HypothesisClassification of inflammatory arthritis by enthesitis
Section snippets
Other features
If primary enthesitis explains synovitis, could it also explain all musculoskeletal signs of spondyloarthropathy (figure 2)? Dactylitis is a characteristic feature of spondyloarthropathy and is considered to be predominantly a tenosynovitis,24 but if this were the case, dactylitis would be common in rheumatoid arthritis. The circumferential digital thickening of dactylitis could be related to inflammatory epicentres located at several digital entheseal insertions not only resulting in synovitis
Pathogenic implications
The pathophysiology of inflammatory enthesitis is poorly understood. One study of synoval joints after inoculation of bacteria and particulate matter intravenously showed that the bone-enthesis interface was a common site of accumulation of bacteria, which may be due to a distinctive vascularity at this site.29 There are some suggestions that bacteria also localise in the enthesis in human beings. Viable micro-organisms have been cultured from SAPHO-associated bone lesions, although this
Diagnostic and prognostic implications
Rheumatoid arthritis is a symmetrical polyarthropathy that generally has a poor prognosis compared with other types of arthritis. Most other polyarthropathies clinically distinct from rheumatoid arthritis, such as remitting seronegative arthritis with pitting oedema (RS3PE),35 polymyalgia rheumatica-associated polyarthritis,36 and viral polyarthritis generally have a better prognosis and share other features with spondyloarthropathies including acute onset of symptoms, prominent extracapsular
Hypothesis
A primary inflammatory enthesitis could explain all of the rheumatic signs of spondyloarthropathies. An understanding of spondyloarthropathy pathogenesis will come from assessing the role of micro-organisms and HLA B27 at the enthesis.
This hypothesis could be tested with specialised MRI techniques and high-resolution ultrasonography to show that enthesitis is ubiquitous in all skeletal lesions of early spondyloarthropathies and by similarly assessing other hitherto unclassified arthropathies.
New classification of arthritis
On the basis of these concepts, joint inflammation may be classified as primarily synovial based like rheumatoid arthritis or entheseal based like spondyloarthropathy. This would not only provide a unifying classification for spondyloarthropathies, but also a framework for the classification of other arthropathies. The demonstration of primary synovial or entheseal abnormalities in various arthropathies including isolated inflammatory monoarthritis,40 Be¸het's syndrome, psoriatic arthritis,
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