Physically demanding work may increase effects of inflammation in ankylosing spondylitis

Ankylosing spondylitis, or AS, is a type of inflammatory arthritis characterized by joint stiffness, pain and new bone formation that can result in partial or complete fusion of the spine. AS tends to develop in young adults, and tends to affect young men more often than young women. The prevalence of AS in the United States is estimated at 0.5 percent.

A previous study in animal models showed that mice exposed to less mechanical stress developed less new bone formation. Researchers in the Netherlands, Belgium and France set out to investigate the same in humans and to learn if mechanical strain from physical activity might worsen bone formation in patients with AS.

Using data from 184 AS patients taken from the 12-year OASIS study, the researchers looked into the complex relationship between inflammation, mechanical stress and radiographic progression of disease in AS patients. Key factors in the study included job type, including sedentary white collar versus physically demanding blue collar, as well as smoking and socioeconomic indicators like personal income, family income and post-secondary education.

Patients had assessments with X-rays and disease activity, at baseline and at twice-yearly intervals throughout the study. Structural damage was measured using the modified Stoke Ankylosing Spondylitis Spine Score, or mSASSS and disease activity by the Ankylosing Spondylitis Disease Activity Score, or ASDAS. Seventy percent of the patients studied were male, 83 percent were HLA-B27 positive, 39 percent were smokers and 48 percent were blue-collar workers.

"We hoped to assess whether mechanical stress plays a role in new bone formation in AS. From a previous study we had conducted, we already knew that inflammation leads to radiographic damage. However, patients without inflammation also have some level of residual damage progression, so we were interested in other potentially explanatory factors, and that was where mechanical stress came into the scene," said Sofia Romero, MD of the Amsterdam Rheumatology Center/University of Amsterdam in the Netherlands and a lead author on the study.

The results showed that both physical work and smoking amplified the effects of disease activity on C-ray progression. Among blue-collar workers versus white-collar workers, every additional unit of disease activity (one ASDAS unit) resulted in an increase of 1.2 vs 0.2 mSASSS-units per two years. In smokers versus non-smokers, every additional unit of disease activity resulted in an increase of 1.9 vs 0.4 mSASSS-units per two years. Personal income also significantly modified the relationship between disease activity scores and X-ray progression of disease, but education and family income did not.

The study's authors concluded that strenuous physical activity in physically demanding, blue-collar jobs may amplify some of the effects of inflammation on disease progression in AS. They further concluded that this data may support the theory that mechanical stress leads to bone formation in AS. If further studies confirm this connection, rheumatologists may want to rethink the common recommendation for AS patients to do strenuous exercise, the authors noted.

"We need more research into this topic to unravel the precise role of mechanical stress and to disentangle this from the potential effect from smoking. Further research is also needed before recommendations are issued for patients, but it may be that certain types of exercise are more beneficial than others for patients with AS," said Dr. Romero.