Psoriasis

Common chronic inflammatory dermatosis

Aetiology

  • Genetic predisposition + environmental triggers
    • Associated with specific HLA types
  • Equal sex incidence
  • Two peaks in incidence - 20s and 50s

Precipitating factors

  • Stress
  • Trauma
  • Alcohol and smoking
  • Infection - strep throat
  • Drugs - β-blockers, lithium, anti maliarial drugs, swift withdrawal of topical or systemic steriods ('rebound' psoriasis)

Pathophysiology

  • Triggered by environmental factors in genetically susceptible individuals
    • Bacterial pharyngitis, Koebner phenomenon (mild trauma to skin), interferon therapy, drugs
  • Hallmark of skin lesions is inflammation
  • Plaques are reversible
 
  1. Keratinocytes under stress (environmental factors) release factors that stimulate plasmacytoid DC to produce IFN⍺, and also release Il-1β/IL-6 and TNF
  1. Chemical signals activate DC, which migrate to skin draining lymph node and present to and activate T cells (TH1 and TH17)
  1. T cells stimulate an inflammatory cascade in the dermis involving anti-microbial peptide release and neutrophil-attracting chemokines
    1. Complement attracts neutrophils to keratin layer → Munro micro abscesses
  1. Results in keratinocyte proliferation
  1. CD8+ cells also contribute to pathogenesis
  1. Dermal fibroblasts become involved, which release keratinocytes and epidermal growth factors

Clinical presentation

Symptoms

  • Symmetrically distributed, red scaly plaques with well defined edges
    • Common sites - scalp, elbows and knees
    • Scale is typically silvery white
    • Plaques may have a moist peeling surface
  • Itching - mild or severe
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Signs

  • Scratching can lead to lichenification
  • Koebner phenomenon: psoriasis that develops in an area of trauma
  • Auspitz sign: bleeding points where surface scale is removed
  • Nail changes are seen in many patients with psoriasis
    • Nailbed pitting: superficial depressions in the nailbed
    • Onycholysis: separation of nail plate from nailbed
    • Subungual hyperkeratosis: thickening of the nailbed

Classification

  • Chronic plaque psoriasis: this is the commonest type and causes symmetrical plaques on the extensor surfaces of the limbs (knees + elbows), scalp and lower back
  • Flexural (inverse) psoriasis: smooth, erythematous plaques without scale in flexures and skin folds, colonised by candida yeasts
  • Guttate psoriasis: multiple small, tear-drop shaped, erythematous plaques occur on the trunk after a Streptococcal infection in young adults
  • Pustular psoriasis: multiple petechiae and pustules on the palms and soles
  • Generalised/erythrodermic psoriasis: this is rare but serious form characterised by erythroderma and systemic illness
  • Others:
    • Unstable plaque psoriasis: the rapid extension of existing or new plaques, induced by infection, stress, drugs, or drug withdrawal
    • Sebopsoriasis: overlap of seborrhoeic dermatitis and psoriasis, affects scalp, face, ears and chest, colonised by malassezia
    • Palmoplantar psoriasis: palms and/or soles, keratoderma, painful fissuring

Investigations

Biopsy

  • Rarely needed, may be performed to rule out other diseases
  • Findings:
    • Thickened epidermis with more keratin in the keratin layer
    • Retention of nuclei in keratinocytes (parakeratosis) due to the rapid and abnormal differentiation of keratinocytes
    • Accumulation of inflammatory cells, particularly neutrophils, in the upper epidermis (micro-abscesses)
    • Rete pegs which project down into the dermis are often thicker and longer, often joining together with their neighbours
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Management

Primary care - topical treatment

  • All patients should use an emollient to reduce scale and itch
  • 1st: potent topical corticosteroid OD (eg betnovate) + topical vitamin D OD (eg dovonex) applied at different times
  • 2nd: stop the topical corticosteroid, apply topical vitamin D twice daily
  • 3rd: stop the topical vitamin D, apply potent topical corticosteroid twice daily
  • Other topical treatments:
    • Coal tar preparations - smelly and messy so compliance often poor, most commonly used for scalp psoriasis
    • Dithranol
    • Keratolytics e.g. salicylic acid
    • Retinoids e.g. lazarotene

Secondary care management

  • Phototherapy
    • UVB - classic/plaque and especially for guttate psoriasis
    • PUVA - used in cases not responsive to UVA
  • Systemic therapy - immunosuppressive (e.g. methotrexate), immune modulation e.g. biologics
    • Usually reserved for severe or non-responsive disease