Classification of axial spondyloarthritis
Some patients have features suggestive of axial spondyloarthritis, but do not meet the formal classification criteria for ankylosing spondylitis. While in the past these patients have often been described as having ‘undifferentiated spondyloarthritis’, the term ‘nonradiographic axial spondyloarthritis’ has now been formally defined.
Nonradiographic axial spondyloarthritis is used to classify patients who have evidence of axial inflammation but do not have definite changes on plain X-ray of the sacroiliac joints. Management of nonradiographic axial spondyloarthritis is generally the same as for ankylosing spondylitis (see Management for axial spondyloarthritis); nonpharmacological management is particularly important in these patients.
Ankylosing spondylitis is diagnosed when sufficient changes have occurred in the axial skeleton to be evident on plain X-ray of the sacroiliac joints.
All people with axial spondyloarthritis will have had nonradiographic axial spondyloarthritis as the initial stage of their disease. However, not all people with nonradiographic axial spondyloarthritis will progress to ankylosing spondylitis; the rate of progression is approximately 10% every 2 yearsSampaio-Barros, 2001. There are no reliable clinical indicators of which patients are likely to progress to ankylosing spondylitis, and the symptoms and functional impact of nonradiographic axial spondyloarthritis are generally indistinguishable from ankylosing spondylitis. There is a more even sex distribution in nonradiographic axial spondyloarthritis, in contrast to the male predominance seen in ankylosing spondylitis.