The current concept of spondyloarthritis with special emphasis on undifferentiated spondyloarthritis.

In a recent international workshop for ankylosing spondylitis (AS), a consensus was reached that the term ‘spondyloarthritis’ is preferable to ‘spondyloarthropathy’, to emphasize the inflammatory nature of these diseases [1]. The term is often quoted in the plural form, ‘the spondyloarthritides’, accentuating that this is a group of similar diseases with distinct clinical features and a common genetic predisposition, rather than one disease with different clinical presentations. The main association recognized is with HLA B27 but it is clear that there are other genes involved, including the interleukin-1 family gene cluster [2, 3]. Irrespective of the subtype of spondyloarthritis (SpA), the main clinical manifestations of this group are inflammatory back pain (IBP), peripheral arthritis, enthesitis and anterior uveitis, while other organ manifestations are rare. Psoriasis and inflammatory bowel disease (IBD)-associated colitis should be considered as basic subtype-defining entities with their own genetic background, rather than as disease manifestations. Five major subtypes of SpA are recognized on the basis of recently proposed classification criteria (European Spondyloarthropathy Study Group [ESSG]). In recent years we and others have defined the subgroups as AS, psoriatic arthritis (PsA), reactive arthritis (ReA), arthritis associated with inflammatory bowel disease (AIBD) and ‘undifferentiated’ SpA (uSpA). This terminology suggests that it is intended to, for example, include the whole group of patients with psoriasisassociated arthritis. Even on the basis of the ESSG criteria, this is certainly not correct and it is well recognized that there are different subtypes of PsA, including patient groups that rather resemble rheumatoid arthritis or osteoarthritis. Therefore, it is much clearer to choose another terminology, which is hereby introduced. The subtypes are ankylosing spondylitis, psoriatic SpA (PsSpA), reactive SpA (ReSpA), SpA associated with inflammatory bowel disease (SpAIBD) and uSpA. Alongside AS, uSpA is the most common subtype of the spondyloarthritides [4, 5] with a prevalence between 0.7% and 2.0% [5–7], and yet we know less about it than the other subtypes. This is often the case when new disease classifications emerge, and it can be difficult to retrospectively assess disease prevalence and associations from studies designed to address different disease classifications. This review aims to outline the current concept of SpA and uSpA, the latter in more detail, looking at its clinical features and natural history and the current evidence for therapy. Classification of spondyloarthritis