Lichen Planus

• CD8+ T-cell–mediated cytotoxicity as primary mechanism: Activated CD8+ cytotoxic T lymphocytes recognize and attack basal layer keratinocytes, resulting in apoptosis and necrosis. This mechanism is mediated by perforin and granzyme B released from T-cell granules, as well as Fas-FasL interactions. The T cells are recruited through production of chemokines (CCL5, CXCL10) and express cutaneous lymphocyte-associated antigen (CLA), directing them to skin and mucosal surfaces. Immunohistochemistry consistently demonstrates CD8+ predominance (>CD4+), and infiltrating T cells express activation markers (HLA-DR, CD25). This explains the characteristic basal layer "saw-tooth" acanthosis with liquefactive degeneration of the basal epidermis—the direct result of keratinocyte destruction by cytotoxic granules.

• Antigen-driven activation and molecular mimicry: A central unresolved question in lichen planus pathophysiology is the triggering antigen. Proposed mechanisms include molecular mimicry, where T cells recognize epitopes on basal keratinocytes that cross-react with exogenous antigens (viral, bacterial, or chemical). Hepatitis C virus infection is strongly associated with lichen planus in certain populations (particularly Mediterranean and Japanese), suggesting viral antigen presentation to T cells and potential cross-reactivity with keratinocyte antigens. Drug-induced lichen planus (lichenoid eruptions) occurs when drugs or their metabolites act as haptens, binding to keratinocytes and rendering them immunogenic. Contact allergens (nickel, mercury) similarly trigger sensitization through MHC presentation. This explains why identical clinical and histologic patterns can arise from diverse triggers—the final common pathway is T-cell–mediated attack on the basal epidermis.

• Disrupted regulatory T-cell (Treg) function and loss of immune tolerance: Lichen planus demonstrates reduced Foxp3+ Treg infiltration and impaired IL-10 and TGF-β production relative to the magnitude of effector T-cell response. This suggests a fundamental imbalance favoring effector over regulatory T cells. Genetic factors may contribute, as subtle variations in genes regulating immune tolerance (e.g., PTPN22) theoretically could predispose to loss of self-tolerance. The chronicity of lichen planus reflects persistent antigen presentation or failure to re-establish immune homeostasis after initial triggering. Longitudinal biopsies show that even clinically resolving lesions retain subclinical inflammation, suggesting incomplete immune reconditioning.

• Keratinocyte activation and epithelial dysfunction: Basal keratinocytes themselves contribute to perpetuation through increased MHC Class I expression (HLA-A, HLA-B, HLA-C), enabling enhanced antigen presentation. Keratinocytes produce pro-inflammatory cytokines (TNF-α, IFN-γ, IL-6) and chemokines that recruit and activate additional T cells. This creates a feedback loop of epithelial damage and immune activation. Oral mucosal keratinocytes show heightened susceptibility to this process, explaining the higher prevalence and severity of oral involvement and erosion formation.

• Collagen deposition and fibrosis: In chronic lesions, the lichenoid inflammation triggers fibroblast activation and excessive type I collagen deposition in the superficial dermis, forming the characteristic "colloid bodies" or Civatte bodies at the dermal-epidermal junction (DEJ). These represent apoptotic keratinocyte remnants engulfed by fibroblasts. Over prolonged disease, this may progress to dermal sclerosis and, in particular variants (lichen planus pemphigoides), to subepidermal fibrosis with clinical erosion formation.

• Idiopathic (primary) lichen planus: The majority of cutaneous lichen planus cases (70–80%) occur without identifiable triggering factor, representing true autoimmune disease with genetic predisposition. Some patients report preceding minor trauma or stress, supporting both mechanical and psychological triggers. HLA associations (HLA-B27 in some populations) and familial clustering support genetic susceptibility. This form typically pursues a chronic course over years to decades.

• Hepatitis C virus (HCV) infection: Strong epidemiologic association exists between HCV seropositivity and oral lichen planus in endemic regions (Mediterranean, Japan, Spain; prevalence 60–90% in some cohorts vs. 5–10% in non-endemic areas). The mechanism likely involves viral antigen presentation or molecular mimicry. HCV-positive patients with lichen planus have higher rates of severe oral disease and potential malignant transformation. Screening HCV serology in patients with lichen planus, particularly oral disease, is recommended, especially in endemic regions. Treatment of HCV with direct-acting antivirals may improve lichen planus.

• Drug-induced lichenoid eruptions: Numerous medications trigger lichenoid reactions clinically and histologically indistinguishable from idiopathic lichen planus. Common culprits include antimalarials (hydroxychloroquine, chloroquine), beta-blockers (propranolol, atenolol), thiazide diuretics, NSAIDs, tetracyclines, gold, and penicillamine. The reaction typically develops weeks to months after drug initiation and may persist after discontinuation. Drug-induced forms tend to present with more erythema and less of the classic violaceous hue compared to idiopathic disease. Complete resolution may require months after drug withdrawal.

• Contact/occupational allergens: Chronic contact with allergens (nickel in jewelry, dental restorations; mercury in amalgam; acrylics in nail polish) induces sensitization and localized lichenoid reactions. This explains oral lichen planus adjacent to mercury-containing amalgam restorations, which resolves after amalgam replacement. Occupational exposures to chemicals (chromium, turpentine) are documented triggers. Patch testing may identify the offending allergen.

• Other systemic associations: Graft-versus-host disease (GVHD), particularly chronic GVHD post-hematopoietic stem cell transplant, presents with lichenoid inflammation clinically and histologically mimicking lichen planus. Systemic lupus erythematosus (SLE) rarely presents with lichenoid lesions overlapping with lichen planus, though full lupus serologies are typically negative in pure lichen planus. Lichen sclerosus coexists or overlaps with lichen planus in some cases. Immunosuppression (HIV, organ transplantation) does not increase lichen planus incidence, distinguishing it from other autoimmune conditions.

• Psychological stress as modifier: Psychological stress is frequently reported as a precipitant or exacerbant by patients, though objective data are limited. Stress may augment Th1/Th17 cytokine responses favoring lichenoid inflammation. This supports inclusion of stress-reduction counseling as adjunctive therapy.

• Cardinal cutaneous presentation—"The 6 P's": Planar, Purple, Polygonal, Papules, Plaques, Pruritic: Classic lesions are flat-topped, violaceous (red-purple), polygonal-shaped papules and plaques with shiny surfaces. The violaceous hue is pathognomonic and results from increased hemoglobin in dilated superficial dermal capillaries surrounded by the inflammatory infiltrate. Lesions are typically firm and may exhibit Wickham's striae—a fine white lacy pattern visible under magnification or dermoscopy representing intraepidermal acanthosis. Pruritus is common (60–70% of patients), ranging from mild to severe and disproportionate to lesion extent, suggesting neuropathic involvement. Lesions often localize to flexural surfaces: inner wrists, forearms, shins, thighs, lower back, and anogenital region, though distribution varies widely.

• Oral mucosal involvement—the most common extracutaneous site: Oral lichen planus affects 20–60% of lichen planus patients (higher prevalence than cutaneous disease in some populations) and may occur in isolation without skin lesions. Presentations range from asymptomatic reticular (lacy white) pattern on buccal mucosa to erosive, ulcerated disease causing severe pain and dysphagia. Reticular form (most common, 60–70% of oral cases) appears as white interlacing lines or patches on buccal mucosa, hard palate, and dorsal tongue, corresponding to hyperkeratotic ridges. Erosive form (20–30% of cases) presents with tender ulcerations and erythematous patches on attached gingiva, alveolar ridge, dorsal tongue, and floor of mouth, resulting from epithelial necrosis and loss. Erosive oral lichen planus is associated with higher malignant transformation risk (1–5% over 5–10 years) and significant morbidity (difficulty eating, altered taste). Desquamative gingivitis presents as diffuse gingival erythema and bleeding, easily confused with periodontitis or other causes of bleeding gums.

• Genital involvement and its complications: Anogenital lichen planus affects 3–15% of patients and causes significant pain, dyspareunia, and functional impairment. Females commonly develop vulvar erosions, scarring, and architectural distortion (loss of labia minora, clitoral involvement, introital stenosis), which may be disfiguring and compromise sexual function. Males present with glans involvement, balanitis, phimosis, and urethral involvement (rare). Anogenital involvement often persists longer than cutaneous disease and has malignant transformation potential. Distinction from lichen sclerosus is important, as lichen sclerosus has different management and prognosis.

• Hair and nail involvement: Lichen planopilaris (scalp variant) causes scarring alopecia (permanent hair loss) through destruction of hair follicle structures. Presents with violaceous papules, erythema, and scale on the scalp, progressing to cicatricial alopecia if untreated. Early intervention with intralesional or systemic corticosteroids may prevent permanent scarring. Nail lichen planus affects 7–12% of patients, manifesting as longitudinal ridging, thinning, onycholysis, pterygium formation (dorsal adhesion of nail plate to nail bed destroying the nail), and permanent nail loss. Nail disease develops insidiously and may progress despite clearing of skin disease.

• Systemic/extracutaneous manifestations: Mucosal involvement extends beyond oral cavity to esophagus (dysphagia, strictures with long-term ulceration), conjunctiva (keratitis, dry eye), and larynx (hoarseness, airway compromise—rare). Hepatic involvement occurs rarely; patients with lichen planus have slightly increased liver enzyme abnormalities, and some develop cirrhosis, though causality is debated and HCV coinfection may confound.

• Morphologic variants with clinical significance:
• Reticular vs. erosive: Reticular lesions are asymptomatic and benign; erosive lesions are painful and carry malignant potential.
• Hypertrophic lichen planus: Verrucous, thickened plaques predominantly on shins; slower to resolve and prone to post-inflammatory hyperpigmentation.
• Atrophic lichen planus: Thin, shiny lesions with loss of skin markings; occurs particularly on shins and may progress to scarring.
• Lichen planus pemphigoides: Variant with subepidermal blistering and IgG/C3 deposits along the basement membrane on immunofluorescence; presents with vesicles and erosions overlying typical lichen planus lesions; has distinct management (may require dapsone or rituximab).
• Generalized vs. localized: Some patients develop widespread eruption (generalized LP) involving >10% body surface area; others have isolated patches or single-site involvement (localized LP, particularly common with oral or genital disease).

• Clinical diagnosis supported by dermoscopy and histopathology: In most cases, lichen planus is diagnosed by clinical presentation plus dermoscopic or histologic confirmation. Clinical hallmarks alone—violaceous polygonal papules with Wickham's striae on flexural surfaces—are highly suggestive but not diagnostic, as lichenoid drug reactions and cutaneous lupus can mimic this appearance. Dermoscopy of skin lesions reveals the characteristic "reticular" or lacy white pattern (Wickham's striae) overlying a reddish or violaceous background, aiding clinical diagnosis. However, skin biopsy remains the gold standard for definitive diagnosis and is indicated when diagnosis is uncertain, in atypical presentations, or to assess malignant potential in oral lesions.

• Histopathologic diagnosis—the "saw-tooth" pattern: The distinctive histologic findings form the basis of diagnosis. Orthokeratosis (preserved stratum corneum without parakeratosis) is seen, with irregular acanthosis of the epidermis creating a "saw-tooth" or "church spire" appearance at the dermal-epidermal junction. Dense, bandlike lymphocytic infiltrate in the superficial dermis ("lichenoid infiltrate") is composed predominantly of CD8+ T cells with admixed CD4+ cells, histiocytes, and occasional plasma cells. Crucially, the infiltrate respects the DEJ—it does not extend deeply into the reticular dermis, distinguishing lichen planus from other lichenoid disorders. Liquefactive degeneration (vacuolization) of the basal layer reflects keratinocyte apoptosis and is a hallmark finding. Civatte bodies (colloid bodies) are seen as eosinophilic, rounded remnants at the DEJ representing apoptotic keratinocytes engulfed by dermal cells; they are not pathognomonic but supportive. Melanin incontinence is observed in the superficial dermis, reflecting melanin released from destroyed basal melanocytes. The combination of these features creates the classic diagnostic pattern. In oral lichen planus, similar histologic features are present, though surface ulceration or parakeratosis may occur if erosion is present.

• Immunofluorescence studies for variant assessment: Direct immunofluorescence (DIF) is not routinely performed for diagnosis of typical lichen planus (findings are nonspecific: fibrin and sometimes IgM/C3 at the DEJ) but is valuable in specific scenarios. Lichen planus pemphigoides shows linear IgG and C3 at the basement membrane zone on DIF, distinguishing it from typical lichen planus and guiding treatment. Pemphigoid-like patterns may warrant DIF to exclude bullous pemphigoid or pemphigus variants. Indirect immunofluorescence (circulating antibodies) is not useful in lichen planus diagnosis.

• Laboratory investigations based on clinical context: Routine bloodwork is not required for diagnosis but should be tailored to clinical suspicion:
• HCV serology (anti-HCV antibodies and HCV RNA if seropositive): Indicated in all patients with oral lichen planus and in cutaneous lichen planus in endemic regions. Positive serology warrants referral for hepatology evaluation and potential antiviral therapy.
• Liver function tests: Mildly elevated alkaline phosphatase or transaminases occur in a subset; significant elevation should prompt HCV investigation and consideration of cirrhosis.
• Lupus serology (ANA, anti-dsDNA, complement): Reserved for clinical features suggesting SLE or atypical presentations; most lichen planus patients are seronegative.
• Syphilis serology: Syph

Stabilise and remove the trigger first

• Airway and swallowing assessment: laryngeal lichen planus with stridor or hoarseness plus airway narrowing is an emergency requiring urgent ENT evaluation; esophageal disease with progressive dysphagia or weight loss warrants gastroenterology referral for endoscopy and dilation of strictures.
• Withdraw the culprit: for a lichenoid drug eruption, stop the suspected agent (thiazide, beta blocker, antimalarial, NSAID, gold, penicillamine); resolution takes months. Replace amalgam restorations only when patch testing confirms mercury sensitivity, per American Academy of Oral Medicine guidance.
• Treat associated HCV: direct-acting antiviral therapy following the AASLD/IDSA HCV guidance; USPSTF supports one-time HCV screening in adults 18–79.

First-line therapy

• Ultrapotent topical corticosteroids (e.g., clobetasol propionate 0.05% ointment): suppress the CD8+ lichenoid infiltrate at the dermo-epidermal junction. Mainstay for cutaneous, genital, and oral disease (gel, adhesive paste, or dexamethasone rinse for widespread mucosal involvement).
• Intralesional triamcinolone acetonide: for hypertrophic plaques, nail matrix disease, and lichen planopilaris, where topical penetration is inadequate.
• Antipruritic adjuncts: sedating antihistamines give limited relief; the itch is largely neuroinflammatory rather than histaminergic.

Escalation and second line

• Topical calcineurin inhibitors (tacrolimus, pimecrolimus): steroid-sparing for oral and genital sites prone to atrophy; carry an FDA boxed warning regarding theoretical malignancy risk and cause transient burning.
• Systemic agents: short-course oral corticosteroids (prednisone) for severe or generalized flares, then transition to acitretin, methotrexate, mycophenolate mofetil, hydroxychloroquine (favored in lichen planopilaris), or cyclosporine. Phototherapy (narrowband UVB or PUVA) is an option for widespread cutaneous disease.
• Lichen planus pemphigoides: dapsone or rituximab, as noted for this variant.

Definitive/procedural and contraindications

• Surgery is reconstructive, not curative: circumcision for phimosis, lysis of vulvar synechiae with dilator use, esophageal dilation — deferred until inflammation is controlled, since trauma provokes Koebnerization.
• Avoid: acitretin and methotrexate in pregnancy or planned conception (acitretin requires prolonged post-therapy contraception); prolonged systemic steroids; continued exposure to the inciting drug.

Disease-related complications

• Squamous cell carcinoma of oral or genital mucosa: chronic epithelial injury with repeated repair drives dysplasia; erosive and atrophic forms carry the risk quoted for oral disease. Signal findings are a non-healing indurated ulcer, an exophytic or speckled red-white plaque, or a lesion that fails to respond to adequate topical steroid — biopsy immediately rather than re-treating. Long-term periodic oral surveillance is recommended for potentially malignant oral disorders.
• Laryngeal or upper airway involvement: rare but an emergency — hoarseness with stridor or dyspnea requires urgent airway evaluation.
• Esophageal lichen planus: submucosal fibrosis produces webs and proximal strictures; signaled by progressive dysphagia to solids, odynophagia, food impaction, and weight loss.
• Ocular cicatrizing conjunctivitis: symblepharon formation and corneal scarring; red eye with lashes turning inward or adhesions warrants urgent ophthalmology referral to prevent vision loss.
• Irreversible scarring: lichen planopilaris destroys the follicular stem cell bulge → cicatricial alopecia with loss of follicular ostia; nail matrix destruction → pterygium and anonychia; vulvar synechiae, introital or vaginal stenosis, and phimosis. All are permanent once fibrosis is established, so early aggressive therapy is the point.
• Nutritional compromise and secondary infection: painful erosions limit intake; superimposed candidiasis or HSV causes sudden worsening of pain with a change in ulcer morphology.
• Post-inflammatory hyperpigmentation: from melanin incontinence; prominent and long-lasting in darker skin, and is not active disease.

Treatment-related complications

• Topical corticosteroids: cutaneous atrophy, striae, telangiectasia, and oral or genital candidiasis; periocular use risks glaucoma and cataract.
• Systemic corticosteroids: hyperglycemia, HPA-axis suppression, bone loss with prolonged use.
• Acitretin: teratogenicity, hyperlipidemia, transaminase elevation, mucocutaneous dryness.
• Methotrexate: hepatotoxicity, cytopenias, hypersensitivity pneumonitis.
• Hydroxychloroquine: retinal toxicity requiring baseline and periodic screening per American Academy of Ophthalmology recommendations — and it can itself cause a lichenoid eruption.
• Cyclosporine and PUVA: nephrotoxicity/hypertension and cumulative photocarcinogenesis, respectively.

• **The 6 Ps plus *Wickham's striae***: planar, purple, polygonal, pruritic papules and plaques with a lacy white surface network on the flexor wrists — recognition alone answers most stems.
• Histology triad: saw-tooth rete ridges, a band-like (lichenoid) lymphocytic infiltrate hugging the dermo-epidermal junction, and basal vacuolar degeneration with Civatte (colloid) bodies. Melanin incontinence explains the residual hyperpigmentation.
• The association examiners test is hepatitis C: oral lichen planus in the stem → obtain anti-HCV antibody (reflex HCV RNA). USPSTF endorses one-time HCV screening in adults 18–79; treatment follows AASLD/IDSA guidance.
• Best next step for a non-healing, indurated oral erosion: biopsy to exclude squamous cell carcinoma — not another course of topical steroid. Erosive oral disease is the variant with malignant potential.
• Koebner phenomenon: new lesions along a scratch or surgical scar. Shared with psoriasis and vitiligo; it is also why elective genital surgery is deferred until disease is quiescent.
• First-line treatment is an ultrapotent topical corticosteroid (clobetasol); topical calcineurin inhibitors are the steroid-sparing second line at mucosal and genital sites.
• Drug-induced lichenoid eruption is the classic distractor: thiazides, beta blockers, antimalarials, NSAIDs, gold, penicillamine. Favor it with photodistribution, eosinophils and parakeratosis on biopsy, and a deeper perivascular infiltrate; management is drug withdrawal, and clearing takes months.

Other distractors to avoid

• Lichen sclerosus, not lichen planus, gives porcelain-white atrophic figure-of-eight anogenital plaques without Wickham's striae or vaginal involvement.
• Oral candidiasis scrapes off leaving a raw base; reticular lichen planus does not.
• Nail pterygium (dorsal pterygium with scarring) is lichen planus; subungual hyperkeratosis with distal onycholysis suggests onychomycosis or psoriasis.
• Lichen planopilaris causes scarring alopecia with loss of follicular ostia — distinguish from alopecia areata, where ostia are preserved and regrowth is possible.