French Cohort of Undifferentiated Spondyloarthritis

Inclusion Criteria: * Patients aged over 18 and under 50 years * Inflammatory back pain (buttock, lumbar or thoracic spine) fulfilling either the Calin orBerlin criteria : * Calin criteria (at least 4 of 5 criteria must be met) (ref. 25): * Insidious onset * Onset before age 40 * Persistence since at least three months * Morning stiffness 30 minutes * Improvement with exercise * Berlin criteria (at least 2 of 4 criteria must be met) (ref. 26): * Morning stiffness 30 minutes * Improvement with exercise and absence of improvement at rest * Nocturnal pain in the second part of the night * Alternating buttock pain * Symptom duration more than 3 months and less than 3 years * Symptoms suggestive of spondyloarthritis according to the local investigator's assessment(e.g. score ≥5 on a 0 to 10 numerical rating scale in which 0 = no suggestive and 10 = verysuggestive of spondyloarthritis) * Realization of a prior medical examination (article L.1121-11 of the Code of the Public health) * Informed consent Dated and signed voluntarily Exclusion Criteria: * Other spinal disease clearly defined (eg symptomatic mechanical discopathy) * Pregnant woman * History of alcoholism, drug addiction, psychological problems, severe co-morbidities which might interfere with the validity of the informed consent and/or prevent an optimal compliance of the patient to the cohort * It was possible to include patients who have received or are receiving a thorough treatment such as Sulfasalazine, Methotrexate or Azathioprine. However any history of treatment with biotherapy including anti-TNF therapy defined exclusion criteria * Corticosteroid intake was permitted only in case of a dose lower than 10 mg prednisoneper day and stable for at least 4 weeks prior baseline * MRI contraindication * No affiliation to the French National Social Security System