Validation of a Surveillance Case Definition of Carpal Tunnel Syndrome

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Jeffrey N. Katz, MD, MS, Martin G. Larson, ScD, Anne H. Fossel

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Matthew H. Liang, MD, MPH

Introdudion Carpal tunnel syndrome is a major problem in occupational health, particularly in occupations requiring highly repetitive and/or forceful hand motions.' Up to 15 percent ofworkers in the highest risk industries are affected annually,2 100 times more than expected in an age- and sex-adjusted population.3 Costs per worker may exceed $15,000, including medical and indemnity expenses.2,4 Accurate and efficient diagnosis of carpal tunnel syndrome is important for research and clinical practice. Nerve conduction testing is generally regarded as the gold standard for the diagnosis of carpal tunnel syndrome.5 The sensitivity of nerve conduction tests has been estimated at over 80 percent when compared with a clinical diagnosis of carpal tunnel syndrome in a community based study6 and over 90 percent when compared with relief following surgery in 292 patients undergoing carpal tunnel release.7 Electrophysiologic tests require 30 to 60 minutes, expensive equipment and experienced operators; thus, they may be inappropriate for many occupational surveillance efforts. Thermography8 andvibrometry9 are noninvasive but costly and have not been evaluated rigorously. The clinical history and physical examination are inexpensive and simple to administer but are of limited value in the diagnosis of carpal tunnel syndrome. For example, nocturnal pain has a specificity of 0.28 and diminished two-point discrimination has a specificity of 0.32.10 The Tinel and Phalen signs have sensitivities of 25-75 percent and specificities of 47-90 percent.'0-'4 These data suggest that screening instruments comprised solely of

historical and physical findings may not accurately identify workers with carpal tunnel syndrome. The National Institute for Occupational Safety and Health (NIOSH) recently approved a surveillance case definition for work-related carpal tunnel syndrome15 (see Figure 1). The NIOSH surveillance case definition of carpal tunnel syndrome requires that workers satisfy three criteria: symptoms of median nerve involvement; a history of occupational risk factors such as repetitive or forceful hand tasks or use of vibrating tools; and either the presence of physical examination findings-including the Tinel or Phalen signs or decreased pinprick sensation in the median nerve distnbution of the hand-or diagnostic nerve conduction studies. To date the validity of NIOSH case definition has not been tested. The performance of the case definition is of particular interest when physical examination findings are used as the objective evidence required to fulfill criterion B (Figure 1). The purpose of this study is to test the NIOSH case definition in a sample of symptomatic workers. Physical examination findings serve as the sole objective evidence of disease (criterion B) in this Address reprint requests to Jeffrey N. Katz, MD, Arthritis Center, Brigham and Women's Hospital, 75 Francis Street, Boston, MA 02115. He is also Instructor in Medicine, Harvard Medical School; Dr. Larson is Associate Professor of Biostatistics, Harvard School of Public Health; Ms. Fossel is Research Associate, Robert Brigham Multipurpose Arthritis Center, Brigham and Women's Hospital; Dr. Liang is Associate Professor of Medicine, Harvard Medical School. This paper, submitted to the Journal February 27, 1990, was revised and accepted for publication July 13, 1990. Editor's Note: See also related editorial p 161 this issue.

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FIGURE 1-NIOSH Surveillance Case DeflnMon of Work-Related Carpal Tunnel Syndrome*

Criteria A, B, and C must be met: A. Symptoms suggestive of carpal tunnel syndrome: paresthesia, hypoesthesia, pain or numbness affecting at least part of the median nerve distribution of the hand. B. Objective findings consistent with carpal tunnel syndrome: Either, (1) One or more of the following physical findings: Tinel sign, Phalen sign, or decreased or absent sensation to pin prick in the median nerve distribution of the hand. or, (2) Electrodiagnostic findings of median nerve dysfunction across the carpal tunnel. C. Evidence of work relatedness: One or more of the following: Frequent, repetitive or forceful hand work on affected side; sustained awkard hand position; use of vibrating tools; prolonged pressure over wrist or base of palm; temporal relabtionship of symptoms to work or association with carpal tunnel syndrome noted in co-workers. *adapted from reference 15

study and nerve conduction tests are used the gold standard for the diagnosis of CTS. as

Meths An evaluation of clinical tests for carpal tunnel syndrome was carried out from January through June 1988. Details of the study have been reported.10 Briefly, consecutive patients over 18 years of age with pain or paresthesia in the upper extremity who were referred to the Brigham and Women's Hospital Neurophysiology Laboratory were studied. A subset of this sample who were working or disabled as a result of their upper extremity condition are the subject of this report. Prior to neurophysiologic testing patients were evaluated by one of four examiners (a rheumatologist and three research assistants) who administered a questionnaire and performed a standardized physical examination. Examiners were blind to the results of nerve conduction tests. Clinical data included demographic information, co-morbid conditions, occupation, and self-report ofwhether patients were exposed in their work to repetitive hand motion, vibration, or forceful wrist flexion or extension. The subjects also completed a hand pain diagram16 which depicts dorsal and palmar views of both hands. Patients were asked to shade in areas corresponding to the location of their symptoms. Diagrams were rated blindly as "classic carpal tunnel syndrome," "probable," "possible," or "unlikely."

Moving two point discrimination17 performed with electrocardiogram calipers with tips set at four millimeters apart. The Tinel sign was performed by

was

190 American Joumal of Public Health

dropping the square end of a reflex hammer on the distal wrist crease. Pain or paresthesia in at least one of the first three digits constituted a positive response.18 The Phalen sign was performed with the dorsal aspects of both flexed wrists in apposition for 60 seconds, and was considered positive if pain or paresthesia occurred in one of the first three digits.19 The presence or absence of CTS was established by the neurophysiologists with nerve conduction testing. The core protocol consisted of bilateral ulnar and median motor and sensory nerve conduction tests and electromyography of the abductor pollicus brevis. (Some subjects refused bilateral testing and/or electromyography.) Additional tests such as palmto-wrist latencies6 were performed according to the clinical circumstances at the discretion of the neurophysiologists. The neurophysiologists were blind to the results of physical examinations performed by the study investigators. The diagnosis of carpal tunnel syndrome required at least one of the following: median motor latency greater than 4.0 m/sec; sensory latency greater than 3.7 m/sec; and/or sensory velocity less than 50 m/sec. Ulnar nerve conduction and electromyography were used to exclude polyneuropathy, radiculopathy, and other diagnoses. Criterion A of the NIOSH case definition (Figure 1) corresponded to a possible, probable, or classic rating on the hand pain diagram.16 Criterion B corresponded to a positive Tinel or Phalen sign or diminished two point discrimination. The nerve conduction criterion was not used because our goal was to evaluate the performance of the case definition specifically when physical examination data are used as the objective criterion. Criterion C

was satisfied if patients reported exposure on their jobs either to vibration, repetitive pinching, grasping, or wrist flexion or ex-

tension. The sensitivity, specificity, odds ratio, positive predictive value (proportion with a positive test result who had carpal tunnel syndrome) and negative predictive value (proportion with a negative test result who did not have carpal tunnel syndrome) were determined for each of the diagnostic tests and for the NIOSH case definition.20 Statistical analyses were performed with the chi-squared statistic using the SAS statistical package2l on an International Business Machines P/S 2 Model 60 computer. Confidence intervals for sensitivity and specificity estimates were performed assuming a binomial distribution22 and confidence intervals for predictive values were calculated with an iterative proportional fitting algorithm assuming a hypergeometric distribution.

Resiu Seventy-eight symptomatic subjects studied. Sixty-six were working at the time of evaluation and 12 were temporarily disabled as a result of their upper extremity problems. Patient characteristics are presented in Table 1. The mean age was 43.4 years. Sixty-three percent of subjects were female and 77 percent were White. Ten percent of patients had diabetes mellitus, 6 percent had thyroid disorders, and 4 percent had rheumatoid arthritis. These conditions were well controlled at the time of evaluation. Ten percent (eight patients) had prior carpal tunnel release surgery. Four of these eight patients were found to have carpal tunnel syndrome; in all four the previously operated hand was involved. Sixty-nine percent of subjects reported that their work involved repetitive hand motion, vibration, or forceful wrist flexion or extension. Thirtyeight percent of subjects had carpal tunnel syndrome documented by nerve conduction; diagnoses in the other patients are listed in Table 1. Table 2 depicts diagnostic performance features of the individual components of the NIOSH case definition and of the case definition itself. In univariate analyses the presence of symptoms in at least part of the median nerve distnbution of the hand was highly sensitive for carpal tunnel syndrome (0.93 [95% CI: 0.87, 0.99]) but non-specific (0.25 [95% CI: 0.15, 0.35]). The presence of occupational risk factors was also sensitive (0.83 [95% CI: 0.75, 0.91]) but nonspecific (0.40 [95% CI: were

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Case Definiiton of Carpal Tunnel Syndrome ease. Carpal tunnel syndrome was more prevalent in our sample (0.38) than in other reported groups of symptomatic workers.26.27 In a hypothetical sample of workers with a prevalence of CTS of 15 percent the positive predictive value of the NIOSH case definition would bejust 0.22. In evaluating the usefulness of the case definition it is important to distinguish between epidemiological surveillance, which provides information about disease prevalence in target populations, and screening, which identifies individuals at risk of disease.28 The NIOSH case definition is a surveillance instrument intended for "use by state health departments to facilitate standardized counting of cases"; it is not intended for clinical use.15 In surveillance programs designed

0.29, 0.51]). The Phalen sign was similarly sensitive (0.73 [95% CI: 0.63, 0.83]) but non-specific (0.36 [95% CI: 0.25, 0.47]), while the Tinel sign was less sensitive (0.62 [95% CI: 0.51, 0.73]) but more specific (0.66 [95% CI: 0.56,0.77]). Two point discrimination was the least sensitive (0.23 [95% CI: 0.14, 0.32]) and most specific (0.82 [95% CI: 0.74, 0.91]) test for carpal tunnel syndrome. The individual criteria were combined according to the NIOSH surveillance case definition and compared to actual diagnoses confirmed by nerve conduction tests. The case definition had sensitivity of 0.67 (95% CI: 0.57, 0.77), specificity of 0.58 (95% CI: 0.47, 0.69), positive predictive value of 0.50 (95% CI: 0.39, 0.61) and negative predictive value of 0.74 (95% CI: 0.64,0.84). Overall, 48 of 78 (62 percent) subjects were classified correctly by the surveillance case definition. Bayes' Theorem20 was used to calculate the predictive values in a population with prevalence of 0.15, as might be found in high-risk industries. In such a population the positive predictive value of the case definition would be 0.22 and the negative predictive value would be 0.91.

Discussion Carpal tunnel syndrome has been associated with a variety of occupations in-

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cluding meat packing,2 aircraft and bearing manufacturing,123 sewing,24 grocery checking,25 and many others. Epidemiologic studies relating carpal tunnel syndrome to specific occupations have been limited by the lack of uniform, valid criteria for case ascertainment. The diagnostic gold standard, nerve conduction testing, is not suitable for many large surveillance efforts. Furthermore, simple clinical tests are of limited diagnostic val-

ue.'0 The proposed NIOSH surveillance case definition is an important step toward standardization of surveillance. The present study is the first validation of the case definition against verified cases of carpal tunnel syndrome. This study suggests that when physical examination findings serve as the objective criterion the case definition has important limitations and must be used carefully. In our sample of workers with upper extremity complaints the positive predictive value ofthe case definition was 0.50 (95% CI: 0.39,0.61) and the negative predictive value was 0.74 (95% CI:

0.64, 0.84). These results indicate that half of workers satisfying the case definition did not have carpal tunnel syndrome while a fourth of subjects not satisfying the case definition did. Overall, 38 percent of subjects were classified incorrectly. It is critical to note that predictive values vary with the prevalence of dis-

solely to identify high-risk workplaces, simplicity and low cost are paramount. False negative and false positive results may compromise estimates ofrisk in these settings but do not adversely affect individual workers.28 Our data suggest that the NIOSH case definition would be suitable for occupational surveillance as it is simple, inexpensive and accurate in 62 percent of subjects in our sample. In contrast, inaccurate results exact higher costs when tests are used to identify possible cases for further testing and treatment. Here the cost of false negative test results is withholding treatment from workers who might benefit. The costs of false positives include worker anxiety, work absenteeism,29 and the expense of disproving the screening diagnosis (e.g. $200-500 for a physician visit and nerve conduction tests). The modest sensitivity (0.67) and specificity (0.58) of the NIOSH case definition suggest that it should be used carefully if at all in screening for affected individuals or in clinical practice. Other combinations of tests performed in parallel or in series would produce different trade-offs between sensitivity and specificity.30 For example, we have demonstrated that the combination of a probable or classic hand diagram or a positive Tinel sign has sensitivity of 0.82 (95% CI: 0.75, 0.89) and specificity of 0.50 (95% CI: 0.41, 0.59), while a probable or classic diagram and positive Tinel sign has sensitivity of0.39 (95% CI: 0.30, 0.48) and specificity 0.89 (95% CI: 0.83, 0.95)."0 However, no individual test or combination of tests offers sensitivity and specificity over 0.70 (Table 2) reflecting inherent limits in the diagnostic value ofclinical tests for carpal tunnel syndrome.10 This study has several limitations. Subjects were referred to a hospital laboAmerican Journal of Public Health 191

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Validation of a surveillance case definition of carpal tunnel syndrome.

The National Institute for Occupational Safety and Health (NIOSH) has proposed a surveillance case definition for work-related carpal tunnel syndrome ...
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