American Journal of Industrial Medicine 21S95-897 (1992)
LETTER TO THE EDITOR
Review of Ergonomic Studies of Carpal Tunnel Syndrome Key words: assessment of validity, ergonomic exposure measurements, repetitive trauma, carpal tunnel syndrome, occupational diseases
Dr. Susan Stock’s review [1991] of studies examining the relationship of occupation and carpal tunnel syndrome (CTS), including our study [Nathan et al., 19881, states that in validity assessment our study “ranked lowest, with serious flaws in the measurement of exposure,” and that the study by Silverstein et al. [1985, 1986, 1987; Armstrong et al., 19871 “ranked substantially higher.” Regarding the measurement of exposure, we originally invested many hours carefully observing hand use at each of the four studied industries as the basis for our five-way occupational categorization by hand resistance (force). Despite their use of video tape and surface electromyography to determine repetition and force very precisely, Silverstein et al. eventually dichotomized both repetition and force measurements to simply “high” or “low.” Thus, their measurement precision may be misleading, given this lack of categorization precision. Dr. Stock also cites major flaws in our assessment of exposure confounders, particularly with regard to gender. The adjustment of latency values to eliminate age as a possible confounding variable also had the effect of removing gender as a confounder, since the increased prevalence of slowing in women had been found to be an effect of age rather than gender in previous analyses of the same data [Nathan et al., 19881. Dr. Stock’s interpretation of our data deserves a response as well. She confirms our report of a statistically significant difference in the percentages of slowing between occupational groups I and V. However, she reports an additional statistical difference in bilateral nerve conduction slowing between these groups, using our reported data. Her calculated odds ratio of 2.5 in support of this statement is incorrect; the correct odds ratio is 1.2 and the chi-square statistic is not significant. Her assessment of our study also dismisses several other important findings. Specifically, there was a notable lack of statistically significant differences between groups I11 and V and between all other paired groups, except VV and UIII. Portland Hand Surgery and Rehabilitation Center, Portland, OR. Address reprint requests to Dr. Peter A. Nathan, Portland Hand Surgery and Rehabilitation Center, 2455 NW Marshall, Suite 1, Portland, OR 97210-2997. Accepted March 5 , 1991.
0 1992 Wiley-Liss, Inc.
896
Nathan
These other paired groups also require strikingly dissimilar hand-use patterns; differences in prevalence of slowing would be expected if hand use were associated with nerve abnormality, yet these differences were not observed. Dr. Stock states that “when one reanalyzes the Nathan data in contingency tables, one can demonstrate increasing odds ratios with increasing exposure,” yet those respective odds ratios for groups 11-V versus group I are 1.0, 2.3, 1.6, and 4.0, an obviously erratic pattern. The differences observed between groups I and I11 and between groups I and V thus do not, as we stated, indicate a consistent association between occupational hand use and slowing when taken in this larger context [Nathan, 19911. Another oversight is reflected in Dr. Stock’s statement that age was not associated with increased prevalence of upper limb disorders in any of the three studies, despite our statement that median nerve sensory latencies were positively associated with the age of the subject. Concerning the validity of the Silverstein study, we emphasize two very significant potential limitations in outcome measurement that Dr. Stock mentions only briefly, i.e., the exclusion of workers who reported their symptoms to have begun prior to the current job, and the omission of nerve conduction studies from the case definition process. Regarding the exclusion of pre-employment symptoms, it may be that a great deal, if not the majority, of CTS is thus ignored and that any conclusions of causality are thus applicable only to a relatively small percentage of CTS cases. It is possible as well that the relatively high prevalence of CTS in the high-force/high-repetition category reflects the reporting bias of those employees toward work-relatedness of symptoms in the currently highly charged medicolegal environment. Regarding the omission of nerve conduction studies, it seems that the lack of this objective and routine diagnostic modality seriously impairs the reliability of the case definition and therefore of the study in general. The support of the NIOSH-funded study by Silverstein et al. for a relationship between occupation and CTS is understandable, given the focus of that governmental agency, which appears to exclude factors other than occupation as possible causes for a wide range of musculoskeletal conditions. However, the study may in fact beg the question, in defining CTS largely by the subject’s sense of job-related symptoms. The precipitation of CTS symptoms by intensive occupational hand use, without definitive electrophysiologic evaluation of the underlying median entrapment neuropathy, does not constitute “strong evidence of a causal relationship between repetitive, forceful work and the development o f . . . carpal tunnel syndrome” [Stock, 19911. Perhaps the most telling aspect of Dr. Stock’s review is that her evaluation of 49 relevant studies (nearly all reporting a positive relationship between exposure and outcome) yielded only three (including our own) that merited careful consideration on the basis of criteria emphasizing adequate definitions of populations, exposures, and outcomes. This seems evident both of the paucity of definitive scientific comment on the question and of the unreliability of a large number of studies frequently used to support a connection between hand use and CTS. Peter A. Nathan, MD Portland Hand Surgery and Rehabilitation Center
Review of Ergonomic Studies of Carpal Tunnel Syndrome
897
REFERENCES Nathan PA, Meadows KD, Doyle LS (1988): Occupation as a risk factor for impaired sensory conduction of the median nerve at the carpal tunnel. J Hand Surg [Br] 13B:167-170. Nathan PA, Meadows KD, Doyle LS (1988): Relationship of age and sex to sensory conduction of the median nerve at the carpal tunnel and association of slowed conduction with symptoms. Muscle Nerve 11:1149- 1153. Nathan PA (1991): Letter to the Editor. J Hand Surg [Br] 16B:231-232. Silverstein BA, Fine LJ, Armstrong TJ (1987): Occupational factors and carpal tunnel syndrome. Am J Ind Med 11:343-358. Stock SR (1991): Workplace ergonomic factors and the development of musculoskeletal disorders of the neck and upper limbs: A meta-analysis. Am J Ind Med 19:87-107.
LETTER TO THE EDITOR
Review of Ergonomic Studies of Carpal Tunnel Syndrome Key words: assessment of validity, ergonomic exposure measurements, repetitive trauma, carpal tunnel syndrome, occupational diseases
Dr. Susan Stock’s review [1991] of studies examining the relationship of occupation and carpal tunnel syndrome (CTS), including our study [Nathan et al., 19881, states that in validity assessment our study “ranked lowest, with serious flaws in the measurement of exposure,” and that the study by Silverstein et al. [1985, 1986, 1987; Armstrong et al., 19871 “ranked substantially higher.” Regarding the measurement of exposure, we originally invested many hours carefully observing hand use at each of the four studied industries as the basis for our five-way occupational categorization by hand resistance (force). Despite their use of video tape and surface electromyography to determine repetition and force very precisely, Silverstein et al. eventually dichotomized both repetition and force measurements to simply “high” or “low.” Thus, their measurement precision may be misleading, given this lack of categorization precision. Dr. Stock also cites major flaws in our assessment of exposure confounders, particularly with regard to gender. The adjustment of latency values to eliminate age as a possible confounding variable also had the effect of removing gender as a confounder, since the increased prevalence of slowing in women had been found to be an effect of age rather than gender in previous analyses of the same data [Nathan et al., 19881. Dr. Stock’s interpretation of our data deserves a response as well. She confirms our report of a statistically significant difference in the percentages of slowing between occupational groups I and V. However, she reports an additional statistical difference in bilateral nerve conduction slowing between these groups, using our reported data. Her calculated odds ratio of 2.5 in support of this statement is incorrect; the correct odds ratio is 1.2 and the chi-square statistic is not significant. Her assessment of our study also dismisses several other important findings. Specifically, there was a notable lack of statistically significant differences between groups I11 and V and between all other paired groups, except VV and UIII. Portland Hand Surgery and Rehabilitation Center, Portland, OR. Address reprint requests to Dr. Peter A. Nathan, Portland Hand Surgery and Rehabilitation Center, 2455 NW Marshall, Suite 1, Portland, OR 97210-2997. Accepted March 5 , 1991.
0 1992 Wiley-Liss, Inc.
896
Nathan
These other paired groups also require strikingly dissimilar hand-use patterns; differences in prevalence of slowing would be expected if hand use were associated with nerve abnormality, yet these differences were not observed. Dr. Stock states that “when one reanalyzes the Nathan data in contingency tables, one can demonstrate increasing odds ratios with increasing exposure,” yet those respective odds ratios for groups 11-V versus group I are 1.0, 2.3, 1.6, and 4.0, an obviously erratic pattern. The differences observed between groups I and I11 and between groups I and V thus do not, as we stated, indicate a consistent association between occupational hand use and slowing when taken in this larger context [Nathan, 19911. Another oversight is reflected in Dr. Stock’s statement that age was not associated with increased prevalence of upper limb disorders in any of the three studies, despite our statement that median nerve sensory latencies were positively associated with the age of the subject. Concerning the validity of the Silverstein study, we emphasize two very significant potential limitations in outcome measurement that Dr. Stock mentions only briefly, i.e., the exclusion of workers who reported their symptoms to have begun prior to the current job, and the omission of nerve conduction studies from the case definition process. Regarding the exclusion of pre-employment symptoms, it may be that a great deal, if not the majority, of CTS is thus ignored and that any conclusions of causality are thus applicable only to a relatively small percentage of CTS cases. It is possible as well that the relatively high prevalence of CTS in the high-force/high-repetition category reflects the reporting bias of those employees toward work-relatedness of symptoms in the currently highly charged medicolegal environment. Regarding the omission of nerve conduction studies, it seems that the lack of this objective and routine diagnostic modality seriously impairs the reliability of the case definition and therefore of the study in general. The support of the NIOSH-funded study by Silverstein et al. for a relationship between occupation and CTS is understandable, given the focus of that governmental agency, which appears to exclude factors other than occupation as possible causes for a wide range of musculoskeletal conditions. However, the study may in fact beg the question, in defining CTS largely by the subject’s sense of job-related symptoms. The precipitation of CTS symptoms by intensive occupational hand use, without definitive electrophysiologic evaluation of the underlying median entrapment neuropathy, does not constitute “strong evidence of a causal relationship between repetitive, forceful work and the development o f . . . carpal tunnel syndrome” [Stock, 19911. Perhaps the most telling aspect of Dr. Stock’s review is that her evaluation of 49 relevant studies (nearly all reporting a positive relationship between exposure and outcome) yielded only three (including our own) that merited careful consideration on the basis of criteria emphasizing adequate definitions of populations, exposures, and outcomes. This seems evident both of the paucity of definitive scientific comment on the question and of the unreliability of a large number of studies frequently used to support a connection between hand use and CTS. Peter A. Nathan, MD Portland Hand Surgery and Rehabilitation Center
Review of Ergonomic Studies of Carpal Tunnel Syndrome
897
REFERENCES Nathan PA, Meadows KD, Doyle LS (1988): Occupation as a risk factor for impaired sensory conduction of the median nerve at the carpal tunnel. J Hand Surg [Br] 13B:167-170. Nathan PA, Meadows KD, Doyle LS (1988): Relationship of age and sex to sensory conduction of the median nerve at the carpal tunnel and association of slowed conduction with symptoms. Muscle Nerve 11:1149- 1153. Nathan PA (1991): Letter to the Editor. J Hand Surg [Br] 16B:231-232. Silverstein BA, Fine LJ, Armstrong TJ (1987): Occupational factors and carpal tunnel syndrome. Am J Ind Med 11:343-358. Stock SR (1991): Workplace ergonomic factors and the development of musculoskeletal disorders of the neck and upper limbs: A meta-analysis. Am J Ind Med 19:87-107.
