Lichen planus (LP) is a disease of the skin and/or mucous membranes that resembles lichen. The cause is unknown, but it is thought to be the result of an autoimmune process with an unknown initial trigger. There is no cure, but many different medications and procedures have been used to control the symptoms.
The term lichenoid reaction (or lichenoid lesion) refers to a lesion of similar or identical histopathologic and clinical appearance to lichen planus (i.e. an area which looks the same as lichen planus, both to the naked eye and under a microscope). Sometimes dental materials or certain medications can cause a lichenoid reaction. They can also occur in association with graft versus host disease.:258
'Ring-shaped' lesions that develop gradually from single small pigmented spots into circular groups of papules with clear, unaffected skin in the center. Annular lesions occur in approximately ten percent of lichen planus cases. The ring-like lesions may very slowly enlarge, co-join and morph into larger irregular (serpentine) bands, sometimes accompanied by lines (See Linear, below).
Papules are arranged in a line (the "Blaschko line"). This pattern may develop secondary to trauma (koebnerization) or, uncommonly, as a spontaneous, isolated eruption, usually on the extremities, and rarely on the face.
This pattern usually occurs on the extremities, especially the shins and the interphalangeal joints, and tends to be the most pruritic variant of lichen planus. Also known as "Lichen planus verrucosus".
This morphology is characterized by the presence of a few well-demarcated, white-bluish papules or plaques with central superficial atrophy. This is a rare variant of lichen planus.
This morphology is characterized by the development of vesicles and bullae with the skin lesions. This is a rare variant of lichen planus, and also known as "Vesiculobullous lichen planus".
This morphology is characterized by chronic, painful bullae and ulceration of the feet, often with cicatricial sequelae evident. This is a rare variant of lichen planus.
This morphology is characterized by hyperpigmented, dark-brown macules in sun-exposed areas and flexural folds. This is a rare variant of lichen planus.
Occasionally, lichen planus is known to occur with other conditions. For example:
Lupus erythematosus overlap syndrome. Lesions of this syndrome share features of both lupus erythematosus and lichen planus. Lesions are usually large and hypopigmented, atrophic, and with a red-to-blue colour and minimal scaling. Telangectasia may be present.:366,470–1
Although lichen planus can present with a variety of lesions, the most common presentation is as a well-defined area of purple-coloured, itchy, flat-topped papules with interspersed lacy white lines (Wickham's striae). This description is known as the characteristic "6 Ps" of lichen planus: planar (flat-topped), purple, polygonal, pruritic, papules, and plaques. This rash, after regressing, is likely to leave an area of hyperpigmentation that slowly fades. That said, a variety of other lesions can also occur.
Cutaneous lichen planus
Cutaneous lichen planus on the shin.
Lichen planus involving the nails.
Variants of cutaneous lichen planus are distinguished based upon the appearance of the lesions and/or their distribution. Lesions can affect the:
Extremities (face, dorsal hands, arms, and nape of neck).[a] This is more common in Middle Eastern countries in spring and summer, where sunlight appears to have a precipitating effect.:468:223
Nails characterized by irregular longitudinal grooving and ridging of the nail plate, thinning of the nail plate, pterygium formation, shedding of the nail plate with atrophy of the nail bed, subungual keratosis, longitudinal erthronychia (red streaks), and subungual hyperpigmentation.:781 A sand-papered appearance is present in around 10% of individuals with nail lichen planus.
Scalp. This is also known as lichen planopilaris, acuminatus, follicular lichen planus, and peripilaris,:467 characterised by violaceous, scaly, pruritic papules. Scalp lichen planus can cause scarring alopecia if it is untreated.:223
Hair. This variant causes inflammation of hair follicles and gradual replacement with scarring. About 10% of people with lichen planus have the scalp or nail variants of the condition.
Other variants may include:
Lichen planus pemphigoides characterized by the development of tense blisters atop lesions of lichen planus or the development vesicles de novo on uninvolved skin.:471
Keratosis lichenoides chronica (also known as "Nekam's disease") is a rare dermatosis characterized by violaceous papular and nodular lesions, often arranged in a linear or reticulate pattern on the dorsal hands and feet, extremities, and buttock.:472:224
Lichenoid keratoses (also known as "Benign lichenoid keratosis," and "Solitary lichen planus") is a cutaneous condition characterized by brown to red scaling maculopapules, found on sun-exposed skin of extremities.:473 Restated, this is a cutaneous condition usually characterized by a solitary dusky-red to violaceous papular skin lesion.:639
Lichenoid dermatitis represents a wide range of cutaneous disorders characterized by lichen planus-like skin lesions.:473
Mucosal lichen planus
Lichen planus on the lips and the lateral border of the tongue
Lichen planus affecting mucosal surfaces may have one lesion or be multifocal. Examples of lichen planus affecting mucosal surfaces include:
Genital lichen planus, which may cause lesions on the glans penis or skin of the scrotom 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.
Oral lichen planus
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 are recognized:
Reticular, the most common presentation of oral lichen planus, is characterised by the net-like or spider web-like appearance of lacy white lines, oral variants of Wickham's straiae. This is usually asymptomatic.
Erosive/ulcerative, the second most common form of oral lichen planus, is characterised by oral ulcers presenting with persistent, irregular areas of redness, ulcerations and erosions covered with a yellow slough. This can occur in one or more areas of the mouth. In 25% of people with erosive oral lichen planus, the gums are involved, described as desquamative gingivitis (a condition not unique to lichen planus). This may be the initial or only sign of the condition.
Atrophic, appearing as areas. Atrophic oral lichen planus may also manifest as desquamative gingivitis.
Bullous, appearing as fluid-filled vesicles which project from the surface.
These types often coexist in the same individual. Oral lichen planus tends to present bilaterally as mostly white lesions on the inner cheek, although any mucosal site in the mouth may be involved. Other sites, in decreasing order of frequency, may include the tongue, lips, gingivae, floor of the mouth, and very rarely, the palate.
Generally, oral lichen planus tends not to cause any discomfort or pain, although some people may experience soreness when eating or drinking acidic or spicy foodstuffs or beverages. When symptoms arise, they are most commonly associated with the atrophic and ulcerative subtypes. These symptoms can include a burning sensation to severe pain.
It has also been suggested that mercury exposure may contribute to lichen planus 
Lichen planus lesions are diagnosed clinically by their "lichen-like" appearance. A biopsy can be used to rule out conditions that may resemble lichen planus, and can pick up any secondary malignancies.
Lupus erythematosus, with lesions more commonly occur on the palate and appear as centrally ulcerated or erythematous with radiating white striae. In contrast, OLP and lichenoid reactions rarely occur on the palate, and the striae are randomly arranged rather than radial.
There is no cure for lichen planus, and so treatment of cutaneous and oral lichen planus is for symptomatic relief or due to cosmetic concerns. When medical treatment is pursued, first-line treatment typically involves corticosteroids, and removal of any triggers. Without treatment, most lesions will spontaneously resolve within 6–9 months for cutaneous lesions, and longer for mucosal lesions 
Cutaneous lichen planus
Many different treatments have been reported for cutaneous lichen planus, however there is a general lack of evidence of efficacy for any treatment. Treatments tend to be prolonged, partially effective and disappointing. First-line treatments may include retinoids such as Acitretin, or corticosteroids
In contrast to cutaneous lichen planus, lichen planus lesions in the mouth may persist for many years, and tend to be difficult to treat, with relapses being common. Atrophic/erosive lichen planus is associated with a small risk of malignant transformation, and so people with OLP tend to be kept on long term review to detect any potential change early. Sometimes OLP can become secondarily infected with Candida organisms.
The overall prevalence of lichen planus in the general population is about 0.1 - 4%. It generally occurs more commonly in females, in a ratio of 3:2, and most cases are diagnosed between the ages of 30 and 60, but it can occur at any age.
Oral lichen planus is relatively common, It is one of the most common mucosal diseases. The prevalence in the general population is about 1.27-2.0%, and it occurs more commonly in females than males, and in middle aged people. OLP in children is rare. About 50% of females with oral lichen planus were reported to have undiagnosed vulvar lichen planus.
^ abcdefghiBolognia, edited by Jean L.; Joseph L. Jorizzo; Ronald P. Rapini (2008). Dermatology (2nd ed. ed.). [St. Louis, Mo.]: Mosby/Elsevier. ISBN1416029990.
^ abGordon, KA; Vega, JM; Tosti, A (Nov–Dec 2011). "Trachyonychia: a comprehensive review.". Indian journal of dermatology, venereology and leprology77 (6): 640–5. doi:10.4103/0378-6323.86470. PMID22016269.
^Cevasco NC, Bergfeld WF, Remzi BK, de Knott HR (2007). "A case-series of 29 patients with lichen planopilaris: the Cleveland Clinic Foundation experience on evaluation, diagnosis, and treatment". J. Am. Acad. Dermatol.57 (1): 47–53. doi:10.1016/j.jaad.2007.01.011. PMID17467854.
^ abEbrahimi, M; Lundqvist, L; Wahlin, YB; Nylander, E (October 2012). "Mucosal lichen planus, a systemic disease requiring multidisciplinary care: a cross-sectional clinical review from a multidisciplinary perspective.". Journal of lower genital tract disease16 (4): 377–80. doi:10.1097/LGT.0b013e318247a907. PMID22622344.
^Panagiotopoulou, N; Wong, CS; Winter-Roach, B (April 2010). "Vulvovaginal-gingival syndrome.". Journal of obstetrics and gynaecology : the journal of the Institute of Obstetrics and Gynaecology30 (3): 226–30. doi:10.3109/01443610903477572. PMID20373919.
^Alam, F; Hamburger, J (May 2001). "Oral mucosal lichen planus in children.". International journal of paediatric dentistry / the British Paedodontic Society [and] the International Association of Dentistry for Children11 (3): 209–14. PMID11484471.
^ abcdefgScully C (2008). Oral and maxillofacial medicine : the basis of diagnosis and treatment (3rd ed.). Edinburgh: Churchill Livingstone. pp. 192–199. ISBN9780702049484.
^Roopashree, MR; Gondhalekar, RV; Shashikanth, MC; George, J; Thippeswamy, SH; Shukla, A (November 2010). "Pathogenesis of oral lichen planus--a review.". Journal of oral pathology & medicine : official publication of the International Association of Oral Pathologists and the American Academy of Oral Pathology39 (10): 729–34. doi:10.1111/j.1600-0714.2010.00946.x. PMID20923445.
^ abcdDavidson's principles and practice of medicine. (21st ed. ed.). Edinburgh: Churchill Livingstone/Elsevier. 2010. pp. 1265–1266. ISBN978-0-7020-3085-7.
^ abIssa, Y; Brunton, PA; Glenny, AM; Duxbury, AJ (November 2004). "Healing of oral lichenoid lesions after replacing amalgam restorations: a systematic review.". Oral surgery, oral medicine, oral pathology, oral radiology, and endodontics98 (5): 553–65. doi:10.1016/j.tripleo.2003.12.027. PMID15529127.
^Lichenoid reactions of murine mucosa associated with amalgam. Dunsche A, Frank MP, Luttges J, Acil Y, Brasch J, Christophers E, Springer IN. Br J Dermatol. 2003 Apr;148(4):741-8.
^ abcdeKerawala C, Newlands C (editors) (2010). Oral and maxillofacial surgery. Oxford: Oxford University Press. pp. 412–413. ISBN9780199204830.