A study [1] by researchers at the Mayo Clinic, recently received a lot of media attention when it concluded that there is no association between computer use and carpal tunnel syndrome (CTS).  However, the methodology used in the study was not appropriate to evaluate whether or not computer use causes carpal tunnel syndrome.

Few if any media outlets actually reviewed the entire study, but instead took the authors’ findings at face value.  Opponents of an ergonomics standard have used this study as “proof” that the relationship between work and musculoskeletal disorders (MSDs) is fuzzy at best.  For this reason it is important to take a closer look at the study to determine what it actually tells us about the relationship between computer use and carpal tunnel syndrome.  To that end, experts in the fields of occupational medicine and occupational epidemiology with extensive occupational safety and health research experience reviewed the Mayo Clinic study.  This fact sheet reflects the major problems these experts found with the Mayo Clinic CTS study and explains why the authors of the study are wrong when they claim that there is no association between computer use and CTS.

What is Carpal Tunnel Syndrome?

The carpal tunnel is a tunnel inside the hand, formed by carpal bones of the hand and a tough fibrous band through which nerves, tendons and blood vessels run to and from the hand.  Carpal tunnel syndrome occurs when a swelling within the carpal tunnel puts pressure on the median nerve.  This causes tingling, pain and numbness to occur in particular fingers and sections of the hand. 

The Study

The authors conducted a cross-sectional evaluation of the prevalence of CTS among health care workers at the Mayo Clinic Facility in Scottsdale, Ariz. with self-reported use of computers at work.  Symptoms of CTS were identified among 27 of 257 participants.  Twenty-four of those people underwent the authors’ gold standard evaluation for CTS—electrodiagnostic studies; 3 people refused electrodiagnostic testing.  The authors reported a prevalence of CTS of 3.5 percent (a total of 9 cases with electrodiagnostic evidence of CTS) among the study population of 257 employees and concluded that because this is comparable to estimates of the frequency of CTS in the general population, this demonstrated a lack of a connection between the occupational use of computers and CTS.

Methodological Flaws in the Mayo Clinic Study

The methodology employed in this study was not appropriate for the evaluation of causality.  Overall, this study is nothing more than a description of the frequency of CTS (one of many possible upper extremity health outcomes) among a group of computer users.  However, because of the lack of a control (unexposed) group, potential selection bias, inadequate exposure assessment and likely confounded comparisons to other study populations, the Mayo Clinic study is severely methodologically limited in its ability to assess the presence or absence of causal relationships between computer use and CTS.

Lack of Control Group—No unexposed group was surveyed, a serious weakness.  Instead the authors made reference to the prevalence of CTS observed in other studies in other locations.  Sometimes called historical controls, this method is generally considered a potentially biased form of comparison.

Selection Bias—The primary weakness of the study is that it only included current workers—thus, any worker who was out of work because of CTS would not have been included in the study.  This weakness—called the survivor effect (or the healthy worker effect)—is particularly problematic in a study of a highly disabling disorder such as CTS, in which the symptoms are likely to interfere with and be aggravated by repetitive manual work.  Because the number of CTS cases in the population was only nine—if only three workers were out of work because of CTS, the authors would have failed to identify 33 percent of the cases. 

Another methodological concern involves subject selection and exclusions from the study population.  The authors stated that they surveyed all Mayo Clinic "employees who use a computer" but did not explain how these persons were identified nor how they could be sure that they correctly included all appropriate employees and no others.  An unknown number of subjects did not undergo nerve conduction studies (NCS) because "it was obvious that some other condition was responsible for the paresthesia" [pins and needles sensations or numbness in the hands].  (These “other conditions” were not specified in the article, although in a recent issue of CTD News, Dr. Stevens mentioned other potentially work-related disorders such as ulnar nerve entrapment).  Three subjects with possible CTS refused to undergo electrodiagnostic testing, and eight had been previously diagnosed "but were not currently symptomatic" (whether because of treatment or episodic symptoms is not known).  It should be noted that subjects who denied hand paresthesias on the questionnaire "were assumed not to have CTS," even though the authors themselves cited a frequency of 18.4 percent of median neuropathy in asymptomatic persons.

Inadequate Exposure Assessment—Another serious weakness is that the authors conducted no formal assessment of exposure to ergonomic hazards from the computers used by the subjects.  Job title, amount of keyboard and mouse use and years of keyboard work were apparently obtained from questionnaire responses, although this was not stated.  The type and amount of use likely varied widely among the 314 persons to whom the questionnaire was sent. 

One-third of respondents gave a job title that was classified as "other," obscuring the nature of the work demands in these jobs, which may have included relatively little computer use.  Specific features of computer work such as type of task performed (data entry, interactive use, etc.) and work postures have been shown to affect the risk of upper extremity disorders.  Failure to assess these characteristics obscures potential exposure-response relationships and could have led to unmeasured confounding.

Confounded Comparisons—The studies offered for comparison with the workers in the Mayo Clinic study were all sampled from the general population and thus were unlikely to have been affected by the healthy worker effect in the same way.  Another working population would have been a far more appropriate basis for comparison with the group of Mayo Clinic workers.

The authors cited prevalence ranges from several other articles, with little discussion of the populations studied, the varying study methods used, or why such a wide range of values was obtained.  Of particular concern here are the noncomparability of case definitions among these studies, including the present report, and lack of attention to the age-gender distribution of each population, although Dr. Stevens himself earlier showed age and gender to be associated with CTS.

Of the comparison studies listed, the only one with even generally comparable data collection methods (mailed survey, nerve conduction study offered to symptomatic cases) was that by Atroshi and colleagues.  This paper gave a prevalence of 3.8 percent for clinically certain CTS and 2.7 percent for electrophysiologically confirmed CTS, which Stevens et al. astonishingly described as "similar," even though it was less then one-third as high as their estimate of 10.5 percent for clinically possible and definite CTS and 3.5 percent for electrophysiologically confirmed CTS. 

Conclusion

The methodology used in the study was not appropriate to evaluate whether computer use causes carpal tunnel syndrome.  The Mayo Clinic study is simply a description of the prevalence of one type of musculoskeletal disorder—CTS—among a group of computer users and no generalizations should be made based on this study.  Finally, it is of note that the authors do, in fact, describe a substantial proportion of computer workers experienced pins and needles sensations or numbness in the hands (paresthesias).  These paresthesias are not in themselves inconsequential in their effect on people's lives and may be indicative of other upper extremity musculoskeletal disorders. 

This fact sheet is based on information provided by Robin Herbert, M.D., Medical Co-Director, Mount Sinai Center for Occupational and Environmental Medicine, and Laura Punnett, Sc.D., Professor, Department of Work Environment, University of Massachusetts Lowell.