In 1997, the National Institute of Occupational Safety and Health (NIOSH) published a systematic review of the more recent literature that purports to offer insights into the epidemiology of regional musculoskeletal disorders in the context of workplace exposures. Any such exercise commences with two strategies: one to select studies and one to synthesize the results. There is no set formulation for either of these strategies. Furthermore, the conclusions of any such review are determined to a large extent by the formulations chosen. For several musculoskeletal disorders covered in the document, I take strong issue with the formulations. But for Chapter 5c on "Hand-Arm Vibration Syndrome," or HAVS,1 my issue is far more with the inferences drawn than with the strategies that were employed to provide the basis for the inferences.
In Chapter 5c, the NIOSH reviewers single out 20 studies, all but one published after 1986, that they felt were adequate in design and detail to be informative. Based on their analysis, they assert that there is "strong evidence" that "high level exposure to hand-arm vibration" is associated with the "vascular symptoms" of HAVS. I will not quarrel with this conclusion. My reservations relate both to the definition of "high level" of vibration and to the seriousness of these "vascular symptoms." In this editorial, I will rely on the same studies to argue that there was precious little that was "high level" enough to provoke much morbidity. Furthermore, the symptoms of HAVS pale next to other regional musculoskeletal symptoms, which, in the aggregate, impugn the context of the work rather than its content.
A Very Brief Historiography of HAVS
I am not the first to argue so. In 1960 Pecora and his colleagues,2 writing for the US Public Health Service's Occupational Health Program, estimated that at least 2 million Americans were exposed to vibration by virtue of using "beating or rotating" tools. They surveyed the states, queried the medical departments of automobile and airplane manufacturers, visited 11 relevant plants employing between 1500 and 25000 workers, and gathered data from the Air Material Command of the US Air Force. They concluded that "Raynaud's phenomenon of occupational origin may not be completely eradicated but that it may have become an uncommon occupational disease approaching extinction in this country." Such optimism was no match for the convictions of the investigators and policy makers committed to the eradication. They argued that many workers are afflicted who feel no need to complain, know not to complain, or are thwarted from registering their complaints. Such arguments are no longer tenable.
But in the 1960s they carried the day. In fact, such was the conviction as to pathogenesis that by 1970, occupational Raynaud's could be designated "vibration white finger" (VWF) by the British Industrial Injuries Advisory Council. Given that the blanched finger is nearly always dysesthetic, it was subsequently argued that the sensorineural component might best be appreciated as a separate health effect and, further, that there may be other consequences of exposure to hand-transmitted vibration so that VWF was too restrictive a rubric. "Hand-arm vibration syndrome" was born at a workshop in London in 1983.3 However, I will restrict my comments in this editorial to the component of HAVS that is VWF because VWF is the focus of most of the relevant literature and because it was also the focus of the conclusion of the NIOSH reviewers.
The Challenge of VWF for Clinical Epidemiology
For over 20 years, there has been a concerted effort to define the hazard for VWF so that engineering standards could be instituted to preclude noxious levels of hand-transmitted vibration. To this day, inability to precisely define either the exposure or the health effect has stymied this effort. The NIOSH reviewers were seduced by the 13 of the 20 studies that attempted to objectify the health effect with at least a physical examination. Why? The hands of thousands of exposed and unexposed subjects were examined in these series; there were no specific abnormalities! One study4 of chain-saw workers and community controls in Gifu Prefecture, Japan, went so far as to bring in two dermatologists to quantify "sclerodactylia... by pinching the skin of the dorsa of fingers." The result is presented in Table 1. Whether this "sclerodactylia" is nothing more than the calluses and skin thickening that protects the hands of any of us who pursue forceful vocational or avocational manual activities, or not, the health effect on examination is indeed "slight."
Several investigations include specialized testing to try to discern abnormality: five performed cold provocation tests; three, nail compression tests; three, some form of neurophysiologic assessment; and one timed the recovery of perfusion in Allen's test. The last has unacceptable inter-observer reliability to be clinically or epidemiologically useful5 as would the nail compression test, I would predict, if it were ever tested. Cold provocation testing is controversial to this day. For patients with connective tissue disorders, it is neither necessary nor safe. For worker with putative VWF, certainly when the prevalence is low (<25%), cold provocation testing as practiced in the papers that seduced the NIOSH reviewers cannot be relied upon for case definition.6 As for sensory perceptions and electrodiagnostic neurologic testing, these reservations are even more strident, not just by me, but by leading investigators in the field,7 "The electrodiagnostic tests...can identify carpal tunnel syndrome reasonably well, whereas the sensitivity, specificity and predictive value of other tests...are low or unknown."
So, like it or not, the health effect at issue remains the recall of vasospastic episodes. Furthermore, there is still limited consensus, let alone analysis, as to the reliability of recall, the fashion in which it is to be elicited, and the basis for quantification.8 For example, should frequent episodes involving a single digit be considered more severe than less frequent episodes involving multiple digits? And if there have been no further episodes in, say, 2 years, is VWF still present?
The scale of symptoms for VWF that has been widely utilized by investigators in the field was drawn up at a workshop in Stockholm in 1986 (Table 2).9 VWF was defined as "attacks of well-demarcated local blanching and accompanying numbness of the affected parts of the finger skin triggered by exposure to environmental cold...with a distribution over the hands and fingers that agrees well with the strongest vibration exposure." This was to be distinguished from "constitutional Raynaud's, a tendency towards a feeling of cold and diffuse pallor (rather than localized blanching) in the fingers and toes (which) may simulate white fingers and give the false impression of an initial phase of Raynaud's phenomenon. Inclusion of a patient with such symptoms in Stage 1 of the scale will be confounding and should be avoided... no clear-cut white fingers will ever develop." In operation, this distinction regarding Stage 1 is blurred at best. Realize that in community-based surveys in varying locales, the prevalence of memorable vasospasm, akin to VWF Stage 1 or greater, varies between 4% and 20%.10 Furthermore, workers with Stage 4 are so rare that they are probably workers with systemic rheumatic diseases rather than VWF.
The challenge of defining VWF as a health effect for the sake of epidemiologic deductions is considerable. The challenge of defining the exposure, hand-transmitted vibration, is also considerable. Vibration is a vector quantity with periodicity; the tool moves from rest some distance, the displacement or amplitude, in a given time. Displacement/time is velocity. However, the velocity is not constant over the displacement; the time rate of change of velocity is acceleration, distance/time/time. The periodicity, or frequency, of the vibration is the time it takes to complete one cycle measured in cycles/second or hertz (Hz.) For a tuning fork, the measurements are straight-forward: the frequency is the pitch; the acceleration, the loudness. But tools vibrate in three axes, with accelerations and frequencies that are not fixed over time or along the geometry of the tool. It has proven a monumental engineering task to capture this complexity as some quantity. Often, particularly for impulse tools such as the air hammer, "transients" are difficult to factor. In practice, for each axis, at each grip site on the tool, acceleration is measured as a function of frequency by a small accelerometer mounted on the tool. By accumulating acceleration across the range of frequencies over time, a Fourier spectrum is computer-generated, from which the averaged acceleration over time, the root-mean-squared (or rms) acceleration, is derived. To reduce the spectrum to a convenient quantity, it is convention to denote the rms for the 1/3-octave-band, center-frequency-weighted acceleration. The frequency weighting harkens to old psychophysical data suggesting that the unpleasantness of touching a vibrating surface is independent of frequencies between 6.3 and 16 Hz and decreases with higher frequencies between 16 and 1500 Hz. Most hand-held tools vibrate with frequencies in the 30-60 Hz range, although exposures between 5 and 5000 Hz are relevant.11 At any given frequency, the higher the acceleration, the less the displacement. For most tools with high accelerations and high frequencies, displacement is minuscule, much less than 1 mm.
Even if one could reliably quantify these parameters, which are intrinsic to the tool, are they relevant to the myriad of uses to which the tool might be put or to the hand/handle interface? And how does one validate the history of exposure time?
The Precedent for Engineering Standards to Diminish the Risk for VWF
It is not possible to ban vibration exposure; society would grind to a halt. It is not even desirable to ban vibration, unless we want all music synthesized, for example. Certainly we should ban any physical insult that is directly traumatic, or even particularly uncomfortable. That's not the issue. Rather, how can we limit exposures that are only hazardous as they accumulate over great periods of time? To do so requires a dose/response curve based on sound epidemiology so that committees can set limits that will protect as many as is feasible. For VWF, no such curve exists today. Given the inherent limitations to the accuracy and validity of measures of either the health effect or the exposure, no such curve may ever exist.
Nonetheless, in the late 1980s, several professional organizations had the temerity to rise to this challenge. The resulting work products still have great influence, having been incorporated into the guidelines of most of the countries that have written such. The most influential document was written by the international Organization for Standardization, published in 1986, and designated ISO 5349.12 By relying on the methodology just discussed for measuring exposure, the following dose-response relationships were generated: the prevalence of finger blanching is directly proportional to the square of the vibration magnitude, the square of the lifetime exposure, and the daily vibration exposure. But the prevalence is inversely proportional to the square of the vibration frequency above 16 Hz. This means that if the acceleration is halved, the daily exposure can be increased by a factor of 4 and life-time exposure by a factor of 2. But if the frequency is halved, the daily exposure must be decreased by a factor of 4, or lifetime exposure by a factor of 2. These dose-response relationships were offered as guidance "to protect the majority of workers against serious health impairment and to assist in the development of hand-operated tools the use of which will reduce the risk of disorders...." However, the measures of health effect that are the basis for these relationships were cross-sectional surveys which were most remarkable for enormous variability in prevalence and latency of VWF from study to study, worksite to worksite, and task to task. ISO 5349 is an enormous leap of faith, ie, lots of assumptions and approximations and unjustifiable selection of data were necessary to generate the dose-response curves.
There is considerable pressure today to revisit these exercises so as to produce new engineering guidelines. Realize that the belief in the hazard and the pressure for regulations and any consensus regarding tolerability of risk are all politicized;13 such is the human predicament. Is the science of the past decade sufficiently informative so that another round of these exercises will prove less fatuous? The 20 studies chosen by the NIOSH reviewers, when clustered by industry (as follows) rather than by methodology as the NIOSH authors chose, are enlightening.
The Hazards of Vibration to Quarry Drillers and Stone Carvers
Professor Massimo Bovenzi, along with his colleagues in Northern Italy, has been a leader in investigating HAVS for 2 decades. Of his three articles chosen by the NIOSH reviewers, two relate to the prevalence of VWF in quarry drillers and stone carvers. Some of the quarries he studied have been producing ornamental stone for centuries. It follows that the surrounding communities are home to families whose men have long provided the necessary skilled labor. Such is the case for the Rapolano travertine quarry in Tuscany. Some of the work involves using tools that have been in use for much of the century and that produce considerable vibratory motion, which Bovenzi quantified according to ISO 5349. The rock drills and chipping hammers were found to generate some 10 times the weighted rms accelerations of the vertical grinders and hand cutters. Bovenzi et al undertook a cross-sectional survey14 of nearly all of the men using these tools (n = 76) contrasted with 60 otherwise comparable laborers whose work entailed no exposure to vibrating tools. Of the controls, 8.5% reported vasospasm. Of the 76 exposed quarrymen, 35.5% recalled vasospasm: 14 were Stage 1; 8, Stage 2; and 5, Stage 3. The prevalence of VWF was not influenced by the particular tools used or the magnitude of daily exposure. The latency was 10 years. There was a relationship between magnitude of exposure and Stage, but the dose-effect curve documented far less risk than was purported in the ISO 5349.
This was also the conclusion of a subsequent multi-site study involving 828 quarrymen.15 Of these, 258 were manual polishers and machine operators not exposed to hand-transmitted vibration; these 258 served as the referent group in this cross-sectional survey. The exposed stone workers had a mean daily exposure of 4.5 hours and a total mean duration of exposure of 17.4 of their 39.1 years. These exposed workers were divided into three categories, based on the measured magnitude of vibration of the tools they used; drillers and the stone carvers using chipping hammers were exposed to 4- to 5-fold greater weighted rms acceleration than were the group of stone carvers who used rotary grinders exclusively. The prevalences of symptoms of HAVS are presented in Table 3. The risk ratio for the stone carvers who used rotary grinders exclusively is slightly less than 4; for the drillers and the stone carvers using chipping hammers it is much more impressive at 15.1 and 12.8, respectively. Further analysis demonstrates a linear correlation between magnitude of daily exposure or duration of exposure and the log odds of risk. However, 2 decades or more of continuous exposure is required before these levels of risk are manifest. This is far less risk than had been predicted when the ISO 5349 was formulated.
The Hazards of Vibration to Lumberjacks Using Chain Saws
It is not surprising that a preponderance of the surveys in the NIOSH review involved lumberjacks. This population has long been identified as at high risk for VWF; the early estimates of prevalence ran as high as 90% and the early estimates of latency were part of the basis for the dose-response curves put forth in ISO 5349. The surveys chosen by NIOSH document far less risk, call for a major reappraisal of these dose-response curves, and offer insights as to the reasons for the diminished hazard.
NIOSH holds up a study of sawyers by Professor Bovenzi and his colleagues as an exemplar for the modern HAVS literature.16 True, great effort was made to quantify exposure and to move beyond the report of symptoms by including a measure of the recovery of digital blood pressure from cooling in the definition of the health effect. As already discussed, effort or not, such attempts to quantify exposure and health effect have serious inherent limitations. The study population included all of the active forestry workers in three Tuscan districts who had >400 hours of exposure and were otherwise well. Actual exposure proved highly variable both in duration and the fact that not all chain saws were modified to dampen vibration. The referent population was recruited from shipyard workers "engaged in manual work" that entailed no exposure to hand-transmitted vibration. The case definition required a history in the prior 2 years of cold provoked episodes of well-demarcated blanching. A history of vasospasm prior to occupational exposure was grounds for elimination as an instance of "primary Raynaud's disease." The crude risk for vasospasm is presented in Table 4. It is important to realize that the 2.6% prevalence of symptoms of VWF in this particular control population is peculiarly low. There is no ready explanation unless the interviewers applied particularly stringent criteria in scoring any response as positive. Regardless, this low prevalence in the controls will serve to inflate the odds ratios. Table 5 provides more illuminating descriptive statistics and some results of a logistic regression probing for a dose-response relationship between exposure and any degree of VWF. It is clear that there is little increment in risk for any symptoms of vasospasm until there has been considerable exposure-on the order of 4 hours per day of using a chain saw for a decade! ISO 5349 overestimated the risk by a factor of 4. The symptoms are not disabling, as "no claim for VWF" had been registered with the National Institute for Insurance of Occupational Accidents "by retired or migrated chain saw operators who had worked in the surveyed forest districts."
A cohort study from Finland reached a similar conclusion17: after allowing for age, there was no difference in the incidence of "finger blanching" between lumber-jacks with less than 15 years of exposure and peat workers not exposed to hand-arm vibration. After 25 years of exposure, the risk ratio was 6.8 (95% confidence interval, 3.0-16.8). Again, symptoms of blanching were not disabling, as they did not predict the 9% of lumberjacks who would change professions. There are corollary inferences to be drawn from a surveillance program carried out in the Suomussalmi parish in northeastern Finland between 1972 and 1990. In that program, cross-sectional surveys of foresters with >1500 hours of chain saw operating time in the 3 prior years were undertaken serially as part of compulsory health examinations.18 The prevalence of VWF symptoms diminished from 40% to 5% over this time. The incidence has declined so that only 3 new cases have been recorded since 1972 in a workforce of about 200 with modest turnover; 57 participated in every survey, 102 participated only once. The dramatic improvement was ascribed to the introduction in the 1970s of lighter, faster saws modified to dampen vibration. But don't conclude that this is now easy work: about 40% experience some degree of work incapacity and 15% are disabled by musculoskeletal symptoms-usually neck and back pain, rarely HAVS. Furthermore, this is hazardous work, possibly unconscionably hazardous work; in some settings, nearly all lumberjacks have sought care at some time for traumatic injuries.19
If the experiences with lumber-jacks in Tuscany and Finland are reassuring, that in Japan is even more so. The lumber industry is divided between state-owned and private forest preserves. In the 1970s, multiple surveys documented prevalences of VWF at 20% or higher in chain saw operators. Since 1978, the state-owned industry has been regulated so that no sawyer is older than 55 years of age or works more than 2 twice a day, 2 days continuously, 32 hours a month, or 120 days a year. The prevalence of any symptoms of VWF in sawyers in the state forest of the northern island of Hokkaido had fallen to 7.7% by 1984 and 11% in those exposed for greater than a decade.20 In the private forestries, not only are there no such stringent regulations on work practices, but remuneration is based on felling time, which encourages exposure. Surprisingly, the prevalence of any stage of VWF is similar to that in the state-owned forest workforce, 9.6%.21 What accounts for this dramatic reduction in VWF in Japanese sawyers since the mid-'70s? The improved work practices enforced in the state forests may be operating to some degree in the private sector as well. However, the authors point out that there are other aspects of work practices that have been improved in both settings, such as commuting in heated cars. What has not changed since the mid-'70s is the task of chain-sawing itself.
The Hazards of Pneumatic and Other Vibrating Tools in Industry
The remainder of the literature in the NIOSH review draws from exposures that generally are less homogeneous and more intermittent than quarrying and lumbering present. Several studies compare workers exposed in other settings with chain saw operators as well as with unexposed referents. Generally, the risk for VWF in the sawyers is as illustrated above, while the hazard for workers who assemble aircraft, work in shipyards, or operate sewing machines or dental drills is substantially less, approaching that of the prevalence of vasospastic symptoms in the unexposed referents.22-24 When the exposures are more finely dissected, the prevalence of VWF symptoms does correlate with the intensity and duration of particular exposure but also independently with age and often inversely with body mass index. Furthermore, the hazard, overwhelmingly, is for minimal symptoms, generally VWF Stage 1, even after decades of exposure.25 With rare exception,26 it is shown that ISO 5349 greatly overestimates the risk.
One study is particularly instructive. This was a cross-sectional survey comparing a sample of 118 male shipyard workers who use vibrating tools with 53 who don't for VWF defined by the Stockholm Scale.27 The measure of exposure was duration in years; the authors found that they had no choice. Of the 51 pneumatic tools in use, quantification of the rms acceleration was attempted for the six most commonly used tools by three different techniques. To the researchers' consternation, "... extreme variability between acceleration measures found on the same tools in different studies and even within studies..." compromised this quantification. Furthermore, the differences were accentuated if the measures were weighted or if the accelerometer was attached to the "anti-vibration" gloved hand rather than to the tool itself! The results of the survey were analyzed by polychotomous logistic regression to probe for a relation between duration of exposure and Stage, as well as for the effects of age, smoking status, and race. The model that used the log-transformed estimate of cumulative exposure produced the best fit with Stockholm Staging (Table 6). This cross-sectional survey is almost certainly rife with recall and reporting biases. After all, this was a troubled worksite at the time, embroiled in labor-management turmoil, as a result of which many workers had previously been evaluated by these authors for symptoms of HAVS.28 That may account for the exceptionally high prevalences of symptoms of VWF, including symptoms of higher stages of VWF. Nonetheless, the risk for experiencing VWF as a function of vibration exposure is not impressive and is of the same order of magnitude as the risk associated with smoking or working in awkward postures.
Putting the NIOSH Conclusion Into Perspective
As I said at the outset, I agree with the NIOSH reviewers that there is "strong evidence" that "high level exposure to hand-arm vibration" is associated with the "vascular symptoms" of HAVS. Few occupations require as concerted and consistent an exposure to hand-transmitted vibration as the work of the quarryman and the sawyer. I would argue that these are worst-case occupational exposures. I would further argue that the modern literature is reassuring; even such exposures present little hazard in terms of discomfort from VWF and none in terms of damage from VWF, even after decades on the job. But there are far more important lessons to be culled from this literature:
It may be possible to further reduce the risk for VWF. It can't be eliminated, since cold-induced digital vasospasm is a personal trait of a significant minority of people.10 But it might be possible to reduce it to this background prevalence. To do so with further engineering standards and regulations is illusory; such would require torturing data and lead to modifications that cannot be validated. New standards and modified tools can be studied-the science is a match for the testing. However, given the precedent in the literature, one would have to follow many workers for years and years to document any benefit-the science is no match for this reality. Besides, the ergonomic theory is no match for the possibility that modifications might be harmful.
That new engineering controls are fatuous and the risk-to-benefit ratio of any new tools or modifications of existing tools indeterminate does not mean that improvement cannot be sought. The literature offers guidance in that regard, but it is guidance that moves beyond the entrenched ergonometrics of the past. Most tasks that involve considerable hand-transmitted vibration are physically demanding, some exceedingly so. Certainly engineering initiatives should be encouraged to make these tasks easier and more comfortable. Certainly, some may be supplantable by new machines. But even more certainly, work practices need to be altered: rotations to less-demanding tasks, limitations in duration of exposure in terms of hours per week and years per worker, modifications in coincident physical exposures such as thermal challenge. The literature singled out by NIOSH impugns archaic work practices far more than exposure to vibration itself as associated with the recall of digital vasospasm.
The literature has another, strident message. Vibration is not the most important hazard to which these workers are exposed. In fact, it is minor, given the prevalence of VWF and the details of the morbidity of VWF. These are jobs that place the workers at risk for violent trauma,19 for lacerations and amputations and fractures. Let's not miss the forest for the trees!
And finally, what emerges from this literature, if you are prepared to grasp it, is the idea that the aging worker pursuing demanding tasks such as those with vibrating tools is more and more likely to recall and decry regional musculoskeletal morbidities such as back, neck, and extremity pain. These symptoms are far more their challenge than is the occasional vasospasm.18,29,30 These are symptoms that can be rendered less tolerable because of the demands of workers' tasks, because of the context in which they labor, because they fear redundancy, or all.31 Isn't it time for us to realize that there are jobs that may not provide lifelong pursuits and that preparation for such an eventuality is a hallmark of any enlightened labor-management contract?
Nortin M. Hadler, MD
Department of Medicine; School of Medicine; University of North Carolina at Chapel Hill; Chapel Hill, NC 27599-7280
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