Contact dermatitis

Also called allergic contact dermatitis, irritant contact dermatitis

Eczema caused from the outside: irritant (direct chemical injury, no sensitisation needed) or allergic (type IV hypersensitivity). The geometry gives it away, rashes shaped like watchbands, waistbands, or leaf-brush lines.

Try the case

How it looks

Sharply demarcated erythematous, vesicular, weeping plaques in a linear array confined to the exposed site.

Vesicle Erythema Plaque Crust Linear Localized

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

Across skin tones

Acute vesiculation reads similarly across skin tones, but the erythema may be subtle in deeply pigmented skin; chronic irritant dermatitis often presents as hyperpigmented lichenified plaques.

Symptoms & course

Itch (allergic) or burning (irritant)
Onset hours to days after exposure

Diagnostic approach

History is the investigation, map exposures to the rash's shape
Patch testing for recurrent or occupational cases

Differential, and how to separate them

Atopic dermatitis
Favours it: Flexural, symmetric, lifelong history
Against it: Contact lesions are asymmetric and geometry-bound
Cellulitis
Favours it: Fever, tenderness, spreading unilateral warmth
Against it: Contact dermatitis itches more than it hurts and is afebrile

Management principles

Identify and remove the culprit
Topical corticosteroids for the eruption
Barrier protection to prevent recurrence

Clinical pearl: A rash with straight edges or perfect geometry was made by the world, not by biology.

Clinical pearl: Nickel, fragrance, and preservatives are the perennial patch-test culprits.

Keep going

Try the case: Streaks after the weekend Compare with Cellulitis Back to the Atlas