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CHAPTER 11  |  Juvenile Idiopathic Arthritis

A B Figure 11-2.  Juvenile psoriatic arthritis. A: Nail pitting associated with psoriasis. B: Swelling of a single DIP joint in a child with juvenile psoriatic arthritis.

Etiology.  The etiology of psoriatic arthritis is unknown but genetic associations with HLA-Cw6, DRB1*01, and DQA1*0101 have been demonstrated (14, 43). There is often a strong family history of psoriasis or psoriatic arthritis in affected children. Clinical Features.  The arthritis in psoriatic JIA is often an asymmetric mono- or polyarthritis affecting both large and small joints. At onset, patients may have pitting of the nails (67%) (Fig. 11-2) and a family history of psoriasis (69%) or dactylitis (39%), while less than one-half of the children have the rash of psoriasis (13% to 43%) (25, 44, 45). JIA criteria do not require the development of psoriasis to confirm a diagnosis of psoriatic arthritis (Table 11-2) (46). In children younger than 5 years, the presentation is often characterized by the ­involvement of a small number of fingers or toes that are relatively asymptomatic, but leading to marked overgrowth of the digit(s). Children with psoriatic arthritis may have chronic life- long arthritis that follows a relapsing and remitting course. Arthritis mutilans and severe distal interphalangeal (DIP) joint disease are unusual. However, many of the children will have prolonged polyarthritis that may result in irreversible joint damage (47). Amyloidosis has been reported in the European literature as having resulted in the deaths of at least three chil- dren (47, 48). Chronic anterior uveitis has been observed in up to 17% of the children (44, 45) and is associated with a positive ANA titer; the uveitis associated with psoriatic JIA is clinically indistinguishable from the uveitis in oligoarticular and polyarticular JIA. Enthesitis-Related Arthritis Definition.  The JIA criteria for classification of ERA describe a group of arthritides that includes undifferentiated spondyloarthritis, JAS, and IBD-associated arthritis. The JIA criteria include many of the children who were previously diagnosed with a syndrome of seronegativity, enthesopathy,

than that seen in Europe. The reason for this discrepancy remains unclear. Macrophage activation syndrome (MAS), also termed hemophagocytic lymphohistiocytosis , is a severe, potentially life- threatening complication seen nearly exclusively in systemic arthritis. It is characterized by macrophage activation with hemophagocytosis and is associated with hepatic dysfunc- tion, disseminated intravascular coagulation with a precipitous fall in the erythrocyte sedimentation rate (ESR) secondary to hypofibrinogenemia, and encephalopathy (36). It has been suggested that anti-inflammatory medications and viral infec- tions can induce this syndrome. High-dose corticosteroids, cyclosporine A, and IL-1 inhibition have been shown to improve the ­outcome of MAS (37–39). Psoriatic Arthritis Definition.  Psoriatic arthritis is defined as the presence of arthritis and psoriasis, or arthritis and at least two of the fol- lowing: (a) dactylitis, (b) nail pitting or onycholysis (Fig. 11-2), and (c) psoriasis in a first-degree relative. Exclusions to a diag- nosis of psoriatic JIA include the following: (a) arthritis in a first-degree relative after the age of 6 years; (b) AS, enthesitis- related arthritis sacroiliitis with IBD, reactive arthritis, or acute anterior uveitis, or a history of one of these in a first-degree relative; (c) presence of IgM RF on at least two occasions, mea- sured 3 months apart; and (d) systemic JIA (8). Epidemiology.  Psoriasis occurs in approximately 0.5% of the population (40), 20% to 30% of whom have associated arthritis (41, 42). There is a bimodal distribution of age of onset with a peak in the preschool years and again around 10 years of age. Girls are slightly more affected than boys. Psoriasis often begins after the onset of arthritis, usually within 2 years. The prevalence of psoriatic JIA is estimated at 15 per 100,000 children (15). Psoriatic arthritis accounts for 5% to 7% of JIA (13).