Objective: To evaluate the course of impaired spinal mobility in patients with long‐standing well‐defined ankylosing spondylitis (AS ).
Methods: Data from 232 patients with AS (186 men, 46 women) and 3,849 clinical measurements performed between February 1980 and June 2016 were analyzed. Lateral spinal flexion (LSF ), the 10‐cm Schober test, chest expansion (CE ), and cervical rotation measurements were stratified by disease duration at 10‐year intervals and compared with published age‐ and height‐adjusted spinal mobility reference intervals as well as with fixed reference values commonly used in clinical practice.
Results: After 10 years of AS , most patients exhibited at least 1 measurement, most commonly LSF , that was under the 2.5th percentile of the adjusted reference interval (53% of men, 65% of women). In all measurements except CE , there were significant linear increases in the proportion of patients during 40 years of disease duration who exhibited impaired mobility. Measured LSF values <2.5th percentile (mean 14.8 cm) after 10 years were associated with further spinal mobility impairments later in the disease course. Fixed reference values yielded higher proportions of patients with impaired mobility compared with adjusted reference intervals.
Conclusion: Impaired spinal mobility in AS is common after a 10‐year disease duration. LSF below the 2.5th percentile at 10 years appeared to be associated with a worse prognosis. Fixed reference values overestimated spinal mobility impairments in AS and should be avoided.