Biology 202
1998 Second Web Reports
On Serendip

As far as neurological diseases go, Tourette's is relatively common: between 2.9 and 4.5 children out of every 100,000 children have it, and that figure may be higher due to mild cases which go undiagnosed (1). The condition is much more prevalent in males than females, and Tourette's is most common present in young people - the condition often disappears in post-pubertal years. The onset of vocal tics most always succeeds motor tics, though most individuals with Tourette's eventually present both. Tics can sustained (and are then called tonic or dystonic), which can result in abnormal posture or gait for Tourette's patients. Behaviors associated with Tourette's include copropraxia, echopraxia (both motor behaviors), coprolalia, palilalia, and echolalia (all verbal) (2).

The neurological causes of Tourette's are not fully understood. About half of patients have "non-localizing, so-called 'soft,' neurological findings suggesting disturbances in the body scheme and integration of motor control". When EEG's are performed on people with Tourette's, the results are often vagule abnormal, yet provide little concrete information to practitioners; the same is true of CAT scans. Additionally, these abnormalities may be the result of drugs taken to counteract Tourette's symptoms (3).

Neurotransmitters, particularly dopamine, serotonin, norepinephrine, and choline, are suspected to be the key to Tourette's. There is evidence that these chemicals, which are natural stimulants in the human nervous system, are metabolized differently in Tourette's patients. Genes which help establish these patterns of metabolism may cause this (4). Experiments involving medications that are dopamine agonists show that hyperkinesia, uncontrolled movements like those seen not only in Tourette's but in other neurological conditions like Huntington's disease, decreases when dopamine levels are lower (5). However, research has yet to fully explain these relationships between genetics and these neurotransmitters: in a study of mutations in genes coding for dopamine and serotonin receptors, subjects with Tourette's, bipolar affective disorder, and schizophrenia, no higher incidence of mutations was found in the experimental group (6).

Based on its disproportionate frequency in men, there is an idea that hormones may also play a role in the onset and severity of Tourette's. Whether they influence gene expression or inhibition systems, androgens definitely seem to affect the tics. Some researchers postulate that spikes of these hormones during pregnancy (when the fetus' nervous system is developing) make the fetus more likely to develop Tourette's some time after birth (2). The fact that the severity of Tourette's often lessens as patients age also lends credence to the theory that androgens (whose levels wane as people mature) interfere with the mechanisms that would ordinarily control tics (7).

Structural analyses of the brains of individuals have also yielded some ideas about the causes of Tourette's. Researchers have looked at the basal ganglia, a region of the brain beneath the cortex, and discovered that several parts - the putamen, the globus pallidus, and the renticular region- are smaller in people with Tourette's; blood flow and glucose usage are also decreased. The basal ganglia plays a role in controlling movement, so that this lessening of its functions could result in the inability of individuals with Tourette's to control some of their muscle movements. The basal ganglia is part of the corticostriatothalamocortical pathway, which is a pathway which inhibits the basal ganglia; this pathway is regulated by dopamine, serotonin, and norepinephrine (2). In studies of monozygotic twins, the twin with the lower birth weight presented more serious symptoms, supporting the hypothesis that in utero development affects Tourette's, particularly since the basal ganglia is especially sensitive to environment at this point (7). Additionally, these brains have shown less asymmetry than brains of people not diagnosed with any neurological disorder. Usually, the left side of the basal ganglia is larger; this phenonmenon has been observed as less pronounced in people with Tourette's. Other researchers have noticed that the corpus callosum is more rounded and that this may affect the connection between the two hemispheres of the brain. They support this theory with the fact that Tourette's onset is most common during the phase of development (early latency) when the corpus callosum is developing most rapidly (2).

The cingulate cortex may also be involved in Tourette's, as patients who had these cortices removed showed lessened symptoms of Obsessive-Compulsive Disorder, which is often also present in people with Tourette's. There are several ideas about how the cingulate cortex may cause Tourette's:the pre-frontal cortex which usually regulates the cingulate cortex may malfunction so that the cortex fires uncontrollably, or the failure to regulate may be caused by increased orbito-frontal metabolic rates (2).

Several other conditions currently understood to have similar genetic and neurological causes are often comorbid with Tourette's. Obssessive-Compulsive Disorder and Attention Deficit Hyperactivity Disorder are far and away the most common; the two share many pharmacological treatments with Tourette's. In their study of twins Hyde and Weinberger found that when they included all these conditions together as "tic disorders," genetic analysis showed 80% concordance in monozygotic twins and 25% in dizygotic twins. Yet twins showed wildly varying symptoms, showing that environment cannot be discounted in determining the causes of these diseases conditions (7).

The realtionship of Tourette's, OCD, and ADHD, and the current thought on their causes calls of course for more research, but also for a new way of treating the conditions. Since they cannot be fully extricated from each other nor can their treatments be isolated, less emphasis needs to be put on diagnosing people, especially children, with a specific disorder and consequently pigeon-holing their case and their treatment. Until a better understanding of the conditions is developed, practitioners need to look at the entire related group of tic disorders as one and find a manner of treatment which works best for the individual patient (as opposed to the course presribed for someone with x syndrome).

RESOURCES

1. American Academy of Neurology - Tourette's Syndrome

2. Macalaster College Behavioral Neuroscience Tourette's Syndrome Page

3. Internet Mental Health

4. National Institute of Neurological Disorders and Stroke - "What Cuases TS?"

5. Department of Neurobiology, Baylor College of Medicine - Movement Disorders Research: Tetrabenazine Study

6. HUM-MOLGEN [Human Molecular Genetics] European Server

7. "Tourette's Syndrome: A Model Neuropsychiatric Disorder" - Grand Rounds at the Clinical Center of the National Institutes of Health,


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