Psoriasis

Also called plaque psoriasis, psoriasis vulgaris

A chronic immune-mediated disease of keratinocyte hyperproliferation. The classic plaque form is instantly recognisable once you know its four-part signature: well-demarcated, erythematous, silvery-scaled, extensor.

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How it looks

Multiple well-demarcated erythematous plaques with adherent silvery scale, symmetrically distributed over the extensor elbows and knees.

Plaque Scale Erythema Discrete Koebnerized (isomorphic)Extensor

Rose is the lesion, grey the pattern the lesions make, green where on the body it lives.

Across skin tones

In deeply pigmented skin the erythema reads violaceous or grey rather than salmon-red, and plaques resolve with striking post-inflammatory pigment change that patients may find more distressing than the plaques.

Forms & variants

Plaque psoriasis (psoriasis vulgaris)
The classic form: fixed, well-demarcated, silvery-scaled extensor plaques.
Guttate psoriasis
A sudden shower of drop-like scaly papules over the trunk, classically 2-3 weeks after streptococcal pharyngitis; often the first presentation in the young.
Pustular psoriasis
Sterile pustules on inflamed skin, localized to palms and soles or generalized (von Zumbusch) with fever; the generalized form is an emergency.
Inverse (flexural) psoriasis
Smooth, shiny, sharply bordered red plaques in the axillae, groin, and gluteal cleft; moisture strips the scale, so the razor-sharp edge carries the diagnosis.
Erythrodermic psoriasis
Confluent redness and scaling over nearly the whole body surface; temperature and fluid regulation fail, so this form is admitted, not prescribed for.
Psoriatic arthritis
Inflammatory arthritis in up to a third of patients; dactylitis, enthesitis, and nail disease are the tells, and joints ask about themselves at every visit.

Symptoms & course

Variable itch
Bleeding points when scale is picked (Auspitz sign)
Joint pain in a subset

Diagnostic approach

Clinical diagnosis in typical cases
Biopsy only when atypical
Screen for psoriatic arthritis symptoms at every visit

Differential, and how to separate them

Nummular eczema
Favours it: Coin-shaped, intensely itchy, weepy surface
Against it: Lacks the micaceous scale and extensor preference
Tinea corporis
Favours it: Annular with central clearing
Against it: Psoriatic plaques are filled in, not rings; KOH negative
Seborrheic dermatitis
Favours it: Greasy yellow scale in seborrheic zones
Against it: Psoriatic scale is dry and silvery, sites are extensor

Management principles

Topical corticosteroids and vitamin D analogues first-line for limited disease
Phototherapy for extensive disease
Systemic and biologic therapy for severe disease or arthritis

Clinical pearl: Nail pitting plus a scaly scalp margin often clinches an uncertain plaque.

Clinical pearl: Guttate psoriasis after a sore throat is the classic exam vignette, discrete drop-like papules on the trunk.

Keep going

Try the case: Silver on the elbows Compare with Atopic dermatitis Back to the Atlas