Acne, Rosacea, and Disorders of the Pilosebaceous Unit
One unit, several diseases
The pilosebaceous unit underlies acne vulgaris, acneiform eruptions, rosacea, folliculitis, hidradenitis suppurativa, and follicular occlusion disorders. Distinguish them by lesion type, distribution, and trigger.
Concepts to master
- Acne vulgaris: comedones (open and closed) are the sine qua non; inflammatory papules, pustules, nodules, and scars follow. Pathogenesis combines follicular hyperkeratinization, sebum, Cutibacterium acnes, and inflammation.
- Acne grading drives therapy: topical retinoid for comedonal disease; add benzoyl peroxide and topical antibiotic for inflammatory disease; oral isotretinoin for nodulocystic or scarring disease.
- Rosacea: no comedones. Subtypes are erythematotelangiectatic (flushing, telangiectasia), papulopustular, phymatous (rhinophyma), and ocular. Triggers include heat, sun, alcohol, and spicy food.
- Hidradenitis suppurativa: recurrent nodules, abscesses, sinus tracts, and scarring in intertriginous areas (axillae, groin, inframammary).
- Drug-induced acneiform eruptions: monomorphic pustules without comedones, often with corticosteroids, lithium, EGFR inhibitors, or anabolic steroids.
Traps
The most common exam trap is calling papulopustular rosacea "acne" - the absence of comedones and the presence of flushing and telangiectasia settle it. Another is missing acne fulminans or acne conglobata in a patient with systemic symptoms and destructive lesions; these require urgent isotretinoin and sometimes corticosteroids. Also remember that folliculitis is infection or inflammation of the follicle itself, so lesions are centered on hairs.
Rule of thumb
Comedones present → acne. Comedones absent with flushing and central facial erythema → rosacea. Recurrent intertriginous abscesses and sinus tracts → hidradenitis suppurativa.