Psoriasis overview

Learning objectives
Introduction
Histology of psoriasis
Clinical features of psoriasis
PASI score
Activity

Learning objectives

Describe the pathogenesis and histology of psoriasis
Outline clinical presentations of psoriasis and their differential diagnosis
List options for treatment

If an adult or child presents with an acquired predominantly scaly eruption, the most likely explanation is psoriasis, which is estimated to affect 1-4% 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 inflammatory arthropathy.

Genetic predisposition
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), interleukins 2 & 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.

Precipitating causes
Although there is often no obvious cause, the following may induce or aggravate psoriasis:

Physical and emotional stress
Infections
- Streptococcal tonsillitis – guttate psoriasis
- Candida – flexural psoriasis
- Malassezia – scalp psoriasis
Physical, chemical, electrical, infective and inflammatory injury (Koebner phenomenon)
Hormones: post-pubertal onset is common; pregnancy may improve or worsen
Medications: withdrawal of the responsible drug may lead to improvement but does not always do so
- Lithium
- Interleukin II
- Interferon
- Beta blockers
- Anti-malarials
- NSAIDs (uncommon)
- Withdrawal from corticosteroids
Ultraviolet radiation
Excessive alcohol
Smoking

Characteristics of psoriatic plaque

Symmetrical distribution in typical sites
Well-demarcated borders
Pink to red colour
Plate-like scale that is easy to peel off
Bleeds easily e.g. on scratching
Linear plaques at sites of skin trauma (Koebner phenomenon)

Psoriatic plaques

PASI score

The PASI score (Psoriasis Area and Severity Index) is used to evaluate severity in plaque-type psoriasis, and ranges from 0 to 72. The PASI evaluates the degree of erythema, thickness, and scaling of psoriatic plaques, and estimates the extent of involvement of each of these components in four separate body areas (head, trunk, upper and lower extremities). In general terms, PASI >12 defines severe, PASI 7-12 moderate and PASI <7 mild chronic plaque-type psoriasis.

The PASI score is imprecise and there have been various attempts to improve on it. However, it remains the most commonly used assessment of the effectiveness of treatment by its ability to result in 75% or, more realistically, 50% reductions in PASI score at certain timepoints.

Histology of psoriasis

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
Thick stratum corneum and projections of epidermis	Note inflammatory infiltrate	Parakeratosis (cell nuclei within thickened stratum corneum)

Clinical features of psoriasis

The skin disease may present as one or other form of acute psoriasis, chronic psoriasis or localised to hands and feet. These subtypes may overlap. Types of psoriasis include:

Acute guttate psoriasis
Chronic plaque psoriasis
Scalp psoriasis
Palmoplantar psoriasis
Flexural psoriasis
Generalised pustular psoriasis
Psoriatic arthropathy
Nail psoriasis

Koebner phenomenon

Psoriatic plaques may develop at the sites of skin trauma, including scratches, sunburn, tight clothing, surgery
The healing skin in those genetically predisposed triggers excessive keratinocytes proliferation
Koebnerised lesions are frequently linear in shape

Koebnerised psoriasis

Psoriatic arthropathy

Several distinct forms including spondyloarthropathy, arthritis mutilans, oligoarthropathy and a rheumatoid-like pattern
All forms of psoriatic arthritis are rare in children.
Although more common in those with severe psoriasis, it may develop in those with mild psoriasis and may flare independently of the skin disease
Systemic treatment with immunomodulating drugs such as methotrexate, ciclosporin, etanercept and infliximab may improve both skin and joint manifestations

PASI score

The PASI score is the ‘Psoriasis Area and Severity Index’, and is mainly used for evaluating the effect of interventions in clinical trials. It is calculated as follows:

Differential diagnosis

Plaque psoriasis may be confused with:

Discoid dermatitis (more itch, vesicles, dry rather than plate-scale)
Tinea corporis (elevated border, slowly extending edge, positive mycology)
Pityriasis rosea (herald patch, fir-tree distribution of oval plaques, trailing scale, duration less than 3 months)
Seborrhoeic dermatitis (thin, paler patches with smaller flakes of scale on scalp, face and sometimes upper trunk)
Psoriasiform drug eruption (unusual features, clears with drug withdrawal)
Pityriasis rubra pilaris (cephalo-caudal spread, follicular prominence)

Generalised pustular psoriasis may be confused with:

Acute generalised exanthematic pustulosis (drug eruption that settles with withdrawal of drug)
Bacteraemia and bacterid: cutaneous pustules arising during gram positive and gram negative septicaemia
Infected generalized atopic and/or seborrheic dermatitis
Pityriasis rubra pilaris
Reiter syndrome
Folliculitis

Investigations

In most cases of psoriasis, no specific investigations are required. However, on occasion the following may be useful:

Skin swabs for bacteriology: to identify secondary infection (Staph. aureus and Strept. pyogenes)
Throat swab for beta haemolytic streptococcus
Skin scrapings and nail clippings to rule out dermatophyte infection
Skin biopsy to confirm diagnosis
Blood count, electrolytes, calcium, liver function in acutely unwell patients

Management

It is most important to explain the nature of psoriasis to the patient presenting with psoriasis for the first time. Provide them with written material and refer them to DermNet (http://www.dermnetnz.org/) for further information. They need to be aware that there is no known cure and that psoriasis may persist lifelong. Treatment rarely results in complete clearance but reduces the severity and extent of the disease.

Topical agents include: