Psoriasis is a chronic, non-infectious, inflammatory condition of the skin, characterised by well-demarcated erythematous patches and silvery scaly plaques.
It affects 3% of the population worldwide.
Peak of onset in teens and early 20s and late onset 55–60 years.
M = F
Less common in Africa and Japan.
The aetiology is not fully understood but genetic environmental and immunological components are suggested.
· Psoriasis has a familial tendency with 40% of patients having a first-degree relative affected. There is concordance in monozygotic twins and a suggestion of genes located within the major histocompatibility complex close to the class I HLA locus.
· Immunological mechanisms include inflammatory infiltrates within the dermis, activation of growth factors (TGF-α and -β) and expression of various cytokines.
· There is a suggestion of environmental components. Group A streptococcal sore throat can lead to guttate psoriasis, psoriatic lesions occur at sites of trauma and damage (the Koebner¨ phenomenon) and certain drugs may exacerbate psoriasis (β blockers, lithium, antimalarials).
The epidermis is thickened with increased epidermal stem cells and keratinocytes. There is increased cellular DNA synthesis, shortened cell cycle and rapid epidermal turnover (turnover time is reduced from 28 to 4 days).
Psoriasis varies in severity. Typical lesions are clearly demarcated erythematous patches 1–10 cm in diameter. There is a thick silvery scale, which when lifted off characteristically reveals small areas of punctate bleeding. Different distribution patterns are recognised.
Plaque psoriasis is the most common form. It usually affects extensor surfaces especially the elbows and knees, scalp and hair margin or sacrum.
Guttate (drop-like) psoriasis is an acute onset of multiple small psoriatic lesions on the trunk often in a child or young adult with no previous history of pso-riasis. It often follows a streptococcal pharyngitis. It is usually self-limiting.
Pustular psoriasis is the most severe form and can be life-threatening. There is acute onset of diffuse erythema and scaling with sheets of superficial non-infected pustules. If the entire skin is affected, it is termed erythrodermic (the von Zumbusch variant). This may be associated with systemic upset (malaise, fever, diarrhoea) and is potentially life-threatening. Localised forms of pustular psoriasis also occur, such as palmoplantar pustulosis.
Flexural or inverse psoriasis affects the inguinal region, axillae and submammary areas. There may not be scales visible due to moisture, the plaques therefore appear erythematous and smooth.
Nail involvement includes pitting, ridging and ony-cholysis. Nail involvement is specifically associated with psoriatic arthropathy.
Psoriatic arthritis occurs in 5% of patients.
There is infiltration of the strium corneum with neutrophils, epidermal hyperplasia with hyperkeratosis and a thin or absent granular layer. Dilated capillaries are seen in the oedematous papillary dermis.
Psoriasis is a chronic disorder that is managed rather than cured. Treatments are chosen on the basis of disease pattern and severity, patient preference and clinical response.
Emollients both topical and in the bath help reduce the scaling and dryness.
Keratolytic agents (e.g. coal tar) are used both in the form of topical applications and shampoos to remove the scales before applying other treatments.
Topical corticosteroids are often used; however, there is a risk of rebound psoriasis on stopping treatment.
Calcipotriol, a vitamin D analogue, is increasingly used either as single therapy or in combination with topical steroids.
Phototherapy with ultraviolet B (UVB), or with UVA light and an oral psoralen (PUVA), is used in patients with extensive refractory disease. These treatments are expensive and increase the risk of skin cancer. An alternative may be the use of a high-energy laser that treats only the affected skin.
Systemic therapy is used in life-threatening or refractory psoriasis including methotrexate, ciclosporin and retinoids all of which have systemic toxicity requiring monitoring.
Psoriasis is a lifelong disease with variability in severity over time.