Impaired lateral spinal flexion at 10 years of ankylosing spondylitis (AS) disease duration was associated with a worse prognosis, according to research published in Arthritis Care and Research.

The results also indicated that fixed reference values — commonly used in clinical practice — often overestimate the proportion of patients with AS with impaired spinal mobility. The use of age- and height-adjusted reference intervals could improve these assessments.

Researchers analyzed data from patients with AS to evaluate the progression of spinal mobility impairment in patients relative to the duration of their disease. Patients were located in Västerbotten County in northern Sweden and were offered treatment and regular assessment, including spinal measurements, at the Norrland University Hospital Department of Rheumatology.

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Data from lateral spinal flexion, 10 cm Schober test, chest expansion, sitting cervical rotation, and intermalleolar distance were assessed, stratified by 10-year disease duration intervals, and compared with age- and height-adjusted reference intervals.

In total, 3849 measurements from 232 patients (186 men and 46 women) were analyzed (mean measurements per patient 16.6, range 1 to 65 and mean measurements per year 1.2; range, 0.2 to 5.1). Mean disease duration in the patient population was 29.5 years (range 3 to 59).

Researchers found that spinal mobility was lower than the reference values for the corresponding healthy population for all evaluated measurements; this was most noted in lateral spinal flexion, with a mean value below the 2.5th percentile, and least noted in chest expansion. Over 40 years of disease, there was a decrease in mean spinal mobility on all measured parameters with the exception of chest expansion.

In general, the proportion of patients with impaired spinal mobility increased over time, particularly in relation to the adjusted reference intervals and fixed reference values. Patients with impaired lateral spinal flexion exhibited worse absolute mobility and more frequently impaired mobility after a mean 30±5.8 years of disease duration.

In their discussion, the investigators noted that the application of fixed reference values to their data resulted in an “obvious” risk for misclassification of patients.

“Fixed reference values, which are frequently used in the clinic setting and [are] not adjusted for age, could introduce a major risk of misclassifying normal spinal mobility,” they wrote. In fact, 6 of 8 patients classified with impaired chest expansion mobility by the fixed reference values were reclassified with normal mobility when adjusted reference intervals were applied. Similarly, this misclassification affected 2 of 5 patients’ 10 cm Schober test results.

“The lack of specificity when using fixed reference values urges the need for better tools for an accurate identification of impaired spinal mobility early in the disease course,” the researchers wrote.

One noted study limitation was the lack of adjustment made for pharmacological therapy or physiotherapy, which may have affected patient outcomes.

“Early impairment of spinal mobility…is associated with a worse prognosis,” the researchers of the study concluded. “The use of age-and height-adjusted reference intervals could significantly improve spinal mobility assessment during the course of AS disease.”

Lotta Ljung, MD, reports receiving lecture fees from Pfizer and Solveig Wållberg-Jonsson, MD, reports receiving lecture fees from MSD.

Reference

Sundström B, Ljung L, Hörnberg K, Wållberg-Jonsson S. Long-term spinal mobility in ankylosing spondylitis: a repeated cross-sectional study [published online May 17, 2019]. Arthritis Care Res. doi: 10.1002/acr.23929