Pathology

In lichen planus, the granular layer is thickened, basal cells show liquefaction degeneration and lymphocytes infiltrate the upper dermis in a band-like fashion (Fig. 1).

Fig. 1 Histopathology of lichen planus.

Clinical presentation

Two-thirds of cases occur in the 30–60-year-old age group. It is uncommon at the extremes of age, and the sex incidence is equal. Lichen planus tends to start on the limbs. It may spread rapidly to become generalized within 4 weeks, but the commoner localized forms progress more slowly. Typical lesions are very itchy flat-topped polygonal papules, a few millimetres in diameter, which may show a surface network of delicate white lines (Wickham’s striae). Initially, the papules are red, but they become violaceous (Fig. 2).

Fig. 2 Typical violaceous papules of lichen planus at the wrist.

The eruption is symmetrical and affects:

Mucous membrane involvement, especially of the buccal mucosa, occurs in up to two-thirds of cases, and may be present without skin lesions (Fig. 3). Lichen planus also shows the Koebner phenomenon (p. 19) which may explain some linear lesions. Follicular and other variants are found (see below). In most cases, papules flatten after a few months to leave pigmentation, but some become hypertrophic. Half of all patients are clear within 9 months, but 15% have continuing symptoms even after 18 months. Up to 20% have a further attack. Lichen planus may be confused with other conditions, as shown in Table 1.

Fig. 3 White lace-like Wickham’s striae on the buccal mucosa.

Table 1 Differential diagnosis: lichen planus

Variants of lichen planus

A number of variants of lichen planus exist: