Psoriasis

Overview

•Chronic, immune-mediated inflammatory skin disease affecting ~2-3% of the UK population

•T-cell mediated; IL-23/IL-17 axis drives keratinocyte hyperproliferation - turnover compressed from 28 days to 3-5 days, producing parakeratosis and silvery scale

•Bimodal onset: 15-35 years and 55-60 years; strong genetic link (HLA-Cw6, ~30% have first-degree relative affected)

Presentation

•Plaques - well-demarcated, raised, erythematous with thick adherent silvery-white scale

•Distribution - extensor surfaces (elbows, knees), scalp/hairline, sacrum, periumbilical; symmetrical

•Auspitz sign - pinpoint bleeding on removal of scale (dilated dermal capillaries exposed)

•Koebner phenomenon - new plaques at sites of skin trauma

•Nail changes - pitting (most common), onycholysis, subungual hyperkeratosis, oil-drop discolouration

•Psoriatic arthritis - present in ~30%; asymmetric oligoarthritis most common; dactylitis ('sausage digit') and enthesitis are characteristic

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Nail psoriasis is present in ~80% of patients who develop psoriatic arthritis. Nail changes + skin disease = actively screen for joint symptoms and refer promptly.

Investigations

•Clinical diagnosis in the vast majority - history and examination sufficient

•Guttate psoriasis - throat swab and ASOT titre to confirm preceding streptococcal infection

•Prior to systemic therapy - FBC, LFTs, U&Es, fasting glucose, lipids

•Suspected psoriatic arthritis - X-ray affected joints (pencil-in-cup deformity in advanced disease); refer to rheumatology

•Gold standard (atypical cases) - skin biopsy; histology shows acanthosis, parakeratosis, elongated rete ridges, Munro microabscesses

Differential Diagnosis

Key differentials
ConditionDistinguishing features
Seborrhoeic dermatitisGreasy scale, scalp/face, less well-demarcated, responds to antifungals
Atopic eczemaFlexural (antecubital/popliteal), intense itch, atopy, finer less adherent scale
Tinea corporisAsymmetric, annular with active scaly border, positive skin scraping
Pityriasis roseaHerald patch then 'Christmas tree' distribution, self-limiting
Lichen planusPurple, polygonal, pruritic papules; Wickham's striae; flexor wrists/oral mucosa
Secondary syphilisCan mimic guttate psoriasis; palmoplantar involvement, systemic features

Condition	Distinguishing features
Seborrhoeic dermatitis	Greasy scale, scalp/face, less well-demarcated, responds to antifungals
Atopic eczema	Flexural (antecubital/popliteal), intense itch, atopy, finer less adherent scale
Tinea corporis	Asymmetric, annular with active scaly border, positive skin scraping
Pityriasis rosea	Herald patch then 'Christmas tree' distribution, self-limiting
Lichen planus	Purple, polygonal, pruritic papules; Wickham's striae; flexor wrists/oral mucosa
Secondary syphilis	Can mimic guttate psoriasis; palmoplantar involvement, systemic features

Management

Step 1 · Mild disease (all stages: add emollients)
1First-line: calcipotriol (vitamin D analogue) - inhibits keratinocyte proliferation; OD-BD on trunk/limbs
2First-line: betamethasone (potent topical corticosteroid) - combined with calcipotriol (Dovobet) commonly used; avoid long-term on face/flexures (atrophy risk)
3Scalp: coal tar or salicylic acid preparations - keratolytic, removes thick scale
4Difficult sites (face/flexures/genitals): tacrolimus or pimecrolimus (calcineurin inhibitors) - off-label where steroids not suitable
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Step 2 · Moderate disease / topical-refractory
1Narrowband UVB (NB-UVB) - preferred phototherapy; 2-3x/week; effective for plaque and guttate psoriasis
2PUVA (psoralen + UVA) - for localised pustular or NB-UVB failure; higher long-term skin cancer risk
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Step 3 · Moderate-severe (PASI ≥10 or DLQI ≥10) - systemic
1Methotrexate - inhibits folate metabolism; give with folic acid 5 mg weekly; monitor LFTs and FBC; contraindicated in pregnancy
2Ciclosporin - rapid onset; useful for severe flares; monitor BP and renal function; not for long-term use
3Acitretin - oral retinoid; best for pustular/erythrodermic psoriasis; highly teratogenic (avoid pregnancy for 3 years after stopping)
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Step 4 · Severe disease failing conventional systemics - biologics
1Anti-TNF-alpha: adalimumab, etanercept - screen for TB and hepatitis B before starting
2Anti-IL-17: secukinumab, ixekizumab - avoid in inflammatory bowel disease
3Anti-IL-23: guselkumab, risankizumab - excellent efficacy and tolerability
4Apremilast - oral PDE-4 inhibitor; for patients unsuitable for biologics; useful for nail and scalp disease

⚠️
Systemic corticosteroids are generally avoided in psoriasis - withdrawal can precipitate life-threatening pustular or erythrodermic flare.

Complications

•Psoriatic arthritis - ~30%; irreversible joint damage if untreated

•Cardiovascular disease - systemic inflammation drives accelerated atherosclerosis; elevated MI and stroke risk; annual cardiovascular risk assessment recommended

•Metabolic syndrome - obesity, hypertension, dyslipidaemia, type 2 diabetes all more prevalent

•Depression and anxiety - quality of life impact is severe and often underestimated

•Erythrodermic / generalised pustular psoriasis - life-threatening; risks include high-output cardiac failure, hypothermia, secondary infection; requires hospital admission

•Uveitis and inflammatory bowel disease - occur at increased frequency (shared immunological pathways)

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Erythrodermic and generalised pustular psoriasis are medical emergencies - admit immediately.

Risk Factors and Triggers

•Drugs - lithium, beta-blockers (propranolol), antimalarials (hydroxychloroquine/chloroquine), NSAIDs, interferon

•Systemic corticosteroid withdrawal - classic trigger for pustular or erythrodermic flare; key reason oral steroids are avoided in psoriasis

•Other triggers: streptococcal infection (guttate), stress, smoking, alcohol, obesity

Severity Assessment

•PASI <10 and DLQI <10 - mild disease; manage in primary care

•PASI ≥10 or DLQI ≥10 - moderate-to-severe; refer to dermatology for systemic therapy