Carpal tunnel syndrome


Epidemiology of carpal tunnel syndrome

Carpal tunnel syndrome (CTS) is one of the most common upper limb compression neuropathies  CTS account for approximately 90% of all entrapment neuropathies. It is due to an entrapment of the median nerve in the carpal tunnel at the wrist.

The surgical decompression rates for UK are 43 to 74 per 100,000 per year. The incidence and prevalence varies, 0.125% – 1% and 5 -16%, depending upon the criteria used for the diagnosis. It is a condition of middle-aged individuals and affects females more often than males.

It is one of the most widely recognised occupational health conditions; particularly in industries where work involves high force/pressure and the repetitive use of vibrating tools. Einhorn and Leddy estimated an incidence of 1% in the general population.

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Aetiology

There are two distinct varieties of CTS – acute and chronic. The acute form is relatively uncommon. This is most commonly associated with a fracture of the radius. It is also associated with burns, coagulopathy, local infection and injections.

The chronic form is much more common and symptoms can persist for months to years. However, in only 50% of cases is the cause identified, and can be divided into local, regional and systemic causes

Local causes

Inflammatory: e.g. tenosynovitis, histoplasma fungal infection, hypertrophic synovium

Trauma: e.g. Colles’ fracture, dislocation of one of the carpal bones

Tumours: e.g. Haemangioma, cyst, ganglion, lipoma, neuroma etc.

Anatomical anomalies: e.g. thickened transverse carpal ligament, bony abnormalities, abnormal muscle bellies.

Regional causes

Systemic causes

Carpal tunnel syndrome and occupation.

At risk occupations include, grinders, cashiers, and meat packers, workers sewing car seats, aircraft engineers, grocery store workers, and small part assembly liners.

The physical factors implicated and extensively studied in relation to occupational CTS include repetition, force, posture, external pressure, and vibration. Repetition is the most widely recognized risk factor for occupational CTS. In epidemiological studies high repetition is defined either by the frequency of the task or the percentage of time spent on repetitive work. A high repetitive job is defined as one which involves the repetitive use of awkward wrist movements lasting less than 30s or when more than 50% of work time is spent performing tasks that involve repetitive awkward wrist movements.

Experimental studies have shown a higher incidence of CTS in workers who are involved in high force and repetitive work compared to workers who are not.  Silverstein et al examined the association between high force / repetitive movements and CTS among 652 workers from 39 jobs from seven different industrial areas28. The authors noted a prevalence of 5.6% among workers in high force and high repetitive jobs compared to 0.6% among workers in low force and low repetitive jobs. The authors showed occupation to be a risk factor only when high force and high repetition are present, but the accuracy of their estimated ratio suffered from a small sample size. High repetitiveness seems to be a greater risk factor than high force but neither was statistically significant alone.

The most recent systematic literature review on the role of occupation in carpal tunnel syndrome by Palmer et al, found that the regular use of hand-held vibrating tools increased the risk of CTS by more than 2-fold. The authors also found substantial evidence for high risk of CTS in occupations requiring high repetitive flexion and extension at wrist and also forceful grip. However, the authors did not find evidence between the work on keyboard and computers and CTS.

Other studies show that occupational risk factors alone do not explain the occurrence of CTS and it is proposed that a combination of several factors is involved. The majority of CTS is attributable to patient related factors (intrinsic risk factors). Several studies have noted that the occurrence of CTS is correlated with unhealthy habits and lifestyle. This was supported by an analysis that showed that 81.5% of the explainable variation in electro-physiologically defined CTS was attributable to body mass index, age, and wrist depth to width ratio, whereas only 8.29% was due to job related factors.

In a series of 654 hands with CTS, Phalen did not observe any relation between CTS and occupation. Furthermore, he argued that occupational trauma is seldom the precipitating factor in the production of CTS. It is important to establish the nature of risk factor and the interaction between intrinsic and extrinsic factors. In a longitudinal study of predictors of CTS in industrial workers over a period of 17yrs, Nathan et al did not find an obvious relationship between the incidence of carpal tunnel syndrome and repetitive work. However, the authors noted high incidence of carpal tunnel syndrome in overweight people and in females.

PATHOPHYSIOLOGY

The exact pathogenesis of CTS is not clear. Several theories have been put forward to explain the symptoms and impaired nerve conduction studies. The most popular ones are mechanical compression, micro-vascular insufficiency, and vibration theories. According to mechanical compression theory, symptoms of CTS are due to compression of the median nerve in the carpal tunnel.

The diagnosis of CTS should be based on symptoms and signs and nerve conduction studies. Surgery is the only treatment that provides cure in moderate to severe cases.