PEDIATRIC DERMATOLOGY

Blisters on a boy's shins:
What's your Dx?

By Bernard A. Cohen, MD

After complaining several days earlier of itchy, red bumps on both legs, a 6-year-old boy awoke one morning to find large blisters on his shins.

The boy was otherwise well, and within the normal range for growth and development. He took no oral or topical medications, and his parents and older siblings did not have a rash.

1. What's the diagnosis?

2. What would you consider in the differential?

3. What would you do for further evaluation?

4. How would you treat this boy?

Diagnosis: Papular urticaria

This boy's rash is an example of papular urticaria, an insect-bite hypersensitivity reaction that was, in this case, probably caused by fleas. Edematous papules in linear groupings of three or four lesions on the lower extremities illustrate what is known as the "breakfast, lunch, and dinner" pattern typical of "fleabitis" in young children.

Course. Papular urticaria is an allergic reaction, usually caused by the feeding bites of a blood-sucking insect. The condition occurs most often in children 2 to 10 years of age.^1–5 It is rare in children younger than 1 year; in infants, a petechia or small crust is usually all that is seen after an insect bite, and no other signs or symptoms occur.

After a number of bites by a particular species of insect, sensitization may occur, with an immediate or a delayed reaction.² The immediate reaction appears five to 20 minutes after the bite, but rarely persists beyond one hour. The urticaria of an immediate reaction consists of a round or irregular white wheal with a surrounding red flare. The immediate reaction is replaced by a delayed reaction, which occurs as early as 20 minutes after a bite, although onset may be delayed for several hours. As sensitized children get older, the delayed reaction tends to begin hours, even days, after the bite.

Tolerance with little or no delayed reaction develops by 10 to 12 years of age. If a delayed bite reaction does occur, it is characterized by itchy, firm, round 5- to 10-mm red edematous papules with a central punctum. Primary lesions may be obliterated by excoriations that lead to lichenification, crusting, and erosions. The primary lesions usually heal within two to four weeks, but, when pruritus and scratching is severe, lesions may persist for months. In some children, inflammation and edema can be potent enough to trigger blisters, as happened in our patient.

Differential diagnosis. Although bite reactions are seldom characteristic enough to suggest a specific insect, you may find some diagnostic clues to the culprit.^1,3–5 Bites made primarily on the lower legs point to the cat flea (Ctenocephalides felis), found on cats and dogs in temperate climates. In nonambulatory, sensitized children, the bites may appear on any unclothed site.

Papular urticaria from bird fleas (Ceratophyllus gallinae) are usually found on the head, shoulders, and arms, especially when infested birds nest in roof spaces above bedrooms. The human flea, Pulex irritans, now rare in developed countries, often travels under clothing and produces bites in groups of two or three from a single insect's interrupted feedings. The cat flea may also produce grouped linear papules, but these tend to be restricted to exposed sites.

Papular urticaria rarely causes an anaphylactic reaction; the most common complication is secondary bacterial infection. When purulent drainage, pain, erythema, and edema spread beyond the primary papule, consider bacterial infection. Expanding superficial bullae with central crusting suggest bullous impetigo from toxin-producing Staphylococcus aureus; occasionally, staphylococcal scalded skin syndrome may erupt in toddlers and preschool children. Postinflammatory hyperpigmentation can be prominent and occasionally disfiguring, particularly in darkly pigmented individuals.

Our patient's papular urticaria demonstrates the typical linear grouping of edematous papules. The lesions on the lower extremities and the absence of a reaction in other family members are also classic.

Evaluation. Although clinical findings usually point to the diagnosis, skin biopsy is often useful when incredulous parents push for laboratory confirmation. Histologic findings are not always diagnostic, however; an intense mix of dermal infiltrate and eosinophils is characteristic.

The differential diagnosis includes other external injuries (hot water burn, contact with a caustic agent, mechanical trauma) that cause urticarial papules or blisters. Allergic contact dermatitis appears as acute linear red papules, plaques, and blistering in highly sensitized people, but the pattern and course of the rash usually allow you to distinguish it from papular urticaria.

Early lesions of varicella may also be confused with papular urticaria, especially when they group around areas of external trauma. Varicella vesicles have a characteristic evolution. They are widely disseminated on the skin and mucous membranes, and usually are accompanied by constitutional symptoms.

How to treat. The critical component of treatment is prevention.^3–5 Susceptible children should wear protective clothing with long sleeves and pants whenever possible. Animal sources of fleas and outdoor areas such as sandboxes should be identified and disinfected. An insect repellent with a low concentration (less than 10%) of DEET (N,N-diethyl-m-toluamide) may help, and can be applied to clothing and, with care, directly to the skin.

Localized care with cool tap water compresses, formulations of calamine lotion, and a moderate or high-potency topical steroid may relieve symptoms. Antihistamines may soothe pruritus, particularly at bedtime. When secondary bacterial infection is suspected, a topical or oral antibiotic should be started.

REFERENCES

1. Maunder JW: Papular urticaria, in Harper J, Oranje A, Prose N (eds): Textbook of Pediatric Dermatology. London, Blackwell Science, 2000, pp 549–554

2. Stibich AS, Carbonaro PA, Schwartz RA: Insect bite reactions: An update. Dermatology 2001;202:193

3. Stibich AS, Schwartz RA: Papular urticaria. Cutis 2001;68;89

4. Howard R, Frieden IJ: Papular urticaria in children. Pediatr Dermatol 1996;13:246

5. Millikan LE: Papular urticaria. Semin Dermatol 1993;12:53

For this case and related images or to contribute new images from your own cases, see Dr. Cohen's Web site at www.med.jhu.edu/peds/dermatlas .

DR. COHEN, who serves as section editor for Pediatric Dermatology: What's your Dx?, is director, Pediatric Dermatology and Cutaneous Laser Center, and associate professor of pediatrics and dermatology at Johns Hopkins University School of Medicine, Baltimore. He is a contributing editor for Contemporary Pediatrics.

 

Pediatric Dermatology: What's your Dx?. Contemporary Pediatrics 2002;1:111.