Juvenile idiopathic arthritis, the most common type of arthritis in kids and teens, comprises seven sub-types: oligoarthritis, polyarthritis (RF-positive and RF-negative), systemic, psoriatic arthritis (PsA), enthesitis-related, and undifferentiated.
In the United States, juvenile arthritis occurs in an estimated 20 to 45 children per 100,000 children. Of those children who develop juvenile arthritis, as many as 5 percent have PsA, making it relatively rare. In fact, PsA is one of the least common categories of juvenile arthritis, explains Pamela Weiss, M.D., associate professor of pediatrics and epidemiology at Perelman School of Medicine at the University of Pennsylvania, and attending physician and clinical research director of the Division of Rheumatology at Children’s Hospital of Philadelphia in Pennsylvania.
Why certain juvenile patients develop PsA is a question that is still being researched, although genetics may play into the risk factors. “More than half of children with psoriatic arthritis have at least one family member affected by psoriasis,” says Dr. Weiss. Genetic studies have shown associations with certain genes, she says, although these genetic associations are not used to guide diagnosis or therapy in clinical practice.
Up to one-third of adults with psoriasis may develop PsA, but estimates of risk in children with psoriasis are lacking.  “Signs of inflammatory joint disease (arthritis) in children may include stiffness in the mornings that gets better with activity, swollen or puffy joints, fingers or toes that look like sausages,” says Dr. Weiss. “Onset of psoriatic arthritis seems to be most common during the toddler years or during early adolescence. It is slightly more common in girls than boys.”