The spondyloarthritides (SpA) comprise ankylosing spondylitis (AS), reactive arthritis (ReA), arthritis/spondylitis with inflammatory bowel disease and arthritis/spon-dylitis with psoriasis. All these diseases are associated with HLA-B27, although to a different degree (Tab. 1). Further links between these diseases is the possible occurrence of similar clinical symptoms such as inflammatory back pain (predominant in the morning/at night and improvement with exercise) reflecting sacroiliitis/spondyli-tis, a similar pattern of peripheral joint involvement with an asymmetrical arthritis predominantly of the lower limbs, and in about 20% of patients the manifestations of enthesitis and uveitis. Furthermore, a substantial proportion of the HLA-B27-positive patients with reactive arthritis, arthritis/spondylitis associated with inflammatory bowel disease or psoriasis do develop ankylosing spondylitis (see below). AS is regarded as the SpA with the most severe outcome; it normally starts in the second decade of life. The male to female ratio has more recently been estimated to be around 2:1 . The disease normally starts with inflammation in the sacroiliac joint but can affect the whole spine, with progressive spinal ankylosis as a potential outcome in a subgroup of patients. The association of HLA-B27 with AS/SpA is the highest known MHC Class I association for human diseases and the most relevant single factor for the pathogenesis of SpA. However, other less well defined genetical factors also contribute to the overall genetic risk.
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