Contact dermatitis - allergic and irritant

contact dermatitis · allergic contact dermatitis · irritant contact dermatitis · contact eczema · occupational dermatitis

The distribution is the diagnosis - if a rash has a shape that matches an object, look for what touched it.

GPBand D

Red flags

Facial or eyelid involvement with significant swelling - consider angio-oedema as an alternative
Widespread spread beyond the contact area with systemic upset - reconsider a drug eruption
Secondary bacterial infection with weeping, crusting or cellulitis
Occupational disease that is threatening someone's employment - needs proper patch testing and occupational health input, not repeated steroid creams
Severe blistering after plant exposure and sun - phytophotodermatitis, which can be extensive

Recognise

On light skin

Red, scaly, itchy patches with a geometric or object-shaped outline; acute allergic disease may vesiculate and weep, irritant disease tends to be drier and fissured.

On brown and black skin

Erythema is again unavailable, and affected skin may look darker, violaceous or ashen. The shape is the diagnosis and shape is independent of pigment - look for a sharply outlined area that matches a watch strap, a waistband, a glove line, a nickel stud or a cosmetic application. Follicular and papular patterns are commoner in brown and black skin and may not look like the scaly plaque expected. Post-inflammatory hyperpigmentation in the exact shape of the contact is a frequent and useful residual clue, and it persists for months - patients often present because of the pigmentation rather than the dermatitis.

In any skin tone

A rash with an outline that matches an object or an exposure pattern
Itch dominates in allergic disease; soreness, dryness and fissuring in irritant disease
Allergic disease has a delayed onset - typically one to three days after exposure - which is why the culprit is often missed
Irritant disease is dose-dependent and improves away from the exposure; ask about weekends and holidays
Hand dermatitis in a healthcare, catering, cleaning or hairdressing worker is occupational until proven otherwise

Distribution

Follows the pattern of contact. Irritant disease is worst where exposure is greatest, classically the hands. Allergic disease may extend beyond the contact area and can spread to distant sites.

Photographs

Allergic contact dermatitis on the forearm a week after contact with a plant. Grouped vesicles and papules on an inflamed base, weeping and crusting in the centre, and - the detail that gives the cause away - **the lesions run in streaks** where the plant was brushed against the skin. A rash in an odd straight line, or in the exact shape of a watch strap, a waistband or a glove, is contact dermatitis until proven otherwise. On darker skin the inflamed base shows as violet or dark brown and the vesicles and the pattern become the main clues. The only openly licensed photograph found for this entry shows light skin. **That is a limitation of this page, not of the disease** - a darker-skin photograph is still being sought.
Fitzpatrick II (Fitzpatrick type estimated from the photograph, not stated by the source) · Nunyabb at English Wikipedia · Wikimedia Commons - Urushiol-induced contact dermatitis 7 days after contact.jpg · Public domain

Provenance for every photograph in this library: image licences. How much of the corpus still has no darker-skin photograph: skin tone coverage.

Mimics

Atopic eczema and infected eczema — The two coexist frequently; atopic disease is flexural and lifelong, contact disease matches an exposure
Tinea corporis, cruris and pedis — An asymmetrical scaly patch may be fungal - check for a raised active edge before prescribing a steroid
Angio-oedema - allergic, ACE-inhibitor and hereditary — Facial swelling without scaling is angio-oedema, not dermatitis
Drug eruptions - morbilliform and fixed — A widespread symmetrical eruption after a new drug is a drug eruption, not contact dermatitis
Pompholyx — Allergic contact dermatitis of the hands can be vesicular and look identical; distribution follows the exposure, and patch testing is what separates them

What to do in the ED

Take the exposure history properly - work, hobbies, cosmetics, jewellery, topical medicines including over-the-counter preparations, and plants
Identify and remove the cause; this is the treatment, and no topical will overcome continued exposure
Topical corticosteroid of potency matched to the site and severity, plus generous emollient
Treat secondary bacterial infection if present
Refer for patch testing where allergic contact dermatitis is suspected, particularly occupational disease - this is the only way to identify the allergen and it is frequently never arranged
Advise on barrier measures and hand care for irritant disease, including glove use and soap substitutes
Document the occupational link clearly if there is one, because it matters for the patient's employment and for any claim

Disposition

Topical treatment and avoidance advice with GP follow-up. Dermatology referral for patch testing where an allergen is suspected, and occupational health involvement for work-related disease.

Safety-netting

The most important treatment is working out what is touching your skin and stopping it - the cream will not win against continued contact. Return if the skin becomes hot, swollen, weeping or crusted or you develop a fever, or if it spreads well beyond the original area. If this is related to your work, ask for a dermatology referral for patch testing rather than repeated creams.

Local variation

Access to patch testing varies considerably and waiting times can be long; refer early rather than after several failed topical courses.

Sources

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