November 2007

This 3-year-old girl presented to the office with multiple lesions on her trunk for several days. She was asymptomatic and otherwise well. She had no sick contacts and no pets. What is your diagnosis?

Answer

The diagnosis is pityriasis rosea. Typically, the initial lesion, a herald patch, can measure as large as two to five centimeters. Within a few days to several weeks, crops of smaller oval lesions (3-5 mm) appear. The distribution tends to be on the trunk and proximal extremities. The pattern on the back is described as a “Fir Tree” pattern, while the lesions on the flanks are arranged like a “school of minnows.”

The lesions themselves are skin-colored or somewhat erythematous with scale. The scale can be located just inside the rim of the lesion (a collarette of scale). In more darkly pigmented children, the lesions may have a more papulovesicular quality in addition to a tendency for more crusting. Pruritus is a variable finding. When the lesions resolve, dyspigmentation may result.

Pityriasis rosea has long been considered to be due to an infectious agent. The evidence for this theory includes: 1) the fact that most patients have pityriasis rosea only once; 2) seasonal variation (more cases in the spring and fall); 3) clustering of cases; and 4) some documented association with respiratory infections.

The differential diagnoses for the herald patch include tinea corporis, erythema migrans, nummular eczema, and fixed drug eruption. The more numerous lesions can be incorrectly diagnosed as secondary syphilis, guttate psoriasis, or nummular eczema.

The lesions can last for six to eight weeks. Pityriasis rosea will typically resolve spontaneously. Topical corticosteroids may reduce any erythema. If pruritus is present, topical anti-pruritics or antihistamines may be used. There are some reports of more rapid resolution when erythromycin is prescribed. Other therapies with anecdotal successes include: azithromycin, ultraviolet B, and acyclovir.

For more information:

• Amer A, et al: The natural history of pityriasis rosea in black American children: how correct is the “classic” description? Arch Pediatr Adolesc Med. 2007;161:503-506.
• Chuh AA, et al: Interventions for pityriasis rosea. Cochrane Database Syst Rev. 2007;(2):CD005068.
• Cook B, et al: Pityriasis rosea. Dermatol Nurs. 2006;18:370.
• Dyer JA: Childhood viral exanthems. Pediatri Ann. 2007;36:21-29.