Psoriatic arthritis (PsA) is a long-term inflammatory arthritis that occurs in people affected by the autoimmune disease psoriasis. The classic feature of psoriatic arthritis is swelling of entire fingers and toes with a sausage-like appearance ("sausage digit"). This often happens in association with changes to the nails such as small depressions in the nail (pitting), thickening of the nails, and detachment of the nail from the nailbed. Skin changes consistent with psoriasis (e.g., red, scaly, and itchy plaques) frequently occur before the onset of psoriatic arthritis but psoriatic arthritis can precede the rash in 15% of affected individuals. It is classified as a type of seronegative spondyloarthropathy.
Genetics are thought to be strongly involved in the development of psoriatic arthritis. Obesity and certain forms of psoriasis are thought to increase the risk.
Psoriatic arthritis affects up to 30% of people with psoriasis and occurs in both children and adults. Approximately 40–50% of individuals with psoriatic arthritis have the HLA-B*27 genotype. The condition is less common in people of Asian or African descent and affects men and women equally.
Signs and symptoms
Pain, swelling, or stiffness in one or more joints is commonly present in psoriatic arthritis. Psoriatic arthritis is inflammatory, and affected joints are generally red or warm to the touch. Asymmetrical oligoarthritis, defined as inflammation affecting two to four joints during the first six months of disease, is present in 70% of cases. However, in 15% of cases, the arthritis is symmetrical. The joints of the hand that is involved in psoriasis are the proximal interphalangeal, the distal interphalangeal, the metacarpophalangeal joint, and the wrist. Involvement of the distal interphalangeal joints is a characteristic feature present in many cases. Nail pitting often accompanies distal interphalangeal joint involvement and may be essential in differentiating psoriatic arthritis from other diseases.
In addition to affecting the joints of the hands and wrists, psoriatic arthritis may affect the fingers, nails, and skin. Sausage-like swelling in the fingers or toes, known as dactylitis, may occur. Psoriasis can also cause changes to the nails, such as pitting or separation from the nail bed, onycholysis, hyperkeratosis under the nails, and horizontal ridging. Psoriasis classically presents with scaly skin lesions, which are most commonly seen over extensor surfaces such as the scalp, natal cleft and umbilicus.
In psoriatic arthritis, pain can occur in the area of the sacrum (the lower back, above the tailbone), as a result of sacroiliitis or spondylitis, which is present in 40% of cases. Pain can occur in and around the feet and ankles, especially enthesitis in the Achilles tendon (inflammation of the Achilles tendon where it inserts into the bone) or plantar fasciitis in the sole of the foot.
Along with the above-noted pain and inflammation, there is extreme exhaustion that does not go away with adequate rest. The exhaustion may last for days or weeks without abatement. Psoriatic arthritis may remain mild or may progress to more destructive joint disease. Periods of active disease, or flares, will typically alternate with periods of remission. In severe forms, psoriatic arthritis may progress to arthritis mutilans which on X-ray gives a "pencil-in-cup" appearance.
Because prolonged inflammation can lead to joint damage, early diagnosis and treatment to slow or prevent joint damage is recommended.
Causes
Psoriatic arthritis is an inheritable polygenic disease, with many genes known or theorized to contribute to its clinical presentation (or lack thereof). When someone with the genes for psoriatic arthritis comes into contact with certain substances, these substances may induce an autoimmune reaction, causing the immune system to target normal tissues in the body. The exact strength, location, and clinical effects of this reaction depend on which genes are involved for each individual. The substance that triggers the reaction is typically not known.
Genomic analysis has identified several genes involved in some patients, notably genes related to class I MHC including HLA-B*08, HLA-B*27, HLA-B*38, and HLA-B*39. Other genes relating to the immune system and central tolerance may also be involved, such as interleukin receptor genes. Thematically, these genes are often those that identify human tissues as normal and healthy, or the genes in immune cells designed to recognize those identifiers. If the genes are functioning abnormally, then the immune system has a higher risk of attacking normal tissues.