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Deep Brain Stimulation in Tourette Syndrome
Introduction
Diagnosis and Prevalence
Tourette syndrome (TS) is a chronic neuropsychiatric disorder with an unknown etiology characterized by motor and vocal tics. The onset of tics occurs most commonly in early childhood around the age of 5–7 years, and the disorder follows a typical waxing and waning pattern over the years ( ). The severity and frequency of tics increase during the prepubescent years and decline most commonly by early adulthood ( ). The majority of TS patients will experience either a stabilization or remission of tics by adulthood ( ). TS is considered an inherited disorder, although the exact genetic abnormality remains unknown. The fact that the beginning of symptoms is present during the course of maturation suggests that it is a developmental disorder ( ). TS is considered a relatively common disorder affecting an estimated 1% of the population and occurring four or five times more commonly in males than in females ( ). Diagnosis is performed during childhood after the documentation of persistent involuntary motor and phonic tics over a period of at least 1 year without a symptom-free period of three consecutive months ( ). Tics in TS are sudden, rapid, repetitive, stereotyped, and preceded by a premonitory urge, which consists of muscle contractions (motor tics) or outbursts of sound by moving air through the mouth, nose, and throat (vocal tics) ( ). The presence of this premonitory sensation enables the individual to suppress tics, a typical characteristic of TS that usually does not occur in other psychiatric disorders ( ). Tics can be classified as simple when they involve an individual muscle or a small group with discrete tonic or clonic contractions, such as eye blinking, coughing, sniffing, throat clearing, grunting, or nose twitching. Complex tics involve the coordination of several muscle groups to produce movements that often appear purposeful, such as gesturing, scratching, head or extremity jerking, jumping, making animal sounds, or uttering short phrases ( ). The vociferation of short phrases or words (echolalia and palilalia) is more characteristic of TS than the more widely known association of uttering obscene words (coprolalia), which occurs only in a very few TS patients ( ). Normally, cognition is not directly affected by TS, but these abilities may be limited in many patients due to a functional social impairment ( ). In addition to the presence of tics, patients with TS are at risk for comorbidities, including attention deficit hyperactivity disorder (ADHD), obsessive–compulsive disorder (OCD), anxiety, depression, and affective disorders ( ). These comorbid disorders make the therapeutic strategy for TS difficult since they add another layer of complexity to the treatment of tics. Pathologic involvement of cortico–striatal–thalamo–cortical (CSTC) pathways is supported by neurophysiological, brain-imaging, and postmortem studies ( ). However, the exact primary site responsible for the pathophysiology remains controversial. Although several neurotransmitters are involved in the signaling of the CSTC circuitry, dopamine is considered a key candidate responsible for the dysfunction ( ). The dysregulation of dopamine is supported by multiple clinical and preclinical studies. For instance, suppression of tics has been observed after the administration of drugs that block dopaminergic D2 receptors, or after the administration of dopamine uptake inhibitors ( ). Furthermore, exacerbation of tics has been observed after withdrawal of neuroleptics or administration of drugs that increase central dopaminergic activity like l -Dopa ( ).
Treatment of Tourette Syndrome
There is currently no cure for tics in TS, but since in most of the mild cases all symptoms will disappear by early adulthood, not all patients will require long-term treatment. Behavioral therapy can be sufficient treatment for the majority of these patients. Since no side-effects are observed with behavioral therapy, it can be implemented simultaneously with other treatments. Pharmacological treatment may be required in moderate to severe cases when symptoms interfere with the social, academic, or professional performance of the individual. The most common group of drugs is the classic dopamine antagonists (D2 receptor antagonists), such as haloperidol or pimozide. Risperidone, clozapine, quetiapine, and other second-generation neuroleptics are also widely used and very effective on tics ( ). The use of a third group may be considered in case of severe side-effects, mainly extrapyramidal symptoms. These drugs include noradrenergic agents like clonidine, and others such as benzodiazepines. In case of comorbidity the use of psychostimulants such as methylphenidate or selective serotonin reuptake inhibitors might be an option for the treatment of ADHD and OCD, respectively ( ). However, these drugs may not be helpful for the treatment of tics.
Surgical Treatment of Tourette Syndrome
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