Conceptual and methodological issues in studies of obsessive-compulsive and Tourette's disorders.

An association between recurrent motor and phonic tics and obsessive-compulsive behaviors has been noted since Tourette's Syndrome (TS) was first described. Obsessive-compulsive disorder (OCD) until recently was considered a rare disorder with poor prognosis. Currently, OCD is considered among the most common psychiatric diagnoses, and new treatments have spurred the development of considerable clinical, epidemiological, genetic, and biological research. Recent studies suggest a high rate of obsessive-compulsive symptoms in Tourette's Syndrome patients. A high rate of OCD among relatives of TS probands, both with and without OCD symptoms, suggest that some forms of OCD may represent an alternative expression of factors responsible for TS and/or chronic motor tics. Areas of conceptual controversy in the differentiation of tics, impulsions, and compulsions are discussed, confusing aspects of differential diagnosis are explored, and the relationship of diagnostic issues to clinical and familial studies are highlighted. There is considerable evidence for neuropsychiatric abnormalities in both OCD and TS; however, no studies have directly compared both disorders with similar methodological design. While studies of neurotransmitter function have primarily implicated dopaminergic dysfunction in TS and serotonergic function in OCD, other systems may be involved in each disorder, and neurotransmitter systems may be tightly linked, such that alterations of one system will affect other systems. This article reviews and discusses some of the conceptual and methodological issues associated with clinical, familial, neuropsychiatric and biological studies attempting to elucidate the association among tics, obsessive-compulsive disorder, and Tourette's Syndrome.