Tic Disorders: Etiology

Etiology

Genetics

Comparison of the concordance rates for Tourette’s disorder in monozygotic and dizygotic twins identifies Tourette’s disorder as an inherited condition. The twin studies, however, are unable to identify a particular mode of genetic transmission or to identify the breadth of the clinical phenotype. To answer these questions, other research methods are required. Segregation analyses of family study data have been used to identify the pattern of ge-netic transmission and alternative phenotypes of the Tourette’s disorder genetic diathesis. Linkage studies of Tourette’s disorder based on the assumption of Tourette’s disorder as an autosomal dominant condition have been undertaken but to date have not been successful. Candidate gene studies based on the neurotrans-mitter hypotheses of the etiology of Tourette’s disorder have also not been successful in identifying the Tourette’s disorder gene(s). Recently, a large federally funded sibpairs study of Tourette’s disorder has published encouraging results.

Twin Studies

Evidence from twin studies suggests an important role for both ge-netic and nongenetic factors in the development of Tourette’s dis-order. Two large twin studies have shown high concordance rates in monozygotic twins for Tourette’s disorder (both twins have Tourette’s disorder) and for tic disorders (one twin has Tourette’s disorder, the other has tics but not Tourette’s disorder). In both of the studies, the concordance rate for Tourette’s disorder in monozy-gotic twins was more than 50%. When the concordance rates were calculated for the presence of any tic disorder, they approached 100%. By comparing the concordance rates of monozygotic twins with dizygotic twins, one can separate the role of genetic factors from other environmental factors. In the one study in which such a comparison was done, the concordance rate for Tourette’s disorder in monozygotic twins was significantly higher than the concord-ance rate in dizygotic twins (Price et al., 1985), further suggesting a powerful role for genetics in Tourette’s disorder.

Pathophysiology

In Tourette’s disorder, the complex clinical presentation suggests several neuroanatomical sites of disease as well as neurochemical substrates including the basal ganglia and their interconnections with the frontal cortex and limbic system. Abnormalities in these structures could readily cause the wide variety of motor, sensory– motor, cognitive and affective symptoms seen in patients with Tourette’s disorder. The complex phenotypic presentation seen in Tourette’s disorder could also be produced by a neurochemical abnormality at various locations within this circuitry. Reports of group A beta-hemolytic streptococcus-related antineuronal an-tibodies being associated with the development or exacerbation of tics and OCD suggest a role for infectious agents and autoim-mune processes in the etiology of these complex disorders.

Anatomical and Biological Abnormalities

Neuroanatomical Abnormalities in Tourette’s Disorder

Increasingly sophisticated imaging methods, such as volumetric magnetic resonance imaging (MRI) and functional neuroimag-ing, have identified subtle abnormalities in the basal ganglia and its interconnections with cortical and limbic regions of the brain.

Two volumetric magnetic resonance studies identified the absence of the usual left–right asymmetry in the basal ganglia, leading to speculation of hypoplasia or atrophy of the left basal ganglia in Tourette’s disorder. Areas associated with tic suppres-sion may reflect brain areas involved in central nervous system dis-inhibition and ultimately tic symptoms. Functional neuroimaging studies, such as single-photon emission computed tomogra-phy, have identified decreased blood flow to the basal ganglia, specifically the left lenticular region. Positron emission tomog-raphy identified similar decrements of glucose use in the basal ganglia. Areas associated with increased functioning in Tourette’s disorder include the midbrain, lateral premotor and supplemental motor cortexes and areas associated with sensorimotor, executive and paralimbic functioning. Areas associated with decreased func-tioning include the circuitry involving the caudate and thalamus, and their interconnections with the cortical and limbic areas.

Neurochemical Abnormalities

A number of neurochemical abnormalities have been proposed in Tourette’s disorder, in large part on the basis of responsiveness of symptoms to specific pharmacological agents. Tic suppression with dopamine blockers such as haloperidol and beta-adrenergic agonists such as clonidine have implicated the dopamine–acetyl-choline and adrenergic systems, respectively. The serotonin sys-tem has been implicated because of the association of Tourette’s disorder with OCD and the positive therapeutic effect of serot-onin reuptake inhibitors in OCD.

Other Biological Causes

In several reports the development of tics as well as obsessive– compulsive symptoms in children and adolescents has been as-sociated in time with group A beta-hemolytic streptococcal in-fection. The underlying mechanism is proposed to be similar to that involved in the development of Sydenham’s chorea, in which antibodies developed in the course of infection cross-react with basal ganglia tissues, resulting in the characteristic choreiform movement disorder of Sydenham’s. Case reports have described subjects with the abrupt onset or exacerbation in symptoms occur-ring in parallel with antibody increases and with MRI changes in caudate size. These cases have been given the acronym PANDAS for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infection. These preliminary findings link the development of a movement disorder and psychiatric symptoms to an infectious agent and autoimmune processes, and suggest new and alternative treatments including the potential for vaccines for Tourette’s disorder and OCD, though the possibility of chance as-sociation is high given that tics, obsessive–compulsive symptoms and streptococcal infections are common events in childhood.

Environmental Causes of Tourette’s Disorder

To date, studies have not identified any specific factors that cause Tourette’s disorder, yet it is increasingly clear that environmental factors have an impact on tic severity and, perhaps, even on the types of symptoms expressed. Clinical wisdom suggests that tic severity increases in response to stressful (e.g., examinations) or exciting life experiences (e.g., amusement parks). It is also not uncommon for persons with Tourette’s disorder to be able to identify a particular environmental stimulus that initiated either a bout of symptoms or a new tic symptom.

Environmental factors associated with increases in symp-tom severity can occur early in development, including prena-tal (intrauterine) development. In a study comparing groups of Tourette’s disorder subjects with severe versus mild tics, pro-tracted vomiting by a subject’s mother during her pregnancy with the subject was a risk factor for increased tic severity (Leckman et al., 1990). Because of the male preponderance of Tourette’s dis-order, it has been postulated that intrauterine exposure to andro-genic hormones may be a factor in the development of tics and in tic severity. An open-label study of flutamide, an antiandrogenic hormone, identified significant but transient tic reduction in adult men, suggesting at least a partial role for sex hormones in tic severity (Peterson et al., 1994). Family-genetic and twin studies have also been useful for identifying factors associated with tic severity and have found an association between birth weight and tic severity suggesting that differences in intrauterine environ-ment may be associated with tic severity.

Psychosocial Aspects of Tourette’s Disorder

Although psychosocial issues do not play a large etiological role in the development of tic disorders they do play a major role in adaptation and impairment and are often the focus of treatment and rehabilitative efforts. Clinical work that involves the family, friends, school and workplace is often the bedrock of treatment in a patient with Tourette’s disorder.

Children

For children with Tourette’s disorder, the onset of symptoms oc-curs early in development and directly affects family life and re-lationships with peers and schoolmates. The diagnostic label of Tourette’s disorder can be helpful for understanding the nature of a youngster’s problems and can communicate the need to protect the youngster from excessive adversity. The diagnostic label can, however, be a problem. There is a tension between protecting a child with Tourette’s disorder from adversity while ensuring that the child encounters and masters life’s challenges. With too much protection, a child may run the risk of not developing a strong and complex identity adequate for the rigors of adult life. Support from parents for mastering the challenges of development is key to long-term functioning of children with Tourette’s disorder.

Young Adults

The transition to adulthood is difficult enough for most young peo-ple, but young adults with Tourette’s disorder have a particular chal-lenge. The transition to adulthood often occurs when an important component of their early experience and identity (i.e., Tourette’s disorder) begins to show some improvement. Young adults most vulnerable during this transition are those who, as a result of their Tourette’s disorder, did not develop the foundations of an adult iden-tity as a child. These adults often face the rigors of adult life without the necessary skills to manage, but also without the presence of tic symptoms of sufficient severity to explain their impairment.

Adults

Today’s adult with Tourette’s disorder belongs to a different co-hort than today’s child with Tourette’s disorder. Most adults with Tourette’s disorder were not diagnosed in childhood. They did not have the “protection” of the diagnosis and often experienced significant confusion, isolation and discrimination. Some adults with Tourette’s disorder have significant anger, resentment and distrust related to their early life experiences including ineffec-tive treatments, which can have an impact on current function-ing. Many adults who appear to function well in spite of their Tourette’s disorder may be doing so at an emotional cost.