• Lichen planus (LP) is a relatively uncommon cutaneous inflammatory disorder.

• “Classic” LP is a pruritic, idiopathic eruption with characteristic shiny, flat-topped (planus is Latin for “flat”) papules on the skin and mucous membranes.

• The papules are characterized by their violaceous color, polygonal shape, and, sometimes, fine scale and are most commonly found on the extremities, genitalia, and mucous membranes. Less commonly, lesions can also involve the hair and nails.

• LP is seen predominantly in adults—greater than two-thirds of patients are between 30 and 60 years of age—but it can occur at any age.

• Women are more commonly affected than men.

The “Seven Ps”:

• Lesions are often pruritic, but can be asymptomatic.

• Lesions are typically purple (can be red to violet).

• Lesions tend to be planar (flat-topped) (Figs. 15.12 and 15.13).

• Lesions form papules or plaques.

• Lesions are often polygonal (Fig. 15.12).

• Lesions are frequently polymorphic (Fig. 15.13) in shape and configuration—that is, oval, annular, linear, confluent (plaque-like), large, and small, even on the same person.

• Lesions tend to heal with residual postinflammatory hyperpigmentation, leaving darkly pigmented macules in their wake (Fig. 15.14).

Variations in LP include the following:

• Hypertrophic: These extremely pruritic lesions are most often found on the extensor surfaces of the lower extremities (Fig. 15.20), especially around the ankles. Hypertrophic lesions are often chronic; and residual pigmentation and scarring can occur after lesions clear.

• Linear: Most often arise on extremities (Fig. 15.21).

• Atrophic (rare): Atrophic LP is most often the result of resolved lesions (Fig. 15.22).

• Erosive: These lesions are found on the mucosal surfaces.

• Follicular: Also called lichen planopilaris; typically seen on the scalp, can lead to scarring alopecia

• Bullous (rare): Intense inflammation in the dermis leads to blistering of epidermis.

• Despite its range of clinical presentations, LP is often diagnosed by its characteristic clinical appearance including the presence of Wickham striae, the Köebner reaction, and the characteristic oral lesions.

• Skin biopsy may be necessary if clinical presentation is atypical.

• LP is a cell-mediated immune response of unknown origin. An association has been noted between LP and hepatitis C infection, chronic active hepatitis, and primary biliary cirrhosis.

• The flexor areas such as the wrists, forearms, dorsal hands and feet, pretibial shafts, scalp, trunk, sacrum, glans penis, and labia minora are most often affected.

• Hypertrophic (verrucous) lesions tend to occur on the lower legs.

• Lesions may also become generalized (Fig. 15.15).

• Mucous membrane involvement is common and may be found without skin involvement. Lesions typically seen on the tongue and buccal mucosa but may also be noted on the gingiva, palate, or lips.

• Mucous membrane involvement may become erosive and painful, particularly if ulcers are present. Rarely, malignant transformation to squamous cell carcinoma has been documented.

• Genital involvement is common in men with cutaneous disease. Typically, an annular configuration of papules is seen on the glans penis (Fig. 15.16). Less commonly, linear white streaks (Wickham striae) can be seen on male genitalia. Vulvar involvement can range from reticulate papules to severe erosions.

• Vulvar lesions can result in dyspareunia, burning, and pruritus.

• Nail lesions may exhibit symptoms ranging from a mild dystrophy to a total loss or absence of the nails (twenty-nail dystrophy of childhood).

• Scalp lesions result in a permanent, patchy, scarring follicular alopecia (lichen planopilaris; see Chapter 19: Hair and Scalp Disorders Resulting in Hair Loss).

• The course of LP is unpredictable. The onset may be abrupt or gradual. The lesions may resolve spontaneously, recur intermittently, or persist for many years. Chronicity is especially likely when hypertrophic lesions appear.

• New lesions may be noted at sites of minor trauma such as scratches or burns (the Köebner reaction [isomorphic response]) (Fig. 15.17).

• Wickham striae are characteristic white, lacelike streaks that are best visualized on the surfaces of lesions after mineral oil is applied. If present, they are virtually pathognomonic of LP (Fig. 15.18).

• Mucous membrane lesions may be characterized by white lacy streaks in a netlike pattern (Fig. 15.19) or by atrophic erosions or ulcers.

• Pruritus, which may be severe, and the cosmetic appearance of lesions are the major concerns to patients.