Pityriasis rosea
Also called PR
A self-limited eruption of adolescents and young adults: a single larger herald patch, then days later a crop of smaller oval scaly plaques along the rib lines, the fir-tree pattern.
How it looks
A single 3 cm oval erythematous patch with fine collarette scale on the trunk, followed by multiple smaller oval plaques with their long axes along skin cleavage lines.
Rose is the lesion, grey the pattern the lesions make, green where on the body it lives.
Across skin tones
In deeply pigmented skin PR is often papular, may involve the face and scalp, and commonly leaves marked post-inflammatory pigment change; the erythema itself may be subtle.
Symptoms & course
- Mild itch in about half
- Sometimes a viral prodrome
Diagnostic approach
- Serologic testing for syphilis when palms or soles are involved or the history fits, secondary syphilis is the great PR mimic
- Otherwise clinical
Differential, and how to separate them
- Tinea corporis
Favours it: One or few expanding rings, KOH positive
Against it: PR's many oval plaques follow cleavage lines; the herald patch predates the crop
Management principles
- Reassurance, resolves in 6–10 weeks
- Emollients and antihistamines for itch
Clinical pearl: A 'PR' involving the palms and soles is syphilis until the serology says otherwise.
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