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Case Note

Case Note: Papular Eruption on the Limbs

History

A 12-year-old female with no significant past medical history presents for management of pruritic papules on the legs and arms.

Initial evaluation

  • Tired but otherwise healthy appearing female
  • Takes no medications
  • Review of systems is notable only for pruritus and occasional lack of sleep in association with the rash
  • Presents with firm erythematous papules with central punctum scattered in a symmetric eruption on the arms and legs, with a few scattered on the trunk without mucosal lesions
    • No linear burrows or stigmata of scabies
    • No evidence of lice on the body or on clothing
    • Started in the springtime and recur in crops
    • No other family member affected
  • There is a cat living with the family
  • Image 1
    Image 1

Enter your diagnosis

Diagnosis

  • A diagnosis of papular urticarial is favored

Initial Treatment

  • No evidence of bacterial superinfection of the skin
  • A regimen of non-sedating antihistamines during the day (loratadine 10 mg twice daily), sedating antihistamine (hydroxyzine 10 mg) at bedtime and topical corticosteroid cream (fluocinonide cream 0.05%) is prescribed
  • The family is counseled and is advised to seek consultation with a veterinarian for evaluation of their pet cat and also a professional exterminator to determine whether the home is infested by fleas
    • Strategies for avoiding bites are discussed
  • Follow up in 4 weeks

Follow-up evaluation strategy

4-week follow-up evaluation:

  • Rash is much improved
    • The family reports that the lesions are less frequent in number and less intense in symptoms
  • The family reports that the veterinarian and exterminator identified a flea infestation involving the pet cat and decontamination of the pet and household environment was ongoing
  • Follow up in 8 weeks

Subsequent 8-week follow-up evaluation:

  • The rash has mostly resolved, with only few lesions now present
  • The daytime non-sedating antihistamines are continued until full resolution of the recurrent lesions

Further recommendations:

  • If secondary infection occurs, treat with appropriate antibiotics (e.g., oral dicloxacillin or cephalexin 250 mg four times daily for 7 days)
  • Make the patient less attractive to biting arthropods or insects by instructing the them to apply an insect repellant containing diethyltoluamide (DEET) daily
    • Permethrin 5% may also be used as an insect repellent - it repels many biting arthropods (but not fleas) for about 1 week
    • If condition is linked to outdoor arthropod/insect biting, instruct the patient to cover as much exposed skin as possible when outside
  • As discussed in this Case Note, exercise appropriate measures to eliminate the presence of fleas within the household

Subsequent treatment steps:

  • Although rare, patients may require a short course of systemic steroids for severe reactions
  • More potent antihistamines (e.g., doxepin 25-50 mg at bedtime) may be helpful
    • They should be continued as suppressive therapy for as long as lesions persist
    • Doxepin is a tricyclic antidepressant, and if used in higher doses (typically above 75 mg daily in adults), cardiac arrhythmias may result
  • Phototherapy (broadband UVB, narrowband UVB, PUVA) 2-3 times weekly can be effective in patients in whom topical steroids and antihistamines are inadequate
    • Response is noted after about 10 treatments
    • Phototherapy can be continued as maintenance treatment

General discussion

Papular urticaria refers to a clinical presentation of hypersensitivity reactions to insect bites that results in a distinct cutaneous morphology and natural history. The classic morphology is a symmetric urticarial to papular or papulovesicular eruption, each lesion usually 0.3-1 cm in diameter, which is intensely pruritic, occurs in crops, and lasts a few days to a week. They may have a central punctum and typically occur on exposed areas of skin (e.g., arms, legs). Though they can be caused by a broad spectrum of insect causes, these lesions are termed papular urticaria, independent of the inciting organism. They are commonly caused by arthropod bites. Other common causes include bites by mosquitos, followed by bites by fleas, chiggers, bedbugs, and black flies. When caused by arthropods, pets or other animals are often the primary host (e.g.,Cheyletiella, animal scabies, mite dermatitis, fleas). Exposure to penetrating body parts (setae) of certain moths and caterpillars may cause similar reactions. This Case Note discusses those hypersensitivity reactions to arthropod bites and exposures from species that do not infest (e.g., live on) humans.

There is increased seasonal incidence of papular urticaria during the spring and summer months. It is important to note that the bites of many insects can trigger hypersensitivity reactions only in certain individuals. This contributes to a key clue to the natural history of this process - that only certain individuals within a household will be affected by papular urticaria, though all household individuals are exposed to the inciting bites. Thus often only a single individual within a household is affected. Children are most commonly affected. They typically present without systemic symptoms and are well appearing. Over time, with repeated exposure, the hypersensitivity reaction wanes and the skin eruption spontaneously resolves or ceases to flare; this may take weeks, months, or even years. In rare cases, resensitization - marked by increased numbers of lesions and symptoms such as pruritus - may occur.

The therapeutic strategy is to identify and eliminate the arthropod source and to suppress the resultant inflammatory reaction. The most difficult step in therapy is identifying the offending arthropod; three factors are needed in this identification:

  • Know the offending arthropods in your geographic region
  • Know the distribution pattern of bites (i.e., lower leg: fleas, chiggers; grass-exposed areas of arms and face: biting flies, mosquitos)
  • Take a careful history from the patient or family members
    • Are there pets in the home?
    • Have arthropods or potential arthropod vectors been seen or collected?
    • Is there occupational, travel, or recreational exposure?
    • In difficult cases, this information is critically important and aids in identifying unusual causes of papular urticaria (e.g., rodent mite dermatitis)
  • Exclude human scabies and lice infestation by careful examination of the patient's body and clothing

Because papular urticaria can clinically mimic other inflammatory conditions, such as pityriasis lichenoides, urticaria, and may have a widespread eruption, many patients will undergo extensive diagnostic evaluation. It is therefore critical for clinicians to have a high level of suspicion for this entity and recognize the typical morphology and natural history. In classic cases, the diagnosis is clinical and additional diagnostic evaluation is not needed.

Further reading

Hernandez RG, Cohen BA. Insect bite-induced hypersensitivity and the SCRATCH principles: a new approach to papular urticarial. Pediatrics 2006;18:e189-96.

Shmidt E, Levitt J. Dermatologic infestations. Int J Derm 2012;51:131-41.

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