Current Medical Diagnosis & Treatment in Psychiatry

Obsessive-Compulsive Disorder

Population Genetics

Family Studies
One study evaluated 145 first-degree relatives of 46 children with obsessive-compulsive disorder (OCD). The investigators found that of the 90 parents evaluated in person, 15 (17%) received a diagnosis of OCD, compared to 1.5% of the parents of 34 children with conduct disorder who served as a control group. This 17% rate is also significantly higher than the prevalence rate of 2% in the general population. Fathers were three times as likely as mothers to receive a diagnosis of OCD. Of the 56 siblings evaluated personally, three (5%) met criteria for OCD. When data were corrected for age, the morbid risk was 35% in siblings. This result should be viewed with caution because of the magnitude of the age correction for siblings. The probands all had severe childhood-onset OCD and were referred to the investigators for treatment protocols. It is possible that childhood-onset OCD represents a more severe form of the illness. Nevertheless, this carefully conducted family study reveals an increased risk for OCD among the first-degree relatives of OCD probands.

Twin Studies
There are no large twin studies of OCD. A review of reported series noted that 63% of MZ pairs were concordant. However, assignment of diagnosis and zygosity has been questioned in some of these cases. There is general agreement on 13 concordant MZ twin pairs and 7 discordant MZ twin pairs. There are several unquestioned reports of discordant MZ twin pairs for OCD, implicating some nongenetic factors in incidence or age at onset.

Genetically Related Disorders
When OCD occurs in the familial context of Tourette’s syndrome, the OCD may be considered part of the spectrum of Tourette’s syndrome. However, most OCD occurs in individuals who have no first-degree relatives affected by Tourette’s syndrome. Occasionally, an individual destined to develop Tourette’s syndrome will present with symptoms of OCD, and the motor tics appear subsequently. These patients are often diagnosed as having OCD until the motor tics develop.

There is limited evidence from family, twin, and adoption studies regarding the inheritance of OCD. Although OCD may be familial, there is insufficient data from twin studies and no data from adoption studies. Several avenues of research suggest a serotonergic abnormality in OCD patients.

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