GPAtlas🧭 When to suspect
Pityriasis rosea is an acute, self-limiting papulosquamous eruption, most likely caused by endogenous reactivation of human herpesvirus 6 and 7 (HHV-6/HHV-7). It is not contagious. It is largely a disease of teenagers and young adults – most cases occur between 10 and 35 years – with a slight female excess and a recognised increase in pregnancy.
The classic sequence is a single herald patch followed 1–2 weeks later by a symmetrical eruption of smaller oval, scaly, pink patches across the trunk and proximal limbs, aligning along the lines of cleavage to give the characteristic ‘Christmas tree’ pattern on the back. Diagnosis is clinical.
The two jobs in primary care are to recognise the pattern and reassure, and to actively exclude its important mimic, secondary syphilis – while remembering the one group in whom pityriasis rosea is not a trivial diagnosis: pregnant women, particularly in the first 15 weeks.
| Key differential | How to distinguish |
|---|---|
| Secondary syphilis | Palms and soles involved, mucosal lesions, generalised lymphadenopathy, sexual-health risk – the must-not-miss mimic; check treponemal serology. |
| Guttate psoriasis | Smaller ‘tear-drop’ scaly papules, often post-streptococcal, with scalp/extensor involvement and no herald patch. |
| Tinea corporis | A single annular plaque with a leading scaly edge – the solitary herald patch is the classic mimic; confirm with skin scrapings if unsure. |
| Nummular (discoid) eczema | Coin-shaped, more eczematous and crusted, intensely itchy, with no ‘Christmas tree’ distribution. |
| Pityriasis versicolor | Hypopigmented or hyperpigmented macules on the upper trunk; Malassezia hyphae and spores on microscopy. |
| Drug-induced PR-like eruption | Often no herald patch, atypical distribution, and fails to clear until the responsible drug is withdrawn. |
|
⚠️ Common pitfall Committing to a diagnosis on the solitary herald patch. In isolation it is frequently mislabelled as tinea corporis or eczema and treated accordingly. The diagnosis declares itself 1–2 weeks later when the symmetrical ‘daughter’ lesions erupt along the lines of cleavage – so resist anchoring on a single annular patch. |
Source: DermNet · British Association of Dermatologists
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