Allergic skin rash is an acute, generalised eczema/dermatitis that arises in response to a prior localised inflammatory skin disease.
It is also called an id reaction, autosensitisation dermatitis and autoeczematisation.
Id reaction
What causes id reaction?
The cause of id reaction. Theories have suggested it is an immune response to some component of the skin and/or to circulating infectious agents or cytokines (messenger proteins).
Who gets id reaction?
Id reaction can occur in children and adults, but is more often diagnosed in the elderly with a neglected primary rash on the lower leg.
The most common types of eczema / dermatitis that precede id reaction — an eczematid—are:
- Chronic venous eczema
- Acute contact eczema
- Acute or chronic discoid eczema
Infections preceding id reaction include:
- Fungal infection, eg inflammatory tinea pedis or animal kerion—a dermatophytide
- Bacterial infection, eg wound infection or thermal burn—a bacterid
- Viral infection, eg molluscum contagiosum
- Arthropod infestationeg scabies or lice —a pediculid
What are the clinical features of id reaction?
Id reaction presents as an acute, symmetrical, generalised acute eczema. It tends to be extremely itchy, disturbing sleep.
- Forearms, lower legs, thighs and trunk are commonly affected.
- Appearance varies and includes blisters, bumps, crusted plaques (discoid eczema), follicular papules, morbilliform eruption, targetoid lesions and pompholyx(blisters on palms and soles).
- Occasionally, the patient may feel unwell with fever and loss of appetite.
Non-eczematous id reactions include erythema nodosum, Sweet syndrome, guttate psoriasis and blistering diseases.
How is id reaction diagnosed?
The clinical features of id reaction are characteristic. Finding the cause depends on taking a careful history of the initial site of a skin problem. Sometimes the patient does not associate a chronic minor rash with their current widespread and symptomatic eruption.
Additional investigations that may be considered include:
- Dermatoscopyof hair shafts for nits (head lice) and burrows for scabies mites
- Swabs of crusted areas or pustules for bacteriology
- Scrapings of scaly annular or hairless plaques for mycology
- Skin biopsyof primary lesion and/or secondary rash (histology is spongiotic dermatitis)
- Blood count in an unwell patient
- Referral for patch tests, if there is suspicion of contact allergy
Patch testing should not be undertaken during the acute phase of id reaction, but may be planned in several months when it has settled.
What is the treatment for id reaction?
The primary rash needs to be treated vigorously. This may require systemic therapy, eg antibiotics for bacterid or oral antifungal for confirmed dermatophytid.
The secondary eczema is often extensive and highly symptomatic. Treatment may entail:
- Referral for specialist assessment and treatment, including admission to hospital
- Wet wrapsor dressings for weeping eczematous plaques
- Potassium permanganate1:10,000 soaks for localised oozing, infected areas
- Potent topical corticosteroidcreams for 1-3 weeks
- Systemic corticosteroids, eg prednisone or prednisolone for several weeks
- Oral sedating antihistaminesat night