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The exact cause of obsessive-compulsive disorder (OCD) is unknown. However, a combination of psychological, biological and environmental factors may be responsible. Heredity is believed to play a strong role in the development of the disorder.

Growing evidence indicates that chemical imbalances may be common in the development of OCD. Various studies have indicated that low levels of the chemical serotonin (a neurotransmitter that helps nerve cells communicate) may contribute to OCD. Many patients with OCD who take medications that boost serotonin levels show improvement in symptoms. In addition, people who have brain injuries sometimes develop OCD, which further suggests that physical damage in the brain can cause the disorder.

OCD also appears to increase metabolism in the basal ganglia (a region of specialized nerve cells that is mainly involved in body movement) and frontal lobes (the upper brain area, which is mainly involved in emotions and personality) of the brain. This may cause repetitive movements, rigid thinking and lack of spontaneity. Finally, people with OCD have been shown to have higher levels of the hormone vasopressin (which raises blood pressure and reduces excretion of urine).

People with close family members (e.g., parents) with a history of OCD have a higher risk for developing the condition themselves. Researchers have identified a previously unknown gene variant that makes a person twice as likely to develop OCD. A gene variant, or allele, is an alternate form of a gene that often affects appearance. For instance, different alleles are responsible for variations in eye color. The allele is part of the human serotonin transporter gene (hSERT) that is affected by selective serotonin reuptake inhibitors (SSRIs), commonly prescribed medications for OCD, other anxiety disorders and depression. A recent study found that patients with OCD were twice as likely to have the hSERT genetic variant. In a subsequent, related study, researchers found that the newly-discovered gene variant was twice as likely to be passed down from a parent to a child with OCD. Those who are shown to have this variant are likely to respond well to medication.