What is psoriasis?
Psoriasis is a chronic inflammatory skin condition characterised by clearly defined, red and scaly plaques. It is classified into a number of types.
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 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 hyperproliferation of the skin in 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.
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 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.
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%) vs late age of onset > 50 years
- Acute eg guttate psoriasis vs chronic plaque psoriasis
- Localised eg, scalp, palmoplantar psoriasis vs generalised psoriasis
- Small plaques < 3 cm vs large plaques > 3 cm
- Thin plaques vs thick plaques
- Nail involvement vs no nail involvement
Types 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
Chronic plaque psoriasis
Unstable plaque psoriasis
- The 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 (inverse psoriasis)
- Often the first or only site of psoriasis
- Overlap of seborrhoeic dermatitis and psoriasis
- Affects scalp, face, ears and chest
- Colonised by malassezia
- Palms and/or soles
- Painful fissuring
- Pitting, onycholysis, yellowing, and ridging
- Associated with inflammatory arthritis
- 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
There is some controversy as to whether generalised pustulosis and localised palmoplantar pustulosis are classified as being within the psoriasis spectrum.
How do clinical features vary in differing types of skin?
Plaque psoriasis is the most common type 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
Factors that aggravate psoriasis
- Streptococcal tonsillitis (strep throat) and other infections
- Injuries such as cuts, abrasions, sunburn (koebnerised psoriasis)
- Sun exposure in 10% (sun exposure is more often beneficial)
- Dry skin
- Excessive alcohol
- Medications such as lithium, beta-blockers, antimalarials, nonsteroidal anti-inflammatories, and others
- Stopping oral steroids or strong topical corticosteroids
- Other environmental factors such as a stressful event.
Health conditions associated with psoriasis
Patients with psoriasis are more likely than others to have associated health conditions such as are listed here.
- Inflammatory arthritis “psoriatic arthritis” (an autoimmune disease) and spondyloarthropathy can be seen in up to 40% of patients with early-onset chronic plaque psoriasis.
- Inflammatory bowel disease (Crohn disease and ulcerative colitis).
- Uveitis (a form of inflammation of the eye).
- Coeliac disease.
- Metabolic syndrome: obesity, hypertension, hyperlipidaemia, gout, cardiovascular disease, type 2 diabetes.
- Localised palmoplantar pustulosis, generalised pustulosis, and acute generalised exanthematous pustulosis.
- Non-alcoholic fatty liver disease [see Liver problems and psoriasis].
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 the effect of psoriasis on daily life, and evaluation of comorbid factors.
Validated tools used to evaluate psoriasis include:
- Psoriasis Area and Severity Index (PASI)
- Self-Administered Psoriasis Area and Severity Index (SAPASI)
- Physicians/Patients Global Assessment (PGA)
- Body Surface Area (BSA)
- Psoriasis Log-based Area and Severity Index (PLASI)
- Simplified Psoriasis Index
- Dermatology Life Quality Index (DLQI)
The severity of psoriasis is classified as mild in 60% of patients, moderate in 30% and severe in 10%.
Evaluation of comorbidities may include:
- Psoriatic Arthritis Screening Evaluation (PASE) or Psoriasis Epidemiology Screening Tool (PEST)
- Body Mass Index (BMI ie, height, weight, waist circumference)
- Blood pressure (BP) and electrocardiogram (ECG)
- Blood sugar and glycosylated haemoglobin
- Lipid profile, uric acid
Treatment of psoriasis
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.
- Coal tar preparations
- Salicylic acid
- Vitamin D analogue (calcipotriol)
- Topical corticosteroids
- Combination calcipotriol/betamethasone dipropionate ointment/gel or foam
- Calcineurin inhibitor (tacrolimus, pimecrolimus)
Most psoriasis centres offer phototherapy (light therapy) 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. They can also be used to treat concurrent psoriatic arthritis. These treatment include:
- Tumour necrosis factor inhibitors (anti-TNFα) infliximab, adalimumab and etanercept
- The interleukin (IL)-12/23 antagonist ustekinumab
- IL-17 antagonists such as secukinumab and bimekizumab
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