Carpal tunnel syndrome (CTS) is a condition that typically results in some combination of numbness, tingling, pain, and weakness in the affected hand. It occurs secondary to damage or irritation of the median nerve at the wrist [Gelberman et al., 1981], and therefore is a common work-related condition in manual labor occupations requiring repetitive use of the hands [Frost et al., 1998]. It is estimated to affect 2.7% of the general population, results in $500 million in healthcare costs in the United States yearly, and is a leading cause of workers’ compensation claims [Atroshi et al., 1999, Stevens et al.,1988, Herbert et al.,1999]. The Bureau of Labor statistics reports that the incidence of CTS among workers in the manufacturing industry is 2.4/10,000 [BLS, 2011]. Although significant, the incidence rate may be underestimated by 40% or more [Leigh, 2012]. Workers often do not report musculoskeletal disorders, and physicians are often not trained to identify musculoskeletal disorders as occupational disorders, therefore contributing to underreporting of these conditions [Azaroff et al.,, 2002].

Poultry processing, one segment of the manufacturing industry, typically requires repetitive hand movements to hang, kill, pluck, clean, eviscerate, cut, package, and box poultry at a rapid pace. Workers also frequently clean and repair equipment, assemble boxes, and move heavy pallets [U.S. Department of Labor, 2001; Fink, 1998]. Throughout the United States many poultry processing workers are now immigrant Latinos [Fink, 1998], and this group of workers faces unique challenges because of language and cultural barriers and reluctance to complain about work conditions [Marín et al., 2009a; Marín et al., 2009b]. It has recently been demonstrated that Latino poultry processing workers have a high prevalence of CTS, which exceeds the prevalence of CTS in other manual laborers and affects between 6.5 and 59.2% of poultry processing workers, depending on the methods used to define CTS [Cartwright et al., 2009b]. However, there are no data on incidence of CTS in this population.

Latino workers represent 15% of the United States workforce, [US. Dept. of Labor, 2012] yet they have higher than average injury and fatality rates [Smith, 2012]. Latino workers, especially those who are foreign born, are concentrated in occupations such as manufacturing with the highest prevalence of labor law violations [Pinedo et al., 2011]. While it is widely agreed in the literature that reported injury rates do not reflect real injury rates due to underreporting [Smith, 2012], underreporting is especially problematic among Latino workers. These workers belong to a vulnerable population, which is often hidden and afraid to report their injuries because they fear retaliation [Smith, 2012]. Furthermore, many of the new settlement areas where Latinos have migrated in the past two decades and where the majority of poultry production takes place are located in the southern United States [Passel et al., 2010; National Chicken Council, 2010]. Most southern states have statutes that are unfavorable to collective bargaining and unions do not have a strong presence, further contributing to the underreporting of injuries. Because these workers are hard to reach, and there is often no access for outside groups to conduct occupational health research at these worksites, the literature addressing prevalence and incidence of injuries among Latino workers is limited.

As Latino workers are projected to become one of the fastest growing groups in the US workforce [Toossi, 2012], it is important to have better data regarding the incidence of injury rates among this group of workers. The purpose of this study is to assess the incidence of CTS development over one year in Latino poultry processing workers, and to identify factors associated with incident CTS.

The baseline personal characteristics for the poultry processing workers and non-poultry workers are described in Table I, and there were no statistically significant differences between the poultry and non-poultry workers in regards to age, BMI, gender, spoken language, and level of education. The one-year incidence of CTS, in all participants, poultry processing workers, and non-poultry workers is described in Table II. At the wrist level, 19.8% of poultry workers developed possible or definite CTS at one year compared to 11.7% of non-poultry workers (p = 0.022). At the individual level, the increased incidence of CTS in the poultry workers compared to the non-poultry workers did not reach statistical significance. However, statistical significance was approached when evaluating the development of bilateral CTS using the less strict definition (12.0% v. 4.9%, p = 0.095) and when evaluating the development of unilateral CTS using the strict definition (4.0% v. 0.0%, p = 0.082).

Table IIIa includes the adjusted odds ratios for the development of CTS at the wrist level when controlling for type of work, age, BMI, and gender. Of these variables, only poultry work was associated with an increased odds ratio that approached statistical significance at 1.89 (p = 0.089). Table IIIb is similar, but includes the adjusted odds ratios for the development of CTS at the participant level. Similar to the wrist level results, the highest odds ratio is for poultry work (1.81), but it does not reach statistical significance (p = 0.139). Finally, when only those in poultry work were assessed to determine factors that may increase the incidence of CTS, none of the assessed variables (age, BMI, gender, poultry task) approached statistical significance (Table IV).

One of the challenges of CTS, from both a clinical and research standpoint, is defining the presence of the condition. The diagnosis can be based on symptoms, examination findings, nerve conduction studies, neuromuscular ultrasound, response to surgery, or a combination of these parameters [Rempel et al, 1998; Keith et al., 2009; Stevens, 1997; Cartwright, 2012a]. In research settings, in particular, defining the presence of CTS may be limited by time and financial constraints. In addition, CTS can also be described on either a wrist or individual level, and different cut-offs can be applied to alter the accuracy of each diagnostic test. Given these diagnostic challenges, the incidence data in this study are presented in a variety of manners to increase clinical relevance and allow for comparison to previous studies. Using a strict definition of CTS, in which both clinical and electrodiagnostic abnormalities must be present, two wrists on two separate poultry workers went from no evidence of CTS at baseline to definite CTS at one year, and none of the non-poultry manual workers developed definite CTS. This resulted in 1.9% of wrists and 4.0% of individuals developing CTS after one year in poultry processing. When a less strict definition of CTS was applied, 19.8% of wrists and 28.0% of poultry workers developed CTS over one year, compared to 11.7% of wrists and 17.9% of individuals in non-poultry manual laborers, with the data based on the wrist level reaching statistical significance (p = 0.022). Based on these findings, it is possible that Latinos employed in poultry processing have a higher one-year incidence of CTS than Latinos employed in other manual labor positions.

There are only a few prospective studies of CTS incidence in the literature. Silverstein and colleagues examined workers in manufacturing at baseline and one year, using symptoms and nerve conduction studies to define CTS, and found that at one year 1.05% of 479 wrists developed CTS [Silverstein, 2010]. This is slightly less than the 1.9% (strict definition) and certainly less than then 19.8% (less strict definition) of wrists in poultry workers that developed CTS at one year. On an individual rather than wrist level, Werner and colleagues found that 4.5% of auto assembly workers developed CTS over one year, using clinical symptoms and nerve conduction studies (latency 0.5 ms or greater in the median compared to ulnar sensory response) to define CTS [Werner, 2005]. Using similar criteria, Gell and colleagues found a 7% incidence of CTS over an average of 5.4 years in industrial and clerical workers, or 1.2% per year [Gell at al., 2005]. Direct comparison of the incidence in auto assembly, industrial, and clerical workers to poultry workers is challenging because the case definitions of CTS differ in these studies, but on an individual level our study found between 4.0% (strict definition) and 28% (less strict definition and similar to the definition used by Werner and Gell) of poultry workers developed CTS over one year. Other studies, using only clinical symptoms to define CTS, have detected higher incidence rates at one year, but not as high as the 19.8% of wrists and 28.0% of individuals detected amongst poultry workers in this study. For example, Andersen and colleagues used symptoms obtained through surveys to determine that 5.5% of 5,658 computer users in Denmark developed CTS at one year. Of interest, Nathan and colleagues examined 148 industrial workers at baseline and again 17 years later, using symptoms and nerve conduction studies to define CTS, and found that 28% of workers developed CTS over this extended follow up [Nathan, 2005].

Another finding of note in this study is that traditional risk factors for CTS, such as higher age, higher BMI, and female gender did not predict the development of CTS over one year in this population of manual workers. Our previous examination of the prevalence of CTS in Latino manual workers did show a modest association between CTS and higher age and BMI, but not female gender [Cartwright et al., 2012b]. The reason these traditional risk factors do not appear to carry as much importance for the incidence of CTS in this population is not known, but it is possible that the manual labor performed by this group is a greater risk factor than higher age and BMI and female gender, as has been suggested in other studies of CTS incidence in occupations requiring forceful exertion [Burt et al., 2013].

While this study is one of the few prospective investigations of CTS, and it provided significant insight into this condition in poultry processing workers, it did have some limitations. First, although the study initially started with a large number of participants, the final analyses included just 56 poultry workers and 134 non-poultry workers with no CTS at baseline and full one-year follow up data. A larger study population would have increased the power to detected statistically significant differences at the individual level. Second, there are certain types of nerve conduction studies, such as palmar mixed comparison studies, that are more sensitive for the diagnosis of CTS than the antidromic sensory responses used in this study. We did not use the more sensitive studies because they are more technically challenging, especially in the field setting. Using these more sensitive nerve conduction studies might have increased the incidence of CTS in both groups slightly. Third, the comparison group in this study also had a relatively high one-year incidence of CTS, which likely occurred because some were involved in occupations requiring repetitive wrist movements, such as landscaping, construction, restaurant work, and manufacturing. While this is an appropriate comparison group, the high incidence of CTS in this group made it more challenging to document statistically significant increases in the poultry workers. Fourth, the length of time each worker was employed in their current position was not included in the analyses. It is difficult to speculate how this may have affected the results, since it is possible that a longer employment might lead to more cumulative trauma and a higher likelihood of CTS, or conversely a worker that is intrinsically more resistant to the development of CTS might stay in the same job longer, so they are less prone to develop CTS over the length of the study. Either way, this is a limitation of the current study. Finally, we used a hand diagram, rather than a detailed history and physical examination, to diagnose CTS. This accepted approach was used because a detailed clinical evaluation would not have been feasible with the number of participants in this study, but a detailed history and examination might have slightly increased our diagnostic accuracy for CTS.[Rempel et al, 1998]

Given the high one-year incidence of CTS amongst poultry workers, the increased incidence compared to other manual laborers, and our previous finding of an increased prevalence of CTS in poultry workers compared to non-poultry manual laborers [Cartwright et al., 2012b], it is possible that poultry processing predisposes workers to the development of CTS. The current study did not identify any specific job tasks that significantly increased the risk of CTS, but the study of CTS prevalence in poultry workers did identify an association with performing multiple jobs (odds ratio = 2.66, p = 0.0035) and a trend towards a positive association in those that were involved in cutting, eviscerating, wash-up, trimming, and deboning (odds ratio = 1.57, p = 0.0661).

The increased risk of CTS in the current group of poultry workers, with at least 4.0% of workers developing CTS over one year, likely results from the strenuous and repetitive nature of poultry processing. Employers and regulators should consider this risk in an effort to improve the overall health of this vulnerable population. While the benefit of specific interventions to decrease the incidence of CTS has not been proven, policies to provide more rest from repetitive hand movements, improve ergonomics, and increase screening for CTS should be considered to help decrease the high incidence of CTS in this group of workers [Dick et al., 2011].