Undifferentiated spondyloarthritis (uSpA), also known as undifferentiated arthritis, is a non-specific mono- or polyarthropathy that lacks the clinical, serological and radiological features that would allow specific diagnosis. It often turns out to be an early presentation of a more well-known form of arthritis.
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Epidemiology
The prevalence of undifferentiated spondyloarthritis is estimated at ~1% 4.
Clinical presentation
Back pain, enthesitis, and peripheral arthritis are common, as well as extra-skeletal manifestations such as dactylitis and fatigue 1.
Pathology
Depends on the underlying cause. Most cases reveal themselves to be either:
Markers
HLA-B27 positive in ~75% (range 70-84%) 4
Only 50% of presentations have abnormal levels of acute-phase reactants (CRP, ESR), and only 10% are positive for rheumatoid factor despite many eventually declaring themselves as rheumatoid arthritis 3.
Treatment and prognosis
A specific diagnosis is usually reached within three months, commonly:
rheumatoid arthritis (30%) 2
ankylosing spondylitis: undifferentiated spondyloarthropathy is often considered an early manifestation 4
A small number will remain "undifferentiated", and ~45% will recover without a more specific diagnosis 1.
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