What is psoriatic arthritis?
Psoriatic arthritis is an inflammatory arthritis associated with psoriasis, a chronic skin and nail disease. There are five types of this disease:

• Arthritis involving primarily the small joints of fingers or toes
  
  
• Asymmetrical arthritis, which involves joints of the extremities
  
  
• Symmetrical polyarthritis, which resembles rheumatoid arthritis
  
  
• Arthritis mutilans, which is rare but very deforming and destructive
  
  
• Arthritis of the sacroiliac joints and spine (psoriatic spondylitis)

The exact prevalence of each of these forms of arthritis is difficult to establish. Patterns may themselves change with time in individual patients, and some patients may show overlapping features or more than one type. Sometimes arthritis is associated with inflammation of the eyes, or inflammation at the bony sites of attachment of ligaments and tendons, causing local pain, for example at the heels.

Cause
The exact cause is unknown, but an interplay of immune, genetic, and environmental factors are suspected. Up to 40% of patients with psoriatic arthritis may have a history of psoriasis or arthritis in family members. Both psoriasis and psoriatic arthritis flare up in the presence of immunodeficiency due to HIV infection (AIDS).

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