Psoriasis is a chronic inflammatory skin disease characterised by erythematous, circumscribed scaly papules, and plaques and a relapsing-remitting course. It can cause itching, irritation, burning, and stinging.

Epidemiology:

• affects 1-3% of the worlds population
• chronic plaque psoriasis accounts for 80-90% of cases
• Approx. 30% of people with cutaneous psoriasis also have psoriatic arthritis

Pathophysiology/aetiology:

Multifactoral and not yet fully understood

• A combination of genetics, immunological, environmental factors
• genetic: associated HLA-B13, -B17, and -Cw6. Strong concordance (70%) in identical twins
• immunological: abnormal T cell activity stimulates keratinocyte proliferation. There is increasing evidence this may be mediated by a novel group of T helper cells producing IL-17, designated Th17. These cells seem to be a third T-effector cell subset in addition to Th1 and Th2
• environmental: it is recognised that psoriasis may be worsened (e.g. Skin trauma, stress), triggered (e.g. Streptococcal infection) or improved (e.g. Sunlight) by environmental factors

Risk factors:

• Genetic - Most people have a positive family Hx
• Infection - Streptococcal infection, HIV, viral infection - may trigger guttate psoriasis.
• local trauma (Koebner phenomenon)
• drugs: B-blockers, Anti-malarials, Lithium, Indomethacin/NSAIDs (BALI)
• Withdrawl of steroids