Papulosquamous and Eczematous Disorders
Two families, one interface
Papulosquamous disease (psoriasis, lichen planus, pityriasis rosea, pityriasis rubra pilaris, secondary syphilis) is defined by papules and plaques with scale. Eczematous disease (atopic dermatitis, contact dermatitis, seborrheic dermatitis, nummular eczema, stasis dermatitis, dyshidrotic eczema) is defined by spongiosis histologically and by pruritus clinically.
Concepts to master
- Psoriasis: well-demarcated erythematous plaques with silvery scale on extensor surfaces, Auspitz sign, nail pitting and onycholysis, and the Koebner phenomenon. Recognize variants: guttate, inverse, pustular, erythrodermic.
- Lichen planus: the six Ps - pruritic, purple, polygonal, planar, papules, and plaques - with Wickham striae and oral involvement.
- Pityriasis rosea: herald patch, collarette scale, Christmas-tree distribution on the trunk.
- Atopic dermatitis: flexural distribution, personal or family atopy, and the itch-scratch cycle producing lichenification.
- Contact dermatitis: geometric or exposed-site distribution; distinguish irritant from allergic.
Traps
Inverse psoriasis in flexures mimics intertrigo and candidiasis; look for sharp borders and absence of satellite pustules. Seborrheic dermatitis of the scalp is often mislabeled psoriasis, but psoriasis crosses the hairline and seborrheic dermatitis spares it. Secondary syphilis is the great imitator - consider it in any new widespread papulosquamous eruption, especially with palmoplantar involvement.
Worked example
A young adult with a single annular scaly patch on the back, followed two weeks later by smaller oval patches along skin tension lines. Diagnosis: pityriasis rosea. The herald patch plus collarette scale plus truncal distribution is sufficient; no biopsy is needed unless the eruption is atypical or persistent.