Validation of a Personalised Axial Spondyoarthritis Metrology Index Using Data from Clinical Trials.

Validation of a Personalised Axial Spondyoarthritis Metrology Index Using Data from Clinical Trials.

Radiographic axial spondyloarthritis (r-axSpA), also known as Ankylosing Spondylitis, is a disease in which inflammation in the spine can lead to bony fusion between vertebrae and arthritis in the hip joints, leading to a temporary or permanent loss of mobility in these joints. With the aid of a tape measure and a simple goniometer (an instrument for the precise measurement of angles), clinicians started tracking these changes in mobility over the course of the disease by measuring the maximum angle of rotation in the neck (cervical rotation), the flexion in the lumbar spine (Schober's test), side flexion of the lumbar spine (lateral spinal flexion), abduction of the hips (intermalleolar distance) and flexion deformity of the spine (originally occiput to wall distance, then tragus to wall distance -distance from the occiput and tragus to the wall respectively). The Bath Ankylosing Spondylitis Metrology Index (BASMI) was then developed as a composite index of these spine and hip mobility measures and it has been widely used in r-axSpA research ever since it was first published in 1994 (1). Since then, the BASMI formula has undergone minor modifications ,from nominal scales (BASMI2 and BASMI10) to a continuous scale (BASMI Linear)(2) the last one being the version most commonly used in recent clinical trials due to its higher practicability in computer evaluations. The population studied in these trials has also evolved. Over the past 15 years, the use of MRI rather than x-ray to achieve an earlier diagnosis has led to the introduction of a new and more inclusive term for the disease ‘axial spondyloarthritis' (axSpA). Although the BASMI is still widely used, those clinical trials that have focused on patients with early axSpA have often reported a lack of responsiveness of BASMI, suggesting that this outcome tool may no longer be appropriate in this group of patients(3). Indeed, the Assessment of SpondyloArthritis international Society (ASAS) has recently reviewed the core set for axSpA and determined that the BASMI should no longer be recommended as a core outcome domain for inclusion in all axSpA therapeutic studies (4). Among the reasons for this change are the floor effect seen in patients with short disease duration, and poor sensitivity to change (5). A floor effect exists when a significant number of participants have a score at or near the bottom of the scale, meaning that the scale is unable to measure any improvement in mobility in these patients. Similarly, a ceiling effect in an outcome measure exists when a significant number of participants have a score at or near the top of the scale, meaning that the scale is unable to detect any worsening of mobility in these patients. Sensitivity to change means that the scale should be able to detect small but significant changes in spinal mobility.At the time the BASMI was developed, it had not been tested on a group of normal individuals. When this was systematically studied in the MOBILITY study of healthy individuals, it was found that the BASMI was strongly influenced by factors such as height, age and gender. Previous versions of the BASMI had no way of determining the appropriate normal/lower end of the reference range for each measurement. These limits can now be defined more precisely using the 50th/75th percentile values from the MOBILITY study but the choice of the best ‘cut off' points also requires analysis of axSpA datasets. We therefore hypothesized that an adjusted version of the BASMI Linear, taking age, height and gender into account would have better psychometric properties than the ones from the original BASMI Linear. As a first step of this project, the Personalised Axial Spondyloarthritis Metrology Index (PASMI) was developed. The PASMI allows, by adjusting each individual spinal mobility measure for age, height and gender, as appropriate, to deliver a more ‘truthful' representation of spinal mobility for each individual.

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