Predictors of the progression of functional disability in patients with ankylosing spondylitis

To identify patient characteristics that predict the short term (5 yr) rate of progression of functional disability in ankylosing spondylitis (AS).

In a prospective longitudinal study, 212 patients with AS reported information on health status on biannual mailed questionnaires. The median duration of followup was 5 yrs (range 1-7.5 yrs). Functional disability was measured using the Health Assessment Questionnaire for the Spondyloarthropathies (HAQ-S; possible range 0-3). Predictors of the rate of change in the HAQ-S over time included demographic characteristics, number of comorbid conditions, age at onset of AS, peripheral arthritis, smoking, body mass index, social support (measured by the Interpersonal Support Evaluation List, ISEL), current and past physical activity at work, current and past recreational activity, severity of pain and stiffness, nonsteroidal antiinflammatory drug use, and frequency of back exercise. Associations between the predictors and the rate of progression of functional disability (HAQ-S units/yr) were tested using pooled time series regression analysis.

Patients were mostly middle-aged (mean 48 yrs), male (70%), white (86%), and well educated (mean 15.5 yrs), with an average duration of AS of 20 yrs and little functional disability at study entry (mean HAQ-S 0.66). Over a median of 5 yrs, the HAQ-S increased at an average rate of 0.0168 units/yr. In multivariate analyses, older age (change in slope +0.0007 units/yr of age; p = 0.0008) and smoking (change in slope +0.0313 units/yr; p < 0.0001) were associated with more rapid progression, while more frequent back exercise (change in slope -0.0019 units/yr per each additional day/week; p < 0.0001) and better social support (change in slope -0.003 units/yr per 10 point increase in ISEL score; p = 0.05) were associated with improvement in functional disability over time. Smoking and lack of social support were also associated with the progression of functional disability in the subgroup of 58 patients who had AS < 10 yrs at entry.

Functional disability in AS progresses more rapidly in older patients and smokers, and less rapidly in those who regularly do back exercises and have better social support.