Epidemiology

How common is it?

There have now been several studies of the population incidence of CTS and it is clear that it is an extremely common disorder. A summary of the main findings appears below.

As always, when you ask a simple question 'how common is CTS?' - the answer turns out to be more complicated than one might expect. Firstly there are two ways of answering the question 'how many people get CTS?' The number of people in a population who have a disorder at any given time is known as the prevalence and can be ascertained by a survey at one point in time. Figures derived in this way are shown in the table below as unbracketed numbers. The other way of looking at it is to ask how many people in a given population will develop the disorder during a set time interval, usually one year. This is known as the incidence and is more difficult to measure as it involves studying a population over a period rather than just doing a single survey. Incidence figures are shown below in brackets. Both are given as rates per 100,000 people. In order to make fair comparisons between studies one also has to know something about the other characteristics of the population being studied. As can be seen from the above CTS is generally found to be commoner in women than men. If one study looks at a population which has 70% women and a second a population with 40% women then the first study is going to find a higher overall incidence and prevalence of CTS than the second. Similar considerations apply to age. The european studies below have generally had their results adjusted to indicate what they expect they would find in the 'european standard population' - which is a european average age and sex mixture.

These studies show very marked differences in results, most of which probably originate in differences in the criteria used to define a subject as having CTS. In some studies it is little more than a self-report by the patient, at the other end of the scale the most rigorous standard of diagnostic proof is represented by requiring both a confident clinical diagnosis by a specialist clinician and an abnormal result consistent with CTS on nerve conduction studies. 

A great deal is known about the population characteristics of CTS patients and the main findings on which there is general agreement are:

Sex

Almost everywhere in the world CTS is commoner in women than in men though the ratio of female to male varies from study to study. The exceptions are one or two American studies which found a relatively equal sex incidence but these were probably carried out in atypical restricted populations. One might immediately ask - do women, being generally smaller, have smaller carpal tunnels? - and the answer to this is yes, but.... they also have smaller carpal tunnel contents so that the ratio of contents to tunnel dimensions is similar in men and women (Bower 2006). In a study carried out entirely in women, subjects with CTS had similar carpal tunnel dimensions to control subjects without CTS suggesting that tunnel size itself may not be a major factor (Pierre-Jerome 1997), though other studies disagree about this - see below under anthropometry. Furthermore, CTS is especially seen in women during pregnancy and around the time of the menopause. Presumably any given individual’s carpal tunnel does not change in size as a result of pregnancy or menopause and this raises the possibility that female hormones may play a role by affecting the carpal tunnel tissues in some way. Some further clues to the possible importance of female hormones in CTS are appearing in the literature. Firstly CTS has recently been found to be common in women taking the drug Exemestane for breast cancer (see the section on breast cancer and CTS). Exemestane reduces the amount of circulating oestrogen in post-menopausal women. Secondly tissue samples from CTS patients have been found to contain more receptors for oestrogen and progesterone than control subjects (Toesca 2008).
 

Age

CTS gets commoner with age - to some extent, like many other disorders, it appears to be a ‘wearing out’ phenomenon. It can however occur at any age and one should never discount the diagnosis purely on grounds of age.

Weight

CTS incidence is higher in obese individuals. This effect may be more pronounced in younger patients. (Bland 2005)

Anthropometry

Several studies have agreed that physical measurements of the wrist size are different in CTS patients. The usual observation is that patients have a ‘squarer’ wrist profile. (This needs an illustration if anyone would care to draw one). Internally there are studies which suggest that patients with CTS tend to have smaller carpal tunnels than asymptomatic controls (Dekel 1980, Horch 1997) or that CTS patients have relatively less 'free space' in the carpal tunnel (Cobb 1997).

One further epidemiological aspect of CTS has received an enormous amount of attention in the medical, lay and legal press but remains controversial:

Occupation

There is some reasonably convincing evidence that CTS may be commoner in people who use the hands for strenuous, repetitive tasks and in those making extensive use of vibrating tools. With lesser degrees of use, and especially in relation to typing/keyboard use, the evidence is not convincing, indeed some recent evidence even suggests that regular keyboard use may be protective against CTS (Atroshi 2007 Stevens 2001 Mattioli 2009).

There are so many studies of CTS and occupational exposure that they have been a productive vein to be mined for systematic reviews. The most recent meta-analysis of 37 studies concludes that exposure to vibration and high grip forces are significant risk factors for the development of CTS with use of vibrating tools in particular being associated with an approximately five-fold increase in risk, but this study did not look at keyboard use and it does not allow one to quantify how much exposure to these risk factors is a hazard. (Barcenilla 2012). A slightly older systematic review of 44 studies (van Rijn 2009) also concluded that high grip forces, vibration exposure and repetitiveness of work were associated with an increased risk of CTS and specified what degree of exposure they were talking about - though the units make it hard to comprehend quite what these figures mean. For reference, the levels of exposure which they felt increased risk were:

Grip force > 4kg

Repetitiveness - cycle time < 10 seconds or 50% of cycle time performing same movements

Vibration - 8-hour energy equivalent frequency weighted acceleration 3.9 m/sec/sec

Yet another review (Palmer 2007) of 38 primary studies, estimated that prolonged use of vibrating tools increased the risk of CTS >2-fold and "similar or even higher risks from prolonged and highly repetitious flexion and extension of the wrist, especially when allied with a forceful grip". One should remember that all three of these reviews are essentially drawing on the same source material, though more studies are being added every year.

A completely different way of synthesising the mass of data from epidemiological studies was attempted by an American group (Lozano-Calderon 2008) who gathered together 117 primary articles describing risk factors for CTS applied to each of them a scoring system designed to give a numerical estimate of the strength of evidence indicating a causative link between the factors being studied and the disease. This scoring system is based on criteria for causation suggested by Sir Austin Bradford Hill in 1965 and though the Bradford Hill criteria have been subject to some criticism since, it at least provides a consistent way of scoring the strength of evidence for causation, rather than simply strength of association, found in these epidemiological studies. Using this scoring system they were able to perform two analyses. First of all they grouped together all studies looking at occupational risk factors and all studies looking at biological risk factors (age, sex, genetics etc) and measured the overall average strength of evidence for all papers in each group. For the biological factors group the score was 12.2 while for occupational factor studies it was 5.2 - suggesting that in general, studies of biological factors provide rather more convincing evidence of causation than do studies of occupational factors. Secondly they were able to look at individual risk factors and pool the Bradford Hill scores for all papers in which each factor had been examined. This gave the highest scores for:

Genetics - 14.2

Race - 11.7 (only 3 studies)

Anthropmetric measures (wrist size etc) - 11.3

Hormonal status - 10.9

Obesity - 10.6

Diabetes - 10.5

Age - 10.4

Gender - 10.4

In contrast the highest scoring occupational factor, repetitive hand use, scored only 6.5 points. There are fairly serious methodological issues with this paper - in particular it takes little interest in the methodological flaws of the studies which it uses as its source material, but within its limitations it is an interesting set of observations.

One should be wary of increased risk figures in epidemiological studies - an increase in incidence from 1 per thousand per year to 2 per thousand per year is a doubling of risk, but even the doubled risk is still a very small one. What is undoubtedly true however is that once you have CTS, using the hands heavily for a day is likely to be followed by an exacerbation of symptoms that night.

Whether CTS is considered an industrial disease or not depends on where you live. In some countries, notably the USA, there is a small legal and insurance industry servicing compensation claims for carpal tunnel syndrome allegedly caused by work. In the UK most cases of CTS are not considered to be occupationally caused and the government department of work and pensions applies fairly strict criteria in deciding what the worker must have been exposed to in order to qualify for industrial injuries diablement benefit - see this dwp website. (Section A12)

Family History

As is becoming evident for most diseases nowadays your susceptibility to illness is to some extent determined by your parents - genetics

Revision date - 24th October 2012