The symptoms of OCD are varied, but the most common are irrational doubt causing excessive verification (doors or spigots, position of objects, etc.) and fears, such as contamination by microbes, symptoms that prevent touching things or require washing (of the hands, showering, house cleaning) often ritualized. Today, 2-3% of the population suffer from OCD, children, adults, men, women.

The doubts and fears of people with OCD go far beyond the intrusive thoughts that we all have from time to time; they are invasive, uncontrollable, and can severely perturb daily life. They do not constitute a psychosis, since the patients are aware that their fears are unjustified, but they can’t manage to suppress them. The symptoms are sometimes present several hours a day, which seriously hampers the ability to work, to interact with others, to perform even simple tasks. Relatives of persons with OCD are often ill at ease, hesitating between an excessive desire to help and discouragement and incomprehension leading to total rejection. More than half of those with OCD become depressive. Two types of treatment are effective: antidepressants that act on serotonin metabolism – even in the absence of a depressive state –and cognitive or behavioural therapies (CBT). In the most severe cases, drugs that block dopaminergic transmission can also be beneficial. These methods can help the majority of patients, but the treatments are often long and difficile and are unfortunately insufficient in about 20% of patients.

There is no one cause that explains OCD. Pychological, biological and social factors are involved. The psychological factors can be related to experiences and a certain fragility of the personality. However, diverse brain dysfunctions can underlie the onset of OCD. Many studies have sought to identify them in order to develop new treatments. The efficacy of treatments that are used suggest that the dopaminergic and serotoninergic systems are implicated in the physiopathogy of OCD. Neuroimaging has also demonstrated an excessive activation of certain neural circuits (the orbito-frontal, dorsolateral-prefontal and anterior cingulare cortex) and the basal ganglia (notably the ventral striatum). One hypothesis that is currently being explored is a dysfunction of the cerebral networks involved in the detection of errors made in the evaluation of risks; this would explain the patients' repetitive doubts and uncontrollable verifications. These neuroimaging studies add to the results obtained by deep brain stimulation, an innovative technique that modulates electrically the activity of brain networks, and which is proposed in severe cases of OCD that are resistant to all conventional treatments. The ultimate objective is to understand the brain mechanisms at the origin of OCD to better treat the patients who have it.