Epidemiology
The tics of Tourette syndrome begin in childhood and tend to remit or subside with maturity; thus, a diagnosis may no longer be warranted for many adults, and observed prevalence rates are higher among children than adults. As children pass through adolescence, about one-quarter become tic-free, almost one-half see their tics diminish to a minimal or mild level, and less than one-quarter have persistent tics. Only 5 to 14% of adults experience worse tics in adulthood than in childhood.
Tourette syndrome is found among all social, racial and ethnic groups and has been reported in all parts of the world; it is three to four times more frequent among males than among females. The reported prevalence of TS varies "according to the source, age, and sex of the sample; the ascertainment procedures; and diagnostic system" from a low of .05% in a 1993 study to a high of 2.9% in a 1998 study.
Up to 1% of the overall population experiences tic disorders, including chronic tics and transient tics of childhood. Chronic tics affect 5% of children, and transient tics affect up to 20%. Robertson (2011) suggests that the prevalence of Tourette syndrome alone in the general population is also 1%, with a range reported between .4% and 3.8% for children ages 5 to 18. Singer (2011) states the prevalence of TS in the overall population at any time is .1% for impairing cases and .6% for all cases, while Bloch et al. (2011) state the overall prevalence as between .3 and 1%. According to Lombroso and Scahill (2008), the emerging consensus is that .1 to 1% of children have Tourette's, with several studies supporting a tighter range of .6 to .8%. Bloch and Leckman (2009) and Swain (2007) report a range of prevalence in children of .4 to .6%, while Du et al. (2010) report that 1 to 3% of Western school-age children have Tourette's. Prevalence rates in special education populations are higher. Using year 2000 census data, a prevalence range of .1 to 1% yields an estimate of 53,000–530,000 school-age children with Tourette's in the US, and a prevalence estimate of .1% means that in 2001 about 553,000 people in the UK age 5 or older would have Tourette's. Most cases would be mild and almost unrecognizable in older individuals.
Tourette syndrome was once thought to be rare: in 1972, the US National Institutes of Health (NIH) believed there were fewer than 100 cases in the United States, and a 1973 registry reported only 485 cases worldwide. However, multiple studies published since 2000 have consistently demonstrated that the prevalence is much higher than previously thought. Discrepancies across current and prior prevalence estimates come from several factors: ascertainment bias in earlier samples drawn from clinically referred cases, assessment methods that may fail to detect milder cases, and differences in diagnostic criteria and thresholds. There were few broad-based community studies published before 2000 and until the 1980s, most epidemiological studies of Tourette syndrome were based on individuals referred to tertiary care or specialty clinics. Children with milder symptoms are unlikely to be referred to specialty clinics, so these studies have an inherent bias towards more severe cases. Studies of Tourette syndrome are vulnerable to error because tics vary in intensity and expression, are often intermittent, and are not always recognized by clinicians, patients, family members, friends or teachers; approximately 20% of persons with Tourette syndrome do not recognize that they have tics. Newer studies—recognizing that tics may often be undiagnosed and hard to detect—use direct classroom observation and multiple informants (parent, teacher, and trained observers), and therefore record more cases than older studies relying on referrals. As the diagnostic threshold and assessment methodology have moved towards recognition of milder cases, the result is an increase in estimated prevalence.
Tourette's is associated with several comorbid conditions, or co-occurring diagnoses, which are often the major source of impairment for an affected child. Most individuals with tics do not seek medical attention, so epidemiological studies of TS "reflect a strong ascertainment bias", but among those who do warrant medical attention, the majority have other conditions, and up to 50% have ADHD or OCD. One author reports that a ten-year overview of patient records revealed about 40% of patients with Tourette's have "TS-only" or "pure TS", referring to Tourette syndrome in the absence of ADHD, OCD and other disorders. In children with tics, the additional presence of ADHD is associated with functional impairment, disruptive behavior, and tic severity. Other comorbid conditions include self-injurious behaviors (SIB), anxiety, depression, personality disorders, oppositional defiant disorder, and conduct disorders.
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