Epidemiological Investigation of Rheumatoid Arthritis Links Manual Labor to the Disease

APRIL 17, 2017
Dava Stewart
Prolonged, repetitive physical workload is a risk factor for rheumatoid arthritis (RA), according to the results of a recent study. Pingling Zeng, PhD, of the Institute of Environmental Medicine at the Karolinska Institute in Stockholm, Sweden, and colleagues conducted the study with two objectives. First, they wanted to determine if physical workload is a factor for RA and secondly to learn about the possible interactions between physical workload and the genes located in the human leucocyte antigen (HLA) region.
 
The authors note that several environmental risk factors for RA have been identified, including smoking. They explain, “Some identified important environmental factors (particularly smoking) have been shown to interact with the major histocompatibility complex class II alleles, initially defined by the classic HLA-DRB1 shared epitope, which is a genetic risk factor for anticitrullinated protein antibodies (ACPA)-positive RA”. They go on to say that smoking and HLA polymorphisms react through specific amino acid sequences and this interaction and others like it appear to vary, depending on the subtype of RA.
 
Information from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA) provided the data for this population-based case-control study. There were 3724 participants with RA who completed the questionnaire for the study; 99% of them provided blood samples. There 5935 controls who completed the questionnaire and 55% of them provided blood samples. The questionnaire included questions regarding specific work postures and seven types of physical workloads, with emphasis on the length of time spent doing different activities. ACPA was assessed using the blood samples.
 
After exclusions, the researchers analyzed data for 3150 cases and 5130 controls. In participants with RA, 66% were ACPA-positive, and the researchers report, “More RA cases were smokers compared with controls”.
 
Each type of physical workload in the questionnaire was associated with an elevated risk of developing RA. Those with the strongest risk were participants who did work that required their hands to be above shoulder level. Additionally, the risk for developing RA was increased with physical workload exposure. In other words, the more of the seven types of physical workload the participant was exposed to, the greater the risk of RA. The researchers note, “This trend remained significantly significant after adjustment for potential confounding factors”. The RA cases were stratified by ACPA status, and there was no significant or substantial change in the odds ratio, even adjusting for potential confounders.

The researchers say that “repetitive bending/turning, repetitive hand/finger movements, lift or carry more than 10 kg, hands below knee level, hands above shoulder level and vibration are associated with an increased risk of RA,” and that risk is similar for ACPA-positive and ACPA-negative RA.
 
The questionnaire used in this study included questions concerning participants’ activities in the five previous years, the authors suggest that pain and fatigue, which can precede the development of RA, may have motivated participants to reduce their exposure to physical workloads. As a result, the number of people exposed to particular types of physical workloads may be lower. The authors point out, however, that the results of their analyses of the previous five years are “quite consistent”.
 
The authors conclude that prolonged repetitive physical workload is a novel environmental risk factor for RA. The full study can be found in the BMJ Journal RMD Open Rheumatic & Musculoskeletal Diseases.
 
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