Thorough examination may lead to the detection of extraoral manifestations of LP that provide additional support for the diagnosis or the identification of clinical findings that suggest another diagnosis. Tissue biopsies of oral LP help to confirm the diagnosis and are particularly of value for erythematous and erosive LP, which share features with multiple other mucosal disorders, including oral malignancy. Biopsies to confirm oral LP are less essential in patients who present with classic reticular LP, particularly in patients in whom a diagnosis of LP has already been confirmed through biopsy of an extraoral manifestation of this disorder. Differential diagnosis
Skin
Lichenoid drug eruption
The cutaneous manifestations resemble idiopathic lichen planus. Chronic graft-versus-host disease
The history of preceding hematopoietic cell transplant is helpful for diagnosis
Psoriasis
Atopic dermatitis
Cutaneous lupus erythematosus
Discoid lupus erythematosus

Mouth
Oral lichenoid drug reaction
Lichenoid drug eruptions may be caused by a variety of systemic medications and share clinical features with oral LP. Histologic findings of a deep mixed infiltrate with lymphocytes, plasma cells, and neutrophils (with or without eosinophils) and perivascular inflammation favor this diagnosis. Oral lichenoid contact reaction (allergic contact mucositis)
Oral lichenoid contact reactions may be caused by a variety of substances. The clinical and histologic features of oral lichenoid contact reactions are similar to oral LP. Patch testing and recognition of the proximity of an offending substance to the eruption can aid with diagnosis. Autoimmune blistering diseases
Mucous membrane pemphigoid and other autoimmune blistering diseases may present with oral erosions and desquamative gingivitis similar to that seen in erosive LP.
Restated, this is a cutaneous condition usually characterized by a solitary dusky-red to violaceous papular skin lesion. Lichenoid dermatitis represents a wide range of cutaneous disorders characterized by lichen planus-like skin lesions. Mucous membranes
Lichen planus affecting mucosal surfaces may have one lesion or be multifocal. Examples of lichen planus affecting mucosal surfaces include:
Esophageal lichen planus, affecting the esophageal mucosa. This can present with difficulty or pain when swallowing due to oesophageal inflammation, or as the development of an esophageal stricture. It has also been hypothesized that it is a precursor to squamous cell carcinoma of the esophagus. Genital lichen planus, which may cause lesions on the glans penis or skin of the scrotum in males, and the vulva or vagina in females. Symptoms may include lower urinary tract symptoms associated with stenosis of the urethra, painful sexual intercourse, and itching. In females, Vulvovaginal-gingival syndrome, is severe and distinct variant affecting the vulva, vagina, and gums, with complications including scarring, vaginal stricture formation, or vulva destruction. The corresponding syndrome in males, affecting the glans penis and gums, is the peno-gingival syndrome. It is associated with HLA-DQB1. Mouth
Oral lichen planus (also termed oral mucosal lichen planus), is a form of mucosal lichen planus, where lichen planus involves the oral mucosa, the lining of the mouth. This may occur in combination with other variants of lichen planus. Six clinical forms of oral lichen planus (OLP) are recognized:
These types often coexist in the same individual.