DermNet NZ

Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


What is psoriasis?

Psoriasis is a chronic inflammatory skin condition characterised by clearly defined, red and scaly plaques (thickened skin). It is classified into several subtypes.

Who gets psoriasis?

Psoriasis affects 2–4% of males and females. It can start at any age including childhood, with peaks of onset at 15–25 years and 50–60 years. It tends to persist lifelong, fluctuating in extent and severity. It is particularly common in Caucasians, but may affect people of any race. About one third of patients with psoriasis have family members with psoriasis.

What causes psoriasis?

Psoriasis is multifactorial. It is classified as an immune-mediated inflammatory disease (IMID).

Genetic factors are important. An individual's genetic profile influences their type of psoriasis and its response to treatment.

Genome-wide association studies report that HLA-Cw6 is associated with early onset psoriasis and guttate psoriasis. This major histocompatibility complex is not associated with arthritis, nail dystrophy or late onset psoriasis.

Theories about the causes of psoriasis need to explain why the skin is red, inflamed and thickened. It is clear that immune factors and inflammatory cytokines (messenger proteins) such is IL1β and TNFα are responsible for the clinical features of psoriasis. Current theories are exploring the TH17 pathway and release of the cytokine IL17A.

What are the clinical features of psoriasis?

Psoriasis usually presents with symmetrically distributed, red, scaly plaques with well-defined edges. The scale is typically silvery white, except in skin folds where the plaques often appear shiny and they may have a moist peeling surface. The most common sites are scalp, elbows and knees, but any part of the skin can be involved. The plaques are usually very persistent without treatment.

Itch is mostly mild but may be severe in some patients, leading to scratching and lichenification (thickened leathery skin with increased skin markings). Painful skin cracks or fissures may occur.

When psoriatic plaques clear up, they may leave brown or pale marks that can be expected to fade over several months.

How is psoriasis classified?

Certain features of psoriasis can be categorised to help determine appropriate investigations and treatment pathways. Overlap may occur.

Early age of onset <35 years (75%) Late age of onset >50 years
Acute eg guttate psoriasis Chronic plaque psoriasis
Localised eg scalp, palmoplantar Generalised
Small plaques < 3 cm Large plaques > 3 cm
Thin plaques Thick plaques
Nail involvement No nail involvement

Types of psoriasis

Typical patterns of psoriasis
Post-streptococcal acute guttate psoriasis
  • Widespread small plaques
  • Often resolves after several months
Small plaque psoriasis
  • Often late age of onset
  • Plaques <3 cm
Chronic plaque psoriasis
  • Persistent and treatment-resistant
  • Plaques >3 cm
  • Most often affects elbows, knees and lower back
  • Ranges from mild to very extensive
Unstable plaque psoriasis
  • Rapid extension of existing or new plaques
  • Koebner phenomenon: new plaques at sites of skin injury
  • Induced by infection, stress, drugs, or drug withdrawal
Flexural psoriasis
  • Affects body folds and genitals
  • Smooth, well-defined patches
  • Colonised by candida yeasts
Scalp psoriasis
  • Often the first or only site of psoriasis
  • Overlap of seborrhoeic dermatitis and psoriasis
  • Affects scalp, face, ears and chest
  • Colonised by malassezia
Palmoplantar psoriasis
Nail psoriasis
  • Pitting, onycholysis, yellowing and ridging
  • Associated with inflammatory arthritis
Erythrodermic psoriasis (rare)
  • May or may not be preceded by another form of psoriasis
  • Acute and chronic forms
  • May result in systemic illness with temperature dysregulation, electrolyte imbalance, cardiac failure
Psoriasis Scalp psoriasis
© Dr Ph Abimelec – dermatologue
Plaque psoriasis
Guttate psoriasis Nail psoriasis
© Dr Ph Abimelec – dermatologue

Generalised pustulosis and localised palmoplantar pustulosis are no longer classified within the psoriasis spectrum.

Aggravating factors

Health conditions associated with psoriasis

Patients with psoriasis are more likely than other people to have other health conditions listed here.

How is psoriasis diagnosed?

Psoriasis is diagnosed by its clinical features. If necessary, diagnosis is supported by typical skin biopsy findings.

Assessment of psoriasis

Medical assessment entails a careful history, examination, questioning about effect of psoriasis on daily life, and evaluation of comorbid factors.

Validated tools used to evaluate psoriasis include:

The severity of psoriasis is classified as mild in 60% of patients, moderate in 30% and severe in 10%.

Evaluation of comorbidities may include:

Treatment of psoriasis

General advice

Patients with psoriasis should ensure they are well informed about their skin condition and its treatment. There are benefits from not smoking, avoiding excessive alcohol and maintaining optimal weight.

Topical therapy

Mild psoriasis is generally treated with topical agents alone. Which treatment is selected may depend on body site, extent and severity of the psoriasis.


Most psoriasis centres offer phototherapy with ultraviolet (UV) radiation, often in combination with topical or systemic agents. Types of phototherapy include

Systemic therapy

Moderate to severe psoriasis warrants treatment with a systemic agent and/or phototherapy. The most common treatments are:

Other medicines occasionally used for psoriasis include:

Systemic corticosteroids are best avoided due to risk of severe withdrawal flare of psoriasis and adverse effects.


Biologics or targeted therapies are reserved for conventional treatment-resistant severe psoriasis, mainly because of expense, as side effects compare favourably with other systemic agents. These include:

Related information


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Author:Hon A/Prof Amanda Oakley, Hamilton, New Zealand. Revised and updated, August 2014.

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If you have any concerns with your skin or its treatment, see a dermatologist for advice.