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Occupational Medicine Advance Access published online on April 11, 2007
Occupational Medicine, doi:10.1093/occmed/kqm012
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© The Author 2007. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Nerve conduction studies and current perception thresholds in workers assessed for hand–arm vibration syndrome

Lina Lander1,2, Wendy Lou1 and Ron House1,3

1 Department of Public Health Sciences, University of Toronto, Toronto, Ontario, Canada
2 Department of Environmental Health, Harvard School of Public Health, 665 Huntington Avenue, Building I, Room 1418A, Boston, MA 02115, USA
3 Occupational and Environmental Health Clinic, St Michael's Hospital, Toronto, Ontario, Canada

Correspondence to: Ron House, Occupational and Environmental Health Clinic, St Michael's Hospital, 30 Bond Street, Toronto, Ontario M5B 1W8, Canada. Tel: +1 416 864 5074; Fax: +1 416 864 5421; e-mail: houser{at}smh.toronto.on.ca


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Background Workers exposed to hand–arm vibration are at risk of developing the neurological abnormalities of hand–arm vibration syndrome (HAVS). The Stockholm classification of the neurological component of HAVS is based on history and physical examination. There is a need to determine the association between neurological tests and the Stockholm scale.

Aims The main objective of this study was to compare the Stockholm neurological scale and the results of current perception threshold (CPT) tests and nerve conduction studies (NCS).

Methods Detailed physical examinations were done on 162 subjects referred for HAVS assessment at a specialist occupational health clinic. All subjects had NCS and measurement of CPT. The Stockholm neurological classification was carried out blinded to the results of these neurological tests and compared to the test results.

Results The nerve conduction results indicated that median and ulnar neuropathies proximal to the hand are common in workers being assessed for HAVS. Digital sensory neuropathy was found in only one worker. Neither the nerve conduction results nor the current perception results had a strong association with the Stockholm neurological scale. Exposure to vibration in total hours was the main variable associated with the Stockholm neurological scale [right hand: OR 1.30, 95% CI (1.10–1.54); left hand: OR 1.18, 95% CI (1.0–1.39)].

Conclusion Workers being assessed for HAVS should have nerve conduction testing to detect neuropathies proximal to the hand. Quantitative sensory tests such as current perception measurement are insufficient for diagnostic purposes but may have a role in screening workers exposed to vibration.

Keywords      Current perception; HAVS; nerve conduction; neurological; vibration


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Occupational exposure to hand–arm vibration is common in industrialized countries [1,2]. Such exposure has been associated with neurological, vascular and musculoskeletal abnormalities of the upper extremities that are collectively referred to as hand–arm vibration syndrome (HAVS) [3]. This paper will focus on the neurological component of HAVS.

The occurrence of HAVS is related to the duration of vibration exposure [4], types of work processes and types of tools used [5,6]. The initial neurological symptoms consist of numbness and tingling in the fingers and subsequently there is diminished sensation in the fingers and reduction in manipulative dexterity [7]. Early detection of sensory loss in the fingers is important because the recovery may depend on the severity of the neurological damage at the time of intervention [8].

The specific pathological mechanism of the neurological component of HAVS has not been established [9,10]. Injury to peripheral nerves may result in Wallerian degeneration, segmental demyelination, axonal atrophy and degeneration and primary disorders of cell bodies [11]. Finger biopsy specimens have revealed fibrosis, and proliferation of Schwann cells [12–14] and injury to sensory receptors may also occur [15].

The Stockholm scale used to describe the neurological component of HAVS was initially described by Brammer et al. [16] and is based on history and physical examination. According to this classification, sensory abnormalities on examination would occur in Stages 2 and 3. Therefore, if the Stockholm sensorineural scale (SSN) is to be used in a clinical context, it would be helpful if clinical tests could distinguish these stages from the milder stages.

Quantitative sensory tests (QSTs) measuring vibration and temperature perception have been used to assess a large volume of compensation claims for HAVS in the UK [17]. QST involves measurement of the sensory perception of vibration, electric current, temperature or pressure [15]. The test requires a subject to accurately report a physical sensation in the tested region, and hence is not entirely objective. The measurements depend on the subject's alertness, concentration, cooperation, pain and mood as well as other factors such as age and skin temperature [8,18]. The magnitude of reduced sensation tends to correlate with the severity of patients' symptoms, impairment in grip strength, sensitivity to cold, tendency to drop things as well as the presence of numbness and tingling [19–24]. In a recent review, however, Lundstrom [8] stated that the sensitivity, specificity and reliability of different QST methods for HAVS are still very much unknown. Vibration perception threshold and temperature perception threshold are often performed in field studies when screening workers for HAVS. Other QSTs, however, may be used for workers exposed to HAV such as current perception threshold (CPT). For example, Kurozawa and Nasu [25] reported that 59 male subjects with HAVS had significantly increased CPTs in both the index and little fingers in comparison to 20 male controls.

Nerve conduction studies (NCS) provide objective and quantitative assessment of peripheral nerve function independent of the subject's feedback [26]. They are considered the gold standard by neurologists assessing peripheral nerve damage. These studies, however, are insensitive to abnormalities in the small anatomic area of the fingertips and, thus, may be less sensitive than QST in detecting early effects of vibration. Temperature, age, anthropometric factors and gender affect nerve conduction results and should be considered during testing [27–31].

The objective of this study was to evaluate the relation between clinical tests for the assessment of the neurological component of HAVS and the SSN. In the setting in which the patients included in this study were seen, CPT and NCS were done on all patients assessed for HAVS. Therefore, these tests were compared to the SSN.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflicts of interest
 References
 
The study subjects were selected from 172 patients who had been assessed consecutively at the Occupational Health Clinic at St Michael's Hospital in Toronto, Ontario, for possible diagnosis of HAVS between 1995 and 1997. This period was chosen because all patients had the same assessment protocol with the results recorded on standardized forms. After 1997, the forms were changed making recruitment beyond this date more difficult.

Prior to clinical assessment, all patients completed a detailed questionnaire on history of exposure to vibrating tools. Hours of exposure for each tool used was calculated by multiplying the number of years worked by the number of days per year and hours per day. Total hours of exposure to vibrating tools was determined by adding hours of exposure for all tools that were used.

Detailed information on HAVS symptoms was collected including age of onset, frequency, duration and distribution of affected fingers on each hand. Physical examination was carried out in a standardized fashion in all subjects emphasizing the vascular, neurological and musculoskeletal systems. The neurological examination included assessment of sensation and two-point and depth esthesiometry in the upper extremities so that subsequent SSN scoring could be done. The history and physical examinations were done by one of three occupational medicine specialists using a standardized format.

Additional neurological tests consisted of NCS and CPT tests for both upper extremities. All the nerve conduction tests were carried out at St Michael's hospital under controlled conditions in a standardized fashion with measurement of motor and sensory latency, velocity and amplitude in the forearm, wrist and hand for the ulnar and median nerves. The CPT measurements were done using the Neurometer® CPT/C (Neurotron Incorporated) and the normal values established for the Neurometer [32]. The CPT test was chosen because it is a quick and reproducible QST method [33]. Current perception was measured at 5 Hz, 250 Hz and 2 kHz at the index finger for the median nerve and at the little finger for the ulnar nerve. An abnormality at any of the three frequencies resulted in the recording of an overall abnormal CPT result for that nerve. The CPT result at 2 kHz correlates best with vibration perception threshold and the CPT result at 5 Hz correlates best with temperature perception threshold [34].

All the clinical data had been recorded in a standardized fashion which facilitated subsequent chart abstraction. The initial chart abstraction and database creation occurred in 1999. A standardized SSN rating, however, was not provided during the initial clinical assessment. This information was added to the database in 2002 by one of the investigators (L.L.) after evaluation of each patient's chart. The SSN staging was based on the presence of symptoms (numbness and/or tingling) and the results of sensory examination of the hands. Ten subjects had to be removed from the original database because of incomplete documentation of the sensory examination results which prevented proper scoring of the SSN. Therefore, the final sample size for this study was 162 participants. The investigator carrying out the rating of the SSN was blinded to the results of the NCS and the CPT tests. The NCS and CPT results had been blinded to each other during the initial measurements.

Sensory abnormalities are present in Stages 2 and 3 of the SSN but it is clinically difficult to distinguish these stages. Therefore, in our study, Stages 2 and 3 were combined and referred to as Stage 2+. This Stage 2+ included any subjects with numbness and/or tingling and reduced sensory perception in at least one finger as demonstrated by light touch perception, depth-sense discrimination or two-point discrimination measured by esthesiometry.

All data analyses were performed using Statistical Analysis Software (SAS V8, SAS Institute, Cary, NC, USA). Chi-square ({chi}2) tests were used to test associations of categorical variables. t-tests or analysis of variance were used for continuous variables. Logistic regression was used in multivariate modeling to determine the variables that were associated with SSN.

This study was approved by the University of Toronto and the St Michael's Hospital Human Subjects Review committees.


   Results
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 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflicts of interest
 References
 
This cross-sectional study included 162 subjects. Several characteristics of the participating subjects are summarized in Table 1. The workers came mainly from the mining, construction and automotive industries and had been referred for assessment from various parts of Ontario.


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  		Table 1. Study cohort characteristics

 
In this sample, almost all subjects (160 or 99%) complained of numbness and/or tingling. The mean age of onset of these symptoms was 38.4 years (SD 9.0) and the mean duration was 9.0 years (SD 6.5). The age of onset and duration of numbness/tingling and blanching were similar.

SSN staging was assigned for each hand based on the presence of symptoms and sensory testing during initial physical examination. Eight subjects were assigned Stage 0 for both hands including six subjects who complained of transient tingling only during episodes of cold exposure and digital blanching. The majority of subjects were classified as Stage 1 (right hand 124, left hand 128). Stage 2+ classification was assigned to 30 and 26 subjects for the right and left hands, respectively. Substantial agreement ({kappa} = 0.57, 95% CI 0.40–0.74) was found between the SSN for the right and left hands. The association between the SSN and the Stockholm vascular scale (SSV) was highly significant (P < 0.0001) for each hand.

The most prevalent abnormalities found on nerve conduction testing were carpal tunnel syndrome (CTS) and ulnar neuropathy proximal to the hand. CTS was defined as median neuropathy at the wrist on nerve conduction in a worker with hand numbness in a distribution that included the median nerve. CTS was found on the right side in 54 subjects (33%) and on the left side in 35 subjects (22%). Ulnar neuropathy was found in 17 subjects (11%) on the right side and in 14 subjects (9%) on the left side. Digital neuropathy was found in only one subject, with both hands being affected.

The results of NCS were not significantly associated with the SSN except for borderline significance for the ulnar abnormalities in the right hand ({chi}2 = 5.82, P = 0.055). In comparison to the SSN, the specificity (right 63%, left 75%) of the overall NCS results (any abnormalities in the ulnar or median nerve) was higher than the sensitivity in each hand (right 53%, left 35%).

CPT results were abnormal in the right hand overall in 119 subjects (median nerve 108, ulnar nerve 78). CPT results in the left hand were abnormal in 99 subjects (median nerve 84, ulnar nerve 61). There were no statistically significant associations between the CPT results (overall, median and ulnar) and SSN for the right hand. In the left hand, the overall CPT results ({chi}2 = 9.87, P = 0.007) and the results for the ulnar nerve ({chi}2 = 11.27, P = 0.004) were significantly associated with the SSN staging. In comparison to the SSN, the sensitivity of overall CPT abnormalities was higher for each hand (right 80%, left 89%) than the specificity (right 28%, left 44%).

The CPT and NCS results were significantly associated for each of the ulnar, median and overall nerve results in the right hand and the left hand (P < 0.0001 for each of the six comparisons).

Logistic regression was carried out to determine which variables were associated with differences in the SSN. Ordinary logistic regression was used for a dichotomized SSN where Stages 0 and 1 were combined and compared to Stage 2+. Stages 0 and 1 were combined because of the small numbers in Stage 0. Also this division was clinically relevant because there are abnormalities in the sensory examination of the hand in Stage 2 and above, whereas Stages 0 and 1 have no such abnormalities. The following variables were included in the final logistic regression models: age, exposure (total hours), smoking, duration of numbness and tingling, SSV and CPT results (median and ulnar). These variables were selected based on the univariate analyses and clinical significance. Total hours of exposure was treated as a categorical variable with 5000 exposure hour increments between categories. The results are summarized in Table 2 for all observations and after stratification for nerve conduction abnormalities. This stratified analysis was done because the NCS results were strongly associated with the CPT results and were also associated with the SSN (right hand, ulnar nerve).


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  		Table 2. Odds ratios of being classified as Stage 2 of the SSN as a function of various predictor variables

 
The analysis for all observations indicated that total hours of exposure to vibrating tools and age were significant predictors of the SSN for the right hand. CPT for the ulnar nerve and total hours of exposure were significant predictors of the SSN for the left hand. The stratified analysis indicated that for subjects with abnormal NCS, age, exposure (total hours) and CPT results for the ulnar nerve were significant predictors of the SSN for the right hand. These predictors were not significant for the left hand in subjects with abnormal nerve conduction. In subjects with normal NCS results, exposure in total hours was the only significant predictor of the SSN for the right hand and none of the variables were significant predictors of the SSN for the left hand.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflicts of interest
 References
 
Our study found median and ulnar neuropathies proximal to the hand to be common in workers being assessed for HAVS whereas we found virtually no cases of digital sensory neuropathy. Neither the nerve conduction results nor the current perception results had a strong association with the Stockholm neurological scale.

The Stockholm scale for the neurological component of HAVS is based on the history of numbness and abnormalities in the measurement of sensation on physical examination. The underlying pathology is assumed to be digital polyneuropathy [16]. However, the results of our study suggest that the predominant neurological abnormalities that can be measured by nerve conduction in workers using vibrating tools are neuropathies of the ulnar and median nerve proximal to the hand. Indeed, only one subject in our study had digital neuropathy as measured by careful nerve conduction testing. In contrast, the prevalences of CTS (right hand 33%, left hand 22%) and ulnar neuropathy (right hand 11%, left hand 9%) were much higher.

The NCS and CPT results were not very predictive of the SSN. The univariate analysis indicated that the NCS results were not significantly associated with the SSN except for the ulnar abnormalities in the right hand. The univariate analysis for the CPT results indicated a statistically significant association only for the ulnar nerve in the left hand. This finding, however, may reflect a problem related to the SSN which is based only on history and clinical sensory examination rather than the lack of predictive validity of the CPT and the NCS. In the multivariate analysis, the only variable aside from age and total hours of exposure to be significantly associated with the SSN was the CPT in the ulnar nerve. In fact, the highest odds ratio in any of the models was for the CPT in the ulnar nerve.

The study was done in a large specialist Occupational Health Clinic with access to good neurological testing methods, in particular nerve conduction testing. Considerable care was taken to ensure blinding to the neurological tests in the determination of the Stockholm scale. The data, therefore, were of good quality and the sample size was reasonably large. The main weakness was including only vibration-exposed workers with various stages of HAVS. It would have been useful to include a group of manual workers not exposed to vibration. In addition, the study group was predominantly (96%) male.

The results of the nerve conduction tests in our study raise the question of whether CTS is related to vibration exposure. The work relatedness of CTS is somewhat controversial, although recent reviews suggest that ergonomic factors and vibration exposure may both play a role in its development [34–36]. In a cross-sectional study comparing 100 furniture workers using orbital sanders and 100 office worker controls, Bovenzi et al. [37] found a 19% prevalence of CTS in the furniture workers and only 8% in the controls. Multivariate regression analysis indicated that both the vibration exposure and the ergonomic strain index were significantly associated with the increased prevalence of CTS.

There is also human pathology evidence of vibration-induced nerve damage just proximal to the wrist. Stromberg et al. [38] reported structural abnormalities including breakdown of myelin and interstitial and perineural fibrosis in biopsies of the dorsal interosseous nerve just proximal to the wrist in all 10 men who had been exposed to vibration with all but one of the 12 necropsy controls being normal. CTS frequently occurs in association with HAVS and the clinical discrimination of CTS from the diffuse sensorineural impairment in HAVS is difficult [39].

These CPT findings in conjunction with the NCS results suggest the possible presence of two types of lesions—one in the fingertip receptors (identified by CPT) and the other occurring more proximally in the median and/or ulnar nerves (identified by NCS). In this respect, our findings are similar to those of Stromberg et al. [40]. They found that NCS in the median nerve but not the ulnar were abnormal in vibration-exposed workers in comparison to controls but the NCS did not differ significantly between the sensorineural stages of HAVS. In contrast, vibration perception threshold but not temperature perception threshold was significantly elevated in the highest SSN stage in comparison to the other stages.

The American Academy of Neurology cautions against the use of QSTs as the sole basis for diagnosis [41]. The overall CPT test results in our study had reasonable sensitivity (right hand 80%, left hand 89%) to detect Stage 2+ of the SSN although the low specificities indicate that many false-positive results would be expected to occur. These results suggest that the CPT test would be more useful as a screening test than as a diagnostic test.

In our study, NCS and CPT results were significantly associated for the overall results and for the ulnar and median results in each hand. Therefore, if only QSTs are done in vibration-exposed workers, the abnormalities might be attributed to vibration-induced digital polyneuropathy rather than the more prevalent proximal neuropathies of the median and ulnar nerves. Our results highlight the importance of doing careful nerve conduction testing in workers being assessed for possible neurological problems related to the use of vibrating tools.

Key points

  • CTS and ulnar neuropathy are common in workers using vibrating tools.
  • Workers being assessed for HAVS require NCS.
  • The current perception tests suggest that there may be a second more distal type of lesion in the sensory receptors and/or very distal nerve fibers in the fingers of workers with HAVS.


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
References
 
None declared.


   References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 

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Re: Nerve conduction studies (NCS) and current perception thresholds (CPT) in workers assessed for hand-arm vibration syndrome (HAVS)
Occup. Med., March 1, 2008; 58(2): 147 - 147.
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