Neural manifestations
The increased prevalence of neurosensory symptoms in fingers among carpenters is well in line with the results from a previous questionnaire survey in the UK in 1997 of over 1000 men of working age [15]. Neural symptoms were found to dominate over vascular symptoms, which is also in agreement with previous studies [11, 34, 35]. Increased sensation of cold, which may be an important indication of vibration injury [36], was more common among carpenters than among painters. As many as 35% of the carpenters reported increased sensation of cold. This symptom has also been associated with medical conditions such as diabetes mellitus and rheumatic disease, but the difference remained after the exclusion of participants with these diseases. Thus, the difference is probably due to the use of vibrating tools. Cold sensitivity/cold intolerance/cold hypersensitivity is defined as abnormal aversion to cold with pain, sensory alterations, stiffness and/or colour change, but with no observable vasospasm. Pain in the fingers/hands when cold was reported among 24% of the carpenters, while the prevalence among painters (8%) was in agreement with that in the general population (5–15%) [34]. Concerning the clinical findings, we found no statistically significant differences between carpenter and painters, although the ORs were high for impaired 2PD and VPT in the carpenters. The difference between groups was greater at 250 Hz than at 125 Hz.
The ability to perceive vibrations, i.e. vibrotactile sense, is dependent on the function of cutaneous receptors and large-diameter (Aβ) afferent nerves. Some studies indicate that that larger myelinated nerves (Aβ) seem to be more vulnerable to compression than smaller myelinated nerve fibres (Aδ) and the small, unmyelinated (C) nerve fibres seem to be the most resistant to external stress [18, 37, 38]. Animal studies on vibrated rat tail have shown structural changes in blood vessels, reduced nerve fibre density and demyelisation of myelinated nerve fibres [39–41]. Nerve biopsies from vibration-exposed workers (with neural symptoms) have revealed structural changes and low myelinated nerve-fibre density [37, 42–44]. Epidemiological studies have also revealed correlations between thermal sensory impairment and cumulative exposure to vibration [45], where the perception of cold seems to be more affected than the perception of warmth [46]. In fact, the threshold for the perception of cold has been suggested as the best indicator of early neurosensory impairment since it showed greater sensitivity and specificity than the warm threshold and VPT in fingers with numbness or tingling [47]. However, in another longitudinal study on vibration-exposed workers, a low cumulative vibration dose did not significantly affect thermal perception thresholds, whereas age did [20].
In our study, impaired perception of touch and increased VPT were the two most common clinical findings among carpenters with neurosensory affection. In a study by Rolke et al., VPT was found to be the best method to capture neurosensory impairment in vibration-exposed workers, compared to controls [18]. The experts behind ICC suggest examination at 32 Hz and 125 Hz, but other studies have shown a greater increase in the threshold at 150 Hz than at 20 Hz among patients with HAVS [24]. In our study, VPT was more affected at 250 Hz than at 125 Hz. We therefore suggest that examination at 250 Hz be included in screening.
We found no difference in thermal perception between the carpenters and the painters, but impaired perception of cold was twice as prevalent as impaired perception of warmth among carpenters with neurosensory affection, when tested with the RollTempII® instrument. However, this method has not been validated, and these results should be interpreted with caution. Furthermore, the conduction velocity is much slower in the thin unmyelinated C fibres (which detect warmth) than in the myelinated Aδ fibres (which detect cold), which could have affected our results, leading to under-reporting of the true impairment of warmth perception.
The validity of 2PD has been questioned [48], but in the present study the prevalence of impaired 2PD was three times higher among carpenters than among painters, (although the difference was not statistically significant, due to few individuals), indicating that it may provide valuable information.
About one third of the carpenters fulfilled our criteria for neurosensory affection, which is of the same magnitude as reported previously for forestry workers [9]. Neurosensory affection was much more common among carpenters than among painters, which we attribute to the exposure of the carpenters to vibration. This difference was neither detected by the SWC nor the ICC score, although SWS stage ≥ 2 was more prevalent than the corresponding ICC score. These scales are based solely on the symptom of numbness, but the dominating symptom among carpenters was increased sensation of cold (67%), followed by numbness (55%), and then pain in fingers/hands when cold (45%). Therefore we choose to set a clinical condition that we called neurosensory affection including impairment/sensation in Aβ, Aδ, and/or C fibres. We did not include symptoms of impaired grip strength or neuro-vegetative effects (shaking, tremors and sweating), since it has been suggested that nerve fibres other than the cutaneous afferents are involved in these conditions [26]. Only a few individuals exhibited ICC or SWS neurosensory scores indicating stage 2 or higher. Yet half of the carpenters with neurosensory affection reported symptoms on a daily basis, and one fifth that they interfered with daily life activities
Both hand-intensive work and vibration exposure are risk factors for carpal tunnel syndrome [7, 49]. A prevalence of 7–35% has been reported among vibration-exposed workers in various epidemiological studies [11, 18, 50], with an elevated POR of 3.4 (1.4–8.3) among stoneworkers, compared to controls [51] [52]. In the clinical setting it is often difficult to distinguish neuropathy caused by compression in the carpal tunnel from more distally distributed neuropathy, as in vibration injury. Vibration perception thresholds have also been found to be increased in a group of subjects with carpal tunnel syndrome not exposed to vibration, compared to controls [53]. In our study, the prevalence of carpal tunnel syndrome was the same in both groups, and higher than expected for men in the general population [54, 55].
Vascular manifestations
The prevalence of white fingers among carpenters was 8%. As expected, it was higher than in among men in the general population [33]. This is in line with a nationwide study in UK, where 14% of carpenters reported white fingers [56], and with a Swedish cross sectional study, where 13% of construction workers reported white fingers [57]. In a cross sectional study on vibration-exposed stoneworkers the prevalence of white fingers was found to be 30%, compared to 4% in non-exposed controls [51]. We found no difference concerning white fingers between the carpenters and painters, which is surprising. We cannot rule out that some painters may have developed white fingers as a result of working with vibrating tools. There may also be some selection bias among the painters. To the best of our knowledge, the prevalence of white fingers among painters has not been studied previously.
Both carpenters and painters exhibited an elevated prevalence of carpal tunnel syndrome, which is risk factor for white fingers [58]. However, we found no correlation between fulfilling the criteria for carpal tunnel syndrome and reporting of white fingers.
One third of the carpenters and one tenth of the painters had noise-induced hearing loss. Other studies have shown an increased prevalence of white fingers among workers with hearing loss [59]. Although hearing loss was more common among the carpenters than the painters in the current study, we found no difference in the prevalence of white fingers between the groups.