*1.3. Systemic JIA*

Systemic juvenile idiopathic arthritis (sJIA) is a subtype of the disease that occurs in childhood secondary to an immune disorder, which associates arthritis and systemic inflammatory symptoms [17].

sJIA is defined as arthritis accompanied or preceded by daily fever, with a minimum duration of 2 weeks, associated with the following extra-articular symptoms: erythematous rash, lymph node hypertrophy, hepato- and/or splenomegaly, and serositis [18].

The diagnosis is sustained by the prolonged fever for at least two weeks and is accompanied by two major criteria, or by one major criterion and two minor criteria. The major criteria are given by erythematous rash and arthritis. The minor criteria could be the following: generalized adenomegaly and/or hepatomegaly and/or splenomegaly; serositis; arthralgia lasting 2 weeks or more (in the absence of arthritis); and leukocytosis (≥ 15,000 /μL) and increased numbers of neutrophils.

sJIA is similar to Still's disease in adults and for the correct diagnosis, the presence of fever is required alongside at least one major criterion; arthritis would no longer be necessary because, as in adults, it may be initially missing [19].

#### *1.4. Enthesitis-Related Arthritis (ERA)*

The enthesitis-related arthritis (ERA) has also been referred to by ILAR as an undifferentiated spondyloarthropathy, because the symptoms are mostly found in adulthood, but with a higher percentage of non-differentiated forms in children. In the current definition, an imagistic criterion was introduced by radiographic images or magnetic resonance images [20,21].

The diagnosis is supported by peripheral arthritis and enthesitis, or arthritis, or enthesitis, which are associated with ≥ 3 months of inflammatory back pain and sacroiliitis on X-ray or MRI images. Arthritis or enthesitis must persist for at least 6 weeks, to which are added the following symptoms: sensitivity of the sacroiliac joint; back pain, the presence of HLA-B27 antigen; previous acute uveitis (symptomatic); and a history of spondylarthritis in a first-degree relative [19,22,23].

Over the last decade, more and more evidence has been gathered suggesting that some of these categories appear to be quite homogeneous and are present both in children and adults; others are heterogeneous and cannot be better defined [24].

### *1.5. Psoriatic JIA*

Psoriatic juvenile arthritis (psJIA) accounts for up to 10% of all JIA subtypes; it is a type of arthritis that affects both sexes and manifests itself in association with psoriatic skin lesions [25].

Psoriatic arthritis is defined within the ILAR classification as a persistent arthritis of more than 6 weeks and either the presence of a psoriatic rash or, in the absence of rash, at least 2 of the following minor criteria: first degree relative with psoriasis, nail pitting, onycholysis, dactylitis; the forms with positive RF and those associated with systemic manifestations are excluded. The peculiarity of psoriatic rheumatism is represented by the presence of ANA in more than 50% of cases and by its association with uveitis [4,25].
