logo Psoriasis

Scaly skin diseases



  • Describe several clinical types of psoriasis.

Key points

  • About one third of patients with psoriasis have a family history of the disease.
  • Psoriasis is characterised by Th1 (cytotoxic) lymphocyte response.
  • Tumour necrosis factor alpha and other cytokines stimulate keratinocyte proliferation.
  • Guttate psoriasis is an acute eruption of small scaly plaques following haemolytic streptococcal infection.
  • Linear psoriatic plaques may follow trauma (Koebner phenomenon).
  • Psoriasis is characterised by well demarcated erythematous scaly plaques.
  • Inverse psoriasis refers to flexural plaques.
  • Psoriatic nail dystrophy results in ridging, pitting, onycholysis, subungual hyperkeratosis.
  • Erythrodermic psoriasis and generalised pustular psoriasis may result in fluid imbalance, temperature disturbance and high output cardiac failure.
  • Localised polmoplantar pustulosis is associated with smoking


If a patient presents with an acquired predominantly scaly eruption, the most likely explanation is psoriasis, which is estimated to affect 2-4% of the population. Itching may be absent, but tends to be mild to moderate. Psoriasis is a chronic dermatosis of genetic origin, often precipitated by an event such as an infection, an injury or psychological stress. It is associated with arthropathy.

There appear to be at least seven psoriasis susceptibility genes. Abnormal expression of one or more of these leads to infiltration of psoriatic plaques by Th1 or cytotoxic lymphocytes and subsequent overproduction of certain pro-inflammatory cytokines, particularly tumour necrosis factor alpha (TNF-α), interferon gamma (IFN-γ), interleukin 2 and 12. The cytokines stimulate keratinocyte proliferation. Epidermal cells take about a week instead of the normal month or two to transit through the skin, with an increased number of actively dividing cells and an increased rate of reproduction. This is a similar pattern to healing wounds.

Histology of psoriasis is characterised by parakeratosis (cell nuclei within stratum corneum) and thickened projections of the prickle cell layer of keratinocytes (psoriasiform hyperplasia). There is no granular layer. Polymorphonuclear leukocytes and lymphocytes infiltrate dermis (CD8+) and epidermis (CD4+).

Histology of psoriasis:

Thickened stratum corneum and projections of epidermis

Note inflammatory infiltrate

Parakeratosis (cell nuclei within markedly thickened stratum corneum)

There are several ways psoriasis may present. The main features are listed below.

Acute guttate psoriasis

Guttate psoriasis


Child with guttate psoriasis

Koebner phenomenon

Koebnerised psoriasis

Chronic plaque psoriasis

Range of appearance of chronic plaque psoriasis

Scalp psoriasis

Scalp psoriasis

Flexural psoriasis

‘Inverse’ psoriasis

Nail psoriasis

Subungual hyperkeratosis



Complete nail destruction

Mixed psoriatic nail dystrophy

Erythrodermic psoriasis

Erythrodermic psoriasis

Generalised pustular psoriasis

Generalised pustular psoriasis

Note superficial pustules

Pustules within plaque psoriasis

Palmoplantar pustulosis

Plantar pustulosis

Acute palmar pustulosis

Palmoplantar keratoderma

Drug-induced psoriasis

Psoriatic arthropathy