Review article
The prevalence and epidemiology of Gilles de la Tourette syndrome: Part 2: Tentative explanations for differing prevalence figures in GTS, including the possible effects of psychopathology, aetiology, cultural differences, and differing phenotypes

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Abstract

As has been demonstrated, Gilles de la Tourette Syndrome (GTS) occurs in at least 1% of the population worldwide. However, earlier studies suggested a lower prevalence. In addition, the prevalence figures for different studies very between 0.4% and 3.8%. Moreover, the prevalence appears to vary in some parts of the world and races, with a lower rate in Afro-Americans and sub-Saharan black Africans. In this the second part of the review, possible reasons for the differences in prevalence and epidemiology are discussed. Tentative explanations for differing prevalence figures in GTS include problems with the diagnosis of GTS, the multidimensional nature of tics, as well as other tic factors including the waxing and waning of symptoms and the suppressibility of symptoms. Other factors inherent to GTS include the fact that there is no diagnostic test and indeed no definitive diagnosis other than clinical, the fact that psychosocial stresses can lead to increased tic severity, and that comorbid disorders may mask tics. The varying methods of study employed can also effect prevalence. There may be some regional differences in GTS as well, which may be due to a lack of awareness of GTS, or it may be a true reflection of low prevalence as in some populations GTS does appear rare. With regard to the sub-Saharan Africa data and possibly the African American data, matters are much more complex than meets the eye. The following reasons are all possible for the apparent rarity in these populations and include (i) other medical priorities and less propensity to seek health care, (ii) lack of awareness of GTS, (iii) chance, (iv) ethnic and epigenetic differences and reasons, (v) genetic and allelic differences in different races, and (vi) an admixture of races. The aetiology of GTS is also complex, with influences from complex genetic mechanisms, pre- and perinatal difficulties and, in a subgroup, some infections, possibly by epigenetic mechanisms. These may well affect phenotype and, thus, prevalence. There have even been suggestions that people with GTS are increasing. Recent data suggests that GTS is not a unitary condition and that there may well be different types of GTS. The prevalence of GTS in these individual subtypes is unknown. It is suggested that a new nomenclature be adopted for GTS in future, pending further genetic and phenomenological studies. To what extent the aetiology affects the phenotype and, thus, the prevalence is still unclear.

Introduction

The generally accepted international diagnostic criteria for Gilles de la Tourette Syndrome (GTS), a childhood-onset neuropsychiatric disorder, include multiple motor tics and one or more phonic (vocal) tics or noises, lasting longer than a year [1], [2]. The prevalence and epidemiology of GTS are more complex than was once thought. In a recent study and review by the present author, Robertson (Part 1 of this duo [3]) collected the data from 14 studies undertaken in mainstream school and school-age youngsters in the community using similar multistage methods, scrutinized the data, and reported prevalence figures for GTS of between 0.4% and 3.8% for youngsters between the ages of 5 and 18 years. Of the 420,312 young people studied internationally, 3989 (0.949%) were calculated as having been diagnosed as having GTS. It was therefore suggested that a figure of 1% would be appropriate for the overall international GTS prevalence figure. There were, however, “outliers” to the figure: for instance, GTS does seem to be substantially rarer in the Xhosa youngsters (Black South African people). With regards epidemiology, GTS is found in all cultures, although to differing degrees, and is not common in the North American “African-American” and has not been reported in sub-Saharan black Africa countries, that is, other than a few GTS individuals identified in the South African Xhosa study. In all cultures where GTS has been reported, the phenomenology is similar, highlighting the biological underpinnings of the disorder [3].

In order to understand the prevalence and epidemiology of GTS, one must take the disorder in context, with regard to clinical phenomenology, psychopathology, and possible phenotypes, as well as the complex aetiological theories, as they almost certainly all affect the prevalence data.

Section snippets

Tentative explanations for differing prevalence figures in GTS

What are the reasons for these differing results in prevalence? Suggestions have included problems with the diagnosis of GTS; the fact that tics are probably multidimensional in nature; which dimensions run along varying continuums including intensity of symptoms (from mild to severe), frequency of symptoms (from rare to constant), a variety of symptoms (single and/or multiple tic groups), complexity of tics (simple to highly complex), and comorbid psychiatric disorders (from none to multiple:

Acknowledgments

This paper was first read as the keynote lecture at the 3rd International Symposium of the Canadian Tourette Syndrome Foundation, Edmonton, September 2006 [2]. I would also like to thank: Dr. Andre van Rensburg for allowing me access to the unpublished data from his PhD thesis and for translating Dr. Pelser's PhD thesis data [3]; Dr. Hugh Rickards as well as Professors Ken Kidd, Roger Kurlan, Brad Peterson, and Chris Stringer, F.R.S., for their helpful comments [4]; and Professor George Gericke

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