Tourette's Syndrome (TS) appears to be familial (i.e., it "runs" in families) in the vast majority of cases, suggesting a genetic component or an inherited vulnerability to it, but our understanding of the mode of genetic transmission is incomplete. At the present time, it appears that there is likely to be a significant genetic factor, although prenatal, perinatal, autoimmune, and environmental factors may contribute to or modulate the severity of symptoms.
Gender is also linked to expression of TS: boys are significantly more likely to develop TS than are girls. One question adults often ask when they are thinking of starting a family is whether there are any tests such as amniocentesis that can screen or test a fetus for Tourette's Syndrome. The answer is "not at this time."
In some of the seminal research on this topic, Pauls et al. (1986) reported that male offspring would have almost a 50% chance of having symptoms of TS, and a .99 (almost certain) probability of having either TS, a chronic motor tic disorder, or Obsessive-Compulsive Disorder. Female offspring had lower probabilities of showing symptoms of these disorders, but there was still a fairly high probability (.70) that a female child would have one of these disorders.
In more recent research, however, McMahon, Carter, Fredine, and Pauls (2003) approached the problem somewhat differently by looking at the onset of TS in children whose parents have TS and comparing that to the onset of TS in children whose parents do not have TS. Children who were considered "at-risk" and control children were enrolled between the ages of 3 and 6 years and followed with yearly structured assessments over intervals of 2-5 years. Of the 34 at-risk children who were tic-free at baseline, 10 (29%) subsequently developed a tic disorder; 3 of those 10 met criteria for TS. None of the 13 control children developed a tic disorder, supporting the hypothesis of a significant genetic or familial contribution to the onset of tics. Similarly, Obsessive-Compulsive Disorder (OCD) or features or OCD emerged in 11 of the at-risk cases, but not in any of the controls, while Attention Deficit Hyperactivity Disorder (ADHD) occurred in 14 at-risk children but not in any of the controls.
The data described above provide support for familial transmission of TS as well as an association between tics, obsessions and compulsions, and ADHD. The study also provided additional information on whether having two parents with TS increased the risk compared to cases where only one parent has TS. When the rates in offspring were compared, the investigators found that the rates of TS in children with two affected parents were three times the rate observed for children with one affected parent.
A second common question is whether there is any way to predict how severe a child's symptoms might be based on what the parent experienced if the child does develop TS. Based on available reports, there is currently no clearly identified relationship between the severity of tics experienced by a parent in their childhood and the severity of tics experienced by their child(ren). A parent who has mild symptoms might have a child who has severe systems (Kurlan et al., 1988) and vice versa. Lichter and his colleagues (1999) also found that the presence of family history does not predict the severity of motor symptoms or Attention Deficit Hyperactivity Disorder in Tourette's Syndrome.
While we cannot yet predict the severity of tics based on family, family history appears to be correlated with severity of obsessive-compulsive behaviors that are often seen in conjunction with Tourette's Syndrome. Lichter et al. (1999) reported that more severe obsessive-compulsive symptoms were associated with bilineal transmission (i.e., the presence of probable or definite tics or obsessive-compulsive behaviors in both parents or relatives on both sides of the family) than in unilineal transmission (family history on only one side of the family). Children who had a family history of tics or obsessive-compulsive behaviors on both sides of the family were not only more likely to have more severe obsessive-compulsive behaviors, but they were also more likely to exhibit self-injurious behaviors.
There is no cure for Tourette's Syndrome at the present time.
Lest the preceding sound somewhat alarming, it is encouraging to note that family studies suggest that most cases of TS are "mild." In addition to what appears to be a genetic vulnerability or component, some research also suggests that prenatal events or factors (such as maternal smoking, drug use, exposure to toxins) and perinatal events (such as jaundice, infections) may be associated with severity of TS. While some factors do not necessarily cause TS, they may be correlated with symptom severity in children who do develop TS. Some of the environmental factors that may affect symptom severity are mentioned later in this primer in the section on stress.
While some investigators look to identify specific genetic contributions to TS, other investigators have focused on using neuroimaging techniques to try to identify whether TS is related to a specific abnormality in one or more brain structures or whether it is due to dysfunction in the way the brain's circuitry functions. Some studies do report differences when comparing the brains of those with TS to those who do not have TS. A growing body of research also points to the role of structures in the brain known as the basal ganglia. A recent study by Sowell et al. (2008) used magnetic resonance imaging (MRI) to look at the cortex of individuals with TS. They found cortical thinning in the frontal and parietal lobes compared to non-TS control children. Of particular significance, the thinning was most prominent in the sensorimotor areas of the cortex that control the facial, orolingual and laryngeal musculature. Because facial tics and vocal tics are part of TS, their findings are particularly helpful in explaining some of the neurological bases for tics. Other researchers further expand our understanding by their study of white matter changes in the brain (Siebner et al., 2009).
Other research has addressed the neurochemical transmitters that might be involved in TS. As you read about Tourette's Syndrome, you will also see frequent reference to dopamine, and the symbol "D2," which refers to a particular type of dopamine receptor in the brain. Dopamine has been implicated in other movement disorders, such as Parkinson's Disease, in which there is too little dopamine and movement slows considerably. Studies on dopaminergic function in individuals with TS has produced some fascinating results.