Obsessive-compulsive disorder (OCD) is a mental illness where a person suffers from unwanted, intrusive thoughts and mental or behavioural rituals. OCD causes the patient’s social life to decrease or otherwise harms it.

Cognitive behavioural therapy (CBT) and selective serotonin reuptake inhibitors (SSRIs) have shown significant positive effects in treating OCD. However, despite this success, changes in neurochemistry (including changes in the serotonergic system) in patients has shown to be elusive.

Studies have investigated changes in the serotonergic system during OCD treatment, but there are few articles and findings of these studies have not been replicated. However, there have been documented changes in the brain after treatment using SSRIs. This is so, particularly, in the cerebrospinal fluid 5-HT metabolite levels and blood platelet 5-HTT levels. Although, it should be noted that peripheral 5-HT measures cannot be used to study brain regional changes in serotonergic functioning. That being said, 5-HTT imaging is fit to reflect density of innervation, rather than functional status.

Nevertheless, studies testing the effectivity of CBT to treat OCD were not found. This is what this study attempts to do. It shall investigate effects of drug treatment or CBT on brain regional 5-HT synthesis capacity. There are a couple of goals: to compare regional 5-HT synthesis capacity in OCD patients before and after treatment with CBT or sertraline (drug treatment) and to find brain regions where pre-treatment regional 5-HT synthesis capacity is seemed to be the result of treatment.

Participants were 16 patients aged 18 to 45 who were diagnosed with OCD and have not been using medication for 3 weeks minimum. Patients were randomly assigned to undergo either CBT or drug treatment for 12 weeks. PET scans were performed before and after the 12 week period as were MRI scans. The severity of OCD in each patient was tested with the help of two questionnaires and every 2 weeks, the tests were repeated. Patients who underwent CBT treatment had 90-minute sessions during the whole experiment. Those who were given drug treatments started off with a 25mg dose and then increased it to 50mg after the first week, moving to 100mg after the second week. The doses were increased after each week until the maximum dose of 200mg was reached.

All the gathered data was calculated and it was found that both CBT and drug treatments offer significant reduction in OCD symptoms. However, there are actually no significant differences between the two treatment types. It was also found that despite significant progress, there is no full remission or at least it is rarely achieved. Furthermore, increases in global 5-HT synthesis capacity correlated with reductions in OCD symptom severity. This is what causes responses to CBT or SSRIs treatments as there was found correlation between the effectiveness of these treatments and pre-treatment 5-HT synthesis capacity. This capacity is found to be compensatory rather than pathological and the pre-treatment activation of the central serotonergic system might connote an unsuccessful attempt to inhibit obsessive-compulsive symptoms.

However, because of the small number of participants, this study would benefit from being replicated with a much larger sample size. This study also focused on preventing contamination of the biological measure by factors that were not specific, which is always a difficult task even when all factors are known. Hence, the quality is doubted. Another limitation is that OCD patients may benefit from longer CBT treatment than what was used in the study. Some patients were treated using SSRIs prior to study and even though they were free of medication for minimum of 3 weeks, the possibility of facilitation should not be excluded.