Juvenile Idiopathic Arthritis (JIA), the most common rheumatologic chronic disease in children, is defined as arthritis persisting for at least 6 weeks with no known cause in a patient under the age of 16. The term JIA is an umbrella that includes very different diseases. The current International League of Associations for Rheumatology (ILAR) classification divides JIA patients into 7 categories based on number of involved joints and time of involvement, presence of systemic symptoms, psoriatic findings and spondyloarthritis. This classification groups together patients with different disease and divides patients with the same disease. In the first case, unifying distinct diseases could lead to undifferentiated therapeutic choices, moving away from the modern concept of therapeutic personalization. In the second case, similarities between paediatric and adult arthritis could not be found. This involves both a loss of collaboration with the adult rheumatologist and the difficulty in accessing possibly effective therapies approved only for adult arthritis.

In clinical practice, it is increasingly evident that the number of affected joints and the speed of joint involvement are not useful criteria for defining the type and severity of disease. Joint counts lead to underestimate the importance of joint distribution in the identification of distinct forms of arthritis. A recent study found that patterns of joint involvement represent prognostic features, so grouping patients by joint pattern and degree of localization may help clinicians tailor treatments based on predicted disease trajectories. Another important point to differentiate some forms of arthritis is the presence of enthesitis and tenosynovitis. Sometimes tendon inflammation can be not clinically evident, so ultrasound evaluation is useful to detect it. Musculoskeletal ultrasound (MSUS) has been used worldwide by adult rheumatologist, but it is beginning a useful tool also in patients with JIA. Recent studies underline the important role of MSUS findings to assess disease activity and assist disease classification. In recent years, the need has emerged to replace the ILAR criteria with a new nomenclature based on the disease biology. This approach could help clinicians to choose a personalized therapeutic strategy for patients with arthritis.

Inclusion Criteria:

• Subjects under the age of 18 years
• Arthritis persisting for at least 6 weeks with no known cause

Exclusion Criteria:

• No consent from the patients' guardians
• Patients with Systemic onset Juvenile Idiopathic Arthritis
• Patients who developed arthritis on a pre-existing inflammatory disorder such as Inflammatory Bowel Disease, and had received previous treatments

Subjects with JIA during the transition to adult care