Id reactions (syn. ide reactions; autoeczematisation)

An id reaction is a secondary immunologic host response associated with, but usually distant to, another inflammatory or infectious skin condition. The most common associations are gravitational eczema, dermatophyte infections (tinea) of the feet, and contact allergic dermatitis. 

This chapter is set out as follows:

• An id reaction is often intensely itchy

In general, id reactions tend to be more severe near to the primary process and decrease in intensity with distance from the primary process, but there are exceptions. For example, patients with tinea pedis may have id reactions of the palms. Clinical findings include erythematous papules or small plaques, scale, vesicles, and bullae (blisters). Features vary depending on the cause: 

• Gravitational eczema - can give rise to an id reaction 1-2 weeks after the initial inflammation and is most frequently located on the forearms, thighs, trunk, and face

• Dermatophyte infection - crops of vesicles and/or bullae on the palms and backs of the hands resembling pompholyx may arise as a result of a moderate-severe tinea pedis or other dermatophytic infection, this reaction is sometimes referred to as a dermatophytide reaction. Patients can also develop id reactions shortly after the initiation of antifungals, which should not be misdiagnosed as a medication allergy. Patients should continue on their antifungal and be treated symptomatically for the id reaction

• Allergic contact dermatitis - such id reactions occur most frequently in children, often resulting in a reaction distal to the initial site of irritation. Development of an id reaction is particularly common with nickel dermatitis

• The most important aspect of treatment is managing the cause of the reaction (eg tinea)
• In terms of the id reaction, this is generally eczematous and can be managed with emollients and topical steroids if needed