Cornell University Ergonomics Web

CARPAL TUNNEL SYNDROME AND COMPUTER USE - IS THERE A LINK?

According to media reports of a recent research studies by researchers in Denmark and those at the Mayo clinic in Scottsdale, Arizona, there is no association between computer use and carpal tunnel syndrome (CTS) . This report apparently contradicts widely held assumptions about the causes of computer-related injuries, so what's the truth?

What is Carpal Tunnel Syndrome? 

CTS is caused by compression of the median nerve as this passes through the carpal tunnel, a narrow channel in the wrist between the carpal bones of the wrist (tunnel roof) and the transverse carpal ligament (tunnel floor). Sustained compression of the median nerve disrupts the axonal transport of fluids and this causes damage to the nerve fibers, a process termed neuropathy. The median nerve is a mixed nerve with both sensory and motor fibers. The sensory fibers are those first affected, and intermittent paresthesia (numbness, tingling) of the thumb and first two fingers often signifies the early stages of CTS.

The Danish Study [1]

A questionnaire follow-up survey of 5,658 Danish workers was conducted one year after a baseline survey had been completed by 6943 workers. The survey collected self-report data on carpal tunnel syndrome symptoms. The prevalence of self-reported symptoms at the baseline survey was 10.9%. The incidence of new or worsened CTS symptoms at the 1-year survey was 5.5%. The prevalence of median nerve symptoms at 1-year was 1.2%. At the baseline, there was no association of psychosocial factors (high work demands, high job pressure, low job control, low social support) or physical factors and possible CTS. At the 1-year follow-up, the average computer use for men was 8 hours per week and for women was 9.3 hours per week and there was no significant association between keyboard use and possible CTS. At the 1-year follow-up, the average mouse use for men was 12.5 hours per week and for women was 14.7 hours per week and there was a  significant association between mouse use >20 hours per week and possible CTS. The authors concluded that computer use is unrelated to carpal tunnel syndrome.

Limitations with the Danish Study 

The fact that the Danish workers only used their keyboard for an average of 8-9 hours per week is a serious limitation. This can hardly be considered hardly intensive keyboard use, especially  by US standards where workers may be keying intensively for >30 hours per week. A

The Danish workers  did make more intensive use of a mouse for an average of 12-15 hours per week, and that the authors noted a significant association between intensive mouse use and possible CTS, with an odds ratio of 3 for possible CTS for workers with  >20 hours per week of mouse use. Other recent Danish work has also found that mouse use increases musculoskeletal injury risks.[2] Both Danish studies found that there was no association of psychosocial factors with injury risks. Contrary to Andersen et al.'s conclusions, both Danish studies present results suggesting that computer use is related to musculoskeletal injuries, such as possible CTS.

It may be unwise to generalize from computer workers in Denmark to those the USA. In Denmark, by law the work week is shorter and ergonomic working conditions are more regulated than in the USA. Providing a high quality working environment is a legal requirement in Denmark and worker awareness of ergonomics and the use of ergonomically designed furniture is widespread. Ergonomists have always maintained that computer use need not increase musculoskeletal risks if the workstation is ergonomically designed, and Andersen et al.'s study may partly be confirming this. Unfortunately, unlike Denmark, many US companies still fail to grasp the benefits of ergonomics, and ergonomically designed workstations remain the exception rather than the rule.

The Mayo Clinic Study [3]

A questionnaire survey of workers using computers at the Mayo clinic in Scottsdale, Arizona, was conducted. Complete data were received for a sample of 257 respondents (81.8% return rate). Some 95% of respondents were women. Of these, 181 employees (70.4%) reported no symptoms of CTS, but 70 employees (29.6%) did report hand paresthesia. From subsequent interview, 27 employees (10.5%) were classified as having CTS. Of these employees, 9 people (3.5%) met a clinical definition of CTS and showed changes in nerve conduction velocity. Steven et al. conclude that this prevalence of CTS is comparable to that for CTS among the general population, and therefore computer use cannot be associated with the development of this syndrome.

Limitations with the Mayo Clinic Study 

Before finally concluding that there is no association between CTS and computer use it is worth considering the following issues:

• The conclusion of the Mayo study assumes that the 3.5% prevalence of CTS that they reported (9/257) is that of the general population. The researchers cite two studies on CTS prevalence in support of their conclusion. One is a Dutch study [4] from the early 1990s, the other is a more recent Swedish study [5]. The Dutch study did not assess computer use. Given the changes in computer use patterns throughout the 1990s, it is probably wisest to compare results with the Swedish study. In this research, a survey of 2466 Swedes (46% men), aged 25 to74 years was conducted, and 14.4% of respondents reported hand paresthesia. Clinical examination confirmed CTS in 94 symptomatic subjects (3.8%). At first sight, the results from this and the Mayo study seem comparable (3.8% vs 3.5%). However, in the Swedish study, 3.8% is the overall prevalence and there are statistically significant effects of a number of variables, including gender, age and occupational requirements. In the Swedish study, the actually prevalence for CTS among office workers was 1.7%, which is half of that reported by the Mayo clinic for a comparable population.
• The Swedish study shows that CTS prevalence rates peak between 45-65 years of age. The mean age of workers in the Mayo study was 41 years, which suggests that this population had not yet reached peak prevalence. It is uncertain how many of the 25% of those who were hurting but who didn't meet the strict clinical criteria for CTS, may develop this syndrome in the foreseeable future.
• It is debatable whether the population of Sweden and the Netherlands are representative of the U.S. population. A recent U.S. study by Nordstrom et al. [6] found that newly diagnosed probable or definite carpal tunnel syndrome (N = 309) occurred at a rate of 3.46 cases per 1,000 person-years, or 0.35%. Another study [7] reported an industry-wide CTS incidence rate of 1.74 claims/1,000 FTEs. This puts the Mayo data at potentially 10 times the normal population rate, although it is important not to place too much emphasis on comparing prevalence and incidence numbers. Another larger U.S. study of 127 million  workers, found a self-reported CTS prevalence of 1.47% (95% CI: 1.30; 1.65), and 0.53% (95% CI: 0.42; 0.65) for medically diagnosed CTS [8]. Occupational risk factor most strongly associated with medically called CTS were:
  • exposure to repetitive bending/twisting of the hands/wrists at work (OR = 5.2)
  •  race (OR = 4.2; whites higher than nonwhites)
  • gender (OR = 2.2; females higher than males)
  • use of vibrating hand tools (OR = 1.8)
  •  age (OR = 1.03; risk increasing per year).
• CTS is characterized as a progressive and chronic disorder. The Mayo study provides a snapshot of current CTS injury prevalence. It is not clear how this value will change with time, especially given that 1 in 3 workers was experiencing some of the early signs of injury.
• No details are given of the occupational history of employees. Workers who develop CTS either tend to be assigned to less computer intensive work in organizations or they opt out of this type of work. The snapshot of the Mayo workers gives no information on the extent to which this might be a self-selected "healthy sample".
• The Swedish study used for comparison did not gather any data on computer use, but the authors did conclude that occupational factors affected CTS prevalence, especially "working with excessively flexed or extended wrist". The Mayo study gives no information on the occupational risk factors, such as extreme wrist postures, for those workers who did and did not have CTS.
• Carpal tunnel syndrome specifically describes median nerve compression and neuropathy within the carpal tunnel of the wrist. Similar symptoms may occur if nerve compression occurs at other points along the median nerve (e.g. elbow, shoulder, neck). There are also muscle syndromes that can mimic carpal tunnel pain and compression at the neck accounted for 75% of CTS-like symptoms [9].
• The Mayo study did not compare the prevalence rates for CTS among computer users and non-computer users, and that is what needs to be done before any definitive conclusion can be made. Also, the study doesn't say whether those computer workers with and without CTS had received any ergonomics training or whether any ergonomics interventions had been made. No data are presented on the workers' postures or on non-work activities that could have confounded the results. Given that there seems to have been widespread complaints of musculoskeletal discomfort (at least 30% of workers), it is doubtful that any good ergonomics program had been implemented.
• The Mayo study sample is relatively small and the extent to which this might be representative of general office workers is unclear.
• The workers self-reported the hours of computer use. Such self-reports may not be reliable. No measure of work intensity (e.g. number of keystrokes, amount of mouse movements) was taken and this could differentiate those who had the worst symptoms from others. Interestingly, the association of  mouse use and CTS was almost statistically significant (frequent mouse use: CTS cases = 48.1%; non-CTS = 27.9%).
• The use of nerve conduction velocity as the primary diagnostic measure is questionable. The Swedish study showed that there can be changes in median nerve latency among people who otherwise appear asymptomatic. Consequently, the Swedish prevalence data are based on a broader clinical definition. Any change in the clinical criteria obviously changes the prevalence values, and the use of only NCS probably underestimates the actual prevalence..
• The Mayo study news release says "those who had symptoms of numbness or tingling but did not have carpal tunnel had mild symptoms that occurred briefly. Some may have had problems with another nerve in the arm, the ulnar nerve. Carpal tunnel affects the median nerve." The fact that there are other problems confirms what we know, that inappropriate computer work may relate to a variety of musculoskeletal problems.
• Syndromes such as CTS can take a long period of time to develop (several years). The fact is that at the time of the study 30% of the sample were experiencing intermittent symptoms, some 10 times the number with full-blown carpal tunnel syndrome. Who's to say how many of those will develop CTS in the coming years?
• Ergonomists have long maintained that to focus solely of carpal tunnel syndrome is misleading, and that incorrect computer-use is associated with a variety of musculoskeletal concerns. Steven's work seems to confirm this when he says "Of the 257 people studied, 30 percent said they had experienced pins and needles sensations or numbness in their hands." When we assess workplaces we always assess discomfort and try to minimize this, rather than trying to focus solely on an injury such as carpal tunnel syndrome.
• Perhaps the most important statement from the news release from the Mayo Clinic is the last line that says "Stevens said the results shouldn't be interpreted to mean that the repetitive motions involved in using a computer can never lead to problems for people." From personal experience I know they do, and also from experience I know they can be reversed and managed by proper ergonomic intervention.

What does it all mean? 

Does computer use cause CTS? Ergonomists have never claimed that computer use is the sole cause of CTS. Ergonomics research shows that computer users often adopt poor wrist postures, and working in deviated postures is thought to increase the risks of injury. Ergonomists have always maintained that computers can be used safely providing the user works in a neutral posture. No assessment of user posture was made in the Mayo study.

What the Mayo study has shown is the prevalence of CTS, characterized by median nerve latency, among women working at the Scottsdale facility of the Mayo clinic. In that respect the study is fine. However, the authors then proceed to compare their results to prevalence data for a Dutch and a Swedish study, and because the overall percentages look similar they assert that computer use has not played any role in the Mayo workplace. That assertion is flawed for the reasons described above. Thee media have reported this study as "proving" that there is no association between computer use and carpal tunnel syndrome, and that assumption too is flawed for the reasons already noted.

The Mayo study doesn't provide any definitive answer on the association between computer use and CTS, and the truthful answer is that we still don't know precisely what this is. What we do know is that computer-use often is associated with a variety of musculoskeletal complaints, and these adversely affect performance at work. We also know that musculoskeletal complaints can be successfully prevented by working at a computer in a neutral posture.

What the Danish study shows is that intensive mouse use is related to possible CTS, and other variables, such as psychosocial factors, are unrelated.

For information on how to arrange a computer workstation for neutral posture working see the adult workstation guide and the HealthyComputing.com web site.

References

1. Andersen, J.H., Thomsen, J.F., Overgaaard, E., Lassen, C.F., Brandt, L.P.A., Vilstrup, I., Kryger, A.I. and Mikkelsen, S. (2003) Computer use and carpal tunnel syndrome: A 1-year follow-up study, JAMA, 289(22): 2963-2969.
2. Jensen, C., Finsen, L., Søgaard, K., and Christensen, H. (2002) Musculoskeletal symptoms and duration of computer and mouse use, Int. J. Industrial Ergonomics, 30(4-5) , 265-275.
3. Stevens, J.C., Witt, J.C., Smith, B.E. and A.L. Weaver (2001) The frequency of carpal tunnel syndrome in computer users at a medical facility, Neurology, 56, 1568-1570. 
4. De Krom M.C., Knipschild, P.G., Kester, A.D., Thijs, C.T., Boskkooi, P.F. and F. Spaans (1992) Carpal tunnel syndrome: prevalence in the general population. J. Clin. Epidemiol. 45, 373-375. 
5. Atroshi, I., Gummessons, C., Johnsson, R., Ornstein, E., Ranstam, J. and Rosén, I. (1999) JAMA, 282 (2) 153-158. 
6. Nordstrom, D. L., F. DeStefano, et al. (1998). Incidence of diagnosed carpal tunnel syndrome in a general population. Epidemiology 9 (3): 342-5.
7. Franklin, G. M., J. Haug, et al. (1991). "Occupational carpal tunnel syndrome in Washington State, 1984-1988." Am J Public Health 81(6): 741-6.
8. Tanaka, S., D. K. Wild, et al. (1995). "Prevalence and work-relatedness of self-reported carpal tunnel syndrome among U.S. workers: analysis of the Occupational Health Supplement data of 1988 National Health Interview Survey [see comments]." Am J Ind Med 27(4): 451-70.
9. http://danke.com/Orthodoc/carpaltunnel.html