The precise cause of lichen planus is unknown, but the disease seems to be mediated immunologically. Lymphocytes abut the epidermal basal cells and damage them. Chronic graft-vs.-host disease can cause an eruption rather like lichen planus in which histoincompatibility causes lymphocytes to attack the epidermis. Lichen planus is also associated with auto-immune disorders, such as alopecia areata, vitiligo and ulcerative colitis, more commonly than would be expected by chance. Drugs too can cause lichen planus . Some patients with lichen planus also have a hepatitis B or C infectionabut lichen planus itself is not infectious.
Typical lesions are violaceous or lilac-coloured, intensely itchy, flat-topped papules that usually arise on the extremities, particularly on the volar aspects of the wrists and legs (Fig. 6.3).
A close look is needed to see a white streaky pattern on the surface of these papules (Wickham’s striae). White asymptomatic lacy lines, dots, and occasionally small white plaques, are also found in the mouth, particularly inside the cheeks, in about 50% of patients (Fig. 6.4), and oral lesions may be the sole manifestation of the disease.
The genital skin may be similarly affected (see Fig. 13.37). Variants of the classical pattern are rare and often difficult to diagnose (Table 6.2). Curiously, although the skin plaques are usually itchy, patients rub rather than scratch, so that excoriations are uncommon. As in psoriasis, the Köbner phenomenon may occur (Fig. 6.5). The nails are usually normal, but in about 10% of patients show changes ranging from fine longitudinal grooves to destruction of the entire nail fold and bed (see Fig. 13.26). Scalp lesions can cause a patchy scarring alopecia.
Individual lesions may last for many months and the eruption as a whole tends to last about 1 year. How-ever, the hypertrophic variant of the disease, with thick warty lesions usually around the ankles (Fig. 6.6), often lasts for many years. As lesions resolve, they become darker, flatter and leave discrete brown or grey macules. About one in six patients will have a recurrence.
Nail and hair loss can be permanent. The ulcerat-ive form of lichen planus in the mouth may lead to squamous cell carcinoma. Ulceration, usually over bony prominences, may be disabling, especially if it is on the soles. Any association with liver disease is probably caused by the coexisting hepatitis infections mentioned above.
Lichen planus should be differentiated from the other papulosquamous diseases listed in Table 6.1. Lichenoid drug reactions can mimic lichen planus closely. Gold and other heavy metals have often been implicated. Other drug causes include antimalarials,
blockers, non-steroidal anti-inflammatory drugs, para-aminobenzoic acid, thiazide diuretics and penicillamine.
Contact with chemicals used to develop colour film can also produce similar lesions. It may be hard to tell lichen planus from generalized dis-coid lupus erythematosus if only a few large lesions are present, or if the eruption is on the palms, soles or scalp. Wickham’s striae or oral lesions favour the diagnosis of lichen planus. Oral candidiasis can also cause confusion.
The diagnosis is usually obvious clinically. The his-tology is characteristic (Fig. 6.7), so a biopsy will confirm the diagnosis if necessary.
Treatment can be difficult. If drugs are suspected as the cause, they should be stopped and unrelated ones sub-stituted. Potent topical steroids will sometimes relieve symptoms and flatten the plaques. Systemic steroid courses work too, but are recommended only in special situations (e.g. unusually extensive involvement, nail destruction or painful and erosive oral lichen planus). Treatment with photochemotherapy with psoralen and ultraviolet A (PUVA;) or with narrow-band UVB may reduce pruritus and help to clear up the skin lesions. Acitretin (Formulary 2) has also helped some patients with stubborn lichen planus. Antihistamines may blunt the itch. Mucous membrane lesions are usually asymptomatic and do not require treatment; if they do, then applications of a corticosteroid or tacrolimus in a gel base may be helpful.