decades of life. PA, occurring in about 5Ð7% of patients with psoriasis, has an estimated prevalence in the population of 0.10%. Onset is more frequent between 30 and 40 years of age and women are slightly more frequently involved than men (1.04:1). Family history may be obtained in one-third of patients, implying a role for genetic and/or environmental factors. Psoriasis and PA are associated with HLA-B13, -B17, -B27, -Bw37, -Bw38, -Bw39, and -Cw6 genes. The association of HLA-B27 is with psoriatic sacroiliitis and spondylitis (50%) but not with psoriatic peripheral arthritis or psoriasis [226, 260]. The developments of guttate psoriasis following streptococcal infections, and of PA following trauma to the joint, have been recognized for many years [226, 259].

6.1.5.2 Systemic Manifestations

Psoriatic arthritis is characterized by skin and articular involvement. Other systemic Þndings, such as amyloidosis, apical pulmonary Þbrosis, and aortic insufÞciency, are seen only rarely [226]. Pustular skin lesions, due to small-vessel vasculitis, may occasionally appear.

Skin and Articular Involvement

The skin disease usually precedes the articular involvement by many years [261, 262], but in about 10% of patients with PA arthritis appears at the same time as psoriasis. Rarely, arthritis may precede psoriasis.

Skin lesions in patients with PA commonly begin on the elbows, followed by appearance on the legs, scalp, abdomen, and back (Fig. 6.15). Although arthritis is more common in patients with severe skin involvement than in those with mild involvement [263, 264], a careful search for minimal psoriatic lesions in the axilla, under the breast, in the umbilicus, or on the genitalia needs to be made in cases in which PA is suspected from the pattern of arthritis. Nail changes are more frequent in patients with PA (80%) than in patients with psoriasis without arthritis (30%). They are characterized by onycholysis, pitting, ridging, and nail discoloration or fragmentation (Fig. 6.16).

Fig. 6.15 Psoriasis: typical scaly, erythematous dermatitis of psoriasis on the abdomen, with some areas having characteristic silver borders

Fig. 6.16 Nail pitting in a patient with psoriasis; the pattern is nearly pathognomonic for this disease

There are at least Þve patterns of joint involvement in PA [261, 262]: (1) asymmetric monoarticular arthritis (5Ð10%) involves the distal interphalangeal joints of the Þngers and toes, and is often associated with diffuse swelling of the digits (sausage digits) and with nail lesions; (2) chronic asymmetric oligoarticular arthritis (50Ð 70%) affects two or three joints at a time; (3) chronic symmetric polyarthritis (15Ð25%) resembles RA, but the test for rheumatoid factor is negative; (4) spondyloarthritis (20Ð30%) is characterized by sacroiliitis with or without spondylitis, is more common in males than in females, and has strong association with HLA-B27; (5) erosive Òarthritis mutilans,Ó the most uncommon