Abstract
The American Conference of Governmental Industrial Hygienists (ACGIH) proposed a method to assess the hand, wrist and forearm biomechanical overload based on exertions frequency (hand-activity level) and force use (normalized peak force). We applied the ACGIH threshold limit value (TLV)® method to a large occupational cohort to assess its ability to predict carpal tunnel syndrome (CTS) onset. A cohort of industrial and service workers was followed-up between 2000 and 2011. We investigated the incidence of CTS symptoms and CTS confirmed by nerve conduction studies (NCS). We then classified exposure with respect to action limit (AL) and TLV. Cox regression models including age, gender, body mass index, and presence of predisposing pathologies were conducted to estimate hazard ratios (HR) of CTS and population attributable fractions. We analyzed data from 3131 workers [females, N=2032 (65%); mean age at baseline 39.3, standard deviation (SD) 9.4 years]. We observed 431 incident cases of CTS symptoms in 8000 person-years and 126 cases of CTS confirmed by NCS in 8883 person-years. The ACGIH TLV® method predicted both CTS symptoms [HR between AL and TLV 2.18, 95% confidence interval (95% CI) 1.86-2.56; above TLV 2.07, 95% CI 1.52-2.81] and CTS confirmed by NCS (HR between AL and TLV 1.93, 95% CI 1.38-2.71; above TLV 1.95, 95% CI 1.27-3.00). About one third of CTS cases were attributable to exposure levels above the AL. The ACGIH TLV® method predicted the risk of CTS, but the dose-response was flat above the AL; a fine-tuning of the proposed thresholds should be considered.
Highlights
Carpal tunnel syndrome (CTS), determined by the compression of the median nerve within the carpal canal at the wrist, is broadly recognized as the most common mononeuropathy [1]
The hazard ratios (HR) of CTS symptoms and CTS confirmed by nerve conduction studies (NCS) showed a similar pattern in relation to the category of American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV)®: compared to subjects exposed below the action limit (AL), both workers with exposure levels between the AL and the TLV or above the TLV showed a two-fold increase in risk
Biomechanical exposure above the AL established by the ACGIH TLV® method was the most important contributing factor (54% of cases) when investigating CTS symptoms; the proportional contribution decreased dramatically when investigating CTS confirmed by NCS and this exposure was the main contributing factor only in 6% of subjects affected by CTS
Summary
Carpal tunnel syndrome (CTS), determined by the compression of the median nerve within the carpal canal at the wrist, is broadly recognized as the most common mononeuropathy [1]. It may adversely affect the quality of life of the patient (sleep disruption as the symptoms – tingling, burning and pain in the first three fingers of the hand – typically present during the night, and/or loss of sensitivity and force, which may affect manual dexterity) [2]. CTS incidence in the general population has been reported to be as high as 3 cases per 1000 person-years, and CTS surgery is very common as well (1 per 1000 person-years) [3, 4].
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