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Author: Hon A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, 1997. Revised August 2014. Skin of colour update: Dr Chelsea Jones, Resident Medical Officer, John Hunter Hospital, Newcastle, NSW, Australia; Dr Monisha Gupta, Dermatologist, University of NSW and Western Sydney University, Sydney, NSW, Australia. December 2020.
Psoriasis is a chronic inflammatory skin condition characterised by clearly defined, red and scaly plaques. It is classified into a number of types.
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.
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 the histocompatibility complex HLA-C*06:02 (previously known as 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 as IL1β and TNFα are responsible for the clinical features of psoriasis. Current theories are exploring the TH17 pathway and release of the cytokine IL17A.
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 with 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 characterised by thickened leathery skin and 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.
Certain features of psoriasis can be categorised to help determine appropriate investigations and treatment pathways. Overlap may occur.
Post-streptococcal acute guttate psoriasis
Chronic plaque psoriasis
Plaque psoriasis is the most common form of psoriasis in all racial groups. Non-Caucasians tend to have more extensive skin involvement than Caucasians. Asian populations are reported to have the highest percentage of body surface area involvement. In skin of colour the plaques are typically thicker with more pronounced silver scale and itch. The pinkness of early patches may be more difficult to appreciate resulting in a low PASI assessment. The thick plaques may appear violet or dark in colour. Plaque psoriasis commonly resolves to leave hyperpigmentation or hypopigmentation in skin of colour, which further impacts quality of life even after disease clearance.
Other types of psoriasis show variable rates in different skin types. Palmoplantar psoriasis is reported to be most common in the Indian population. Non-Caucasians are more likely to present with pustular and erythrodermic psoriasis than Caucasians, whereas flexural psoriasis is said to occur at a lower rate in skin of colour.
Plaque psoriasis in skin of colour
Patients with psoriasis are more likely than others to have associated health conditions such as are listed here.
Psoriasis is diagnosed by its clinical features. If necessary, diagnosis is supported by typical skin biopsy findings.
Medical assessment entails a careful history, examination, questioning about the 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:
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.
Mild psoriasis is generally treated with topical agents alone. Which treatment is selected may depend on body site, extent and severity of psoriasis.
Most psoriasis centres offer phototherapy with ultraviolet (UV) radiation, often in combination with topical or systemic agents.
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 a risk of severe withdrawal flare of psoriasis and adverse effects.
Biologics or targeted therapies are reserved for severe psoriasis resistant to conventional treatment mainly because of expense, as side effects compare favourably with other systemic agents.
Many other monoclonal antibodies are under investigation in the treatment of psoriasis.
Oral agents working through the protein kinase pathways are also under investigation. Several JAK (Janus kinase) inhibitors are under investigation for psoriasis, including tofacitinib and the TYK2 (tyrosine kinase 2) inhibitor BMS-986165; both are in Phase III
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