Psoriasis (pronounced /s??ra?.?s?s/) is a chronic, autoimmune disease  that appears on the skin. It occurs when the immune system sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious.[1]  It commonly causes red, scaly patches to appear on the skin, although some patients have no dermatological symptoms. The scaly patches commonly caused by psoriasis, called psoriatic plaques, are areas of inflammation  and excessive skin production. Skin rapidly accumulates at these sites which gives it a silvery-white appearance. Plaques frequently occur on the skin of the elbows and knees, but can affect any area including the scalp, palms of hands and soles of feet, and genitals. In contrast to eczema, psoriasis is more likely to be found on the outer side of the joint.

The disorder is a chronic recurring condition that varies in severity from minor localized patches to complete body coverage. Fingernails and toenails are frequently affected (psoriatic nail dystrophy) and can be seen as an isolated symptom. Psoriasis can also cause inflammation of the joints, which is known as psoriatic arthritis. Ten to fifteen percent of people with psoriasis have psoriatic arthritis.[2],

The cause of psoriasis is not fully understood, but it is believed to have a genetic component and local psoriatic changes can be triggered by an injury to the skin known as Koebner phenomenon[citation needed]. Various environmental factors have been suggested as aggravating to psoriasis including stress, withdrawal of systemic corticosteroid, excessive alcohol consumption, and smoking but few have shown statistical significance.[3] There are many treatments available, but because of its chronic recurrent nature psoriasis is a challenge to treat.

Classification

The symptoms of psoriasis can manifest in a variety of forms. Variants include plaque, pustular, guttate and flexural psoriasis. This section describes each type (with ICD-10 code [5]).[4]

Psoriasis is a chronic relapsing disease of the skin that may be classified into nonpustular and pustular types as follows[5]:414:
[edit] Nonpustular psoriasis

* Psoriasis vulgaris (Chronic stationary psoriasis, Plaque-like psoriasis). Plaque psoriasis (psoriasis vulgaris) (L40.0) is the most common form of psoriasis. It affects 80 to 90% of people with psoriasis. Plaque psoriasis typically appears as raised areas of inflamed skin covered with silvery white scaly skin. These areas are called plaques.

* Psoriatic erythroderma (Erythrodermic psoriasis). Erythrodermic psoriasis (L40.85) involves the widespread inflammation and exfoliation of the skin over most of the body surface. It may be accompanied by severe itching, swelling and pain. It is often the result of an exacerbation of unstable plaque psoriasis, particularly following the abrupt withdrawal of systemic treatment. This form of psoriasis can be fatal, as the extreme inflammation and exfoliation disrupt the body's ability to regulate temperature and for the skin to perform barrier functions.[6]

Pustular psoriasis

Pustular psoriasis (L40.1-3, L40.82) appears as raised bumps that are filled with non-infectious pus (pustules). The skin under and surrounding the pustules is red and tender. Pustular psoriasis can be localised, commonly to the hands and feet (palmoplantar pustulosis), or generalised with widespread patches occurring randomly on any part of the body.

* Generalized pustular psoriasis (Pustular psoriasis of von Zumbusch)
* Pustulosis palmaris et plantaris (Persistent palmoplantar pustulosis, Pustular psoriasis of the Barber type, Pustular psoriasis of the extremities)
* Annular pustular psoriasis
* Acrodermatitis continua
* Impetigo herpetiformis

Other psoriasis

Additional types of psoriasis include[7]:191-197:

* Drug-induced psoriasis

* Inverse psoriasis. Flexural psoriasis (inverse psoriasis) (L40.83-4) appears as smooth inflamed patches of skin. It occurs in skin folds, particularly around the genitals (between the thigh and groin), the armpits, under an overweight stomach (pannus), and under the breasts (inframammary fold). It is aggravated by friction and sweat, and is vulnerable to fungal infections.

* Napkin psoriasis

* Seborrheic-like psoriasis

Guttate psoriasis (L40.4) is characterized by numerous small, scaly, red or pink, teardrop-shaped lesions. These numerous spots of psoriasis appear over large areas of the body, primarily the trunk, but also the limbs, and scalp. Guttate psoriasis is often preceded by a streptococcal infection, typically streptococcal pharyngitis. The reverse is not true.

Nail psoriasis (L40.86) produces a variety of changes in the appearance of finger and toe nails. These changes include discolouring under the nail plate, pitting of the nails, lines going across the nails, thickening of the skin under the nail, and the loosening (onycholysis) and crumbling of the nail.

Psoriatic arthritis (L40.5) involves joint and connective tissue inflammation. Psoriatic arthritis can affect any joint but is most common in the joints of the fingers and toes. This can result in a sausage-shaped swelling of the fingers and toes known as dactylitis. Psoriatic arthritis can also affect the hips, knees and spine (spondylitis). About 10-15% of people who have psoriasis also have psoriatic arthritis.