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LICHEN PLANUS

Etiology and pathogenesis
Lichen planus (LP) is a moderately common disorder with a prevalence of about 0.5%, and a slight female predominance but no racial predilection. It is generally viewed as an autoimmune disorder, and is more commonly associated with several other autoimmune disorders than would be expected by chance. Associated diseases include the following.

•	Skin—alopecia areata, vitiligo, dermatitis herpetiformis, pemphigus, and sclerodermas.
•	Gastrointestinal—primary biliary cirrhosis, chronic active hepatitis, and ulcerative colitis.
•	Endocrine—thyroiditis and diabetes mellitus.
•	Hematology—pernicious anemia and thymoma.

    There are also associations with human leukocyte antigen (HLA)-A3, A5, B8, Bw35, and DR1. Familial cases may occur. In some countries, such as Italy, the association with hepatitis (especially chronic hepatitis C) infection appears fairly strong, but this has not been confirmed in other studies (e.g. in the UK). It may be that there is an interaction between HLA and infection that explains these geographic differences. However, treatment for hepatitis C virus (HCV) does not correlate with clearance of LP, and HCV is not found in lesional skin.
The pathogenesis of purely oral LP without skin involvement may be different, for example contact allergy frequently plays a part. This is discussed separately later.

Fig. 8.1 Lichen planus (LP): (a) typical, flat-topped, polygonal papules at a typical site on the wrist. (b) LP lesions with the typical purple color. (c) Semiconfluent papules of LP. (d) Multiple eruptive small spots of LP, with typical purple color.

Fig. 8.2 Examples of Wickham’s striae in lichen planus (a–d). Note the annularity of the lesions in (a) (see also Fig. 8.8). If not readily visible, these can be accentuated by the application of oil (or even water) to the lesion, as shown in (d).

Fig. 8.3 The volar aspect of the wrist is a typical site for lichen planus, and is an area where Koebner reactions are often seen.

Clinical features
Lichen planus may develop at any age, but men are typically affected at a younger age than women are.
     The typical lesion of LP is a violaceous or purple, angulated (polyhedral), flat-topped papule or small plaque (Fig.8.1). These are usually, but not invariably, extremely itchy; however, by contrast with eczemas, scratched lesions are unusual. The surface of the lesions, when viewed carefully, is notable for the presence of a white, lace-like patterning known as Wickham’s striae (Fig.8.2). Application of mineral oil to the surface of the lesion to fill air spaces in the keratin makes the striae more easily visible.
    Lesions generally occur as crops, with a predilection for volar wrists or forearms (Fig.8.1a and Fig.8.3) and around the shins and ankles. New lesions continue to appear for a few weeks to several months in most patients and, if untreated, the eruption generally lasts about 12–18 months.
    Lichen planus is one of several disorders that exhibit the Koebner reaction, in which new lesions develop at the sites of minor injuries, such as scratches or burns (Fig.8.4). A list of causes of this reaction is given in (Chapter.1). A further notable feature, typically seen in resolving postinflammatory LP lesions but occasionally very prominent in new lesions, is a marked muddy-brown pigmentation (Fig.8.5). This may be very intense at flexural sites.
   

Fig. 8.4 The Koebner reaction in lichen planus (LP). (a) In this instance the reaction followed a scratch on the arm. Lesions take about a week to appear, about the time the wound would normally take to heal. (b) Multiple small scratches on the dorsum of the hand, demonstrating the Koebner reaction in LP. (c) Pigmented Koebner reaction of LP due to an abrasion incurred while moving a heavy cabinet..

Fig. 8.5 Postinflammatory pigmentation is often prominent in lichen planus (LP). (a) This illustration of the lower back (also a typical site) shows a mixture of newer purplish-red lesions and fading brown lesions. If using strong topical treatments, it is important to realize that the old lesions do not require treatment. (b) Old lesions of LP on the wrists, demonstrating postinflammatory dermal pigmentation. (c) Scattered and confluent heavily pigmented LP lesions.

    Oral involvement is common, in about 75% of cases, but is asymptomatic in about 75% of those in whom it occurs; it is therefore incumbent on the physician to examine the mouth. The most common site for oral lesions is the buccal mucosa; the esophagus and other mucosae may be affected. These lesions have a white, lace-like pattern, and are discussed further with other oral lichenoid eruptions (later in this chapter).

     There are a number of morphologic patterns and variants that appear different to classic LP; these are discussed here.

Specific patterns and sites of involvement
Generalized or guttate
In some patients, LP can be of dramatic and widespread onset, affecting any part of the body (Fig.8.6). The lesions are often quite small papules and may not be readily recognized as LP, although they are generally purplish in colour. This pattern behaves rather like a viral exanthem, with which it may be confused.

Fig. 8.6 Generalized lichen planus, with abrupt, exanthem-like onset. This pattern is associated with hepatitis infection in some countries, but viral serology was negative in this case.

Follicular (lichen planopilaris)
Follicular LP, consisting of tiny keratotic spots, also tends to be relatively widespread (Fig.8.7) and has a tendency to involve the scalp, where it may cause alopecia.

Fig. 8.7 Follicular lichen planus, demonstrating multiple small conical papules centered on follicular orifices

Annular and atrophic
Annular lesions may be the main morphology present, usually in LP of rather limited distribution (Fig.8.8). They are particularly common on
the penis (Fig.8.9). They may resolve with atrophy, although atrophic lesions of LP may occur de novo as well.

Fig. 8.8 (a,b) Annular lesions of lichen planus may be confused with granuloma annulare or dermatophyte infections, but are usually small and have a narrower border than in either of these disorders.

Fig. 8.9 Annular lichen planus of the penis. Annular lesions are common at this site, and are usually asymptomatic. Note the nail involvement.

Localized or hypertrophic
In some patients, and at some body sites, lesions of LP may become grossly thickened and hypertrophic (Fig.8.10). The shins are the most common site for this to occur. LP may occur as hypertrophic lesions on the
lower legs in the absence of lesions at other sites, and may be confused with lichen simplex (Ch.9). Rarely, keratoacanthoma or squamous cell carcinoma may arise on chronic hypertrophic LP.

Fig. 8.10 Hypertrophic lichen planus (LP). (a,b) The shins are a typical site and may be affected in isolation. This pattern tends to run a protracted course and is relatively unresponsive to therapy. Note the annularity and smaller follicular lesions in (b). (c) Hypertrophic LP, with a more keratotic surface, which may be difficult to distinguish from an epidermal neoplasm if the lesion is solitary.

Palm and sole
Lichen planus of palms or soles (Fig.8.11) may be a very difficult diagnostic and therapeutic challenge, particularly if typical sites are unaffected. Lesions tend to be sheet-like rather than discrete, isolated papules, and are often hyperkeratotic and yellowish colored. They may resemble palmar psoriasis or even xanthomas. Less commonly, plantar LP may
be severely erosive. Drug-related lichenoid reactions often involve palmoplantar skin (see later in chapter).

Fig. 8.11 Palmar lichen planus (LP). (a) In African-Caribbean skin, demonstrating prominent pigmentation. (b) LP on the sole of the foot, with a more confluent pattern. A number of disorders may show this pattern of being confined to the skin below (occasionally above) the Wallace line, which runs a centimeter or so above the weight-bearing area of the true plantar skin. (c) LP of the palm with confluent lesions resembling psoriasis. (d) LP of the palm, in this case with discrete hyperkeratotic lesions that had a yellowish color reminiscent of pustules or xanthomas

Nails and nail fold

Lichen planus may cause nail plate damage (Fig.8.9 and Fig.8.12)or periungual inflammation (Fig.8.12b and Fig.8.13). LP of the nail fold is important, as the inflammatory process may lead to scarring and adherence of the nail fold to the dorsal nail plate, a process known as pterygium. This is irreversible and warrants aggressive therapy to preserve the normal nail.

Fig. 8.12 Lichen planus of nails. (a) Trachyonychia-like longitudinal nail plate ridges in a patient with lichen planus (LP) at typical cutaneous sites. (b) LP of the nail fold with proximal nail plate dystrophy; if not treated aggressively, this may cause permanent damage by scarring to the surface of the nail plate (see also Fig.8.13).

Fig. 8.13 Lichen planus affecting the nails. In the patient shown in (a), treatment with systemic steroids avoided permanent nail dystrophy; normal nail growth is recommencing and just visible beyond the cuticle. (b) A permanent longitudinal nail dystrophy; old changes such as this may be clinically indistinguishable from a traumatic dystrophy.

Scalp
Scalp involvement by LP is relatively uncommon but may occur in isolation (Fig.8.14). It may be primarily around the follicles and hyperkeratotic, producing a nutmeg-grater-like feel to the scalp. This is known as lichen planopilaris. The clinical differential diagnosis is usually from discoid lupus erythematosus. Both of these are scarring disorders at
this site, and carry a risk of permanent alopecia, so prompt diagnosis and aggressive therapy are required.

Fig. 8.14 Lichen planus of the scalp: scarring alopecia with purplish-colored perifollicular inflammation.

Genital, perianal, and flexural
Genital involvement is frequent in LP, and often occurs in isolation.
Again, this is often asymptomatic or less symptomatic compared with other sites, and may not be volunteered by the patient. Penile lesions often have an annular morphology (Fig.8.9) but may be erosive (Fig.8.15). Vulval LP (Fig.8.16) may be of typical LP morphology, but may present as a moist, desquamative vaginitis, which may be confused with candidiasis or atrophic vaginitis; involvement of the female genital tract and the oral mucosa together (vulvovaginalgingival syndrome) is notoriously resistant to therapy.
    Perianal LP (Fig.8.17) may resemble a simple intertrigo; striae, if visible, may be confused with simple maceration of keratin. Flexural LP
is diagnostically difficult; some cases are erosive, while others have pigmented macules without the typical surface changes of LP.

Fig. 8.15 Erosive genital lichen planus may occur in either sex, in this case becoming apparent only following circumcision for treatment of chronic balanitis.

Fig. 8.16 Vulval lichen planus, in this instance with macerated keratin producing a white color but still with a purplish background component

Fig. 8.17 Perianal lichen planus may resemble intertrigo, seborrheic dermatitis, or psoriasis, due to the occluded skin at this site.

Fig. 8.18 Linear lichen planus (LP), shown here along the lateral border of the foot in a patient with no evidence of LP at other sites. Linear LP usually develops as a broad band from the buttock down the posterior aspect of the leg to the foot. A similar pattern may occur with linear psoriasis. (Courtesy of Dr. L. Barco.)

Fig. 8.19 Lichenoid lesions in the lines of Blaschko: this eruptive pattern may be better termed lichenoid blaschkitis rather than lichen planus in Blaschko lines.

Fig. 8.20 Bullous lichen planus (LP) on the leg, within a typically purplish-colored lesion. This variant also occurs in clinically normal skin with LP elsewhere, in the mouth, and occasionally on the scalp.

Fig. 8.21 Lichen planus affecting the eyelids, in a pattern that could be misdiagnosed as a contact dermatitis. Note, however, that the lesions are sharply demarcated, which is unlikely in eczematous processes at this site.

This varies according to the site and morphologic pattern (Table 8.3).

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.