AMERICAN JOURNAL OF EPIDEMIOLOGY
Copyright © 1990 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved
Vol 132, No. 6
Printed in U S A
RISK FACTORS FOR CARPAL TUNNEL SYNDROME M. C. T. F. M. DE KROM,1 A. D. M KESTER,2 P. G KNIPSCHILD,3 AND F SPAANS4 de Krom, M. C. T. F. M. (Dept of Neurology, Maastricht U. Hospital, Maastricht, The Netherlands), A. D. M. Kester, P. G. Knipschild, and F. Spaans. Risk factors for carpal tunnel syndrome. Am J Epidemiol 1990; 132:1102-10. Between September 1983 and July 1985, a case-control study was performed of carpal tunnel syndrome risk factors in the general population of Maastricht, The Netherlands, and some surrounding villages. Twenty-eight of the 501 participants were found to suffer from carpal tunnel syndrome. These 28 were added to a series of 128 consecutive carpal tunnel syndrome patients from the same area. The 156 (131 women and 25 men) subjects in whom carpal tunnel syndrome had been diagnosed on the basis of clinical history and neurophysiologic testing were compared with the remaining 473 (310 women and 163 men) subjects. After adjustment for age and sex, the following carpal tunnel syndrome risk factors could be identified: activities with a flexed wrist or with an extended wrist (exposure-related increased risk), hysterectomy without oophorectomy, last menstrual period In menopausal women 6-12 months ago, height, weight, Quetelet index, slimming courses, and in men, varicosis. Associations between carpal tunnel syndrome and the use of oral contraceptives, age at menopause, diabetes, thyroid dysfunction, rheumatism, typing, and pinch grasp could not be demonstrated. carpal tunnel syndrome; case-control studies; risk factors
Carpal tunnel syndrome is a frequently encountered peripheral nerve lesion, espedaily in women. Nearly half of the cases with brachialgia are diagnosed as carpal tunnel syndrome (1).
Various papers summarizing the putative causes of carpal tunnel syndrome have been published (1-7). However, no epidemiologic studies of the determinants of incidence of carpal tunnel syndrome have so far been performed in the general population. Using Received for publication September 26, 1989, and a c a s e -control design, we studied the role in final form June 19, 1990. . , 1 Department of Neurology, Maastricht University ° f various Suggested Carpal t u n n e l SynHospital, University of Limburg, Maastricht, The d r o m e risk factors. Netherlands. 2 Department of Medical Informatics and Statistics, University of Limburg, Maastricht, The Netherlands. MATERIALS AND METHODS 'Department of Epidemiology and Health Care a i • i J J. I Research, University of Limburg, Maastricht, The Selection of cases and controls ^ S L n t of Clinical Neurophysiologv, Between September 1983 and July 1985, Maastricht University Hospital, University of an age- and sex-stratified random sample Limburg, Maastricht, the Netherlands w a s taken from the population register of Reprint requests to Dr. M. C. T. F. M de Krom, » . , . , , rp, XT ,, . , , Department of Neurology, Maastricht University Maastricht, T h e Netherlands, and some Hospital, P. O Box 1918, 6201 BX Maastricht, The surrounding villages. We chose to have Netherlands. equal numbers in each age group between The authors acknowledge the helpful review of AHA J * • oc a es 2 5 a n d 7 4 vearS M d t w l c e the manuscript by Thera van L.eshout and Tiny g . *» many Ba8tiaens-Aarts. women as men in the sample. Of a total 1102
RISK FACTORS FOR CARPAL TUNNEL SYNDROME
sample size of 715 (232 men and 483 women), 501 persons (164 men and 337 women, response rate, 70 percent) were willing to be interviewed. The goal of the survey was hidden; the survey was presented as a questionnaire about general health and health care. Questions concerned life-style, chronic diseases, potential carpal tunnel syndrome risk factors, and carpal tunnel syndrome complaints. Potential carpal tunnel syndrome risk factors were considered to be: 1) various hand and finger positions, such as working with the extended or flexed wrist, and pinch grasp. We chose to study these activities directly, including their duration ("exposure"), rather than to ask about certain occupations and hobbies (except typing) in order to be able to generalize our results to various working conditions; 2) female-related factors, including menarche, number of pregnancies, age at menopause, use of oral contraceptives, and hysterectomy with or without oophorectomy; 3) height, weight, Quetelet index (body weight (kg)/body height (m)2); and 4) coexisting diseases, the diagnosis of which had been revealed to the subjects by a physician, including fractures of the wrist, thyroid diseases, diabetes, and rheumatism. Among those subjects who reported carpal tunnel syndrome complaints, carpal tunnel syndrome patients were identified by means of a second interview and neurophysiologic testing. We realized that there would be too few (n = 28) of these patients in the general population for a powerful case-control study. Therefore, the number of cases was extended by adding 128 from a total of 180 consecutive carpal tunnel syndrome patients (response rate, 71 percent) of the outpatient department of neurology at the Maastricht University Hospital, the only hospital in the area. The cases were interviewed using the same questionnaire in the same period as the controls. All cases lived in the same area as the controls. The carpal tunnel syndrome of the cases was diagnosed on the basis of clinical his-
1103
tory and neurophysiologic examination. The history was considered to be typical for carpal tunnel syndrome if it included tingling, pain, and/or numbness in the median nerve innervated fingers, with a frequency of twice a week or more, which in most cases woke up the patient at night. The diagnosis of carpal tunnel syndrome had to be confirmed by abnormal findings in a neurophysiologic examination, the methods of which have been described earlier (8). Statistical analysis For each of the potential carpal tunnel syndrome risk factors considered, we calculated the numbers of cases and controls at each level of the factor. For continuous variables such as weight, we calculated the mean and standard deviations. Risk ratios were estimated as odds ratios for each factor separately from a multiple logistic regression model including the stratification variables sex and age (in 5 decade groups) and the interaction between sex and age, in order to enhance the accuracy of the estimates. The risk of activities with the wrist was estimated by fitting the mean duration per week of these activities as continuous variables after truncation at 40 hours per week to avoid undue influence of very large exposures. We then used the estimated coefficient to calculate estimated risk ratios at exposures of 8, 20, and 40 hours per week. For those factors not exclusively applicable to women, the interaction with sex was tested and reported when significant at the 0.05 level. The estimated risk ratios were supplemented with 95 percent confidence intervals, based on asymptotic standard errors of the estimated log odds ratios. For factors with more than two ordered levels, a likelihood ratio chi-square test for trend was performed by fitting the factor as a continuous variable. In general, we tried to evaluate risk factors only for persons who could have been exposed to that factor. In order to take into account possible
1104
DE KROM ET AL.
interactions and other relations between risk factors, a further model was fitted, including the factors that were important in the previous analyses, plus those that were considered to be related to these factors. Stepwise, the factors which did not contribute to the goodness-of-fit and did not substantially influence the estimated coefficients of other factors were deleted from this model. The resulting model gives odds ratios for each included factor ceteris paribus: The odds ratios apply to the situation in which only the factor concerned is different, and all others are equal. RESULTS
Eventually, 156 carpal tunnel syndrome cases (131 women and 25 men) were com-
pared with 473 controls (310 women and 163 men) from the general population with regard to the various risk factors. Tables 1-5 present calculated odds ratios which can be regarded as estimated risk ratios for the various factors considered. The risk ratios are accompanied by their 95 percent confidence intervals. Furthermore, we included the raw odds ratios calculated from the population cases only; because of the small numbers, these have not been analyzed more extensively. Table 1 shows the numbers of cases and controls in categories of exposure to working with various hand and finger positions. The risk of carpal tunnel syndrome was found to increase with the duration of activities with the flexed or the extended
TABLE 1
Classification of cases and controls on the basis of their activities, estimated risk ratios, 95% confidence intervals, and x 2 tests, Maastricht, The Netherlands, September 1983 to July 1985 RRt for upper Mean exposures per week 0-5 years ago*
Cases
Controls
limit of
95% CIt
x'test
1.3-1.9 1.8-4.9 3 1-24.1
17 5, p < 0.001
10-19 10-5.2 1.1-27.4
4 3, p = 0 04
0 9-1.2 0 8-1.7 0.7-2.9
0 9, p = 0 33
0.8-1.1 0.5-1.3 0.3-1.6
0.8, p = 0.36
0 6-1.4 0 3-2.5 0.1-6.0
0.11,p = 0.74
category
Activities with flexed wnst 0-5 years ago (hours) 0 1-7
8-19 20-40 Activities with extended wnst (hours) 0 1-7
8-19 20-40 Activities with extended and flexed wrist in combination (hours) 0 1-7
8-19 20-tO Pinch grasp (hours) 0 1-7
8-19 20-40 Typing (hours) 0 1-7
8-19 20-40
112 (22)$ 17 15 (6) 12
421 25 17 10
1.0 1.5 3.0 (2.2) 87
134 (21)
1
4 (7) 4
417 47 6 3
101 (15) 20 17 (13) 18
290 80 50 53
102 (19) 33 14 (9) 7
275 101 58 39
144 (26) 4 3 (2) 5
416 13 18 26
14
* Truncated at 40 hours/week. t RR, risk ratio; 95% CI, 95% confidence interval. t Numbers in parentheses, population cases.
1.4
2 3 (2 5) 5.4
1
11 1 2 (1.4) 1.4
1.0 0.9
0.8 (0 7) 0.7 10 09
0.8 (0.6) 0.7
1105
RISK FACTORS FOR CARPAL TUNNEL SYNDROME
wrist over the past 5 years. For both activities, a clear dose-effect relation could be shown, the risk ratio increasing at least four- to fivefold for people engaged in these activities for more than 20 hours per week. In contrast, working with the flexed and extended wrist in combination over the past 5 years did not carry a significantly increased risk ratio, and the same is true for pinch grasp and typing. Table 2 shows the female-related carpal
tunnel syndrome risk factors. Women who had had a hysterectomy without oophorectomy appeared to run a risk of carpal tunnel syndrome which was twice as high as that for women who had not been operated upon. Age at menopause for all postmenopausal women and age at menopause for women older than age 59 years, age at menarche, number of pregnancies, and diabetes during pregnancy could not be shown to be risk factors. However, the study indicated
TABLE 2
Classification of female cases and controls on the basis of female-related factors, estimated risk ratios, 95% confidence intervals, and x* tests for trend, when applicable, Maastricht, The Netherlands, September 1983 to July 1985 Hysterectomy None Without oophorectomy With oophorectomy In menopause Age at menopause (years) 50
Cases
Controls
RR*
95% CI*
94 (17)t 27 (8) 8 (1) 2 (1)
268 30 10 2
1 (1) 2 0 (4.2) 2.0 (1.6) 2 9 (7.9)
1.1-3.6 0.7-5 3 0.4-21.9
7 (4) 7 (1) 18 (5)
19 32 66
1.3 (2 8) 0.8 (0 4) 1 (1)
0.5-3 6 0.3-2.1
0 09, p = 0 76
3 (2) 5 (1) 12 (5)
13 18 48
0.9 (1 5) 1 1 (0.5) 1 (1)
0 2-3 8 0.3-3.6
0 003, p = 0.95
12 (0) 7 (0) 25 (10)
9 13 104
6.5 (0) 2.4 (0) 1 (1)
1.9-21.8 0 8-7.5
9 3, p •= 0.00
12 (0) 103 (25) 15 (2)
44 227 37
0.6 (0) 1 (1) 0.8 (0.5)
0.3-1.2
0.49, p = 0.49
19 (3) 54(10) 58(14)
51 143 116
1 (1) 0.8 (1.2) 1 0 (2.0)
0.4-1.5 0 5-1.9
101 (22) 11 (2)
244 15
1 (1) 1.6 (1.5)
0.7-3.7
42 (5) 10 (2) 10 (0)
88 48 15
1 (1) 0 7 (0.7) 2.0 (0)
0.3-16 0 8-5.2
X* test
Age (years) at menopause for women older than age 59 years 50
Years since last menstrual period (menopausal women without hysterectomy) £1 2-5 >5
Age at menarche (years)t 15
No. of pregnancies None 1-2 2:3
0.4-1.6 0.094, p = 0.76
Diabetes during pregnancy No
Yes or possible Years of use of oral contraceptives during last 5 years (last menstrual period less than 1 year ago) None 1-4 2:5
* RR, nsk ratio; 95% CI, 95% confidence interval. t Numbers in parentheses, population cases. % No menarche in one case and two controls.
0.82, p => 0.37
1106
DE KROM ET AL.
an increased risk for the first menopausal year. In women who had their last menstrual period not longer than 1 year ago, the use of oral contraceptives during the last 5 years could not be shown to be related to the risk of carpal tunnel syndrome. Table 3 shows the risk ratios of height, weight, Quetelet index, and slimming courses for carpal tunnel syndrome. The taller a person was the lower was the risk for carpal tunnel syndrome. Slimming courses increased the risk. Table 4 gives the data for coexisting diseases. Wrist fractures, thyroid diseases (only present in women), rheumatism, and diabetes could not be shown to be risk factors for carpal tunnel syndrome. The
same was true for varicosis in women, but in men varicosis was found to result in a greatly increased risk ratio. Table 5 presents the estimated coefficients for the final model, including all important factors. The estimated odds ratios are valid after correction for the other factors included, and for sex, age, and their interaction. No interactions were found between any of the factors included. Weight and slimming courses are both included, although they are seemingly insignificant. This means that for persons with the same weight slimming courses do not represent a significant risk and vice versa. Deleting either of these two factors, however, increases the coefficient of the other; the p
TABLE 3
Classification of cases and controls on the basis of physical parameters and slimming courses, estimated nsk ratios, and 95% confidence intervals, Maastricht, The Netherlands, September 1983 to July 1985 Cases (men = 25 (1); women — 131 (27))*
Controls (men = 163, women ™ 310)
RR*t
95% CIt
0 96 (0.98)
0.93-0.98
1.02 (1.02)
1.00-1.03
1.09(104)
1.04-1.14
2 10 (1.77)
1.41-3.12
Height (cm)t Men
Mean SDj
Women Mean SD Weight (kg)t
173 (194) 6.7
163 (163) 6.2 (6 5)
174 7.4
164 6.7
Men
Mean
78
SD
13.0
Women Mean SD
66
11.6
Quetelet index (kg/m2)^
(111)
75
10.7 (65) (8.0)
64
10 7
Men
Mean SD Women Mean SD Slimming courses No Yes
25.9 (29 5) 3.2
2.9
25 0 (24.5) 4.2 (2 9) 92 64
24.7
(18) (0)
23.8 4.0 360 113
* Numbers in parentheses, population cases t RR, risk ratio; 95% CI, 95% confidence interval; SD, standard deviation. X Physical data are given separately for each sex with their means depending on sex. Relative risks were not found to vary with sex and are given per unit increase of the parameter.
1107
RISK FACTORS FOR CARPAL TUNNEL SYNDROME TABLE 4
Classification of cases and controls on the basis of coexisting disease, estimated risk ratios, and 95% confidence intervals, Maastricht, The Netherlands, September 1983 to July 1985
Vancosist Men No Yes Women No Yes
Cases
Controls
17 (0)t 8 (1)
155 8
88(13) 43(14)
203 107
1 0.9 (2.0)
0.6-1.4
147 (27) 9 (1)
428 45
1 0.7 (0.4)
0.3-1 5
124 (25) 7 (2)
294 16
1 1.0 (1.5)
0.4-2.6
153 (25) 3 (3)
450 23
1 0.3 (2.3)
0.1-1.2
152 (27) 4 (1)
456 17
1 0.6 (1.0)
0.2-2.0
RR*
1 12.0 (°°)
95% CI*
3.6-40.1
Wrist fracture No Yes Thyroid disease (only in women) No Yes Rheumatism No Yes Diabetes No Yes
* RR, risk ratio, 95% CI, 95% confidence intervals. t The risk ratio of vancosis was different for men and women t Numbers in parentheses, population cases.
value for both factors together is 0.013, and there is no way to determine which of the two is really unimportant. The last five entries in the table concern women only. The apparent insignificance of the coefficients is because the reference category (premenopausal, no hysterectomy) has a risk intermediate between those of the other categories. The odds ratio of hysterectomy versus menopause longer than 5 years ago is 3.6 (p = 0.002), but the overall p value for the five categories is 0.02. The log odds may be added from table 5 to obtain the log odds of several factors together. For instance, being 10 cm shorter and 5 kg heavier results in a log odds ratio of -10 x -0.057 + 5 X 0.014 = 0.64. The risk ratio is antilog (0.64) = 1.90. DISCUSSION
So far, to our knowledge, no case-control study of the etiology of carpal tunnel syndrome has been performed using controls from the general population. Neither, as a rule, was the diagnosis of carpal tunnel
syndrome in other studies based on clinical history as well as neurophysiologic testing. Therefore, it is difficult to compare our results with the findings of earlier studies. Furthermore, in our study, controls were neurophysiologically tested if they had nocturnal symptoms in their fingers which woke them up. This yielded 28 carpal tunnel syndrome patients from the general population who were added to a series of 128 patients from the same area. Thus, it might be said that we cleared the general population of carpal tunnel syndrome patients, which enabled us to make the best possible comparison between carpal tunnel syndrome cases and general population controls. Regarding the sex ratios in the general population, which were different from those in the hospital cases, we hypothesize that most women in the Netherlands are housewives who are able to avoid continuous wrist-burdening activities because they may vary their household activities. Men, however, have to perform their professional
1108
DE KROM ET AL. TABLE 5
Factors m final model and estimated (log) odds ratios, Maastricht, The September 1983 to July 1985
Height (cm) Weight (kg)
Slimming courses No Yes Wnst activities (hours/week) Flexion Extension Vancosis (for men only) No Yes
Female-related Premenopausal, no hysterectomy Menopause last year Menopause 2-5 years ago Menopause >5 years ago Hysterectomy
Netherlands,
OR*
log
Copyright © 1990 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved
Vol 132, No. 6
Printed in U S A
RISK FACTORS FOR CARPAL TUNNEL SYNDROME M. C. T. F. M. DE KROM,1 A. D. M KESTER,2 P. G KNIPSCHILD,3 AND F SPAANS4 de Krom, M. C. T. F. M. (Dept of Neurology, Maastricht U. Hospital, Maastricht, The Netherlands), A. D. M. Kester, P. G. Knipschild, and F. Spaans. Risk factors for carpal tunnel syndrome. Am J Epidemiol 1990; 132:1102-10. Between September 1983 and July 1985, a case-control study was performed of carpal tunnel syndrome risk factors in the general population of Maastricht, The Netherlands, and some surrounding villages. Twenty-eight of the 501 participants were found to suffer from carpal tunnel syndrome. These 28 were added to a series of 128 consecutive carpal tunnel syndrome patients from the same area. The 156 (131 women and 25 men) subjects in whom carpal tunnel syndrome had been diagnosed on the basis of clinical history and neurophysiologic testing were compared with the remaining 473 (310 women and 163 men) subjects. After adjustment for age and sex, the following carpal tunnel syndrome risk factors could be identified: activities with a flexed wrist or with an extended wrist (exposure-related increased risk), hysterectomy without oophorectomy, last menstrual period In menopausal women 6-12 months ago, height, weight, Quetelet index, slimming courses, and in men, varicosis. Associations between carpal tunnel syndrome and the use of oral contraceptives, age at menopause, diabetes, thyroid dysfunction, rheumatism, typing, and pinch grasp could not be demonstrated. carpal tunnel syndrome; case-control studies; risk factors
Carpal tunnel syndrome is a frequently encountered peripheral nerve lesion, espedaily in women. Nearly half of the cases with brachialgia are diagnosed as carpal tunnel syndrome (1).
Various papers summarizing the putative causes of carpal tunnel syndrome have been published (1-7). However, no epidemiologic studies of the determinants of incidence of carpal tunnel syndrome have so far been performed in the general population. Using Received for publication September 26, 1989, and a c a s e -control design, we studied the role in final form June 19, 1990. . , 1 Department of Neurology, Maastricht University ° f various Suggested Carpal t u n n e l SynHospital, University of Limburg, Maastricht, The d r o m e risk factors. Netherlands. 2 Department of Medical Informatics and Statistics, University of Limburg, Maastricht, The Netherlands. MATERIALS AND METHODS 'Department of Epidemiology and Health Care a i • i J J. I Research, University of Limburg, Maastricht, The Selection of cases and controls ^ S L n t of Clinical Neurophysiologv, Between September 1983 and July 1985, Maastricht University Hospital, University of an age- and sex-stratified random sample Limburg, Maastricht, the Netherlands w a s taken from the population register of Reprint requests to Dr. M. C. T. F. M de Krom, » . , . , , rp, XT ,, . , , Department of Neurology, Maastricht University Maastricht, T h e Netherlands, and some Hospital, P. O Box 1918, 6201 BX Maastricht, The surrounding villages. We chose to have Netherlands. equal numbers in each age group between The authors acknowledge the helpful review of AHA J * • oc a es 2 5 a n d 7 4 vearS M d t w l c e the manuscript by Thera van L.eshout and Tiny g . *» many Ba8tiaens-Aarts. women as men in the sample. Of a total 1102
RISK FACTORS FOR CARPAL TUNNEL SYNDROME
sample size of 715 (232 men and 483 women), 501 persons (164 men and 337 women, response rate, 70 percent) were willing to be interviewed. The goal of the survey was hidden; the survey was presented as a questionnaire about general health and health care. Questions concerned life-style, chronic diseases, potential carpal tunnel syndrome risk factors, and carpal tunnel syndrome complaints. Potential carpal tunnel syndrome risk factors were considered to be: 1) various hand and finger positions, such as working with the extended or flexed wrist, and pinch grasp. We chose to study these activities directly, including their duration ("exposure"), rather than to ask about certain occupations and hobbies (except typing) in order to be able to generalize our results to various working conditions; 2) female-related factors, including menarche, number of pregnancies, age at menopause, use of oral contraceptives, and hysterectomy with or without oophorectomy; 3) height, weight, Quetelet index (body weight (kg)/body height (m)2); and 4) coexisting diseases, the diagnosis of which had been revealed to the subjects by a physician, including fractures of the wrist, thyroid diseases, diabetes, and rheumatism. Among those subjects who reported carpal tunnel syndrome complaints, carpal tunnel syndrome patients were identified by means of a second interview and neurophysiologic testing. We realized that there would be too few (n = 28) of these patients in the general population for a powerful case-control study. Therefore, the number of cases was extended by adding 128 from a total of 180 consecutive carpal tunnel syndrome patients (response rate, 71 percent) of the outpatient department of neurology at the Maastricht University Hospital, the only hospital in the area. The cases were interviewed using the same questionnaire in the same period as the controls. All cases lived in the same area as the controls. The carpal tunnel syndrome of the cases was diagnosed on the basis of clinical his-
1103
tory and neurophysiologic examination. The history was considered to be typical for carpal tunnel syndrome if it included tingling, pain, and/or numbness in the median nerve innervated fingers, with a frequency of twice a week or more, which in most cases woke up the patient at night. The diagnosis of carpal tunnel syndrome had to be confirmed by abnormal findings in a neurophysiologic examination, the methods of which have been described earlier (8). Statistical analysis For each of the potential carpal tunnel syndrome risk factors considered, we calculated the numbers of cases and controls at each level of the factor. For continuous variables such as weight, we calculated the mean and standard deviations. Risk ratios were estimated as odds ratios for each factor separately from a multiple logistic regression model including the stratification variables sex and age (in 5 decade groups) and the interaction between sex and age, in order to enhance the accuracy of the estimates. The risk of activities with the wrist was estimated by fitting the mean duration per week of these activities as continuous variables after truncation at 40 hours per week to avoid undue influence of very large exposures. We then used the estimated coefficient to calculate estimated risk ratios at exposures of 8, 20, and 40 hours per week. For those factors not exclusively applicable to women, the interaction with sex was tested and reported when significant at the 0.05 level. The estimated risk ratios were supplemented with 95 percent confidence intervals, based on asymptotic standard errors of the estimated log odds ratios. For factors with more than two ordered levels, a likelihood ratio chi-square test for trend was performed by fitting the factor as a continuous variable. In general, we tried to evaluate risk factors only for persons who could have been exposed to that factor. In order to take into account possible
1104
DE KROM ET AL.
interactions and other relations between risk factors, a further model was fitted, including the factors that were important in the previous analyses, plus those that were considered to be related to these factors. Stepwise, the factors which did not contribute to the goodness-of-fit and did not substantially influence the estimated coefficients of other factors were deleted from this model. The resulting model gives odds ratios for each included factor ceteris paribus: The odds ratios apply to the situation in which only the factor concerned is different, and all others are equal. RESULTS
Eventually, 156 carpal tunnel syndrome cases (131 women and 25 men) were com-
pared with 473 controls (310 women and 163 men) from the general population with regard to the various risk factors. Tables 1-5 present calculated odds ratios which can be regarded as estimated risk ratios for the various factors considered. The risk ratios are accompanied by their 95 percent confidence intervals. Furthermore, we included the raw odds ratios calculated from the population cases only; because of the small numbers, these have not been analyzed more extensively. Table 1 shows the numbers of cases and controls in categories of exposure to working with various hand and finger positions. The risk of carpal tunnel syndrome was found to increase with the duration of activities with the flexed or the extended
TABLE 1
Classification of cases and controls on the basis of their activities, estimated risk ratios, 95% confidence intervals, and x 2 tests, Maastricht, The Netherlands, September 1983 to July 1985 RRt for upper Mean exposures per week 0-5 years ago*
Cases
Controls
limit of
95% CIt
x'test
1.3-1.9 1.8-4.9 3 1-24.1
17 5, p < 0.001
10-19 10-5.2 1.1-27.4
4 3, p = 0 04
0 9-1.2 0 8-1.7 0.7-2.9
0 9, p = 0 33
0.8-1.1 0.5-1.3 0.3-1.6
0.8, p = 0.36
0 6-1.4 0 3-2.5 0.1-6.0
0.11,p = 0.74
category
Activities with flexed wnst 0-5 years ago (hours) 0 1-7
8-19 20-40 Activities with extended wnst (hours) 0 1-7
8-19 20-40 Activities with extended and flexed wrist in combination (hours) 0 1-7
8-19 20-tO Pinch grasp (hours) 0 1-7
8-19 20-40 Typing (hours) 0 1-7
8-19 20-40
112 (22)$ 17 15 (6) 12
421 25 17 10
1.0 1.5 3.0 (2.2) 87
134 (21)
1
4 (7) 4
417 47 6 3
101 (15) 20 17 (13) 18
290 80 50 53
102 (19) 33 14 (9) 7
275 101 58 39
144 (26) 4 3 (2) 5
416 13 18 26
14
* Truncated at 40 hours/week. t RR, risk ratio; 95% CI, 95% confidence interval. t Numbers in parentheses, population cases.
1.4
2 3 (2 5) 5.4
1
11 1 2 (1.4) 1.4
1.0 0.9
0.8 (0 7) 0.7 10 09
0.8 (0.6) 0.7
1105
RISK FACTORS FOR CARPAL TUNNEL SYNDROME
wrist over the past 5 years. For both activities, a clear dose-effect relation could be shown, the risk ratio increasing at least four- to fivefold for people engaged in these activities for more than 20 hours per week. In contrast, working with the flexed and extended wrist in combination over the past 5 years did not carry a significantly increased risk ratio, and the same is true for pinch grasp and typing. Table 2 shows the female-related carpal
tunnel syndrome risk factors. Women who had had a hysterectomy without oophorectomy appeared to run a risk of carpal tunnel syndrome which was twice as high as that for women who had not been operated upon. Age at menopause for all postmenopausal women and age at menopause for women older than age 59 years, age at menarche, number of pregnancies, and diabetes during pregnancy could not be shown to be risk factors. However, the study indicated
TABLE 2
Classification of female cases and controls on the basis of female-related factors, estimated risk ratios, 95% confidence intervals, and x* tests for trend, when applicable, Maastricht, The Netherlands, September 1983 to July 1985 Hysterectomy None Without oophorectomy With oophorectomy In menopause Age at menopause (years) 50
Cases
Controls
RR*
95% CI*
94 (17)t 27 (8) 8 (1) 2 (1)
268 30 10 2
1 (1) 2 0 (4.2) 2.0 (1.6) 2 9 (7.9)
1.1-3.6 0.7-5 3 0.4-21.9
7 (4) 7 (1) 18 (5)
19 32 66
1.3 (2 8) 0.8 (0 4) 1 (1)
0.5-3 6 0.3-2.1
0 09, p = 0 76
3 (2) 5 (1) 12 (5)
13 18 48
0.9 (1 5) 1 1 (0.5) 1 (1)
0 2-3 8 0.3-3.6
0 003, p = 0.95
12 (0) 7 (0) 25 (10)
9 13 104
6.5 (0) 2.4 (0) 1 (1)
1.9-21.8 0 8-7.5
9 3, p •= 0.00
12 (0) 103 (25) 15 (2)
44 227 37
0.6 (0) 1 (1) 0.8 (0.5)
0.3-1.2
0.49, p = 0.49
19 (3) 54(10) 58(14)
51 143 116
1 (1) 0.8 (1.2) 1 0 (2.0)
0.4-1.5 0 5-1.9
101 (22) 11 (2)
244 15
1 (1) 1.6 (1.5)
0.7-3.7
42 (5) 10 (2) 10 (0)
88 48 15
1 (1) 0 7 (0.7) 2.0 (0)
0.3-16 0 8-5.2
X* test
Age (years) at menopause for women older than age 59 years 50
Years since last menstrual period (menopausal women without hysterectomy) £1 2-5 >5
Age at menarche (years)t 15
No. of pregnancies None 1-2 2:3
0.4-1.6 0.094, p = 0.76
Diabetes during pregnancy No
Yes or possible Years of use of oral contraceptives during last 5 years (last menstrual period less than 1 year ago) None 1-4 2:5
* RR, nsk ratio; 95% CI, 95% confidence interval. t Numbers in parentheses, population cases. % No menarche in one case and two controls.
0.82, p => 0.37
1106
DE KROM ET AL.
an increased risk for the first menopausal year. In women who had their last menstrual period not longer than 1 year ago, the use of oral contraceptives during the last 5 years could not be shown to be related to the risk of carpal tunnel syndrome. Table 3 shows the risk ratios of height, weight, Quetelet index, and slimming courses for carpal tunnel syndrome. The taller a person was the lower was the risk for carpal tunnel syndrome. Slimming courses increased the risk. Table 4 gives the data for coexisting diseases. Wrist fractures, thyroid diseases (only present in women), rheumatism, and diabetes could not be shown to be risk factors for carpal tunnel syndrome. The
same was true for varicosis in women, but in men varicosis was found to result in a greatly increased risk ratio. Table 5 presents the estimated coefficients for the final model, including all important factors. The estimated odds ratios are valid after correction for the other factors included, and for sex, age, and their interaction. No interactions were found between any of the factors included. Weight and slimming courses are both included, although they are seemingly insignificant. This means that for persons with the same weight slimming courses do not represent a significant risk and vice versa. Deleting either of these two factors, however, increases the coefficient of the other; the p
TABLE 3
Classification of cases and controls on the basis of physical parameters and slimming courses, estimated nsk ratios, and 95% confidence intervals, Maastricht, The Netherlands, September 1983 to July 1985 Cases (men = 25 (1); women — 131 (27))*
Controls (men = 163, women ™ 310)
RR*t
95% CIt
0 96 (0.98)
0.93-0.98
1.02 (1.02)
1.00-1.03
1.09(104)
1.04-1.14
2 10 (1.77)
1.41-3.12
Height (cm)t Men
Mean SDj
Women Mean SD Weight (kg)t
173 (194) 6.7
163 (163) 6.2 (6 5)
174 7.4
164 6.7
Men
Mean
78
SD
13.0
Women Mean SD
66
11.6
Quetelet index (kg/m2)^
(111)
75
10.7 (65) (8.0)
64
10 7
Men
Mean SD Women Mean SD Slimming courses No Yes
25.9 (29 5) 3.2
2.9
25 0 (24.5) 4.2 (2 9) 92 64
24.7
(18) (0)
23.8 4.0 360 113
* Numbers in parentheses, population cases t RR, risk ratio; 95% CI, 95% confidence interval; SD, standard deviation. X Physical data are given separately for each sex with their means depending on sex. Relative risks were not found to vary with sex and are given per unit increase of the parameter.
1107
RISK FACTORS FOR CARPAL TUNNEL SYNDROME TABLE 4
Classification of cases and controls on the basis of coexisting disease, estimated risk ratios, and 95% confidence intervals, Maastricht, The Netherlands, September 1983 to July 1985
Vancosist Men No Yes Women No Yes
Cases
Controls
17 (0)t 8 (1)
155 8
88(13) 43(14)
203 107
1 0.9 (2.0)
0.6-1.4
147 (27) 9 (1)
428 45
1 0.7 (0.4)
0.3-1 5
124 (25) 7 (2)
294 16
1 1.0 (1.5)
0.4-2.6
153 (25) 3 (3)
450 23
1 0.3 (2.3)
0.1-1.2
152 (27) 4 (1)
456 17
1 0.6 (1.0)
0.2-2.0
RR*
1 12.0 (°°)
95% CI*
3.6-40.1
Wrist fracture No Yes Thyroid disease (only in women) No Yes Rheumatism No Yes Diabetes No Yes
* RR, risk ratio, 95% CI, 95% confidence intervals. t The risk ratio of vancosis was different for men and women t Numbers in parentheses, population cases.
value for both factors together is 0.013, and there is no way to determine which of the two is really unimportant. The last five entries in the table concern women only. The apparent insignificance of the coefficients is because the reference category (premenopausal, no hysterectomy) has a risk intermediate between those of the other categories. The odds ratio of hysterectomy versus menopause longer than 5 years ago is 3.6 (p = 0.002), but the overall p value for the five categories is 0.02. The log odds may be added from table 5 to obtain the log odds of several factors together. For instance, being 10 cm shorter and 5 kg heavier results in a log odds ratio of -10 x -0.057 + 5 X 0.014 = 0.64. The risk ratio is antilog (0.64) = 1.90. DISCUSSION
So far, to our knowledge, no case-control study of the etiology of carpal tunnel syndrome has been performed using controls from the general population. Neither, as a rule, was the diagnosis of carpal tunnel
syndrome in other studies based on clinical history as well as neurophysiologic testing. Therefore, it is difficult to compare our results with the findings of earlier studies. Furthermore, in our study, controls were neurophysiologically tested if they had nocturnal symptoms in their fingers which woke them up. This yielded 28 carpal tunnel syndrome patients from the general population who were added to a series of 128 patients from the same area. Thus, it might be said that we cleared the general population of carpal tunnel syndrome patients, which enabled us to make the best possible comparison between carpal tunnel syndrome cases and general population controls. Regarding the sex ratios in the general population, which were different from those in the hospital cases, we hypothesize that most women in the Netherlands are housewives who are able to avoid continuous wrist-burdening activities because they may vary their household activities. Men, however, have to perform their professional
1108
DE KROM ET AL. TABLE 5
Factors m final model and estimated (log) odds ratios, Maastricht, The September 1983 to July 1985
Height (cm) Weight (kg)
Slimming courses No Yes Wnst activities (hours/week) Flexion Extension Vancosis (for men only) No Yes
Female-related Premenopausal, no hysterectomy Menopause last year Menopause 2-5 years ago Menopause >5 years ago Hysterectomy
Netherlands,
OR*
log
