A new study has found that Tourette syndrome/chronic tic disorder (TS/CTD) and obsessive-compulsive disorder (OCD) cluster strongly in families, suggesting a role for shared genes and environmental factors. By analyzing a large national health registry, researchers were able to conclude that people with an oldest sibling or parent affected by either TS/CTD or OCD were at significantly higher risk of being diagnosed with the disorder, as compared to the general population. The relationship was particularly strong for TS/CTD.

Previous research has shown that about 25 to 50 percent of people diagnosed with TS/CTD are also diagnosed with OCD; up to 80 percent of those with TS/CTD show more mild obsessive-compulsive behaviors. As many as 30 percent of people with OCD also have a history of TS/CTD. Previous animal studies as well as neuroimaging in humans suggest that both disorders involve dysfunction in a particular part of the brain––the cortical basal ganglia.

The new findings were published in JAMA Psychiatry on February 18th by 1996 NARSAD Young Investigator grantee Dorothy E. Grice, M.D., of Mount Sinai School of Medicine in New York. Dr. Grice led a team that included 2004 NARSAD Young Investigator Abraham Reichenberg, Ph.D., as well as Joseph D. Buxbaum, Ph.D., a member of the Foundation’s Scientific Council. The scientists analyzed data collected by national health registries in Denmark, which includes all people born in the country from 1980 through 2007. The sample used in the study included over 1.7 million individuals.

Dr. Grice and her team found that people who had an oldest sibling diagnosed with TS/CTD were 18 times more likely to be diagnosed with TS/CTD themselves. For people with an oldest sibling with OCD, the risk of a diagnosis with OCD was five times higher. (TS and CTD together are estimated to affect about 0.6% to 2% of the general population; OCD affects 0.7% to 3%). Those who had a parent diagnosed with a disorder were more likely to have the same disorder. There was also significant cross-disorder risk: if a parent or oldest sibling was diagnosed with TS/CTD, the individual was more likely to be diagnosed with OCD (and vice versa) compared to the general population.

The cross-disorder risk, combined with previous research indicating that the disorders affect the same brain circuitry and the relatively high percentage of people diagnosed with both disorders, suggests that similar genetic causes might underlie both TS/CTD and OCD. This research provides important insights into the causes of these disorders and an improved understanding of who might be at risk of developing them.

Read the abstract.