Psoriasis is a scaly rash that occurs most frequently on the elbows, knees, and scalp, but can cover much of the body. It is a chronic, inflammatory disease of the skin, scalp, nails, and joints. A normal skin cell matures and falls off the body’s surface in 28 to 30 days, but a psoriatic skin cell takes only three to four days to mature and gathers at the surface, thus forming lesions.
Up to 30 percent of people with psoriasis also develop psoriatic arthritis. In most cases (though not always), the psoriasis will precede the arthritis, sometimes by many years. When arthritis symptoms occur with psoriasis, it is called psoriatic arthritis (PsA). In these cases, the joints at the end of the fingers are most commonly affected, causing inflammation and pain, but other joints like the wrists, knees, and ankles can also become involved. This is usually accompanied by symptoms in the fingernails and toenails, ranging from small pits in the nails to nearly complete destruction and crumbling as seen in reactive arthritis or fungal infections.
About 20 percent of patients with PsA will develop spinal involvement, which is called psoriatic spondylitis. Inflammation of the spine can lead to complete fusion, as in ankylosing spondylitis (AS), or affect only certain areas such as the lower back or neck. Patients who are HLA-B27 positive are more likely than others to have their disease progress to the spine.
PsA and AS are considered genetically and clinically related because both are inflammatory rheumatic diseases linked to the HLA-B27 gene. HLA-B27 is a powerful predisposing gene associated with several rheumatic diseases. The gene itself does not cause disease, but can make people more susceptible. While a number of genes are linked to PsA, the highest predictive value is noted with HLA-B27.