Journal of Occupational Rehabilitation, Vol. 1, No. Z 1991
Clinical Determination of Work-Relatedness in Carpal Tunnel Syndrome J. Steven M o o r e a,2
The purpose of this study was to determine the diagnosis and work-relatedness of carpal tunnel syndrome (CTS) using standardized criteria in a series of cases that were referred for an independent medical examination with a prior diagnosis of work-related CTS. Using a liberal case definition, only 65% of cases had CTS. Using Wisconsin's worker's" compensation criteria for work-relatedness, only 55% of the cases had any work-related disorder, while only 37% of the cases" had work-related CTS. Duration of exposure was not significantly associated with work-related vs. non-work-related CTS. It was noted that the development of non-work-related CTS cases occurred uniformly across the various durations of exposure, as if unrelated to exposure. The symmetry of the disorder was unrelated to work-relatedness, provided that the symmetry of the disease matched the symmetry of the exposure. Personal characteristics, such as obesity and diabetes, revealed no statistically significant associations with work-relatedness or CTS. The present findings illustrate application of a standard procedure for determining the work-relatedness of CTS in a series of cases referred for independent medical examination. The results indicate that CTS is often overdiagnosed and inaccurately linked to work. While the results may be limited by the method of determining work-relatedness, the findings indicate the importance of careful consideration of criteria for CTS and work-relatedness. When such an approach is taken, more targeted clinical management of the patient and appropriate intervention in the workplace should result. KEY WORDS: carpal tunnel syndrome (CTS); cumulative trauma disorders (CTDs); ergonomics; work-relatedness; worker's compensation; independent medical examinations.
Cumulative trauma disorders (CTDs), including carpal tunnel syndrome (CTS), are significant contemporary occupational health problems. In cross1Department of Preventive Medicine, Medical College of Wisconsin, Milwaukee, Wisconsin. 2Correspondence should be directed to J. Steven Moore, M.D., M.P.H., C.t.H., Department of Preventive Medicine, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226. 145 1053-0487/91/0600-0145506.50/0 9 1991 PlenumPublishingCorporation
sectional surveillance studies, the prevalence of CTS has been estimated to be as high as 25 cases per 200,000 person-hours per year in high-risk jobs (1, 2). Based on analysis of Wisconsin worker's compensation data, Hanrahan et al. noted that claims reporting for CTS rose from 432 per year in 1983 to 2429 per year in 1988, a five-fold increase (3). This represented an increase from 2.5 to 11.5 cases per 10,000 workers. They also noted that the average cost per case was $4,700, with medical costs contributing $1,400. Despite the significance of these concerns, there are no case series or studies in the literature that have critically examined the diagnosis of CTS and determination of its workrelatedness on a case-by-case basis. In 1976, the National Institute for Occupational Safety and Health (NIOSH) published the first edition of its Guide to Determining the Work-Relatedness of Disease. It published the second edition in 1979 (4). This document presented an algorithm for use by state agencies, physicians, and others concerned with worker's compensation when assessing medical, industrial hygiene, and other evidence to determine the work-relatedness of a given disease. The emphasis was on clinical practice. Other authors have also presented decision frameworks (5-9). All ultimately respond to the question of whether an individual patient's degree of occupational exposure was of sufficient intensity (or dose), frequency, and duration to cause or materially contribute to his or her disease process. In making work-relatedness determinations, physicians must be diligent in obtaining and assessing facts, then judiciously evaluate evidence regarding the disease, the exposure, and other potentially significant factors in the context of current knowledge. Merely echoing an interested party's allegation for or against workrelatedness is unacceptable. A self-reported history suggesting work-relatedness may suggest that a condition is work-related, but does not establish association, causation, or aggravation to a reasonable degree of medical probability.
EVIDENCE OF DISEASE
A clinical impression is generally based on a patient's reported symptoms and the physician's physical findings. An impression represents the development of a hypothesis that the clinical manifestations are mediated by a particular anatomic or physiologic state. A clinical impression should be confirmed or refuted with specific objective testing. If confirmed, the clinical impression becomes a clinical diagnosis. The clinical diagnosis describes the current observations. It does not necessarily describe how the condition developed. Review of the literature will generally reveal contemporary concepts regarding pathogenesis. In addition, epidemiological studies often identify factors associated with a disease, and some of these associations may be causal. The strength of the association, consistency of findings, evidence of dose-response, proper chronological relationships, specificity of the association, biological plausibility, coherence, and experimental support for the association are criteria used to assess the weight of evidence regarding the degree of association between a suspected risk factor and a particular health outcome (10).
Carpal Tunnel Syndrome
The study designs used by the epidemiologists affect the strength of the evidence for causation. Case reports, cross-sectional studies, and case-control studies can establish evidence of an association and, in conjunction with longitudinal prospective studies, assist in establishing causation. If the criteria for causation have been sufficiently met, then the strength of the evidence suggests causation.
EVIDENCE OF EXPOSURE
Once factors associated with the occurrence of the disease or condition have been identified, it is necessary to determine whether these factors are present in the workplace and, if so, whether they are present at levels consistent with those reported in the literature. In the context of musculoskeletal disorders, the varied activities and tasks performed by the individual are reviewed, followed by or in conjunction with a workplace evaluation by a competent occupational health practitioner. Since the severity or risk of a particular effect is proportional to the "dose" reaching the target organ, it is necessary to estimate the employee's degree of exposure to the factors of concern. For some musculoskeletal disorders, there are quantitative measurement techniques that provide estimates of the employee's actual degree of exposure, such as biomechanical or energy expenditure models for manual materials handling tasks (11, 12). When quantitative techniques are not available, as currently with ergonomic evaluation for the upper extremities, qualitative estimation may be the only alternative. The mere presence of a factor is not sufficient evidence of an association. Intensity, frequency, and duration should also be considered (13). Once compiled, the estimated degree of exposure and the evidence of the disease can be compared to known dose-response data. When there are no quantitative exposure methodologies or dose-response curves available, it may be necessary to compare the estimated exposures to the exposures reported in the literature.
OTHER SIGNIFICANT CONSIDERATIONS Usually, there are other factors, such as non-occupational correlates with the disease process, that warrant consideration. There are many personal attributes and non-occupational activities associated with musculoskeletal disorders (14-24). In addition, latent periods and durations of exposure also need to be considered if the association is to be biologically plausible (10). The symptoms of a disease process should not be considered equivalent to its pathology or pathogenesis. Many conditions become symptomatic with activity, but the disease process is not affected. Patients with CI'S often develop paresthesias in association with prolonged grasping (17, 18). If an individual with CTS performs a similar task at work, he or she will likely manifest paresthesias at work. An association between symptoms and work does not confirm or refute a hypothesis of causation between the condition and work.
Finally, there are many instances where one identifies a condition associated with work activities or exposures and the exposure assessment suggests only a small probability of a relationship. In these circumstances, review of the health experience of a similarly exposed cohort may be beneficial. If several co-workers have experienced similar difficulties, the probability of association increases. If co-workers with greater degrees of exposure have not been affected, then the probability of association decreases.
REASONABLE MEDICAL CERTAINTY Given knowledge of the disease, associated factors, exposure, and other data, one must evaluate the hypothesis regarding an exposure--disease association according to some criterion of certainty or probability. In worker's compensation, the reasonable medical certainty criterion, often called the "51% rule," implies that a postulated association must be more than a possibility, but not necessarily known with scientific certainty. Scientific certainty implies a probability of association greater than 0.95. The 51% rule implies a probability of association greater than 0.50. The physician must estimate whether the hypothesized association between the exposure and the disease satisfies this criterion. The purpose of this study was to determine work-relatedness using standardized criteria in a series of cases that were referred for independent medical examination. It was assumed that such an analysis would provide descriptive data regarding the clinical presentation of work-related and non-work-related CTS and would assist in furthering the understanding of the process of determining the link between CTS and work exposure in the clinical setting.
METHODS These cases represent individuals that were referred for diagnostic evaluation of their upper extremity symptoms and determination of the work-relatedness of the identified condition in the context of Wisconsin's Worker's Compensation Act. Referral sources included the Medical College of Wisconsin's Physician Referral Network, community physicians, prior patients, employers, worker's compensation carriers, and attorneys representing both plaintiff and defense interests.
Clinical Assessment Protocol
All patients were evaluated by the author at a clinic located on the grounds of the Milwaukee Regional Medical Center. Each evaluation included an in-depth review of the onset of the condition, prior medical evaluation and treatment, personal medical history, family history, social history, and occupational history. Pertinent positive and negative physical findings were documented. When possible,
Carpal Tunnel Syndrome
previous diagnostic studies, especially electrodiagnostic studies, and reports were reviewed. Based on the available data, a diagnosis was rendered, if possible.
Exposure Assessment Protocol There are multiple ways to characterize exposure for manual hand-intensive tasks (13, 25-28). Variables selected for this study included job classification at the onset of the symptoms, duration of exposure to the task of concern, and qualitative estimation of the presence of the ergonomic factors reported to be associated with upper extremity disorders (repetition, forceful exertions, static exertions, awkward postures, localized mechanical compression, and vibration) (25-27). Use of gloves and work in cold temperatures were considered modifiers of the above factors. Duration of employment was considered, but may not accurately portray a workers' actual duration of exposure to an activity because workers often change job classifications while working for the same employer or have changes in process or productivity, such as overtime, that alter degree of exposure. In addition, workers sometimes perform alternate work tasks without changing job classifications. For these reasons, only duration of exposure to the alleged offending work activity' was analyzed. Rather than attempt to quantitate the duration of exposure precisely, the variable was ordinally ranked as "days," "months," "years," or "decades." The duration of exposure was categorized as "days" if the duration of exposure was less than 1-month, as "months" if greater than 1-month but less than 1 year, as "years" if greater than 1-year but less than one decade, and "decades" if greater than 10 years. Qualitative estimates regarding the ergonomic factors were made by the author. Of the 297 total cases, 123 (41%) were evaluated via plant visit or review of a representative videotape. Less direct methods were used for the remaining 174 cases (58.6%). Most of these latter cases performed jobs that had been previously evaluated by the author and, based on detailed discussion with the patient, did not require reevaluation of their jobs. The others in this g r o u p performed tasks analogous to those previously evaluated, but at a different employer, and, based upon knowledge from the prior objective evaluation and careful inquiry, were assumed to have exposures essentially identical to those previously evaluated. Case Definition for CTS One of the concerns in this series was whether or not these patients had carpal tunnel syndrome (CTS). At present, there is no consensus clinical case definition for CTS. For the purposes of this analysis, the following criteria were selected for confirmed CTS: (1) current or historical symptoms suggestive of paresthesias consistent with median nerve entrapment at the wrist and electrodiagnostic study interpretations that reported evidence of CTS, and (2) historical symptoms suggestive of CTS and a favorable response to surgeu, regardless of
the electrodiagnostic study interpretation. If electrodiagnostic study interpretation did not report evidence of CTS, but the clinical presentation was strongly suggestive, the diagnosis of CTS was neither confirmed nor refuted since electrodiagnostic testing lacks perfect sensitivity. These latter cases were considered "possible CTS" and, solely for the purposes of this analysis, dealt with as if CTS had been confirmed. If clinical and/or ancillary diagnostic assessment identified an alternate condition and the patient did not meet one of the above criteria for CTS, the diagnosis of CTS was rejected, regardless of the electrodiagnostic results, and these cases were labeled "non-CTS." Last, if no satisfactory explanation for the patient's symptoms was identified, the diagnosis was considered "not defined" and dealt with as "non-CTS."
Other Significant Considerations
Since CTS has been associated with many conditions unrelated to employment, data regarding age, sex, hypertension, diabetes, obesity, recent gain in weight, recent or historical trauma to the hands or wrists, various gynecological issues, family history of diabetes, and family history of CTS were obtained (14-24). Miscellaneous conditions, such as other rheumatic or systemic disorders, and smoking status were also noted. The symmetry of the disease was noted because it was considered important for two reasons: (1) systemic medical conditions generally predispose to bilateral disease, and (2) the symmetry of the disease should match the symmetry of the exposure if the condition is work-related.
Criteria for Work-Relatedness
The primary criterion for determining work-relatedness was whether the exposure was believed to cause or contribute to the development of the identified upper extremity disorder. If the exposure was considered significant, based on the presence of one or more of the ergonomic factors associated with upper extremity disorders, the condition was considered "work-related." If the exposure was believed not to cause or contribute to the development of the identified disorder, based on lack of exposure to any of the ergonomic factors, the condition was considered "non-work-related." If work activities only contributed to the manifestation of symptoms, but did not affect the underlying disease, the condition was considered "non-work-related." This latter criterion arises from a Wisconsin administrative law decision (29). This case involved the lower back, but set precedent for determining the work-relatedness of any disease process. There are three aspects of this decision: 1. "If there is a definite 'breakage' while the employee is engaged in usual or normal activities of the job, and there is a relationship between the breakage and the effort exerted or motion involved, the injury is compensable regardless of whether or not the employee's condition was pre-existing and regardless of whether or not there is evidence of prior trouble."
Carpal Tunnel Syndrome
Table I. Work-Related Conditions Other than Carpal Tunnel Syndrome Females (34)
Tendinitis/tenosynovitis Flexors Extensors DeQuervain's Not specified
7 1 5 7
20.6 2.9 14.7 20.6
i 0 0 1
7.7 0.0 0.0 7.7
Epicondylitis Lateral Medial
Localized muscle fatigue/myalgias Hand-ann vibration syndrome Digital nerve compression
9 0 3
26.5 0.0 8.8
4 3 0
30.8 23.1 0.0
2. "lf the work activity precipitate~, aggravates, and accelerates beyond normal progression, a progressively deteriorating or degenerative condition, it is an accident causing injury or disease and the employee shouM recover even if there is no definite 'breakage. "' 3. "If the employee is engaged in normal exertive activity but there is no definite 'breakage' or demonstrable physical change occurring at that time but only a manifestation of a definitely pre-existing condition of a progressively deteriorating nature, recovery should be denied even if the manifestation or symptomization of the condition became apparent during normal employment activity." The first criterion rarely applies to CTS, but may apply to conditions that may develop by either point-in-time traumatic incidents or "cumulative trauma," such as epicondylitis or DeQuervain's stenosing tenosynovitis. The second criterion is usually the one through which cumulative trauma disorders are considered compensable. Note that the conjunction used in this criterion is and instead of or, and requires that the work activity affect the condition as opposed to the symptoms of the condition. The third criterion explicitly clarifies this difference between the condition and its symptoms.
RESULTS Of 297 total cases in this series, there were 200 females and 97 males. Of the 200 females, a work-related upper extremity condition was identified in 109 (55.5%), 75 of whom had CTS (69%). A non-work-related condition was identified in 91 females (45.5%), 50 of whom had CTS (55%). Of the 97 men, a work-related upper extremity disorder was identified in 48 (49.5%). This included 35 cases of CTS (73%). A non-work-related condition was identified in 49 men (50.5%), 28 of whom had CTS (57%). Table I summarizes the major non-CTS work-related
Moore Table II. A g e Distribution of Work-Related and Nonwork-Related Carpal Tunnel Syndrome Work-related
< 30 30-39 40-49 50-59 > 60 Total
27 37 26 14 6
24.5 33.6 23.6 12.7 5.5
11 24 24 11 8
14.1 30.8 30.8 14.1 10.3
Table lIl. Duration of Exposure a Work-related Duration
1-28 days 1-11 m o n t h s 1-9 years 10+ years Total
19 30 36 23
17.6 27.8 33.3 21.3
7 16 28 24
9.3 21.3 37.3 32.0
a T h e total n u m b e r of work-related conditions is less than 110 because two m e n had CTS related to complications of point-in-time incidents. The totals for not-work-related CTS is less than 78 because three cases developed their conditions in circumstances without any relationship to exposure, such as onset after termination of employment.
disorders according to gender. The remainder of this paper focuses on a description of CTS according to work-relatedness. The chi-square statistic (p < 0.5) was used to test for significant differences in the distributions of the variables across categories. Table II presents the age distributions of all CTS cases according to workrelatedness. T h e r e was no statistically significant difference between the age dist r i b u t i o n s . Sixty f o u r (58.1%) cases of w o r k - r e l a t e d CTS o c c u r r e d a m o n g individuals less than 40 years of age, as did 35 (44.9%) cases of non-work-related CTS. Table III presents the distribution of CTS cases across duration of exposure categories for CTS. Note that the total number of work-related CTS was less than 110 because two men had CTS related to complications of point-in-time incidents; therefore, duration of exposure was not a factor. The total for non-work-related CTS was less than 78 because three cases developed their conditions in circumstances without any apparent relationship to exposure, e.g., onset after termination
Carpal Tunnel Syndrome
Table IV. Disease Symmetry in Work-Related and Nonwork-Related Carpal Tunnel Syndrome Work-related Symmetry
of employment. There was no statistically significant difference in the distributions. Of the 108 cases of work-related CTS, 49 (45.4%) developed their condition during the first month or year of exposure, as did 23 (30.6%) of the 75 cases of nonwork-related CTS. Among females, 32 (42.7%) of the 75 cases with work-related CTS developed their condition during the first 12 months of exposure, compared to 15 (31.3%) of the 48 cases with non-work-related CTS. Among males, 17 (51.5%) of the 33 cases with work-related CTS developed their condition within 12 months of exposure, as did eight (29.6%) of the 27 cases with non-work-related CTS. These differences were not statistically significant. When comparing workrelated CTS among female vs. male cases, 30 (40%) of the 75 females developed their condition during the 1-10 years of exposure period, compared to six (18.2%) of the 33 males. Comparison of non-work-related CTS among females and males revealed no significant difference in the distributions. It was noted, however, that approximately 70% of all cases developed after 1 year of exposure, a pattern consistent with the notion that non-work-related CTS would likely occur randomly during employment. Table 1V presents the distribution of bilateral vs. unilateral disease for CTS according to work-relatedness category. The distribution differences were not statistically significant. Approximately half of cases with work-related CTS had bilateral disease, compared to two-thirds of non-work-related CTS eases. Table V presents the distribution of associated conditions for all CTS cases according to work-relatedness. Several conditions might be present in a particular individual. The list of conditions includes hypertension, diabetes, obesity, recent gain in weight, smoking status, family history of CTS or diabetes, recent or historical trauma to the hand or wrist, and miscellaneous disorders. There were no statistically significant associations. Of the 45 miscellaneous conditions among those with workrelated CTS, 17 (37.8%) were other CTDs. Of the 40 miscellaneous conditions in the non-work-related CTS group, six (15%) were other CTDs. Chi-square analysis indicated that this difference was not statistically significant. Table VI presents the data obtained regarding total and specific gynecological conditions or treatments among females with CTS according to work-relatedness and symmetry categories. There were no statistically significant associations. Job classification alone failed to predict work-relatedness or the presence of CTS. Considering the 29 females with the job classification of "assembler," there were 11 ,with work-related CTS (14.7%), eight with work-related disorders other