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Tactile perception in hands occupationally exposed to vibration
Tactile perception in hands occupationally exposed to vibration The sensory changes that occur in hands occupationally exposed to vibration have been assessed clinically by conventional neurologic tests and, independently, by improved techniques for the determination of tactile spatial resolution (gap detection) and vibrotactile perception thresholds at frequencies from 2 to 400 Hz. Data from 10 forest workers who were exposed to chain saw vibration and seven laboratory workers of similar age, all of whom were screened to exclude confounding factors, revealed, for the first time, three patterns of response, two of which are associated with a vibration-induced neuropathy. The first appears to be characterized by normal or better than normal thresholds in SAl, FAI, and FAIl mechanoreceptor types, while a second, extreme response involves elevated thresholds in all three receptor systems (and abnormal twopoint discrimination). The third pattern appears to be characterized by elevated thresholds in SAl and/or FAIl receptor types. (J HAND SURG 1987;12A[2 Pt 2]:870-5.)
A. J. Brammer, Ph.D.,* J. E. Piercy, Ph.D.,* P. L. Auger, M.D.,** and S. Nohara, M.D., D.Med.Sc.,*** Ottawa, Ont., and Ste-Foy, Quebec, Canada, and Kanazawa, Japan
Habitual operation of power tools or industrial processes in which vibration is transmitted to the hands may lead to a complex of peripheral neurologic, vascular, and musculoskeletal disturbances.) The initial symptoms usually consist of episodes of tingling or numbness in the fingers or hands. With further exposure to vibration, these symptoms are frequently complemented by episodic vasospasms characterized by finger blanching. Although the early signs and symptoms of the hand-arm vibration syndrome are commonly
From the Division of Physics, National Research Council of Canada, Ottawa, Canada, Department of Community Health of the Hospital Center of Laval University, Ste-Foy, Canada, and the Department of Public Health, School of Medicine, Kanazawa University, Kanazawa, Japan. Work supported by the Forest Products Accident Prevention Association. Reprint requests: Dr. A. J. Brammer, Division of Physics, National Research Council of Canada, Montreal Rd., Ottawa, Ont. KIA OR6 Canada. *Senior Research Officer, Division of Physics, National Research Council of Canada, Ottawa, Canada KIA OR6. **Physician adviser in Occupational Health, Department of Community Health of the Hospital Center of Laval University, Ste-Foy, Quebec, Canada GIY 2K8. *** Assistant Professor, Department of Public Health, School of Medicine, Kanazawa University, Kanazawa, 920 Japan.
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THE JOURNAL OF HAND SURGERY
considered to be of little consequence, prolonged exposure to vibration may result in sufficient reduction in tactile function to restrict the ability to hold and to manipulate objects. This loss of ability to perform fine work is believed to be directly linked to impaired tactile perception in the fingers. 2. 3 Recognition that the neurologic symptoms may develop independently of the vascular disturbances (which until recently have been considered the dominant phenomena), with the former usually reported first, has rekindled interest in the early detection of sensory changes in the hands by means of objective tests. For the purposes of the present work, the neurologic symptoms have been classified clinically into four stages that are listed by increasing severity in Table I. 3 The symptomatic stages focus on reports of intermittent numbness (stage lSN), reduced sensory perception (stage 2SN), and reduced tactile discrimination and/or manipUlative dexterity (stage 3SN). Only the most severe cases of sensory loss can be detected consistently by neurologic tests that are traditionally employed in clinical medicine. 4 The purpose of this article is to report progress on the detection of changes in tactile sensation in hands occupationally exposed to vibration. This is done by focusing on the measurement and interpretation of vibrotactile and esthesiometer perception thresholds ob-
Vol. 12A, No.5, Part 2 September 1987
Tactile perception in hands exposed to vibration
871
Table I. Proposed sensorineural stages of the hand-arm vibration syndrome Stage
OSN ISN 2SN 3SN
Symptoms
Exposed to vibration, but no symptoms Intermittent numbness, with or without tingling Intermittent or persistent numbness, reduced sensory perception Intermittent or persistent numbness, reduced tactile discrimination, and/or manipulative dexterity
The sensorineural stage is to be established for each hand. (Reprinted with permission from Brammer AJ, Taylor W. Lundborg G. Sen· sorineural stages of the hand-arm vibration syndrome. Scand J Work Environ Health [in press].)
tained from two small groups of forest workers who have been exposed to chain saw vibration, and from comparable persons of similar ages who have not been exposed to vibration. A preliminary analysis of data obtained from the first group of chain saw operators has been reported elsewhere. 5
Subjects and methods Clinical evaluation. Twenty-seven subjects (17 forestry and 10 laboratory workers) completed a questionnaire concerning their present and past occupations, and medical history. They then underwent a physical examination and clinical tests to identify possible causes of peripheral sensory dysfunction other than exposure to vibration. The physical examination included inspection of the hands and tests of peripheral nerve function (perception of 128 Hz tuning fork, stereognosis, static and moving two-point discrimination, and a modified Moberg pick-up test).6 Traditional neurologic tests for light touch (cotton wool), pain (pin prick), and temperature (detection of a surface heated to 40° C) were also conducted, as were tests for carpal tunnel and thoracic outlet syndromes. A differential diagnosis was made on the basis of this information, and the 17 subjects considered free from confounding factors (10 forestry workers and seven laboratory workers) were then classified by sensorineural stage of the hand-arm vibration syndrome. The informed consent of each subject was obtained before participation in the study, and the experiments were approved by the ethics committee of the National Research Council of Canada. Measurement of vibrotactile perception. Vibrotactile perception thresholds were determined at the fingertip (digit 3, right hand) by means of a small vibration exciter (Bruel and Kjaer, type 4810) and an accelerometer (Bruel and Kjaer, type 800l) suspended from a beam
Fig. 1. Photograph of flat-topped, cylindrical, vibrating probe contacting the fingertip of a hand supported and restrained by an armrest.
balance (with counterweights) to provide precise control of the force with which a flat-topped, cylindrical, plastic probe contacted the skin. The fingers were supported in a natural, curved position, with the palm upward, while the subject relaxed in a dental chair with both the hand and forearm restrained (Fig. 1). The threshold was established psychophysically by the method of limits,7 by applying sinusoidal bursts of vibration (duration and separation 1.5 seconds or 10 oscillations, whichever was greater), with the mean of the values recorded using ascending and descending amplitudes being taken as the best estimate of the threshold at each stimulus frequency. Measurement of tactile spatial resolution. The threshold for gap detection was determined at the fingertip (digit 3, right hand) with the esthesiometer that was developed by Marshall et al. s, 9 In this apparatus, both the contact force and the relative motion between the fingertip and a horizontal surface containing the feature to be detected are controlled. The latter consisted of a plastic plate in which a groove of constant depth and progressively increasing width had been machined (Fig. 2). With the palm of the hand supported, the finger was positioned in a "V" -shaped slot oriented at 60° to the horizontal so that it contacted the test surface with the required force (a visual display of contact force is provided, but the subject cannot see the test surface). The threshold was established with the test surface moving at 6.7 mm per second and the finger initially in contact with a flat surface (i.e., zero gap width). The gap width d, at which the subject first sensed a change in the surface, was recorded, and the mean of up to eight such determinations, after a training period, was taken as the threshold for gap detection.
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Fig. 4. Vibrotactile perception thresholds at frequencies of 2 to 400 Hz obtained from five vibration-exposed workers, identified by number, and seven laboratory workers of similar ages (mean values and ± 2 SD shown by triangles and bars) . (Reprinted with permission from Brammer AJ, Piercy JE, Auger PL. Assessment of impaired tactile sensation. A pilot study. Scand J Work Environ Health [in press).)
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Fig. 3. Vibrotactile perception threshold at 100 Hz obtained from five vibration-exposed workers (aged 29 ± 5 years), identified by number, and shown by sensorineural stage of the hand-arm vibration syndrome, and from seven laboratory workers (aged 37 ± 7 years). The upper limit of normality (defined in text), estimated from the data of the laboratory workers and from 55 healthy persons (aged less than 45 years),'O is shown by the continuous and dash-dot lines, respectively.
Results and discussion The relationship between the clinical evaluation of sensory changes in the hands and the vibrotactile threshold at a stimulation frequency of 100 Hz is shown in Fig. 3 for the first group of forest workers. In this diagram the perception threshold is shown as the abscissa and the sensorineural stage as the ordinate, with each vibration-exposed subject identified by a number. In view of the small number of normal hands included in this study, two estimates of the upper limit of normal vibrotactile perception have been calculated (defined as the displacement level below which thresholds for 95% of normal persons would occur). The first is derived
from the laboratory workers by use of the t distribution, with 6 degrees of freedom (indicated by the continuous line in Fig. 3) and the second from a larger group of persons who were free of symptoms (n = 55) drawn from the general population (the dash-dot line) . 10 As the method of measurement employed by Lundborg et a1. 10 involved no control of contact force and a slightly higher stimulation frequency, both of which are expected to reduce perception thresholds compared with those obtained here, it can be seen from Fig. 3 that the two predicted limits of normality are in reasonable agreement. They indicate, at least for this group of workers, that abnormal vibrotactile thresholds could be detected in four of five cases, three of which were in early to moderate sensorineural stages of the hand-arm vibration syndrome and possessed normal two-point discrimination. Note that subject No.2 does not follow this pattern in that he reported numbness in his hands but had an extremely sensitive vibrotactile threshold. Early detection of sensory loss as elevated vibrotactile thresholds at frequencies close to 100Hz before any change in two-point discrimination can be recorded has been reported elsewhere, in experiments that involved acute median nerve compression at the wrist II and, recently, in compression neuropathy. 10 Further information on mechanoreceptor or peripheral sensory nerve dysfunction may be derived from vibrotactile measurements conducted over a broad frequency range. 10. 12. 13 Fig. 4 shows the threshold recorded at frequencies of 2 to 400 Hz, with values for
Vol. 12A, No. 5, Part 2 September 1987
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Fig. 5. Gap-detection threshold for five vibration-exposed workers , identified by number, expressed by sensorineural stage of the hand-arm vibration syndrome, and for seven laboratory workers of similar ages . The upper limit of normality (defined in text), estimated from the laboratory workers and from 61 industrial manual workers (Behrens V, personal communication), is shown by the continuous and dash-dot lines, respectively.
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Fig. 6. Correlation coefficient between gap-detection and vibrotactile perception thresholds shown as a function of frequency for seven laboratory workers . The correlation coefficient at 2 Hz is statistically significant (p < 0.05).
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each forest worker again indicated by a number and values for normal subjects expressed in terms of their mean values (triangles) and ± 2 SD (vertical bars). It can be seen from Fig. 4 that the vibrotactile perception of worker No.5 was abnormally insensitive at all frequencies; in fact, his thresholds were greater than the maximum displacement of the vibrator at frequencies below 20 Hz and so could not be determined. This significant elevation of vibrotactile threshold at frequencies of 2 to 200 Hz (accompanied by two-point discrimination in excess of 10 mm at the fingertip) would appear to be characteristic of one type of vibration-induced neuropathy and is compatible with peripheral nerve degeneration . An analysis of nerve conduction in, and pathologic evidence from, vibrationexposed workers reveals that this neuropathy is confined to the hands and is most commonly associated with demyelination and loss of nerve fibers . 14 • 15 In other workers in this group, elevated vibrotactile thresholds were commonly recorded at frequencies of 50 Hz and above (Fig. 4). It is generally accepted that thresholds determined psychophysically at these frequencies are the result of neural activity in mechanoreceptors of the FAIl type (which have been anatomically correlated with pacinian corpuscles in the hairless skin of the hand). 16. 17 In contrast, the lack of significantly elevated thresholds at lower frequencies suggests that the mechanoreceptor type or types responsible for vibrotactile perception at these frequencies is/are func-
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Fig. 7. Vibrotactile perception thresholds at frequencies of 2 to 400 Hz obtained with reduced probe diameter and contact force from five vibration-exposed workers, aged 30 ± 3 years, and five laboratory workers, aged 35 ± 6 years (mean values and ± 2 SD shown by triangles and bars) .
tioning normally. This pattern of sensory loss in vibration-exposed workers has been commonly observed elsewhere. However, independent measurement of the threshold for gap detection , performed with the esthesiometer, revealed elevated spatial discrimination thresholds in the three workers (Nos. 1, 3, and 4) found to possess abnormal vibrotactile thresholds at high frequencies. These results are shown by sensorineural stage for the first group of forest workers in Fig. 5. As before, the upper limit of normality has been estimated for the laboratory workers and for a second, larger group, in
874
The Journal of HAND SURGERY
Brammer et al.
this case manual workers in a factory who were unexposed to vibration (Behrens V, personal communication) and is shown by the continuous and dash-dot lines, respectively. Now the thresholds for gap detection and vibrotactile perception at very low frequencies are significantly correlated in normal hands, as can be seen from the correlation coefficient in Fig. 6, which has been calculated for the laboratory worker data. Furthermore, parallel psychophysical and neurophysiologic experiments on human subjects and on monkeys have demonstrated that gap detection is mediated by SAl-type mechanoreceptor. 18. 19 Thus, the esthesiometer and vibrotactile measurements at low frequencies are in conflict concerning the functional status of the SAl receptors in two of the five forest workers. In view of the similar ranking of, and shifts in, the thresholds for gap and vibrotactile perception at higher frequencies recorded in these vibration-exposed workers, as evident on comparison of Figs. 3 and 5, it would appear that more sensitivity is needed in the vibrotactile measurements at low frequencies to detect vibration-shifted SAl thresholds. Accordingly, experiments were undertaken to improve the resolution of the vibrotactile measurement technique at low frequencies. This has been achieved by modification of the parameters controlling contact between the vibrating probe and the skin. The vibrotactile thresholds recorded by the laboratory workers and a second group of forest workers with the use of a reduced probe diameter and contact force are shown in Fig. 7, expressed as in Fig. 4. Note that data obtained from two laboratory workers have been excluded to provide a better age match to this group of chain saw operators. A comparison of the mean thresholds for normal hands shown in Fig. 7 with those shown in Fig. 4 confirms that the low-frequency thresholds are now more sensitive (while those at frequencies of 50 Hz and above are less sensitive). The insensitivity of the threshold at 20 Hz to the contact parameters and its lack of correlation with the threshold for gap detection (Fig. 5) support the concept of this vibrotactile threshold being mediated by a third receptor type, which, from both psychophysical and neurophysiologic studies, is believed to be the FAI mechanoreceptors. 12 • 16. 17 Inspection of the data for vibration-exposed hands in Fig. 7 reveals that, as in the first group, one forest worker (No.7) had extremely sensitive thresholds at most frequencies. The thresholds of the other four forest workers in this group were less sensitive than the mean values for normal hands at both high and low frequen-
cies. This suggests that vibration exposure has affected the functioning of both the SAl and FAIl receptors and/or their associated nerve fibers in these workers' hands. In conclusion, the development of improved techniques for measuring vibrotactile and gap-perception thresholds has led to the detection of three patterns of response to vibration exposure, at least two of which are associated with a vibration-induced neuropathy. The first appears to be characterized by normal or better than normal thresholds in SAl, FAI, and FAIl receptor types, and it has been observed in two chain saw operators. A second, extreme response involves abnormally elevated thresholds of similar magnitude in all three receptor systems and thus is suggestive of peripheral nerve pathosis. However, this pattern has been recorded in only one forest worker. The third pattern, which has been observed in seven vibration-exposed workers, appears to be characterized by elevated thresholds in SAl and/or FAIl receptor types. This response is suggestive of pathologic changes occurring at or near the end organs. REFERENCES 1. Taylor W, Brammer AJ. Vibration effects on the hand and arm in industry: An introduction and review. In: Brammer AJ, Taylor W, eds. Vibration effects on the hand and arm in industry. New York: John Wiley and Sons, 1982:1-12. 2. Johansson RS, Westling G. Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects. Exp Brain Res 1984;56:550-64. 3. Brammer AJ, Taylor W, Lundborg G. Sensorineural stages of the hand-arm vibration syndrome. Scand J Work Environ Health (in press). 4. Taylor W, Ogston SA, Brammer AJ. A clinical assessment of seventy-eight cases of the hand-arm vibration syndrome. Scand J Work Environ Health 1986;12: 265-8. 5. Brammer AJ, Piercy JE, Auger PL. Assessment of impaired tactile sensation: A pilot study. Scand J Work Environ Health (in press). 6. Dellon AL. Evaluation of sensibility and re-education of sensation in the hand. Baltimore: Williams & Wilkins, 1981. 7. Gescheider GA. Psychophysics-method and theory. Hillsdale, NJ: Lawrence Erlbaum Associates, 1976: 28-32. 8. Marshall DP. Measurements of tactile spatial resolution of a fingertip [M.A.Sc. thesis]. Windsor, Ontario: University of Windsor, 1986. 9. Reif ZF, Marshall D, Brammer AJ, Piercy JE, Taylor W. Improved esthesiometer for measuring tactile per-
Vol. 12A, No.5 , Part 2 September 1987
ception in hands exposed to vibration . Proceedings of the 12th International Congress on Acoustics. Toronto: Canadian Acoustical Association, 1986:F2-5. 10. Lundborg G, Lie-Stenstrom A-K, Sollerman C, Stromberg T, Pyykko I. Digital vibrogram: A new diagnostic tool for sensory testing in compression neuropathy. J HAND SURG 1986;IIA:693-9. 11. Szabo RM , Gelberman RH , Williamson RV, Dellon AL, Yaru NC, Dimick MP. Vibratory sensory testing in acute peripheral nerve compression. J HAND SURG 1984;9A: 104-9. 12. LOfvenberg J, Johannson RS. Regional differences and interindividual variability in sensitivity to vibration in the glabrous skin of the human hand . Brain Res 1984;301: 65-72. 13 . Piercy JE, Brammer AJ . Development of vibrotactile measurement techniques for assessing mechanoreceptor performance at the fingertip . J Acoust Soc Am 1986; 80(suppl 1):S41. 14. Brammer AJ, Pyykko I. Vibration-induced neuropathy: Detection by nerve conduction . Scand J Work Environ Health (in press).
Tactile perception in hands exposed to vibration
15. Takeuchi T, Futatsuka M, Imanishi H, Yamada S. Pathological changes observed in the finger biopsy of patients with vibration-induced white finger. Scand J Work Environ Health 1986;12:280-3. 16. Vallbo AB, Johansson RS . Properties of cutaneous mechanoreceptors in the human hand related to touch sensation . Hum Neurobiol 1984;3:3-14. 17 . Mountcastle VB , Lamothe RH, Carli G. Detection thresholds for stimuli in humans and monkeys: Comparison with threshold events in mechanoreceptor afferent fibers innervating the monkey hand. J Neurophysiol 1972;35: 122-36. 18 . Johnson KO, Phillips JR . Tactile spatial resolution. I. Two-point discrimination, gap detection, grating resolution, and letter recognition . J Neurophysiol 1981;46: 1177-91. 19. Phillips JR, Johnson KO . Tactile resolution . II. Neural representation of bars, edges, and gratings in monkey primary afferents . J Neurophysiol 1981 ;46:1192-1203.
Carpal tunnel syndrome, pyridoxine, and the work place Carpal tunnel syndrome has become a major source of work-related impairment. Recent studies have suggested pyridoxine deficiency may be a cause of carpal tunnel syndrome, but all studies on this relationship have been flawed by lack of scientific design. Given the available evidence, pyridoxine is not likely to be a significant cause of carpal tunnel syndrome in the work place. Ergonomic measures are more likely to be effective than pyridoxine in reducing the incidence and cost of carpal tunnel syndrome in the work place. (J HAND SURG 1987;12A[2 Pt 2]:875-80.)
Peter C. Amadio, M.D., Rochester. Minn .
Carpal tunnel syndrome is a common clinical problem and one that is frequently identified in the work place. 1-12 Work impairment from carpal tunnel syndrome has become a significant problem, and many programs have been established to reduce the cost of carpal tunnel syndrome in the work environment. I3- 17
From the Department of Orthopedics. Mayo Clinic and Mayo Foundation, Rochester, Minn. Reprint requests: Peter C. Amadio. M.D., Department of Orthopedics, Mayo Clinic and Mayo Foundation, 200 First St., S.W., Rochester, MN 55905.
Recently, emphasis on the prevention of carpal tunnel syndrome in the work place has developed,18-21 and this article will address one aspect of that issue. By definition, carpal tunnel syndrome is a disorder that affects the median nerve at the wrist sufficient to cause impairment of nerve function. Causes of the decreased function are presumed to be associated with increased pressure and subsequent nerve ischemia within the rigid confines of the carpal tunnel. 22, 23 This may be due to either a decrease in the size of the carpal canal 24 or to an increase in the volume of the contents of that canal. 23 Carpal tunnel symptoms can be reliably reproduced in human volunteers by experiments that THE JOURNAL OF HAND SURGERY
875
A. J. Brammer, Ph.D.,* J. E. Piercy, Ph.D.,* P. L. Auger, M.D.,** and S. Nohara, M.D., D.Med.Sc.,*** Ottawa, Ont., and Ste-Foy, Quebec, Canada, and Kanazawa, Japan
Habitual operation of power tools or industrial processes in which vibration is transmitted to the hands may lead to a complex of peripheral neurologic, vascular, and musculoskeletal disturbances.) The initial symptoms usually consist of episodes of tingling or numbness in the fingers or hands. With further exposure to vibration, these symptoms are frequently complemented by episodic vasospasms characterized by finger blanching. Although the early signs and symptoms of the hand-arm vibration syndrome are commonly
From the Division of Physics, National Research Council of Canada, Ottawa, Canada, Department of Community Health of the Hospital Center of Laval University, Ste-Foy, Canada, and the Department of Public Health, School of Medicine, Kanazawa University, Kanazawa, Japan. Work supported by the Forest Products Accident Prevention Association. Reprint requests: Dr. A. J. Brammer, Division of Physics, National Research Council of Canada, Montreal Rd., Ottawa, Ont. KIA OR6 Canada. *Senior Research Officer, Division of Physics, National Research Council of Canada, Ottawa, Canada KIA OR6. **Physician adviser in Occupational Health, Department of Community Health of the Hospital Center of Laval University, Ste-Foy, Quebec, Canada GIY 2K8. *** Assistant Professor, Department of Public Health, School of Medicine, Kanazawa University, Kanazawa, 920 Japan.
870
THE JOURNAL OF HAND SURGERY
considered to be of little consequence, prolonged exposure to vibration may result in sufficient reduction in tactile function to restrict the ability to hold and to manipulate objects. This loss of ability to perform fine work is believed to be directly linked to impaired tactile perception in the fingers. 2. 3 Recognition that the neurologic symptoms may develop independently of the vascular disturbances (which until recently have been considered the dominant phenomena), with the former usually reported first, has rekindled interest in the early detection of sensory changes in the hands by means of objective tests. For the purposes of the present work, the neurologic symptoms have been classified clinically into four stages that are listed by increasing severity in Table I. 3 The symptomatic stages focus on reports of intermittent numbness (stage lSN), reduced sensory perception (stage 2SN), and reduced tactile discrimination and/or manipUlative dexterity (stage 3SN). Only the most severe cases of sensory loss can be detected consistently by neurologic tests that are traditionally employed in clinical medicine. 4 The purpose of this article is to report progress on the detection of changes in tactile sensation in hands occupationally exposed to vibration. This is done by focusing on the measurement and interpretation of vibrotactile and esthesiometer perception thresholds ob-
Vol. 12A, No.5, Part 2 September 1987
Tactile perception in hands exposed to vibration
871
Table I. Proposed sensorineural stages of the hand-arm vibration syndrome Stage
OSN ISN 2SN 3SN
Symptoms
Exposed to vibration, but no symptoms Intermittent numbness, with or without tingling Intermittent or persistent numbness, reduced sensory perception Intermittent or persistent numbness, reduced tactile discrimination, and/or manipulative dexterity
The sensorineural stage is to be established for each hand. (Reprinted with permission from Brammer AJ, Taylor W. Lundborg G. Sen· sorineural stages of the hand-arm vibration syndrome. Scand J Work Environ Health [in press].)
tained from two small groups of forest workers who have been exposed to chain saw vibration, and from comparable persons of similar ages who have not been exposed to vibration. A preliminary analysis of data obtained from the first group of chain saw operators has been reported elsewhere. 5
Subjects and methods Clinical evaluation. Twenty-seven subjects (17 forestry and 10 laboratory workers) completed a questionnaire concerning their present and past occupations, and medical history. They then underwent a physical examination and clinical tests to identify possible causes of peripheral sensory dysfunction other than exposure to vibration. The physical examination included inspection of the hands and tests of peripheral nerve function (perception of 128 Hz tuning fork, stereognosis, static and moving two-point discrimination, and a modified Moberg pick-up test).6 Traditional neurologic tests for light touch (cotton wool), pain (pin prick), and temperature (detection of a surface heated to 40° C) were also conducted, as were tests for carpal tunnel and thoracic outlet syndromes. A differential diagnosis was made on the basis of this information, and the 17 subjects considered free from confounding factors (10 forestry workers and seven laboratory workers) were then classified by sensorineural stage of the hand-arm vibration syndrome. The informed consent of each subject was obtained before participation in the study, and the experiments were approved by the ethics committee of the National Research Council of Canada. Measurement of vibrotactile perception. Vibrotactile perception thresholds were determined at the fingertip (digit 3, right hand) by means of a small vibration exciter (Bruel and Kjaer, type 4810) and an accelerometer (Bruel and Kjaer, type 800l) suspended from a beam
Fig. 1. Photograph of flat-topped, cylindrical, vibrating probe contacting the fingertip of a hand supported and restrained by an armrest.
balance (with counterweights) to provide precise control of the force with which a flat-topped, cylindrical, plastic probe contacted the skin. The fingers were supported in a natural, curved position, with the palm upward, while the subject relaxed in a dental chair with both the hand and forearm restrained (Fig. 1). The threshold was established psychophysically by the method of limits,7 by applying sinusoidal bursts of vibration (duration and separation 1.5 seconds or 10 oscillations, whichever was greater), with the mean of the values recorded using ascending and descending amplitudes being taken as the best estimate of the threshold at each stimulus frequency. Measurement of tactile spatial resolution. The threshold for gap detection was determined at the fingertip (digit 3, right hand) with the esthesiometer that was developed by Marshall et al. s, 9 In this apparatus, both the contact force and the relative motion between the fingertip and a horizontal surface containing the feature to be detected are controlled. The latter consisted of a plastic plate in which a groove of constant depth and progressively increasing width had been machined (Fig. 2). With the palm of the hand supported, the finger was positioned in a "V" -shaped slot oriented at 60° to the horizontal so that it contacted the test surface with the required force (a visual display of contact force is provided, but the subject cannot see the test surface). The threshold was established with the test surface moving at 6.7 mm per second and the finger initially in contact with a flat surface (i.e., zero gap width). The gap width d, at which the subject first sensed a change in the surface, was recorded, and the mean of up to eight such determinations, after a training period, was taken as the threshold for gap detection.
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Fig. 3. Vibrotactile perception threshold at 100 Hz obtained from five vibration-exposed workers (aged 29 ± 5 years), identified by number, and shown by sensorineural stage of the hand-arm vibration syndrome, and from seven laboratory workers (aged 37 ± 7 years). The upper limit of normality (defined in text), estimated from the data of the laboratory workers and from 55 healthy persons (aged less than 45 years),'O is shown by the continuous and dash-dot lines, respectively.
Results and discussion The relationship between the clinical evaluation of sensory changes in the hands and the vibrotactile threshold at a stimulation frequency of 100 Hz is shown in Fig. 3 for the first group of forest workers. In this diagram the perception threshold is shown as the abscissa and the sensorineural stage as the ordinate, with each vibration-exposed subject identified by a number. In view of the small number of normal hands included in this study, two estimates of the upper limit of normal vibrotactile perception have been calculated (defined as the displacement level below which thresholds for 95% of normal persons would occur). The first is derived
from the laboratory workers by use of the t distribution, with 6 degrees of freedom (indicated by the continuous line in Fig. 3) and the second from a larger group of persons who were free of symptoms (n = 55) drawn from the general population (the dash-dot line) . 10 As the method of measurement employed by Lundborg et a1. 10 involved no control of contact force and a slightly higher stimulation frequency, both of which are expected to reduce perception thresholds compared with those obtained here, it can be seen from Fig. 3 that the two predicted limits of normality are in reasonable agreement. They indicate, at least for this group of workers, that abnormal vibrotactile thresholds could be detected in four of five cases, three of which were in early to moderate sensorineural stages of the hand-arm vibration syndrome and possessed normal two-point discrimination. Note that subject No.2 does not follow this pattern in that he reported numbness in his hands but had an extremely sensitive vibrotactile threshold. Early detection of sensory loss as elevated vibrotactile thresholds at frequencies close to 100Hz before any change in two-point discrimination can be recorded has been reported elsewhere, in experiments that involved acute median nerve compression at the wrist II and, recently, in compression neuropathy. 10 Further information on mechanoreceptor or peripheral sensory nerve dysfunction may be derived from vibrotactile measurements conducted over a broad frequency range. 10. 12. 13 Fig. 4 shows the threshold recorded at frequencies of 2 to 400 Hz, with values for
Vol. 12A, No. 5, Part 2 September 1987
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Fig. 5. Gap-detection threshold for five vibration-exposed workers , identified by number, expressed by sensorineural stage of the hand-arm vibration syndrome, and for seven laboratory workers of similar ages . The upper limit of normality (defined in text), estimated from the laboratory workers and from 61 industrial manual workers (Behrens V, personal communication), is shown by the continuous and dash-dot lines, respectively.
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each forest worker again indicated by a number and values for normal subjects expressed in terms of their mean values (triangles) and ± 2 SD (vertical bars). It can be seen from Fig. 4 that the vibrotactile perception of worker No.5 was abnormally insensitive at all frequencies; in fact, his thresholds were greater than the maximum displacement of the vibrator at frequencies below 20 Hz and so could not be determined. This significant elevation of vibrotactile threshold at frequencies of 2 to 200 Hz (accompanied by two-point discrimination in excess of 10 mm at the fingertip) would appear to be characteristic of one type of vibration-induced neuropathy and is compatible with peripheral nerve degeneration . An analysis of nerve conduction in, and pathologic evidence from, vibrationexposed workers reveals that this neuropathy is confined to the hands and is most commonly associated with demyelination and loss of nerve fibers . 14 • 15 In other workers in this group, elevated vibrotactile thresholds were commonly recorded at frequencies of 50 Hz and above (Fig. 4). It is generally accepted that thresholds determined psychophysically at these frequencies are the result of neural activity in mechanoreceptors of the FAIl type (which have been anatomically correlated with pacinian corpuscles in the hairless skin of the hand). 16. 17 In contrast, the lack of significantly elevated thresholds at lower frequencies suggests that the mechanoreceptor type or types responsible for vibrotactile perception at these frequencies is/are func-
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Fig. 7. Vibrotactile perception thresholds at frequencies of 2 to 400 Hz obtained with reduced probe diameter and contact force from five vibration-exposed workers, aged 30 ± 3 years, and five laboratory workers, aged 35 ± 6 years (mean values and ± 2 SD shown by triangles and bars) .
tioning normally. This pattern of sensory loss in vibration-exposed workers has been commonly observed elsewhere. However, independent measurement of the threshold for gap detection , performed with the esthesiometer, revealed elevated spatial discrimination thresholds in the three workers (Nos. 1, 3, and 4) found to possess abnormal vibrotactile thresholds at high frequencies. These results are shown by sensorineural stage for the first group of forest workers in Fig. 5. As before, the upper limit of normality has been estimated for the laboratory workers and for a second, larger group, in
874
The Journal of HAND SURGERY
Brammer et al.
this case manual workers in a factory who were unexposed to vibration (Behrens V, personal communication) and is shown by the continuous and dash-dot lines, respectively. Now the thresholds for gap detection and vibrotactile perception at very low frequencies are significantly correlated in normal hands, as can be seen from the correlation coefficient in Fig. 6, which has been calculated for the laboratory worker data. Furthermore, parallel psychophysical and neurophysiologic experiments on human subjects and on monkeys have demonstrated that gap detection is mediated by SAl-type mechanoreceptor. 18. 19 Thus, the esthesiometer and vibrotactile measurements at low frequencies are in conflict concerning the functional status of the SAl receptors in two of the five forest workers. In view of the similar ranking of, and shifts in, the thresholds for gap and vibrotactile perception at higher frequencies recorded in these vibration-exposed workers, as evident on comparison of Figs. 3 and 5, it would appear that more sensitivity is needed in the vibrotactile measurements at low frequencies to detect vibration-shifted SAl thresholds. Accordingly, experiments were undertaken to improve the resolution of the vibrotactile measurement technique at low frequencies. This has been achieved by modification of the parameters controlling contact between the vibrating probe and the skin. The vibrotactile thresholds recorded by the laboratory workers and a second group of forest workers with the use of a reduced probe diameter and contact force are shown in Fig. 7, expressed as in Fig. 4. Note that data obtained from two laboratory workers have been excluded to provide a better age match to this group of chain saw operators. A comparison of the mean thresholds for normal hands shown in Fig. 7 with those shown in Fig. 4 confirms that the low-frequency thresholds are now more sensitive (while those at frequencies of 50 Hz and above are less sensitive). The insensitivity of the threshold at 20 Hz to the contact parameters and its lack of correlation with the threshold for gap detection (Fig. 5) support the concept of this vibrotactile threshold being mediated by a third receptor type, which, from both psychophysical and neurophysiologic studies, is believed to be the FAI mechanoreceptors. 12 • 16. 17 Inspection of the data for vibration-exposed hands in Fig. 7 reveals that, as in the first group, one forest worker (No.7) had extremely sensitive thresholds at most frequencies. The thresholds of the other four forest workers in this group were less sensitive than the mean values for normal hands at both high and low frequen-
cies. This suggests that vibration exposure has affected the functioning of both the SAl and FAIl receptors and/or their associated nerve fibers in these workers' hands. In conclusion, the development of improved techniques for measuring vibrotactile and gap-perception thresholds has led to the detection of three patterns of response to vibration exposure, at least two of which are associated with a vibration-induced neuropathy. The first appears to be characterized by normal or better than normal thresholds in SAl, FAI, and FAIl receptor types, and it has been observed in two chain saw operators. A second, extreme response involves abnormally elevated thresholds of similar magnitude in all three receptor systems and thus is suggestive of peripheral nerve pathosis. However, this pattern has been recorded in only one forest worker. The third pattern, which has been observed in seven vibration-exposed workers, appears to be characterized by elevated thresholds in SAl and/or FAIl receptor types. This response is suggestive of pathologic changes occurring at or near the end organs. REFERENCES 1. Taylor W, Brammer AJ. Vibration effects on the hand and arm in industry: An introduction and review. In: Brammer AJ, Taylor W, eds. Vibration effects on the hand and arm in industry. New York: John Wiley and Sons, 1982:1-12. 2. Johansson RS, Westling G. Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects. Exp Brain Res 1984;56:550-64. 3. Brammer AJ, Taylor W, Lundborg G. Sensorineural stages of the hand-arm vibration syndrome. Scand J Work Environ Health (in press). 4. Taylor W, Ogston SA, Brammer AJ. A clinical assessment of seventy-eight cases of the hand-arm vibration syndrome. Scand J Work Environ Health 1986;12: 265-8. 5. Brammer AJ, Piercy JE, Auger PL. Assessment of impaired tactile sensation: A pilot study. Scand J Work Environ Health (in press). 6. Dellon AL. Evaluation of sensibility and re-education of sensation in the hand. Baltimore: Williams & Wilkins, 1981. 7. Gescheider GA. Psychophysics-method and theory. Hillsdale, NJ: Lawrence Erlbaum Associates, 1976: 28-32. 8. Marshall DP. Measurements of tactile spatial resolution of a fingertip [M.A.Sc. thesis]. Windsor, Ontario: University of Windsor, 1986. 9. Reif ZF, Marshall D, Brammer AJ, Piercy JE, Taylor W. Improved esthesiometer for measuring tactile per-
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ception in hands exposed to vibration . Proceedings of the 12th International Congress on Acoustics. Toronto: Canadian Acoustical Association, 1986:F2-5. 10. Lundborg G, Lie-Stenstrom A-K, Sollerman C, Stromberg T, Pyykko I. Digital vibrogram: A new diagnostic tool for sensory testing in compression neuropathy. J HAND SURG 1986;IIA:693-9. 11. Szabo RM , Gelberman RH , Williamson RV, Dellon AL, Yaru NC, Dimick MP. Vibratory sensory testing in acute peripheral nerve compression. J HAND SURG 1984;9A: 104-9. 12. LOfvenberg J, Johannson RS. Regional differences and interindividual variability in sensitivity to vibration in the glabrous skin of the human hand . Brain Res 1984;301: 65-72. 13 . Piercy JE, Brammer AJ . Development of vibrotactile measurement techniques for assessing mechanoreceptor performance at the fingertip . J Acoust Soc Am 1986; 80(suppl 1):S41. 14. Brammer AJ, Pyykko I. Vibration-induced neuropathy: Detection by nerve conduction . Scand J Work Environ Health (in press).
Tactile perception in hands exposed to vibration
15. Takeuchi T, Futatsuka M, Imanishi H, Yamada S. Pathological changes observed in the finger biopsy of patients with vibration-induced white finger. Scand J Work Environ Health 1986;12:280-3. 16. Vallbo AB, Johansson RS . Properties of cutaneous mechanoreceptors in the human hand related to touch sensation . Hum Neurobiol 1984;3:3-14. 17 . Mountcastle VB , Lamothe RH, Carli G. Detection thresholds for stimuli in humans and monkeys: Comparison with threshold events in mechanoreceptor afferent fibers innervating the monkey hand. J Neurophysiol 1972;35: 122-36. 18 . Johnson KO, Phillips JR . Tactile spatial resolution. I. Two-point discrimination, gap detection, grating resolution, and letter recognition . J Neurophysiol 1981;46: 1177-91. 19. Phillips JR, Johnson KO . Tactile resolution . II. Neural representation of bars, edges, and gratings in monkey primary afferents . J Neurophysiol 1981 ;46:1192-1203.
Carpal tunnel syndrome, pyridoxine, and the work place Carpal tunnel syndrome has become a major source of work-related impairment. Recent studies have suggested pyridoxine deficiency may be a cause of carpal tunnel syndrome, but all studies on this relationship have been flawed by lack of scientific design. Given the available evidence, pyridoxine is not likely to be a significant cause of carpal tunnel syndrome in the work place. Ergonomic measures are more likely to be effective than pyridoxine in reducing the incidence and cost of carpal tunnel syndrome in the work place. (J HAND SURG 1987;12A[2 Pt 2]:875-80.)
Peter C. Amadio, M.D., Rochester. Minn .
Carpal tunnel syndrome is a common clinical problem and one that is frequently identified in the work place. 1-12 Work impairment from carpal tunnel syndrome has become a significant problem, and many programs have been established to reduce the cost of carpal tunnel syndrome in the work environment. I3- 17
From the Department of Orthopedics. Mayo Clinic and Mayo Foundation, Rochester, Minn. Reprint requests: Peter C. Amadio. M.D., Department of Orthopedics, Mayo Clinic and Mayo Foundation, 200 First St., S.W., Rochester, MN 55905.
Recently, emphasis on the prevention of carpal tunnel syndrome in the work place has developed,18-21 and this article will address one aspect of that issue. By definition, carpal tunnel syndrome is a disorder that affects the median nerve at the wrist sufficient to cause impairment of nerve function. Causes of the decreased function are presumed to be associated with increased pressure and subsequent nerve ischemia within the rigid confines of the carpal tunnel. 22, 23 This may be due to either a decrease in the size of the carpal canal 24 or to an increase in the volume of the contents of that canal. 23 Carpal tunnel symptoms can be reliably reproduced in human volunteers by experiments that THE JOURNAL OF HAND SURGERY
875