1992). Diabetes is well known to be a risk factor for CTS and other compression mononeuropathies (Albers et al. 1996). Stevens et al. (1992) calculated a standardized morbidity ratio for rheumatoid arthritis (3.6), for diabetes (2.3) and for pregnancy (2.5). Thyroid disease and kidney disease also have many connective tissue side effects placing the individual at higher risk for nerve injuries; thyroid disease may also lead to muscle disease. Systemic disease causes the nerves to be more susceptible to compression and ischemia. The biologic plausibility of this association is high and the association is very strong, but these disorders affect a small percentage of active workers. Atcheson et al. (1998) suggest that these disorders are more common among workers diagnosed with CTS compared to other CTDs and may be under recognized in the industrial setting. The studies reviewed in this area use a methodology based upon population based data or large cross-sectional data. There is little bias associated with sample selection and the statistics are appropriate for the sample.
In 1973, Ellis and Presley suggested an association between vitamin B6 deficiency and CTS. Over the next 2 decades, several additional reports appeared which suggest that this association is causal in many cases. The impact of these studies on physician understanding and treatment of CTS is substantial. Vitamin deficiency is mentioned in a major textbook of occupational medicine (Keyserling & Armstrong 1992) as a possible CTS risk factor, implying that such deficiency contributes to CTS among workers.
Unfortunately, the studies which demonstrate an association between vitamin B6 status and CTS usually include small numbers of non-randomly selected subjects, frequently rely on non-standard or entirely subjective measures of outcome, and occasionally suffer from serious design flaws. Recent prospective and population based studies have not borne out this relationship (Folker et al. 1978; McCann & Davis 1978; Ellis et al. 1979, 1981, 1982; Amadio 1985; Franzblau et al. 1996). The recent population based studies and large cross-sectional studies are without the selection bias of earlier studies and use appropriate statistical analysis. The recent study by Kensinton et al. (1998) suggesting a relationship between vitamin B6 deficiency, vitamin C, and CTS (among women but not men) has methodological as well as statistical flaws (Franzblau et al. 1998).
The biologic plausibility is moderate. However, the strength of the relationship is weak except in severely vitamin B6 deficiency (and a severe B6 deficiency is rare). The cross-sectional studies of active workers and population based studies are sound enough to say that there is not a significant relationship between B6 levels and carpal tunnel syndrome.
Pregnancy is considered an independent risk factor (estimated RR of 2.5) for the development of CTS due to increased vasculature and interstitial fluids (Soferman et al. 1964; Gould & Wissinger 1978; Masey et al. 1978; Stevens et al. 1992). These studies have adequate sample size and statistical analysis. This is a strong association with strong biologic plausibility. Fortunately this condition is a time limited and there is usually resolution of symptoms at the end of the pregnancy or shortly thereafter.
Both the use of oral contraceptives and gynecologic surgery have been hypothesized as