[Sacroiliitis: the key symptom of spondylathropathies. 1. The clinical aspects].

Seronegative and HLA B27-associated spondyloarthropathy (SpA) is a heterogeneous disease, so far without a clear origin. The term comprises five clinically defined subcategories: ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis (ReA), inflammatory bowel disease-associated arthritis and undifferentiated spondyloarthropathy (uSpA). Sacroiliitis, an inflammatory involvement of one or both sacroiliac joints, is the key symptom of all spondyloarthropathies. Sacroiliitis is often associated with inflammatory back pain, manifest as deep nocturnal back pain that is improved by exercise. The pathogenesis of SpA and the reason for the tropism for the sacroiliac joints is still obscure. A genetic background of an initially bacterial triggered infection seems to be most likely.