Lichen planus is an inflammatory dermatosis with distinct clinical and histological features. In the majority of cases, the cause is unknown. The term lichen planus is something of a tautology, since lichen in medical tradition seems to refer to a variety of 'lichen-like' skin diseases and planus means flat or flat topped.
The mechanism of inflammation in lichen planus is probably autoimmune.
Two proposed triggers for the condition should be mentioned, although neither is a major cause in the UK. Hepatitis C virus infection seems to be associated with a proportion of cases, and dental amalgam containing mercury appears to be a cause of some cases of oral lichen planus.
History and diagnosis
Most patients with lichen planus describe a relatively acute eruption of intensely itchy, inflamed papules and plaques, appearing out of the blue with no prodromal illness or systemic symptoms.
Unlike eczema and psoriasis, lichen planus tends to be self-limiting, with the lesions resolving after a few months or years. A long-term survey of patients in the UK found that in 68 per cent, it cleared within one year.
Typical lichen planus consists of symmetrical papules on the limbs, concentrated distally with a predilection for the flexor aspect of the forearms. However, there is considerable variability. In some cases, the eruption is less symmetrical, with fewer lesions in an atypical distribution. Other patients do not experience itching and it is not uncommon to find flatter, more pigmented lesions at flexural sites.
In some patients, there can be widespread and generalised eruption, resembling a viral or drug-induced exanthem. In others, the rash is more diffuse, consisting of many small papules that are not immediately recognisable as lichen planus.
Whatever the distribution of the rash, the characteristic morphology of the lesions is usually visible on careful examination.
The physical signs that allow a confident diagnosis are flat-topped papules or plaques with a purplish hue and a whitish, lacy appearance, due to hyperkeratosis, on the surface.
This latter sign is often referred to as Wickham's striae.
Reticular hyperkeratosis in the mouth occurs in about half of patients with a cutaneous eruption and can helichen planus confirm the diagnosis. Marked longitudinal ridging of the nails is also often present. The diagnosis can usually be confirmed histologically.
Variants of lichen planus
Lichen planus has a number of variants, the most common of which are hypertrophic and mucosal lichen planus.
In the hypertrophic variant, the lesions are thickened and there is marked hyperkeratosis, resulting in warty plaques that occur most commonly on the shins. These lesions are often more chronic and difficult to treat than those found in ordinary lichen planus. It is not uncommon for lichen planus to be confined to the mucous membranes of the mouth and genitalia.
The most common appearance of mucosal lichen planus is whitish plaques or the classical reticulate pattern seen in association with cutaneous lichen planus. The condition can also arise on the vermilion of the lips.
Lichen planus lesions occasionally adopt an annular appearance. In some cases the nails or hair follicles are gradually destroyed by the inflammatory process.
Some rarer variants of lichen planus include: erosive lichen planus, usually of the mucous membranes; atrophic lichen planus; bullous lichen planus; and actinic lichen planus, which causes hyperpigmented patches on the face.
How lichen planus is treated
Lichen planus has a tendency to resolve so treatment should not be overly prolonged.
Ordinary cutaneous lichen planus usually responds to potent topical steroids. Hypertrophic plaques can be treated with topical steroids under occlusion or intralesional steroid injections.
Highly potent steroids can be used for a few weeks at a time for stubborn lesions, and topical tacrolimus 0.1 per cent ointment has been found to be a useful non-steroid treatment for some patients.
Symptomatic cases of oral lichen planus can usually be controlled with topical steroid in the form of Adcortyl in Orabase, soluble prednisolone or betamethasone used as a mouthwash or an aerosol device, directed at the oral mucosa.
For severe, widespread lichen planus, prescribing a course of oral steroids is justified, for example prednisolone 30-40mg daily for about two weeks, tapered accordingly. Phototherapy is another option and can certainly be effective.
For difficult, severe and protracted lichen planus, more aggressive immunosuppressive treatments, such as ciclosporin and mycophenolate mofetil, can be tried.