|Home | About | Journals | Submit | Contact Us | Français|
The coexistence of tics and dystonia in the same patient,1 and occurring individually in different members of the same family,2 has been reported in the literature. This has raised the question as to whether patients with Tourette syndrome (TS) could be at a higher risk for dystonia, and if there is a common pathophysiological mechanism for the expression of tics and dystonia.2 We sought to determine the prevalence of dystonia among patients with TS using a large international database.
Using the Tourette Syndrome International Database Consortium (TIC), which is comprised of patients with TS seen by neurologists and psychiatrists worldwide, we searched for cases with a coexisting diagnosis of dystonia. The database was established in 1992 with the aim of learning more about the variability in clinical samples of TS among sites. In all, 28 countries are represented in the consortium, with 52 participating sites. Of the clinicians participating in the consortium, 71% are psychiatrists, 23% are neurologists, 4% are pediatricians and 2% are medical geneticists. At the time of analysis, the database contained 6654 patients, with an average age of 15 years when first seen and entered into the database (range 1–77), and a male predominance of 82%. In all, 695 patients (10%) aged 30 years at the time of registration were in the database. Information collected on the TIC data entry form include the medical specialty of the clinician entering data, family history of TS and related comorbidities, peak tic severity, drugs for tics, perinatal problems, and other neurological, psychiatric or medical diagnoses or problems. Unlisted neurological, psychiatric and medical problems are entered into a text field. There is no specific question addressing the presence of dystonia in the data entry form.
Nine cases of dystonia were found in the database, giving a prevalence rate of 1352 per 1000000. Additional history was available from eight of the nine cases. These eight cases came from two clinicians, a neurologist (J Jankovic) and a neuropsychiatrist (K Muller‐Vahl), both specialised in movement disorders. In one case, the dystonic hand postures had resolved on follow‐up examination 2 years later, making a diagnosis of dystonic tics more probable. In three cases with dystonic hand postures noted on examination, there was a history of significant perinatal complications. In one case, the patient had drug‐induced cervical dystonia. The remaining three cases—one with dystonic hand postures, one with left foot dystonia and one with new onset blepharospasm—had no attributable secondary cause.
Population studies of idiopathic focal and generalised dystonia reveal an estimated prevalence of 330 per 1000000.3 Since probably only three of our nine cases are idiopathic, this would give a prevalence rate of 451 per 1000000 in our sample, which is quite similar to this estimate. More recent epidemiologic assessments of the prevalence of primary dystonia have been divided into early‐onset and late‐onset cases, and have assessed more ethnically homogenous samples. For example, the prevalence estimate for early‐onset dystonia in Ashkenazi Jews from New York is 111 per million, and 600 per million for late‐onset dystonia in northern England.4
Population rates of secondary dystonia are more difficult to find. The only published figure we were able to find was a prevalence rate of 10700 per million in the general community aged 50–89 years.5 The rate of secondary dystonia in the TIC database was 902 per million. It is difficult to make comparisons however, since our clinical sample consists mainly of children in whom one might expect a lower rate of secondary dystonia than in the population of older people.
In addition to clonic tics, the majority of patients with TS also have dystonic tics. Dystonic tics differ from clonic tics in that they are comparatively slow, with sustained twisting or pulling movements, which produce temporarily maintained abnormal postures. Dystonic tics can present diagnostic difficulties to the clinician, but can be distinguished from idiopathic dystonia by history. Diagnostic uncertainty regarding the nature of these movements could lead to both under‐ and over‐representation of dystonia cases in the database.
It is possible, since the data entry form did not specifically ask about the presence of dystonia, that cases were missed and the prevalence rate of dystonia in our TS sample is therefore underestimated. This possibility could further be supported by the fact that the majority of physicians participating in the consortium are psychiatrists rather than movement disorders neurologists. Although more subtle forms of dystonia could not be easily recognised in patients with TS by physicians not specialising in movement disorders, missed cases were probably mild and did not cause significant disability. The clustering of dystonia cases in two clinics specialised in movement disorders could support the possibility of under‐recognition in the total survey population but could just as easily suggest a referral bias of atypical cases to these centres, particularly since no other case of concurrent tics and dystonia were reported from other participating movement disorders specialty clinics. The results of this study give an important first estimate of the prevalence of dystonia in patients with TS.
The authors thank Dr Joseph Jankovic and Dr Kirsten Mueller‐Vahl for their contributions to the paper.
Competing interests : None declared.