Occupation and upper limb disorders

D. Coggon, K. T. Palmer and K. Walker-Bone

MRC Environmental Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.

SIR, Pain in the upper limb is a common symptom in people of working age, with prevalence rates of 5–20% in community-based surveys [1, 2]. In most cases, the pain is not associated with any important limitation of activities, but sometimes it severely impairs people's capacity to work. In Britain, a national survey in 1995 suggested that musculoskeletal disorders of the upper limb and neck were responsible for the loss of at least 3.8 million working days in a 12-month period [3]. The symptom can arise from various pathologies in the neck, shoulder and arm, including cervical spondylosis, capsulitis of the shoulder, epicondylitis, de Quervain's disease and carpal tunnel syndrome. In addition, many patients do not exhibit diagnostic features of any of these disorders, and are classed as suffering from ‘non-specific upper limb pain’.

Painful disorders of the upper limb are often attributed to physical activities, especially in the workplace, and they are sometimes labelled as ‘repetitive strain injury’, ‘cumulative trauma disorders’ or ‘work-related upper limb disorders’. However, these diagnostic terms are unsatisfactory because the link between activity and disease cannot usually be established with confidence in the individual case. For example, although there is consistent evidence that carpal tunnel syndrome is more common in people whose work involves the use of hand-held vibratory tools such as chipping hammers and grinders [4], the increase in risk in some exposed populations has been less than threefold. In this circumstance, for every three cases that occur in exposed individuals, at least one would be expected even in the absence of exposure. It follows that when the disorder occurs in such a person, we cannot be sure that it has resulted from the work. Where relative risks are smaller, the doubt is greater. In investigating and managing upper limb pain, therefore, it is best if the diagnostic classification is based only on clinical features, and not on the presumed cause.

The prevention of painful upper limb disorders is currently one of the major challenges in occupational health practice. At present most emphasis is placed on optimizing ergonomic aspects of work, such as the design of tools and equipment, the use of task rotation to reduce repetitive use of the same muscles and tendons, and the provision of frequent breaks for rest where rotation of tasks is not possible [5]. The basis for this approach lies in observational epidemiology (the disorders have often been found to occur more frequently where work is repetitive and involves awkward postures) and ergonomic theory (repeated movements and unnatural postures place excessive stress on tissues). However, there is little direct evidence from controlled intervention studies on the extent to which these measures reduce the occurrence of disease. Moreover, there are strong indications that the pathogenesis of upper limb symptoms and associated disability is much more complex, and also depends importantly on psychosocial and cultural influences.

One of the most remarkable features of the epidemiology of upper limb disorders is the epidemic that occurred in Australia during the early 1980s. During 1984–85 in South Australia, the number of successful compensation claims by women for soft tissue disorders of the upper limb ascribed to repetitive movement was five times higher than that in 1980–81 [6]; and among keyboard workers employed by one large company, Telecom Australia, the 5-yr cumulative incidence of ‘repetition strain injury’ between 1981 and 1985 was as high as 343 per 1000 [7]. However, by the 1990s the compensation rate in Australia had declined to a much lower level.

There is no evidence that ergonomic practices in Australia during the early 1980s were substantially different from those in other developed countries where no parallel epidemic occurred, and it has been proposed that the phenomenon was driven more by the rules that governed compensation for occupational injuries in Australia at that time, which were particularly favourable to claimants [8]. However, it is also possible that the high incidence among many workforces encouraged a greater awareness of symptoms, and perhaps reinforced symptoms that otherwise would have been ignored [8, 9]. Similar transient epidemics have been observed in other countries at times when compensation was not an issue. For example, there was an outbreak of writer's cramp among British civil servants in 1830 [10]. Whatever the explanation, the Australian epidemic suggests that non-ergonomic factors can have a major influence on reported rates of disability from upper limb disorders.

It is notable also that the problem of upper limb disorders has come to prominence at a time when the physical demands of work have generally been decreasing. High rates have been reported in some manual occupations, for example, meat cutters and packers [11], but large numbers of cases also occur in office workers using keyboards. It is understandable that minor symptoms might have received less attention historically when other, more serious, occupational diseases were more common, but it seems unlikely that a high incidence of disability sufficient to prevent work would have been missed or incorrectly attributed to other types of illness.

One explanation for this paradox could be that more forceful movements of the arm do not necessarily carry the highest risk. Thus, Harris has postulated that non-specific upper limb pain may result from apparent incongruence between motor intention and proprioceptive feedback, particularly in activities such as keyboard work, in which finger movements are of low amplitude and visual feedback is limited [12]. Alternatively, the trend over time may reflect changes in culture and expectations rather than a shift to more hazardous work.

More direct evidence for psychosocial influences on painful upper limb disorders comes from epidemiological studies that have assessed the relation of symptoms and disability to depression and psychological stressors in the workplace [13, 14]. However, most of these have been cross-sectional or retrospective in design, and it is not always clear whether the psychological symptoms preceded or were a consequence of those in the upper limb.

If psychosocial influences are as important as some of the above observations suggest, the optimal approach to the prevention of occupational upper limb disorders may not lie simply in improved ergonomics. Indeed, it is possible that by placing strong emphasis on ergonomics we create a culture in which workers believe that they are at high risk, and that this perception of itself generates disease.

Similar arguments apply to the management of patients who have developed upper limb disorders. In addition to treatments such as physiotherapy, local injections and surgery, advice is often given to rest the arm. However, it is unclear whether restriction of use is always the best approach. For many years, rest was prescribed for low back pain, but evidence has now emerged that the outcome is better if patients remain active within the limits imposed by their symptoms [15]. It is possible that strong emphasis on reducing activities, particularly in patients with non-specific upper limb pain, may reinforce perceptions of injury and encourage long-term disability.

Future research on occupational upper limb disorders should focus on resolution of the major uncertainties that remain in relation to their prevention and management. One immediate requirement is for a satisfactory diagnostic classification that can be applied in epidemiological investigations. A start was made at a meeting sponsored by the UK Health and Safety Executive in 1996 [16]. This defined diagnostic criteria for eight specific disorders, which have since been refined and tested [17]. However, there are important unresolved issues. For example, it is unclear whether, in the absence of diagnostic imaging, a valid distinction can be made clinically between shoulder capsulitis and tendonitis of the rotator cuff. Also, the classification does not cover all relevant diagnoses. Notable omissions include acromioclavicular joint dysfunction, and subacromial and olecranon bursitis.

Once valid and repeatable diagnostic criteria have been defined, it will be possible to explore the occupational and psychosocial determinants of each category of upper limb disorder. For this purpose, longitudinal investigations will in general be more informative than cross-sectional surveys or case–control studies, particularly for the assessment of psychosocial risk factors. In addition, there is an urgent need for controlled intervention studies to assess the impact of ergonomic modifications of the type that are commonly recommended for the prevention of upper limb disorders.

Another unresolved issue that demands research is the pathogenesis of non-specific upper limb pain. If this were better understood, it might be possible to devise more objective diagnostic markers. Also, its potential causes might become clearer, particularly whether repetitive physical activities are likely to have a direct causal role. In addition to incongruence between motor intention and proprioceptive feedback [12], other proposed mechanisms include minor degrees of median nerve compression that fall short of overt carpal tunnel syndrome [18], and reduced pain tolerance as a consequence of functional changes in the central nervous system [19].

Finally, there is a need for randomized controlled trials to assess the benefits of restricting activities in patients with different types of upper limb disorder, particularly in those with non-specific upper limb pain. As a prelude to such research, better information on the natural history of such disorders would be valuable. In particular, we need to know the major determinants of prognosis at the time patients present so that these variables can be taken into account in the design and analysis of trials.

Notes

Correspondence to: D. Coggon. Back

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