Papulosquamous eruptions
Papulosquamous eruptions are raised, scaly and marginated, and include psoriasis, lichen planus and other conditions listed in Table 1. Eczema is not included as it does not usually have a sharp edge. These eruptions are not related aetiologically. Several are characterized by fine scaling and have the prefix ‘pityriasis’, which means ‘bran-like scale’.
Table 1 Papulosquamous eruptions
Pityriasis rosea
Clinical presentation
The generalized eruption is preceded in most patients by the appearance of a single lesion, 2–5 cm in diameter, known as a ‘herald patch’ (Fig. 1). Some days later, many smaller plaques appear, mainly on the trunk but also on the upper arms and thighs. Individual plaques are oval, pink and have a delicate peripheral ‘collarette’ of scale. They are distributed parallel to the lines of the ribs, radiating away from the spine. Itching is mild or moderate. The eruption fades spontaneously in 4–8 weeks. It tends to affect teenagers and young adults. The cause is unknown, but epidemiological evidence of ‘clustering’ suggests an infective aetiology.
Pityriasis (tinea) versicolor
Clinical presentation
The condition is caused by overgrowth of the mycelial form of the commensal yeast Malassezia (previously Pityrosporum ovale) and is particularly common in humid or tropical conditions. In Europe, it mainly affects young adults, appearing on the trunk and proximal parts of the limbs (Fig. 2). In untanned, white caucasians, brown or pinkish oval or round superficially scaly patches are seen, but, in tanned or racially pigmented skin, hypopigmentation is found as a result of the release by the organism of dicarboxylic acids that inhibit melanogenesis.
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