Objective: It has been reported that both obsessive-compulsive disorder (OCD) and attention deficit hyperactivity disorder (ADHD) might be related to impairments in frontal-striatal brain circuits. Besides, these two disorders are frequently comorbid. However, there are no published studies directly comparing the neurobiological findings in these two disorders. The objective of the study was to investigate the differences in cortical blood flow between patients with OCD and ADHD.

Method: Thirteen drug-naive OCD subjects (mean age ± SD: 10.4 ± 2.8), and 13 drug-naive ADHD subjects (mean age ± SD: 10.5 ± 2.2) were included in the study. Cerebral blood flow (CBF) was evaluated with Tc-99m-hexamethylpropyleneamine oxime (Tc^99m HMPAO) brain single photon emission tomography (SPECT) during standard resting conditions in all cases. P value was set to <.006 after Bonferroni correction.

Results: OCD cases had significantly higher right prefrontal (p=.003) CBF than ADHD cases. There were no significant CBF differences in other brain regions. OCD symptom severity was not related to CBF.

Discussion: This is one of the very few studies which directly investigated brain imaging in pediatric OCD subjects. Results are consistent with previous studies reporting increased prefrontal CBF in OCD subjects. We also found that, consistent with the previous literature, ADHD subjects had lower prefrontal cortex CBF. Studies which compare subjects with comorbid ADHD and OCD with pure forms of these disorders are necessary to have a better understanding of similarities and differences of these two disorders.