What Is Lichen Planus: Symptoms, Skin Manifestations, and Clinical Patterns

What Is Lichen Planus

Lichen planus is a chronic inflammatory condition that affects the skin, mucous membranes, nails, and scalp. It is classified as a T-cell-mediated autoimmune disorder in which cytotoxic T lymphocytes target basal keratinocytes, leading to their destruction. The condition is not contagious and does not represent an infection, though its exact trigger remains incompletely understood. It affects approximately 0.5 to 1 percent of the general population, with a peak incidence between ages 30 and 60, and shows a slight female predominance.

The name derives from the resemblance of the skin lesions to lichen — the flat, plant-like growths found on rocks and trees — combined with the Latin word planus, meaning flat. This descriptive term captures the hallmark feature: small, flat-topped papules that may coalesce into plaques. While the cause is not fully established, associations have been documented with certain medications, hepatitis C infection, metal dental restorations, and stress, though many cases arise without an identifiable precipitant.

Diagnosis is primarily clinical, based on the characteristic appearance and distribution of lesions. In uncertain cases, a punch biopsy shows a distinctive band-like lymphocytic infiltrate at the dermo-epidermal junction with degeneration of basal cells (liquefaction degeneration) and saw-tooth rete ridges. Direct immunofluorescence may reveal globular deposits of IgM and complement along the basement membrane zone, supporting the diagnosis.

Classic Cutaneous Lesions and the Six P's

The textbook description of lichen planus skin lesions relies on a mnemonic of six P's: pruritic, planar, purple, polygonal, papules, and plaques. Individual lesions measure 2 to 6 millimeters in diameter, have a flat top, and display a violaceous or reddish-purple hue that distinguishes them from the bright red of eczema or the silvery scale of psoriasis. The surface often shows fine, white, lacy lines called Wickham striae, which are more visible after applying oil or water and represent thickened granular layer overlying areas of basal cell damage.

Lesions are intensely pruritic in most patients, though the degree of itching varies. The polygonal shape arises from mutual pressure as papules enlarge and abut one another. When multiple papules merge, they form larger plaques that retain the violaceous color and may develop central hyperpigmentation as inflammation subsides. The Koebner phenomenon — the appearance of new lesions at sites of skin trauma, scratching, or surgical incisions — is common and can linearize the eruption along scratch marks.

Post-inflammatory hyperpigmentation is a frequent sequela, particularly in darker skin tones, and may persist for months to years after active inflammation resolves. The lesions themselves rarely scar unless they are hypertrophic or ulcerated. Inverse lichen planus affects intertriginous areas such as the axillae and groin, where the typical papular morphology is obscured by moisture and friction, presenting instead as erythematous, sometimes erosive patches without prominent scaling.

Distribution Patterns and Common Sites

Cutaneous lichen planus favors the flexor surfaces of the wrists, forearms, and ankles, as well as the lower back, shins, and sacral region. The extensor surfaces are less commonly involved. The face is typically spared, though the periorbital and perioral areas may be affected in rare cases. Lesions are often symmetrical and bilateral. The genital mucosa — glans penis in men, vulva and vagina in women — is a frequent site of involvement, sometimes occurring in isolation without skin lesions.

The scalp may be affected in a variant known as lichen planopilaris, which presents with perifollicular erythema, scaling, and eventual scarring alopecia. Nail involvement occurs in roughly 10 percent of patients and can range from longitudinal ridging and thinning to complete nail loss (anonychia) with pterygium formation, where the proximal nail fold fuses to the nail bed. Nail changes may be the sole manifestation or accompany skin and mucosal disease.

The distribution often provides diagnostic clues. Involvement of the wrists and ankles in a young adult with violaceous, flat-topped papules is highly suggestive. Isolated shin lesions may mimic hypertrophic lichen planus or lichen simplex chronicus. Genital lesions in women can resemble lichen sclerosus, requiring biopsy for differentiation. Oral involvement — seen in 30 to 70 percent of cases — typically presents as white, reticulated streaks on the buccal mucosa, though erosive and atrophic forms also occur.

Clinical Variants and Atypical Presentations

Several clinical variants of lichen planus exist, each with distinct morphology. Hypertrophic lichen planus features thickened, hyperkeratotic plaques, most often on the shins and interphalangeal joints, and is intensely pruritic. It may resemble prurigo nodularis or lichen simplex chronicus. Atrophic lichen planus presents as whitish, depressed, wrinkled patches, often on the trunk or limbs, representing resolved lesions with epidermal atrophy. Bullous lichen planus is rare, with tense vesicles or bullae arising on existing plaques or normal skin, typically on the lower limbs.

Lichen planus pigmentosus manifests as diffuse, slate-gray to brown macules in sun-exposed areas and flexures, with minimal or no preceding inflammatory phase; it is more common in darker-skinned individuals. Lichenoid drug eruption mimics idiopathic lichen planus clinically and histologically but resolves after discontinuation of the offending agent — commonly antihypertensives, NSAIDs, antimalarials, or checkpoint inhibitors. The temporal relationship to drug initiation aids differentiation, though a latency of months to years can occur.

Annular lichen planus forms ring-shaped lesions with active borders and central clearing, frequently on the male genitalia or in intertriginous zones. Linear lichen planus follows Blaschko lines or appears in a zosteriform pattern, sometimes reflecting the Koebner phenomenon. Actinic lichen planus occurs in sun-exposed areas in subtropical climates, presenting as mildly pruritic, annular or papular lesions on the face, neck, and dorsal hands. Each variant shares the core histopathological features but may require tailored management.

Mucosal, Nail, and Scalp Manifestations

Oral lichen planus is the most common mucosal manifestation, affecting the buccal mucosa, tongue, and gingiva. The reticular form shows white, lace-like striae (Wickham striae) that are often asymptomatic. The erosive and atrophic forms present with erythema, ulceration, and pain, especially with spicy or acidic foods. Gingival involvement (desquamative gingivitis) causes red, shiny, friable gums. Malignant transformation to squamous cell carcinoma occurs in an estimated 1 to 3 percent of longstanding erosive oral lichen planus, warranting periodic monitoring.

Genital lichen planus in women can present as erosive vestibulitis, vaginal stenosis, or the classic white striae on the vulva. In men, the glans penis shows annular or erosive lesions. Scarring can lead to phimosis or urethral stenosis. Both sexes may experience dyspareunia or dysuria. Vulvovaginal-gingival syndrome describes the triad of oral, vulvar, and vaginal involvement. These manifestations significantly affect quality of life and often require multidisciplinary care.

Lichen planopilaris, the scalp variant, causes patchy scarring alopecia with perifollicular erythema and scaling. The frontal hairline may recede in a pattern resembling frontal fibrosing alopecia, now considered a variant of lichen planopilaris. Eyebrow loss can occur. Nail changes include longitudinal striations, thinning, dorsal pterygium (fusion of proximal nail fold to nail bed), and complete nail shedding. Twenty-nail dystrophy — idiopathic nail atrophy affecting all nails — has been linked to lichen planus in some cases.

Disease Course, Associations, and When to Seek Evaluation

A dermatologist should evaluate any persistent, pruritic, violaceous papules or plaques, especially with Wickham striae. Oral lesions that are painful, ulcerated, or fail to respond to topical therapy warrant biopsy to exclude dysplasia. Genital erosions, scarring, or stenosis require prompt assessment. Nail dystrophy without clear cause, particularly with pterygium, should prompt investigation for lichen planus. Scalp scaling with follicular plugging and hair loss merits early biopsy to confirm lichen planopilaris and initiate treatment before irreversible scarring. Primary care clinicians can recognize classic presentations, but referral is appropriate for atypical, extensive, or mucosal disease.