difficult to differentiate between behaviours representing OCD and ASD due to the absence of clear obsessions. Adults with ASD and OCD have more ordering and hoarding compulsions than adults with only OCD.

Differentiation between OCD and ASD based on obsessions is difficult due to the communication difficulties (1), emotion recognition deficits (2), comorbid ID (3) and having caregivers as informants (4) with individuals with ASD.

Treatment includes cognitive behavioural therapy (1), applied behaviour analysis (ABA) (2) and psychopharmacology (3). The CBT techniques need to be modified to meet the cognitive, social and linguistic abilities of the child with ASD. CBT for OCD consists of psychoeducation (1), cognitive training (2), exposure and response prevention (3) and parent training (4).  Response strategies include self-talk (1), setting goals (2) and anxiety management techniques (3).

There are two popular procedures for exposure and response prevention (ERP):

1. Gradually expose the person from low to high anxiety-provoking situations while preventing compulsive behaviour.
2. Gradually increase the amount of time between onset of anxiety provoking situations and opportunity to engage in OCD behaviour.

Applied behavioural analysis focuses on behaviour-environment relationships (1), functional assessment (2) and assessment-derived intervention (3). Medication may be effective but there can be negative side effects (e.g. self-injury).

Children with ASD are at greater risk for anxiety compared to children without ASD. Children with high-functioning autism or Asperger syndrome are more vulnerable than children without Asperger syndrome or low-functioning autism. This may be due to the higher cognitive and linguistic abilities that allow for verbalizing obsessions, thoughts or beliefs. Anxiety can affect quality of life (1), stress levels (2), family functioning and peer acceptance (4). It can contribute to the effectiveness of educational interventions.

Anxiety symptoms for individuals with ASD are similar to those in non-ASD samples. However, anxiety is a common symptom of ASD and it is difficult to know whether it is part of a comorbid anxiety disorder or whether it is a part of ASD itself. The presentation of symptoms may be affected by age and cognitive functioning.

To assess anxiety symptoms in children with ASD, clinicians often rely on caregiver reports. This may be due to the impaired communication and cognitive abilities among individuals with ASD. However, the correspondence between parent-report and self-report is low. The presence of avoidance and negative emotional behaviours should be considered along with the intensity of the emotional response and the continued display of fearful responses after removal of the stimulus.

Treatment for anxiety in ASD consists of pharmacological treatment (1), CBT (2) and ABA (3). CBT for anxiety typically includes psychoeducation (1), creating a fear hierarchy (2), training coping skills (3) and exposure training (4). For individuals with ASD and ID, behavioural approaches such as contingent reinforcement and graduated exposure are effective.