The early clinical recognition of juvenile‐onset ankylosing spondylitis and its differentiation from juvenile rheumatoid arthritis

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Abstract
Objective. To determine which early clinical data differentiate juvenile‐onset ankylosing spondylitis (AS) from juvenile rheumatoid arthritis (JRA). Methods. Medical records of 35 patients with juvenile‐onset AS and 75 with JRA (excluding type II pauciarticular JRA), all of whom had disease onset at age ≤16 years, disease duration of ≤2½ years at the initial visit to the rheumatology clinic, and followup of ≥ 10 years, were analyzed retrospectively with regard to features of disease found 6 months, 12 months, and 10 years after onset. Results. At 6 months, various features appeared more frequently in the juvenile‐onset AS group than in the JRA group, i.e., pauciarthritis (54.3% versus 30.7%; P = 0.03, odds ratio [OR] = 2.7), enthesopathy (82.9% versus 0%; P < 0.0001, OR = 321.4), tarsal disease (71.4% versus 1.3%; P < 0.0001, OR = 185.0), and lumbar/sacroiliac symptoms (11.4% versus 0%; P = 0.02, OR = 11.9). At 12 months, the features found more frequently among juvenile‐onset AS patients than JRA patients were enthesopathy (88.6% versus 4.0%; P < 0.0001, OR = 186.0), tarsal disease (85.7% versus 10.7%; P < 0.0001, OR = 50.3), and knee disease (100.0% versus 82.7%; P = 0.04, OR = 8.0). Involvement of the upper extremities (especially the hands) was found in significantly fewer juvenile‐onset AS patients compared with the JRA group. Definite involvement of the spine and sacroiliitis in juvenile‐onset AS occurred after a mean ± SD of 7.3 ± 2.0 years. Conclusion. Regardless of axial disease, enthes opathy and tarsal disease in children who have arthritis of the lower, but not of the upper extremities differentiate juvenile‐onset AS from JRA within 1 year of symptoms. The discriminative value of these parameters approaches that of axial disease (the gold standard) throughout the followup period.