Ankylosing spondylitis: Assessment and analysis of long-term outcome

Ankylosing spondylitis (AS) is a chronic rheumatic disease, characterized by inflammation of the joints connecting pelvis to spine and/or spine. The studies described in this thesis cover aspects of the assessment of AS and analysis of its long-term outcome. First, several aspects of outcome assessment in AS were discussed and improved, in particular pertaining to disease activity (signs and symptoms resulting from inflammation), spinal mobility and radiographic damage (bone formation). Second, the course of radiographic damage over time was analysed and better insight was obtained into the process of syndesmophyte formation. Finally, longitudinal relationships between disease activity, spinal mobility and radiographic damage and other potential determinants of these outcomes were investigated.
The main research question was whether disease activity leads to radiographic progresssion (new bone formation) in patients with AS. Previous studies suggested that such a relationship was lacking. Given the availability of drugs that are efficacious in controling disease activity, it was of utmost importance to unravel the relationship between disease activity and radiographic damage. It has for the first time been shown that disease activity unequivocally contributes to radiographic progression in the spine in AS: an increase in disease activity is followed by an increase in radiographic progression. The effect is actually rather impressive: an increase of one unit in the Ankylosing Spondylitis Disease Activity Score (ASDAS, measure of disease activity) in an individual patient is expected to lead to an increase of 0.7 units progression in the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS, measure of radiographic damage) over the next 2 years.