Psoriatic arthritis is a chronic inflammatory disease of the joints, spine, and ligaments that often develops in patients with psoriasis.
Psoriatic arthritis  develops in about a third of people with psoriasis. Therefore, all patients with psoriasis should be screened for psoriatic arthritis at least once a year using a self-questionnaire.
The exact causes of psoriatic arthritis have not been fully understood, but it has been proven that the development of the disease is facilitated by the interaction of factors of genetics, immunity and the environment. About half of all patients with psoriatic arthritis have a family history of this disease.
Psoriatic Arthritis Symptoms
As a rule, cutaneous manifestations of psoriasis precede the development of psoriatic arthritis (often for many years), less often arthritis and psoriasis develop simultaneously. In some patients, arthritis occurs earlier than psoriasis. In about 80% of cases, psoriatic nail damage  occurs.
The disease is accompanied by various changes in the skin: the appearance of pink flaky spots, the formation of scars and the appearance of age spots on the nail plates of the fingers and toes. Psoriatic plaques can be very small and can be found on the head near the hairline, under the knees, on the lower leg, elbow, and forearm. Often, such formations spread over the entire area of the body, accompanied by severe itching and discomfort.
Symptoms of psoriatic arthritis are often associated with painful swelling of the joints, especially on the toes and hands, wrists, knees and elbows. The main symptoms of psoriatic arthritis are pain, stiffness and swelling of the joints of the hands and feet.
Other symptoms include inflammation of the fingers like a “sausage” with swelling and purplish-bluish color of the skin, pain in the heels when walking, pain and stiffness in the neck and lower back in the second half of the night which get better after waking up and exercising.
The three main manifestations  are;
Swollen toes and fingers: one major sign  of psoriatic arthritis is the development of a painful, and sausage-like swelling in the toes of finger. Deformities of the hands and feets (such as swelling) may precede the onset of severe joint symptoms.
Foot pain: This disease usually cause the feeling of pain, particularly in your heels (may be due to achilles tendinitis) or the plantar surface of your foot (sole) at the points where tendons and ligaments bind to your bones (plantar fasciitis)
Pain in the lower back: As a result of psoriatic arthritis, some people develop a disease called spondylitis. The intervertebral joint may be inflamed as well as the joints between your pelvis and spine due to spondylitis .
Basically, the disease affects only one joint, but in some cases, several joints can be involved in the inflammatory process. This clinical picture is more similar to rheumatoid arthritis. Signs of psoriatic arthritis may not bother a person for a long time, but in most cases, they appear immediately.