Abstract
Occupational exposures to vibration come in many guises, and they are very common at a population level. It follows that an important minority of working-aged patients seen by medical services will have been exposed to this hazard of employment. Vibration can cause human health effects, which may manifest in the patients that rheumatologists see. In this chapter, we identify the health effects of relevance to them, and review their epidemiology, pathophysiology, clinical presentation, differential diagnosis and vocational and clinical management. On either side of this, we describe the nature and assessment of the hazard, the scale and common patterns of exposure to vibration in the community and the legal basis for controlling health risks, and we comment on the role of health surveillance in detecting early adverse effects and what can be done to prevent the rheumatic effects of vibration at work.
The nature and measurement of vibration at work
Vibration is an oscillatory motion, characterised by the frequency of the oscillatory cycle, its magnitude and its direction. Its potential to cause injury is believed to relate to the average intensity of energy imparted to tissues.
The magnitude of oscillatory motion can be quantified in terms of its maximum displacement or velocity, but it is normally expressed in terms of its acceleration, and time-averaged (the so-called ‘root-mean-square (r.m.s.) magnitude’). The frequency of motion is expressed in cycles per second (hertz). Biodynamic investigations have shown that the response of the body to vibration is frequency dependent, and to account for this, standards for exposure evaluation weight the frequencies of measured vibration according to the assumed effects at each frequency. Frequency weightings are applied to measurements taken in three axes at right angles to one another, sited at the boundary between the body and the vibration (e.g., using accelerometers mounted on the handle of a powered tool or the seat of a vibrating vehicle). ‘Dose’ of vibration is based on specific relationships between the duration of exposure and vibration magnitude defined in the International Organisation of Standardisation (ISO) standards , allowing the daily vibration exposure to be re-expressed in terms of the equivalent acceleration that would impart the same energy over an 8-h reference period (a notional average working day). This is called the A (8) (m/s 2 r.m.s.). Partial doses from several sources can be summed to an equivalent daily dose: inventories of sources, data on vibration magnitude from equipment handbooks or suppliers’ information sheets and online ready reckoner calculators supplied in the UK by the Health and Safety Executive (HSE) allow employers to estimate workers’ exposures relative to control and surveillance standards set in European law.
In practice, two forms of vibrations are distinguished: hand-transmitted vibration (HTV) from handheld powered tools, with potential impacts on the upper limb, and whole-body vibration (WBV) from vehicles, and sometimes platforms, with potential impacts on the spine. For each type of vibration, two exposure limits are specified in the UK and European legislation as follows:
- (1)
An exposure action value (EAV), representing the daily amount of exposure above which employers must act to control exposure. For HTV, this is an A (8) of 2.5 m/s 2 r.m.s., and for WBV, it is 0.5 m/s 2 r.m.s.
- (2)
An exposure limit value (ELV), representing the maximum amount an employee may be exposed to on any given day: 5 m/s 2 r.m.s. for HTV and 1.15 m/s 2 r.m.s. for WBV (Doses of WBV can also be expressed in other units, but for simplicity we ignore this in the present account.).
Health surveillance is required for workers who remain regularly exposed above the EAV. These values have been translated into guidelines based upon typical patterns of exposure. For example, the use of hammer-action tools for >1 h/day, or of some rotary-action tools for >2 h/day regularly is likely to exceed the ELV for HTV, and the EAV may be breached by as little as 15 min/day of exposure to certain hammer-action tools (Some employers employ a ‘traffic-light’ labelling system to identify tools with the worst characteristics.).
This summary of current approaches to risk assessment and control suggests a precise understanding of the exposure–response relationship, and a precise cut-off for safe practice. In fact, the ISO standard is predicated on the assumption that ∼10% of a population exposed at the EAV will still suffer vascular effects over a period of ∼12 years, and other formulae for summing the vibration dose accumulated over a lifetime have been found to approximate cross-sectional and longitudinal data on disease risks more closely than the official assessment standard. Similarly, the limit values for WBV have more to do with discomfort and tolerance than a well-described relationship between vibration dose and health effects on the spine.
Common sources of exposure
Exposure to HTV arises from many sources, including chainsaws, handheld grinders, concrete breakers, metal polishers, power hammers and chisels, needle scalers, scabblers, powered sanders, hammer drills and even powered lawnmowers and motorcycle handlebars ( Fig. 1 ). Exposures are particularly common in the construction industry and in heavy engineering, but significant exposures can arise in many occupations, including builders, metalworking and maintenance fitters, welders, foresters, shipbuilders, foundry workers and labourers. In one survey, it was estimated that about 1.2 million men and 40,000 women in Britain had weekly exposures high enough to justify health surveillance .
The same survey estimated that about 7.2 million men and 1.8 million women in Britain have occupational exposures to WBV on a weekly basis . Thus, WBV and HTV are among the most prevalent of occupational hazards. Common sources of occupational exposure to WBV include cars and vans driven professionally, forklift trucks, lorries, tractors, buses and loaders; exposures also arise among operators of many other vehicles and machines, including excavators, bulldozers, armoured and off-road vehicles . At-risk occupations range from travelling salesmen and delivery men through to bus and lorry drivers, farmers, soldiers, pilots, and police and emergency workers. In another British survey, 12% of men and 1% of women reported their job involving sitting or standing on a vibrating machine or vehicle, with higher prevalences in farming, forestry and road transport .
Common sources of exposure
Exposure to HTV arises from many sources, including chainsaws, handheld grinders, concrete breakers, metal polishers, power hammers and chisels, needle scalers, scabblers, powered sanders, hammer drills and even powered lawnmowers and motorcycle handlebars ( Fig. 1 ). Exposures are particularly common in the construction industry and in heavy engineering, but significant exposures can arise in many occupations, including builders, metalworking and maintenance fitters, welders, foresters, shipbuilders, foundry workers and labourers. In one survey, it was estimated that about 1.2 million men and 40,000 women in Britain had weekly exposures high enough to justify health surveillance .
The same survey estimated that about 7.2 million men and 1.8 million women in Britain have occupational exposures to WBV on a weekly basis . Thus, WBV and HTV are among the most prevalent of occupational hazards. Common sources of occupational exposure to WBV include cars and vans driven professionally, forklift trucks, lorries, tractors, buses and loaders; exposures also arise among operators of many other vehicles and machines, including excavators, bulldozers, armoured and off-road vehicles . At-risk occupations range from travelling salesmen and delivery men through to bus and lorry drivers, farmers, soldiers, pilots, and police and emergency workers. In another British survey, 12% of men and 1% of women reported their job involving sitting or standing on a vibrating machine or vehicle, with higher prevalences in farming, forestry and road transport .
Rheumatic effects of HTV on the upper limb
Some reported effects of HTV have a well-established occupational connection. These include secondary Raynaud’s phenomenon (RP) (vibration-induced white finger (VWF)), digital neuropathy and carpal tunnel syndrome (CTS). For other disorders, the evidence based on occupational causation is less extensive (e.g., Dupuytren’s contracture and tendonitis) or more contentious (e.g., osteoarthritis of the elbow and hand). The disorders of the upper limbs associated with HTV are collectively called the ‘hand-arm vibration syndrome’ (HAVS). In this account, we focus principally on VWF, sensorineural dysfunction and CTS, although other effects are described in passing.
Vibration-induced white finger
VWF, like RP from other causes, is characterised by episodic cold-induced finger blanching. Classically, there is a sharp demarcation between normal and affected skin, the latter becoming numb, cold and sometimes cyanotic with a bluish tinge, but fiery red and tingling in the recovery phase. As with RP more generally, non-classical patterns may also be seen (e.g., blanching affecting only the digit’s lateral or medial aspect rather than its circumference).
A point of distinction is that the disease, initially distal in its development, often comes to affect those areas most closely in contact with the vibrating parts of the worker’s tools. However, this is not a sufficiently reliable basis on which to distinguish VWF from primary RP. In practice, the diagnosis of VWF rests on a history of characteristic colour changes in the digits provoked by cold in a worker with a history of substantial occupational exposure to vibration, and exclusion of causes other than vibration. However, this approach has limitations. It assumes, for example, that every case of RP in an exposed worker is attributable to HTV (which would lead to an overestimation of attributable numbers). More limiting, however, is the subjectivity of the approach: attacks are rarely witnessed by a clinician; workers may have trouble in describing their symptoms and how they have developed; some may have a vested interest in concealing or exaggerating their symptoms, while simple tests of cold challenge (e.g., immersing the hands in cold water) can be painful and unreliable.
Effort has therefore been expended in developing objective assessment methods . Currently available methods for vascular disease include plethysmography, Doppler ultrasonography, direct capillaroscopy, skin temperature and skin re-warming rates after cold challenge and measurements of finger systolic blood pressure (FSBP) during cooling. The last of these has found the widest application. With the history of finger blanching as the reference standard, a ‘positive’ vascular test during finger cooling to 10 °C (FSBP ≥40% below that at 30 °C) has been reported to have a sensitivity of 74–99%, a specificity of 64–98%, a positive predictive value of 75–99% and a negative predictive value of 94–97% for the detection of abnormal cold responsiveness in the digital arteries of vibration-exposed workers .
At this point, an inherent circularity in reasoning can be mentioned. Objective tests have been developed because of the perceived inadequacy of the clinical history; however, their performance is usually gauged using clinical history as the reference standard. In reality, although their specificity can be assessed with confidence (using people with no relevant symptoms), their sensitivity can be gauged only partially (in people with witnessed finger blanching, a minority of those affected): the absence of a positive test in this last situation would denote imperfect sensitivity. Perhaps more usefully, objective tests can generate reference values in populations whose clinical histories are believed, with a view to applying these values in situations where the history may be disputed (e.g., medico-legal cases).
At present, there are limited resources for standardised testing nationally, and these are applied mostly for purposes of research or to help adjudicate such medico-legal disputes. However, some occupational health professionals use them to gauge severity and progression objectively. Another approach has been to adopt simple clinic-based tests of dysfunction and colour charts (picture sets of affected and unaffected digits, Fig. 2 ) to improve history taking. The colour chart method is simple to use and understand independently of language, quick to administer (only 2–3 min), cheap and capable of standardisation. It also performs well and has proved, in a longitudinal study of forestry and stone workers, to be a better predictor of digital arterial hyper-responsiveness to cold than medical history alone .
Estimates of the prevalence of VWF depend on the method of ascertainment and the populations studied. However, from one British population survey in 1997–1998, among almost 20,000 working-aged adults, it was estimated that there were >220,000 cases of VWF nationally with extensive blanching . This figure was considerably higher than had been previously estimated by the HSE using a less representative sample, but not incompatible with the 100,000 medico-legal claims processed among ex-miners from British Coal, many of whom were compensated on the basis of objective testing. High prevalences of VWF have been found in many occupational groups with substantial exposure to HTV ( Table 1 ) .
| Groups | N | White finger (%) | PR (95% CI) |
|---|---|---|---|
| Controls | 455 | 1.1 | 1.0 |
| Grinders | 100 | 9.0 | 8.1 (2.7–24.4) |
| Shipyard workers | 132 | 12.1 | 10.3 (3.8–28.4) |
| Caulkers | 65 | 23.1 | 18.6 (6.7–51.9) |
| Mechanics | 140 | 15.0 | 13.0 (4.9–34.7) |
| Foundry workers | 31 | 51.6 | 39.8 (14.3–111) |
| Construction workers | 148 | 7.4 | 6.1 (2.1–17.8) |
| Quarry drillers | 41 | 36.6 | 31.0 (11.2–85.9) |
| Forestry workers | 165 | 23.0 | 20.0 (7.8–51.2) |
The pathophysiological mechanisms underlying cold-induced RP in vibration-exposed workers are complex. The response may represent an exaggerated central sympathetic vasoconstrictor reflex, triggered by prolonged exposure to vibration, but local alterations in the digital vessels (e.g., thickening of the muscular wall, endothelial damage and functional receptor changes) may also play a role in disease pathogenesis , as may vasoactive substances, including endothelins, immunologic factors and changes in blood viscosity.
Sensorineural effects of HTV
Users of powered vibratory tools often experience tingling in their digits. Initially, symptoms are transient, and they disappear quickly after use. However, if exposures are high enough for a sufficient period of time, then symptoms may develop at other times, initially intermittently and thereafter in troublesome and prolonged spells that affect sleep. Transient and then permanent numbness is also common. In advanced stages, clinical examination may reveal abnormalities of light touch, temperature and pinprick, but before this the clinical approach lacks sensitivity and repeatability.
Electrophysiological tests indicate that a diffuse polyneuropathy of the digits and peripheral nerve entrapment can both arise. Objective methods of assessment include aesthesiometry (to measure two-point discrimination and depth perception), thermo-aesthesiometry and temperature probe tests (to detect thermal thresholds), vibrometry (which measures vibrotactile thresholds using a vibrating probe) and standardised tests of dexterity . Among simple office tests, Semmes-Weinstein’s monofilaments provide a non-invasive controlled reproducible force stimulus, for the evaluation of touch sensation at the palmar surface of the tip of the second and fifth fingers, to assess the median and ulnar nerves. Vibration-associated CTS is assessed in the usual method, by measuring motor and sensory median nerve conduction velocities and latencies.
Affected individuals often complain of clumsiness and impairment of fine finger movements and grip, and this may be a consequence of neuropathic and neuromuscular injury. Sensorineural and neuromuscular effects often coexist with vascular disease, although they can arise and progress independently.
The British National Survey of Vibration estimated that there were 300,000 cases of sensorineural HAVS in the UK , making this one of the more common occupational diseases at a population level. The impact of disease varies from minor and temporary discomfort to permanent incapacity.
Clinical grading and prognosis of HAVS
The vascular and neurological components of HAVS are graded separately according to two international scales, developed by a workshop in Stockholm , and advocated in the UK by the HSE and the Faculty of Occupational Medicine, London ( Tables 2 a and 2b ) . These scales are used for surveillance, research and medico-legal purposes, and especially to frame recommendations on career counselling.
| Stage a | Grade | Description |
|---|---|---|
| 0 | No attacks | |
| 1 | Mild | Occasional attacks affecting only the tips of one or more fingers |
| 2 | Moderate | Occasional attacks affecting distal and middle (rarely also proximal) phalanges of one or more fingers. |
| 3 | Severe | Frequent attacks affecting all phalanges of most fingers |
| 4 | Very severe | As in stage 3, with trophic skin changes in the fingertips |
a The staging is made separately for each hand. In the evaluation of the subject, the grade of the disorder is indicated by the stages of both hands and the number of affected fingers on each hand – for example: ‘2L(2)/1R(1)’, ‘–/3R(4)’, etc.
| Stage a | Symptoms |
|---|---|
| 0SN | Exposed to vibration but no symptoms |
| 1SN | Intermittent numbness, with or without tingling |
| 2SN | Intermittent or persistent numbness, reduced sensory perception |
| 3SN | Intermittent or persistent numbness, reduced tactile discrimination and/or manipulative dexterity |
a The sensorineural stage is established separately for each hand.
There is no established treatment for HAVS that is really satisfactory, although conservative measures (e.g., wearing woollen gloves and warm clothing, and avoidance of wet or draughty conditions) may alleviate symptoms. In lieu of effective therapy, screening, early detection and early withdrawal from exposure are the most important interventions. Additionally, by way of prevention in many cases, the industry has been able to substitute tools that interrupt the pathway of transmission of vibration (by isolation or vibration damping), to improve the maintenance of tools, to redesign them to avoid the need to grip high vibration parts and to restructure work or working patterns to reduce workers’ total exposures. Advice on these issues is available from the HSE .
Until the 1960s, VWF was thought to be irreversible, but studies have now shown that vascular symptoms can improve on withdrawal from exposure, albeit over many years. Workers with advanced disease are less likely to recover. By contrast, the neurological effects of HAVS do not improve with time, and loss of hand function is the main clinical end point to avoid.
Given the lack of treatment options, the poor prognosis in particular of neurological injury and the benefits of withdrawal from exposure, the HSE and the UK’s Faculty of Occupational Medicine have published several recommendations on counselling affected workers . A balance may need to be struck between protecting a worker’s health and limiting their earning opportunities. In some workers, disability will appear slight, and the rate of disease progression will be slow. Thus, advice tends to be titrated to the severity of disease and the rate of progression, and it should also consider the individual’s wishes, the time they are likely to remain in work, the scope to further limit exposures within the same job, the scope for redeployment to another job and the employer’s attitude to medico-legal risk. For those with mild stage 1 disease, work with vibratory tools is not ruled out, provided that health checks and counselling are ongoing, and proper consideration is given to the control of vibration at source; at the other extreme, those with advanced stage 3 disease should discontinue exposure altogether. For those in-between, the best course of action is more finely balanced. At present, most experts feel that the dividing line between an acceptable and an unacceptable outcome lies somewhere along the continuum between early and late stage 2 disease, the challenge being not to allow progression from the former to the latter.
Young workers, even if mildly affected, should be encouraged to explore options for alternative employment; otherwise, they may face many more years of exposure, and they have time in which to forge a different career.
Vibration-associated CTS
There is good evidence that HTV can increase the risk of CTS ( Table 3 ), although rather less information exists on the relationship of risk to levels of exposure. In some studies, a fifth to a quarter of workers complaining of persistent sensory symptoms in the digits/hands were found to have CTS .
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