A scaly and pink rash

A teenager is embarrassed by an itchy rash.

CASE PRESENTATION

A 15-YEAR-OLD male presented with a moderately itchy rash on the trunk of three weeks’ duration. He experienced transient malaise, fever and headache several days before the rash appeared. On further questioning, an initial solitary scaly patch on the shoulder had appeared several days before the current rash. What is the likely diagnosis?

Figure 1. An itchy rash on the trunk and proximal limbs (secondary eruption of PR).

Figure 2. Individual patches and plaques with inward-turning peripheral scales (collarettes).

DIAGNOSIS

Pityriasis rosea (PR) is a descriptive term for scaly (pityriasis) and pink (rosea). PR is an acute, self-limiting skin eruption that typically presents with an initial primary plaque (‘herald patch’) followed by a generalised secondary eruption 1-2 weeks later. In a quarter of cases, a mild prodrome of malaise, fever and headache may accompany the eruption.

The herald patch, when present, greatly assists in the diagnosis. This occurs several days before the onset of the secondary rash. The distribution of the secondary eruption tends to follow the natural cleavage or skin fold lines. On the back, the arrangement resembles a ‘Christmas tree’ pattern. Individual lesions, including the herald patch, typically have fine peripheral scales with the free edge pointing inwards, resembling an inward-pointing collar (collarette).

When all the characteristic PR features are present, the diagnosis is straightforward, and investigation (e.g. skin biopsy) is rarely required. Conditions that may occasionally cause diagnostic confusion include secondary syphilis (check serology if indicated), guttate psoriasis, discoid eczema, tinea corporis and a drug eruption.

Up to a quarter of cases may have atypical features such as absence of the herald patch, peripheral or flexural distribution of the secondary rash, involvement of palms, soles and face, and lesions with atypical morphology (vesicular, pustular, purpuric). Atypical PR cases often require dermatology assessment.

MANAGEMENT

Reassure and educate the patient that the condition is self-limiting and usually clears in 8-10 weeks. Repeat attacks are rare. Asymptomatic patients do not need specific treatment. Pruritus can be relieved with adequate moisturising, topical steroids and antihistamines. Natural sunlight (UV) and phototherapy may be helpful to reduce rash severity but not necessarily the rash duration.

This patient was treated with a moderate-potency topical steroid (Celestone M cream) and sorbolene. He was significantly less itchy one week after starting treatment.

Two weeks later, he returned, concerned the rash was not resolving (although less itchy), and was embarrassed by its appearance at school. With parental consent, he was started on a course of UVB phototherapy. Although PR is only modestly responsive to phototherapy, the secondary tanning effect of phototherapy in this case helped mask existing PR lesions resulting in an improved appearance.

Pityriasis rosea key facts

• Acute self-limiting eruption
• More common in adolescents/young adults
• All races, slight female preponderance
• Herald patch occurs in more than 50% of cases
• Worldwide, including Australia (more common in spring and autumn)
• Probable viral aetiology (unproven)
• Asymptomatic to severe itching
• Usually resolves in 8-10 weeks but may be protracted (> 6 months)
• Topical steroids indicated for symptomatic itching
• Phototherapy useful for recalcitrant cases.

Written by Dr Adrian Lim, Fellow of the Australasian College of Dermatologists.

Dr Stephen Shumack is Honorary Secretary of the Australasian College of Dermatologists.