Chronic inflammatory skin disease
Characterised by erythematous, circumscribed scaly papules, and plaques on elbows, knees, extensor limbs, scalp, and, less commonly, nails, ears, and umbilical region
Typically lifelong, with a fluctuating course of exacerbations and remission.
Causes itching, irritation, burning, and stinging in half the cases
Trauma, such as surgical scars and injection sites, may result in the appearance of new psoriatic lesions at the sites of injury. 
Smokers have a higher risk of psoriasis.
This has been documented in several population studies.
In one study the risk of having psoriasis was 1.7 to 1.9 times more likely in former or current smokers.
The risk of having pustular psoriasis was even higher at 5.3 times. 
Subcorneal pustular dermatosis
The incidence of psoriasis is around 60 per 100,000 people
In general, about 1.5% to 3.5% of the white population has psoriasis. 
The mean age of onset is 28 years, with equal distribution between men and women. 
Asian populations appear to have a very low prevalence of psoriasis (0.3%). 
The incidence of psoriatic arthritis is 1.4 per 100,000 and a prevalence of 24 per 100,000
The aetiology is unknown. Several factors have been suggested:
Psoriasis appeared after cessation of systemic corticosteroids (rebound)
It is aggravated by the use of anti-malarials, lithium, and beta-blocker drugs
Lesions of psoriasis are associated with increased activity of T cells in underlying skin
Biological agents used to treat severe psoriasis directly modify the function of T cells
Typically erythematous, circumscribed scaly papules and plaques
On elbows, knees, extensor surfaces of limbs, scalp, and, less commonly, nails, ears, and umbilical region
In plaque psoriasis, there are raised inflamed plaque lesions with a superficial silvery-white scaly eruption
The scale may be scraped away to reveal inflamed and sometimes friable skin beneath
In guttate psoriasis, there are widespread, erythematous, fine, scaly papules (water drop appearance) on trunk, arms, and legs.
The lesions often erupt after an upper respiratory infection. 
In pustular psoriasis, (von Zumbusch) there are sterile pustules on the hands and feet
Diffuse or circular erythematous lesions with pustules and scaling on the trunk. 
In erythroderma (erythrodermic psoriasis), there is generalised erythema with fine scaling.
It is often associated with pain, irritation, and sometimes severe itching. 
Psoriasis is around twice as common in white populations as in black people
Skin is highly sensitive and itching can be severe.
Bleeding may occur if the lesions are scratched.
The skin can be painful, particularly if joints are involved
Smokers are at higher risk of psoriasis.
Psoriasis is a hyperproliferative disorder, involving a complex cascade of inflammatory mediators
Mitotic activity of basal and suprabasal cells is significantly increased, with cells migrating from the basal layer to the stratum corneum in just a few days
Cytokines, particularly proinflammatory cytokines, T cells, macrophages, and vascular endothelial growth factor are heavily involved in pathogenesis
Tumour necrosis factor-alpha (TNF-alpha), in particular, has been a target of biological therapy.
TNF-alpha is high in serum, synovium and psoriatic plaques.
Human monoclonal antibodies that block TNF-alpha receptors, or inhibit binding or activation of TNF-alpha receptors, have been shown to significantly control psoriasis
Intraepidermal spongiform pustules and Munro neutrophilic microabscess within the stratum corneum
In addition to these classical features, others include focal parakeratosis and epidermal acanthosis with dilated capillaries within dermal papillae.
Skin biopsy should be ordered only when diagnosis is in doubt, but biopsy does not always show classic pathological features.
Ointments (such as Aquaphore) or thick creams (such as Cerave, Nivea, or Eucerin) that are used to reduce scale and irritation. 
They are available as over-the-counter preparations and should be applied at least once a day, preferably twice a day, but can be applied more often if required
Although both preparations are effective, most patients prefer creams to ointments, and compliance tends to be better with cream preparations.
Generally, the lowest potency of topical corticosteroid should be used.
This often means a mid-potency agent for adults and most body areas.
Low-potency treatments are appropriate for lesions on the face or intertriginous areas or for infants.
High-potency topical corticosteroids are usually reserved for adults requiring short-term treatment of thick plaques that are resistant to lower-potency agents. 
Topical vitamin D analogues
Agents such as calcipotriol bind with vitamin D-selective receptors
Have been shown to inhibit the hyperproliferation and abnormal differentiation of keratinocytes characteristic of psoriatic lesions. 
These agents do not smell or stain clothes and may be more acceptable than tar or dithranol products.
Calcipotriol has a relatively slow onset of action and its maximal effect is after 6 to 8 weeks
For patients with scalp psoriasis, combination preparations consisting of a topical vitamin D analogue and corticosteroid (calcipotriene plus betamethasone dipropionate) are a welcome addition to the available topical therapies. 
These drugs (e.g., acitretin) are moderately effective in many cases and are often combined with other treatments.[C Evidence]
Treatment is not recommended for >6 months.
Liver function and blood lipid concentration should be monitored.
Women of childbearing ages are not suitable for this regimen as retinoid agents are teratogenic.
Folic acid antagonist and works as an antiproliferative and anti-inflammatory agent.[C Evidence]
Although effective in most patients, it has the potential for hepatotoxicity. 
Methotrexate is contraindicated in the following groups:
pregnant patients; people with renal impairment, hepatitis, or cirrhosis;
people who abuse alcohol;
patients with leukaemia or thrombocytopenia
Folic acid may be used in addition to methotrexate to minimise adverse effects (such as GI symptoms).
An effective treatment for psoriasis but has significant adverse effects.[C Evidence]
Long-term use (i.e., >12 months) is not recommended.
A break (i.e., drug vacation) is recommended after 1 year, switching to other drugs such as methotrexate.
Ciclosporin can then be restarted
Newer biological therapies are recommended as possible treatment
if the psoriasis is very severe and the disease has not improved with other treatments such as ciclosporin, methotrexate, or PUVA,
or the patients have had adverse effects with these in the past, or such therapy is contraindicated. 
Has a dual mechanism of action that involves induction of T-lymphocyte apoptosis and interruption of T-lymphocyte activation.[A Evidence]
Inhibits tumour necrosis facto-alpha (TNF-alpha), an important cytokine involved in the pathogenesis of psoriasis
Has been shown to significantly reduce the severity of plaque psoriasis.[A Evidence]
Furthermore, etanercept has been demonstrated effective in treating psoriasis in adults, children, and adolescents.
Also inhibits the activity of TNF-alpha and has been shown to have efficacy in the treatment of chronic plaque psoriasis
It has also been demonstrated to improve health-related quality of life in patients with psoriasis. 
Meta-analysis has demonstrated that infliximab is more effective than adalimumab and etanercept. 
Humanized monoclonal antibody
Inhibits Interleukins 12 and 23.
It has been shown to be effective in clinical trial for psoriasis. 
The re-PUVA regimen consists of methoxsalen and ultraviolet A (PUVA) with an oral retinoid
May prevent antigen presentation process by Langerhans cells in the skin
Acitretin is given the day before PUVA therapy, which enhances the efficacy.
Adverse features include inconvenient scheduling (treatment is delivered 3 to 5 times per week), phototoxicity (during and after treatment), and burning if the dose is not adequately controlled
Psoriasis is a chronic illness with a fluctuating course
All treatment options have unwanted factors, and it can be difficult to find a balance between treatment effects and the disease.
No cure is in sight, but long remission is a possibility.