Psoriatic arthritis (PsA) is a chronic inflammatory rheumatic disease, belonging to the wide spectrum of spondyloarthritis, but with the particularity to be associated with personal psoriasis or familial psoriasis. PsA can be a very disabling disease through progressive and irreversible joint damage. Long-term functional prognosis of patients with PsA is correlated with the presence and severity of the radiographic joint lesions of the disease.

However, the proportion of patients who will develop those peripheral joint damages is not yet known and less over the factors which are associated/involved in such an aggressive pattern of the disease. Early identification of this subgroup of patients is particularly important for determining early "intensive" treatment, strict management with a Treat To Target approach, and identification of new treatments with a stronger structural effect.

The main objective of this prospective 10 years cohort is to describe the 5 years structural (radiographic) severity of recent Psoriatic arthritis (PsA)with recent peripheral arthritis.Some of the secondary objectives are to describe the 10 years structural severity within those patients, and to determine the predictive factors of those 5 and 10 years radiographic lesions (genetic, environmental, clinic, therapeutic factors).

APACHE will provide a unique longitudinal standardized database concerning patients with PsA with very recent peripheral arthritis. Research projects which will based on those collected data should allow to identify the mechanisms of aggressive joint damage, to highlight mew treatments targets, to better describe the burden of the disease, to test previous or develop new assessments tolls, to develop early diagnostic criteria

Method:APACHE is a prospective national multicentre (30) cohort including and following during 10 years patients with a first peripheral arthritis in the past year, attributing to a PsA according to the treating rheumatologist. 425 patients will be included during a 4 years period before the follow-up. Standardized visits are planned throughout the 10 years follow-up, permitting the collection of various data:

• clinical data
• Patients Reported Outcome
• usual lab parameters
• plasma, serum and urines
• Baseline and one year RNA, DNA
• peripheral X-Rays, Ultrasounds and MRI at different time points Those visits have non interventional character (no therapeutic decision will be taken) Those collected data will allow submissions of many research projects to the scientific committee of the cohort.

Inclusion Criteria:

• Women or men aged from 18 to 65 inclusive
• First episode of peripheral arthritis in the last 12 months, authenticated by a rheumatologist
• Psoriasis diagnosed by a practitioner or family history of psoriatic arthritis (first-degree relative [parent or sibling] or second degree relative)
• Arthritis most likely recognized as a psoriatic arthritis by a rheumatologist (diagnostic confidence score : ≥ 7 out of 10)
• Signed informed consent form
• Affiliation to a social security system

Exclusion Criteria:

• Formal diagnostic of inflammatory rheumatism other than psoriatic arthritis
• Treatment or history of treatment with a biomedicine
• Patient receiving csDMARDS (methotrexate, sulfasalazine, or leflunomide) or apremilast treatment over the past year for rheumatological symptoms
• Oral steroids in the last 4 weeks, above 10 mg/d of prednisone or with modified dosage
• intravenous or intra articular steroids in the last 4 weeks
• IRM contraindication
• Cognitive, mental or psychic disorders impeding protocol accomplishment
• Difficulties with French language understanding
• Patient under tutorship or curatorship
• Pregnancy

Patients with peripheral psoriatic arthritis diagnosed at an early stage.