Tourette's is a
heterogeneous condition, with waxing and waning symptoms. The inherently changing nature of its core symptoms complicates research design, resulting in questions about medications in clinical practice. Results from case studies may not be borne out by controlled or prospective, longitudinal studies. High-profile media coverage focuses on treatments that do not have established safety or efficacy and alternative therapies involving unstudied efficacy and side effects are pursued by many parents. The direction of current and future research in Tourette's was outlined in a 2005 journal article by the outgoing chairman of the TSA Scientific Advisory Board. Swerdlow divides the research landscape into five broad questions about Tourette's: what is it, who has it, what causes it, how it should be studied, and how it should be (medically) treated. According to Swerdlow, "the 'core' TS conundrum" is a lack of consensus about the definition of Tourette syndrome. Since vocal tics result from a "motor event (ie, a contracting diaphragm moving air through the upper airways)", TS could be defined as a disorder of motor tics, eliminating the distinction between TS and the other
tic disorders. Individuals who have only tics may not be functionally impaired, raising the question of whether Tourette's as currently defined should be a DSM diagnosis. Swerdlow highlights the importance of studies in new areas such as behavioral techniques, and says that "the whole-cloth dismissal of psychologic forces in the pathobiology of TS was a strategic error". Questions remain about whether co-occurring (comorbid) conditions should be part of the core definition, and why
sensory phenomena, which are a core part of Tourette's, are not part of the diagnostic criteria. Dropping the criteria for impairment from the diagnosis resulted in higher estimates of the prevalence of TS (the question of "who has it?"). Older estimates "came from tertiary referral samples, the sickest of the sick"; greater prevalence casts the condition in an entirely new light, and calls for new biological models of the condition and new approaches to addressing a more common disorder. Discovering the causes of Tourette's may help resolve the questions of what it is and who has it. The
autosomal dominant inheritance model has not been validated, and past research has been affected by the problem of referred samples, which may not reflect broader populations of persons with Tourette's. Probabilistic genetic models may yield better clues than the "one gene equals one disorder" approach. One of the most controversial presumed causes, the PANDAS hypothesis, has sparked disagreement. Expanding criteria for the diagnosis, and increasing awareness of the impact of comorbid diagnoses, has resulted in further questions of how to study Tourette's. Tourette's patients are often recruited from sources that introduce
ascertainment bias towards one 'type' of TS. Developing and applying standardized instruments, along with a greater awareness of ascertainment bias in recruitment sources, will be important in genetic studies. It is unknown if "we lose both signals and are just adding noise to the experimental outcome" when comorbid conditions, such as
OCD or
ADHD, are included or excluded from study samples, or samples include/exclude children or adults, or patients with severe symptoms. ==Notes==