These are subsets of patients who have some of the features of seronegative spondyloarthropathy, but do not meet the criteria for the well recognised conditions. They may be an early stage of a known spondyloarthropathy, an atypical variant of a known spondyloarthropathy or an unknown type of spondyloarthropathy. In a cohort of 22 patients followed for 11 years, 15 (68%) developed ankylosing spondylitis, 1 developed psoriatic arthritis, 4 remain undifferentiated and 2 went into remission . Prevalence is not known, but is assumed to be more common than the other seronegative spondyloarthropathy’s.
Can have – sausage digits/dactylitis; insertional tendonitis (eg achilles); asymmetrical sacroilitis; periostitis; spurs; peri-insertional osteoporosis and erosions
Many may have a previously undiagnosed/unsuspected infection as evidenced by presence of an IgA antibody response suggesting that they may be a form of Reiters syndrome/Reactive arthritis. The infection was considered to be asymptomatic or too minor to be noticed.
Seven patients, seen in one clinic over a 3 year period were diagnosed with foot pain due to an undifferentiated form of seronegative spondyloarthropathy. All were positive for HLA-B27