How Are OCD and ADHD Related in the Brain?

What you need to know

• OCD and ADHD are both common neurodevelopmental disorders that frequently co-occur
• Both disorders involve abnormalities in brain circuits connecting the frontal cortex and basal ganglia
• OCD is associated with overactivity in these circuits, while ADHD involves underactivity
• Genetics play a role in both disorders, with some overlapping risk genes identified
• Understanding the neurobiology may help develop better treatments for OCD, ADHD and when they co-occur

Brain structure and function

Neuroimaging studies have found abnormalities in similar brain regions in both OCD and ADHD, particularly in circuits connecting the frontal cortex and basal ganglia. However, the nature of these abnormalities differs between the disorders:

In OCD:

• Increased volume and activity in the basal ganglia
• Overactivity in the orbitofrontal cortex and anterior cingulate cortex
• Enhanced brain response when monitoring for errors

In ADHD:

• Reduced volume in the basal ganglia
• Underactivity in the prefrontal cortex and anterior cingulate cortex
• Diminished brain response when monitoring for errors

These findings suggest OCD involves hyperactivity in frontal-striatal circuits, while ADHD involves hypoactivity. This maps onto the different symptoms - excessive monitoring and repetitive behaviors in OCD versus inattention and impulsivity in ADHD.

Genetics

Twin studies indicate both OCD and ADHD are highly heritable. Some overlapping genetic risk factors have been identified, including genes related to dopamine and serotonin function. However, there are also genetic differences:

• OCD shows stronger links to serotonin-related genes
• ADHD shows stronger links to dopamine-related genes

This aligns with the efficacy of serotonin medications for OCD and stimulant medications for ADHD.

Neuropsychological findings

Both disorders show deficits in executive functions like response inhibition. However, the underlying causes may differ:

• In OCD, poor performance may result from being overly cautious and doubting responses
• In ADHD, deficits stem from difficulties sustaining attention and controlling impulses

OCD is also associated with enhanced error monitoring, while ADHD shows reduced error monitoring.

Conclusions

• OCD and ADHD involve overlapping but distinct patterns of brain dysfunction
• Understanding these similarities and differences may lead to better treatments
• More research is needed on how the disorders interact when they co-occur
• Studying the neurobiology provides insights into the nature of these complex conditions