Spine damage in axial psoriatic arthritis may look remarkably like spondyloarthritis on radiographs. But measuring progression in the former is crucial to clinical trials. What's the best way to interpret the films? A new study offers an answer.
Biagioni BJ, Gladman DD, Cook RJ, et al., Reliability of radiographic scoring methods in axial psoriatic arthritis. Arthritis Care Res (2014) Feb 10. doi: 10.1002/acr.22308. [Epub ahead of print]
A small study from Canada has determined the most reliable method for assessing structural damage in axial psoriatic arthritis (AxPsA), which is crucial to address the unmet need for clinical trials of this condition.
There have been few studies on progression of AxPsA, and scoring structural changes in the axial skeleton is regarded as the gold standard for calculating severity. But it can be easy to mistake radiographic features of new bone formation in AxPsA for ankylosing spondylitis (AS).
More scoring systems have been validated for AS; is any of them also useful in AxPsA? Perhaps not surprisingly, the Psoriatic Arthritis Spondylitis Radiology Index (PASRI) developed specifically for psoriatic arthritis seems to be the best method, this study finds.
To assess the validity of scoring systems for AxPsA, researchers collected readings from four rheumatologists for digital spine radiographs for 40 AxPsA patients and plain spinal X-rays from 18 patients with AS. They then ran the results through a custom computer application that allowed them to compare reliability of the four different scoring systems: the AxPsA-specific PASRI to those done with the Bath AS Radiology Index (BASRI), the modified Stokes AS Spinal Score (mSASSS), and the Radiographic AS Spinal Score (RASSS).
Neither the BASRI, the mSASSS, nor the RASSS has been validated for use in AxPsA, nor has the PASRI been validated for use in AS patients. The four scoring systems performed equally well in AS in this study, but the results for AxSpA were less spectacular: The scoring systems had only moderate intra- and inter-rater reliability in assessing spinal damage and sacroiliitis in AxPsA.
It appears that the PASRI developed specifically for PsA is most useful because it scores the posterior elements of the cervical spine as well as the sacroiliac joints.
Sacroiliitis can be difficult to discern on X-rays in AxSpA, because it may be milder than in AS. Also, psoriatic arthritis patients tend to be older with more osteoarthritic changes in that joint, and new bone formation may mimic the syndesmophytes seen in AS. For all these reasons, say the authors, defining AxSpA solely by radiographic evidence of sacroiliitis is not reliable.
Establishing the sensitivity of tools for measuring change in AsPsA is crucial for future clinical trials of this condition.
The authors say that studies of interest would evaluate whether new bone formation in axial joints parallels that in peripheral joints in PsA, as well as testing the effects of NSAIDs and tumor necrosis factor inhibitors (TNFis) on progression.