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How many people worldwide or, at
least in the USA and the UK have Tourette syndrome?
This seemingly common query for which at least an estimate should be available
somewhere doesn't seem to be thoroughly addressed in any of the published
medical literature or on the Tourette Syndrome Association's (TSA) website.
Admittedly because of the lack of good epidemiological studies, the fact that
tics tend to remit as one passes through adolescence, and many other
complicating factors affecting epidemiological studies of Tourette's (beyond the
scope of this discussion) this answer is hard to come by; the difficulties in
estimating should not prevent an official organization from putting a bracket on
the numbers.
What the TSA and the National Institutes of Health (NIH) say:
Here are the most common (often contradictory or unclear) statements made by the
TSA and the NIH hopefully this article will help you understand how unhelpful
and confusing these numbers are:
"As TS often goes undiagnosed, no exact figure can be given. But authoritative
estimates indicate that some 200,000 in the United States are known to have the
disorder." (TSA
fact sheet PDF)
"Since many people with TS have yet to be diagnosed, there are no absolute
figures. The official estimate by the National Institutes of Health is that
100,000 Americans have full-blown TS. Some genetic studies suggest that the
figure may be as high as one in two hundred if those with chronic multiple tics
and/or transient childhood tics are included in the count." (TSA
FAQ)
"It is estimated that 200,000 Americans have the most severe form of TS, and
as many as one in 100 exhibit milder and less complex symptoms such as chronic
motor or vocal tics or transient tics of childhood." (NIH
FactSheet)
This information leaves a curious reader confused at best. Is it 100,000 or
200,000? Is it one in 100 or one in 200? Does the 200,000 refer to everyone with
TS, or only the most severe? Is "severe TS" the same as "full-blown" TS? And
what about run-of-the-mill, ordinary TS, which usually isn't severe?
If that information is not confusing enough, it actually gets much worse. These
numbers have little relationship to the prevalence numbers cited in the medical
literature, which are much higher, leading an informed reader to wonder why the
TSA hasn't been more effective in putting out better estimates. It should be
possible to disentangle some of this information and merge it with more
scientific knowledge for a more informed guesstimate.
Some definitions and concepts:
There are several issues to be sorted out in understanding epidemiological
estimates of Tourette syndrome.
What is meant by "full-blown" Tourette syndrome or
"most severe form"?
"It has been suggested that the concept of 'full-blown' TS be used to describe
patients who manifest tics as well as significant comorbidities ... " (PMID
16536366)
Full-blown TS includes coprophenomena, echophenomena, and paliphenomena. (eMed)
So, the first problem with the 100,000 and 200,000 numbers is the vaguely
defined terms "full-blown" and "severe," which don't include most people with
Tourette's; it is established that most people with TS don't have "the most
severe form" (PMID
12443611,
Robertson,
TSA).
Prevalence vs. incidence
Prevalence is defined as the total number of cases in the population, divided by
the number of individuals in the population. Incidence is the number of new
cases during a given time interval, usually one year. Both are estimates subject
to wide variability in Tourette syndrome populations because of the difficulties
in identifying persons with tics. In the case of Tourette syndrome, incidence is
rarely used, and prevalence rates are often qualified to a specific age range.
For example, you may see estimates of the prevalence in school-age children, all
children, children in a certain age range, or the overall population; this makes
prevalence estimates confusing for a reader who may not understand why so many
different numbers are cited.
Pediatric vs. adult populations
Another complication comes from the natural course of the condition. Tourette
syndrome begins in childhood, but tics tend to remit or subside as children pass
through adolescence (PMID
9651407); children are 5 to 12 times more likely than adults to be
identified with tic disorders (PMID
9443353). This results in higher prevalence rates among children than
adults, so when estimating populations of persons with TS, one needs to know
what prevalence rate (that is, what age range) is being described.
Difficulty in detecting TS in studies
Tics are intermittent, waxing and waning in location, intensity and frequency,
and are not always recognized by physicians, family members, or teachers (PMID
11077021,
eMed);
according to one estimate, 20% of people do not recognize their own tics (PMID
11077021). Tics are often undiagnosed or hard to detect (PMID
12443611,
PMID 15640424); all of these factors affect prevalence estimates.
Methodology affects how many cases of TS are detected
"Frequency of the disorder varies by age, sex, source of sample, and method of
assessment. For example, studies on direct classroom observation and that use
multiple informants consistently yield substantially higher prevalence estimates
than do other assessment methods. Many patients identified with these techniques
have mild characteristics and do not need long-term intervention apart from
educational and other support." (PMID12443611)
"In the majority, however, it will be probably undiagnosed and mild in the
majority of cases, without distress, impairment or coprolalia." (Robertson)
Other problems and older estimates
Other issues complicate understanding of Tourette's syndrome prevalence
estimates (source: Scahill, 2004 TSA Medical Letter). Earlier estimates were
based on clinically ascertained cases, which may fail to detect milder cases.
Recent studies rely on direct interview or observation to better detect tics.
Studies may also use different diagnostic criteria; some older (and even some
newer studies) required distress or impairment from tics. Moving the threshold
toward milder cases, and detecting more cases with better methodology, results
in higher prevalence estimates.
Current prevalence estimates
"According to current best estimates, TS affects approximately 6 of every
1,000 schoolchildren." (NIH
Conference, 2006)
"... several recent studies have narrowed the estimated prevalence of TS in
children. The emerging consensus ranges from one to eleven per 1,000, with
several studies falling in an even tighter range of three to eight per 1,000."
... "Several other surveys, however, provide good agreement in a much narrower
range from six to eight per 1,000 for TS". (PMID16536365)
"TS is estimated to occur in at least 1% of children." (Zinner SH. "Tourette
syndrome much more than tics". Contemporary Pediatrics. Vol. 21, No. 8,
August 2004: 2249.)
"Although there has been debate as to how common Tourette syndrome is, the
author feels confident stating that the prevalence figure of around 1% in
mainstream schoolchildren (using DSM-IV-TR criteria) is more likely to be
correct than the previous lower figures." (Robertson)
Estimated numbers
Note that a 1% prevalence rate is another way of saying 10 per 1,000 or 1 per
100; the tighter range of 6 to 8 per 1,000 mentioned by Scahill and the NIH
conference is slightly lower than the 10 per 1,000 (1%) prevalence rate
mentioned by Zinner and Robertson.
Here are the only actual numbers I have been able to find I could find no
published numbers for adults, but the numbers of children alone help put the
TSA/NIH numbers in perspective:
"TS is estimated to occur in at least 1% of children. Based on this estimate
and on the 2000 U.S. population census, there are approximately 750,000 children
with TS in the United States." (Zinner SH. "Tourette syndrome much more than
tics". Contemporary Pediatrics. Vol. 21, No. 8, August 2004: 2249.)
"According to the most recent census data, there are 53 million children
between the ages of 5 and 17 years in the United States, a prevalence of one per
1,000 would result in an estimated 53,000 children with TS. By contrast, if the
true estimate is 10 per 1,000, that would increase the tally of TS cases to
530,000". (PMID16536365)
" ... the prevalence figure of around 1% in mainstream schoolchildren (using
DSM-IV-TR criteria) is more likely to be correct than the previous lower
figures. In the UK, the current population between the ages of 518 years is
10,676,000. Thus, between approximately 0.4% (42,700) and 1.76% (187,897) with
an approximate figure of 1% (106,760) of children between the ages of 5 and 18
years in the UK may well have Tourette syndrome." (Robertson)
Putting it all together how many people really have TS?
Putting together the pieces above helps put the 100,000 and 200,000 numbers in
perspective.
Personal bias disclaimer: I believe the 1% for school-age children (10 per
1,000) will turn out to be more correct than the 6 per 1,000. I have never read
a study of the prevalence of TS that I believe would have ascertained a case
like my son's, even with trained observers; his tics have never been detected by
a teacher or independent observer until I pointed them out. I've read every
recent prevalence study, and I can easily see how they would all fail to detect
cases. Also, the prevalence rates in every school he has attended have been
higher than those cited. For example, in his first rigorously academic demanding
private school of 250, there were 5 cases of TS that I knew about, and
this was not a school that tailored to special needs. Anyway ...
If we ignore those who say the 1% (ten per 1000) is a good number and
conservatively accept the six to eight per 1,000, call it seven as an average,
and use the school-age populations given in the sources to do the math, we get:
371,000 school-age children with TS in the United
States, and
74,700 school-age children with TS in the United Kingdom, or
... almost half a million school-age children with TS in the USA and the UK
combined.
How do 371,000 school-age children alone in the U.S. relate to the 100,000 or
200,000 numbers, which are supposed to include adults and pre-school-age
children as well? How do those numbers relate to this "severe" of "full-blown"
number? And what about a worldwide estimate? It would be nice if an official
organization would give us a meaningful estimate of the millions of people
worldwide that have Tourette syndrome. The award-winning HBO documentary,
I Have Tourette's But Tourette's Doesn't Have Me states: "In every school in
America, there's probably at least one kid who has Tourette Syndrome."
Probably and at least are weasly; given the widespread distribution
and heavy airplay this documentary has been given, it could be comforting and
reassuring for children with TS to know that there are hundreds of thousands of
other children with Tourette's.
(Just a note: this website was
designed for newcomers to Tourette's syndrome, to be read through in page order.
Growing up with Tourette's
Syndrome: Information for Kids
HBO Documentary on Tourette's Syndrome Tourette
Syndrome Research Article Summary
Tourette's Syndrome - Now What?
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