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BASICS

  • Generalized skin reaction distant from primary infection/inflammation
  • Commonly referred to with suffixes reflecting cause (bacterid, syphilid, tuberculid)
  • Dermatophytid (fungal-related id reaction) is the most common form
  • Onset usually 1–2 weeks after primary lesion onset or exacerbation
  • Lesions typically localized vesicles, erythema nodosum, erythema multiforme; may include pustules

EPIDEMIOLOGY

  • Affects all ages, sexes, and races equally
  • Incidence unknown; considered common

ETIOLOGY AND PATHOPHYSIOLOGY

  • Exact mechanism unclear; possible immune complex formation, abnormal immune recognition, cytokine-mediated inflammation
  • Infectious triggers:
  • Fungal: Trichophyton mentagrophytes, T. rubrum, Epidermophyton floccosum, Candida spp.
  • Bacterial: Streptococcus pyogenes, Staphylococcus aureus, Mycobacterium tuberculosis
  • Viral: HSV, Molluscum contagiosum, orf, milker's nodules
  • Parasitic: Sarcoptes scabiei, Leishmania spp., Pediculus humanus capitis
  • Allergic causes: Nickel, aluminum allergy, concurrent antibacterial and terbinafine treatment
  • Miscellaneous: retained sutures, cyanoacrylate, radiation, trauma, tattoo ink reactions, intravesical BCG therapy

RISK FACTORS

  • Fungal skin infection, especially tinea pedis
  • Stasis dermatitis

COMMONLY ASSOCIATED CONDITIONS

  • Primary fungal infection
  • Stasis dermatitis

DIAGNOSIS

History

  • Pruritic rash following fungal, bacterial, or other skin infection by days to weeks

Physical Exam

  • Common: Symmetric, pruritic vesicles on palms and lateral fingers
  • Associated findings: tinea infection of feet, eczematous dermatitis, bacterial/fungal/viral infection at primary site
  • Less common: papules, lichenoid eruption, erythema nodosum, eczematoid eruption

Differential Diagnosis

  • Pompholyx (dyshidrotic eczema)
  • Contact dermatitis
  • Drug eruptions
  • Pustular psoriasis
  • Folliculitis
  • Scabies

Diagnostic Tests

  • KOH preparation or fungal culture of primary lesion
  • No fungal elements present at id reaction site
  • Skin testing: positive trichophyton reaction (wheal >10 mm at 20 min; induration >5 mm at 72 hrs)
  • Histology: upper dermal vesicles, superficial perivascular lymphohistiocytic infiltrate, eosinophils, increased granular layer, no infectious agents

TREATMENT

General Measures

  • Treat underlying infection or dermatitis
  • Symptomatic pruritus relief with antihistamines and topical steroids (class 1 or 2)
  • Treat secondary bacterial infections
  • Discontinue causative agents

Medication

First Line

  • Oral antihistamines:
  • Chlorpheniramine 4 mg PO q4-6h PRN (max 24 mg/day)
  • Diphenhydramine 25–50 mg PO q4-6h PRN (max 400 mg/day)
  • Hydroxyzine 25–100 mg PO q6-8h PRN (max 600 mg/day)
  • Topical pruritus treatments:
  • Triamcinolone 0.1% ointment TID
  • Hydrocortisone 0.5%, 1%, 2.5% up to QID
  • Capsaicin cream 0.025%–0.075% TID–QID (EMLA cream pre-application to reduce burning)
  • Doxepin 5% cream QID up to 8 days
  • Permethrin 5% cream for scabies, applied neck down, repeat after 7 days
  • Permethrin 1% rinse for lice, applied for 10 min, repeat after 7 days
  • White petroleum emollients after baths

Second Line

  • Topical/systemic antifungals for fungal infection:
  • Tinea cruris/corporis: topical azoles (econazole, ketoconazole), terbinafine, butenafine
  • Tinea capitis: oral griseofulvin or terbinafine
  • Antibiotics for secondary bacterial infection
  • Antivirals for HSV-associated erythema multiforme

ONGOING CARE

  • Patient education on skin hygiene, drying skin, aeration, wearing loose clothes
  • Prompt treatment of primary dermatitis or fungal infection

PROGNOSIS

  • Complete resolution days to weeks after primary condition treatment
  • Postinflammatory hyperpigmentation common, resolves in ~1 month

COMPLICATIONS

  • Secondary bacterial cellulitis
  • Temporary hyperpigmentation post-dermatophytid

REFERENCES

  1. Ilkit M, Durdu M, Karakaş M. Cutaneous id reactions: a comprehensive review of clinical manifestations, epidemiology, etiology, and management. Crit Rev Microbiol. 2012;38(3):191-202.
  2. Cotes MES, Swerlick RA. Practical guidelines for the use of steroid-sparing agents in the treatment of chronic pruritus. Dermatol Ther. 2013;26(2):120-134.
    Additional reading and case reports as cited in main text.

Clinical Pearls

  • Consider id reaction when a secondary skin eruption follows primary lesion closely in time.
  • Always evaluate for preceding fungal or bacterial infection in itchy rashes.
  • Distinguish from drug-induced allergic eruptions to avoid premature cessation of essential antimicrobial therapy.