In this study, the main factor affecting CPT in the fingers was vibration exposure. In all of the 12 CPT regression models, the backwards stepwise elimination procedure resulted in the inclusion of the daily vibration intensity variable as the key predictive factor. If this daily vibration intensity variable was excluded and the regression modeling was repeated, the key predictor became the duration of vibration exposure. In some of the models, in the left hand where vibration exposureis often less than in the right, proximal neuropathy variables were also found to be statistically significant. In these instances, median or ulnar CPT abnormalities were predicted by the corresponding median or ulnar proximal neuropathies. Proximal neuropathies, in particular median neuropathy at the wrist associated with carpal tunnel syndrome(CTS), commonly occur in workers exposed to hand-arm vibration (Pelmear and Taylor, 1994). As well, proximal neuropathy may be associated with CPT abnormalities in the fingers (Nishimura et aI, 2004). However our findings indicated that the sensory abnormalities measured by CPT in workers exposed to vibration were not due to confounding by proximal neuropathy. All of the fiber types measured by CPT were found to be affected by vibration exposure. The R2 values were highest for the CPT 2000 Hz dependent variable regression models, indicating that vibration had the greatest effect on large myelinated fibres, although all fibre types were affected. This is consistent with previous CPT studies that have shown that CPT thresholds at 2000 Hz are most predictive of the Stockholm sensor neural scale in workers exposed to hand-arm vibration (Kurozawa et al, 2001; House et al 2009). The results are also consistent with animal studies which have shown that acute high exposure of the rat tail to vibration is associated with an increase in CPT thresholds at 2000 Hz. (Krajnak et al, 2007). As well, chronic exposure of the rat tail to vibration has been found to be associated with ultrastructural changes in the myelin sheaths of large myelinated fibres (Chang et al, 1994), and biopsies of workers with HAVS have shown similar lesions (Takeuchi et al, 1988). In conclusion, despite the high prevalence of proximal neuropathy in workers exposed to hand-arm vibration, there is an effect on nerve fibre damage in the fingers due to hand-arm vibration that cannot be explained by common median or lunar neuropathies proximal to the hand.
Arm-injuries; Biological-factors; Biomechanics; Exposure-assessment; Hand-injuries; Laboratory-animals; Laboratory-testing; Mathematical-models; Nerve-fibers; Nervous-system; Neuromuscular-function; Physiological-effects; Physiological-response; Quantitative-analysis; Statistical-analysis; Vibration; Vibration-effects; Vibration-exposure
Engineering and Engineering & Control Technology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, 1095 Willowdale Road, Morgantown, West Virginia, USA