Id Reaction

An id reaction is a symmetrical, sterile, vesicular or eczematous eruption that appears at a site distant from a primary focus of inflammation. The best known is the dermatophytid, which accompanies inflammatory tinea pedis.

It is uncommon but under reported. It occurs in people with an active, inflamed focus: tinea (particularly kerion or vesicular tinea pedis), stasis dermatitis, contact dermatitis, bacterial infection, scabies or other infestation. Children with kerion are particularly prone.

The id lesions contain no organism. They resolve when the primary focus is treated.

Pathophysiology.

The mechanism is not fully defined. Circulating T cells sensitised to antigens from the primary focus, such as fungal antigens, are thought to react with antigens in distant skin, or immune complexes and cytokines cause distant spongiotic dermatitis. The reaction typically begins 1 to 2 weeks after the primary lesion becomes inflamed.

• Sudden, itchy eruption of small grouped papules and vesicles on the sides of the fingers and palms, which may also appear on the trunk and limbs.
• Often symmetrical, with a primary focus at another site: interdigital or vesicular tinea pedis, kerion, an infected eczema or an infestation.
• Intense itch, and sometimes fever or malaise with extensive disease.
• Lesions resolve over days to weeks after treatment of the primary focus, leaving scaling or pigment change.

Red flags.

• High fever, widespread blistering or erythroderma.
• Bullous or necrotic lesions, suggesting a drug reaction or vasculitis.
• Persistence after treatment of the primary focus.

• Clinical, with identification of the primary focus.
• Potassium hydroxide microscopy and fungal culture from the primary site. The same tests from id lesions are negative.
• Swabs for bacteria and herpes simplex PCR when blistering is painful or punched out.
• Skin biopsy when the diagnosis is uncertain: spongiotic dermatitis with a lymphocytic infiltrate and no organisms.
• Review of drugs to exclude a drug eruption.

Differential diagnosis. Dyshidrotic eczema, contact dermatitis, scabies, drug eruption, tinea manuum, hand, foot and mouth disease, herpes simplex, and bullous pemphigoid.

• Treat the primary focus. Tinea pedis: topical terbinafine 1% cream once or twice daily for 1 to 2 weeks, or clotrimazole 1% cream twice daily for 4 weeks. Oral terbinafine 250 mg daily for 2 to 6 weeks is used for extensive or inflamed disease. Kerion in children needs oral terbinafine or griseofulvin, with the dose adjusted to weight.
• Bacterial infection: flucloxacillin 500 mg four times daily for 7 days, or clarithromycin 500 mg twice daily if penicillin allergic.
• Moderate or potent topical corticosteroid (clobetasone butyrate 0.05% or betamethasone valerate 0.1%) once daily on id lesions for 1 to 2 weeks. Avoid steroid on the active fungal focus unless combined with antifungal treatment.
• Cool compresses and potassium permanganate 1 in 10 000 soaks for blistering.
• Sedating antihistamine such as chlorphenamine 4 mg at night for itch.
• Severe disease: prednisolone 0.5 mg/kg/day (usually 30 to 40 mg) for 5 to 7 days.

Monitoring. Review at 1 to 2 weeks for resolution. Persistence needs reassessment for another diagnosis.

Referral and escalation. Refer if the primary focus cannot be identified, the eruption persists, or it is severe.