A 50 y.o. male presents with a pruritic rash on his shoulder
What is your differential?
Our patient had lichen planus.
Lichen planus (LP) is a chronic disease of the skin and mucous membranes. It was first described by Erasmus Wilson in 1869. The term lichen referred to the moss-like lesions and planus to its flat surface. Cytotoxic T lymphocytes are thought to be the primary mediators. While it is immune mediated it is not considered an autoimmune disease.
Clinically, it presents with the “six P’s” since it is described as planar, purple, polygonal, pruritic, papular and plaque-like. The mouth is affected in 50% of all patients with lichen planus. Often this appears as lacy white streaks as shown below. 1-2% of oral lichen planus will undergo malignant transformation to squamous cell ca. In addition to cancer the differential for oral lesions includes pemphigoid, lupus or chronic ulcerative stomatitis.
lichen planus can be caused by mercury in fillings
Lichen planus may be activated by Hep C infection or dental amalgams. In addition, mechanical stress, psychological stress or microbiome changes in the gut have all be associated with LP. It has been reported after vaccinations for flu, varicella, hepatitis, tetanus and CoV-2. It is more common in certain HLA types. Several drugs including: NSAIDS, epilepsy meds, HIV meds, dapsone, and psychiatric medications have been associated with the condition.
Contact allergy to mercury compounds is important in the pathogenesis of oral lichen planus. Half of all dental fillings contain mercury. The chronic inflammation caused by mercury leaching from the amalgams activates the immune system. Removal of the fillings has been shown to improve lichen planus.
lichen planus can also be seen in the genital areas
PATHOPHYSIOLOGY
While the full mechanism of its immunology is not known, LP is caused by cell-mediated cytotoxicity mostly CD4+ T lymphocytes . These CD4+ T cells differentiate into two subsets Th1(promotes cellular immunity) and Th2 ( produces IL-4,5, 12 and 13.) Ultimately, more signaling molecules are present leading to the IL-23/Th-17 axis. While the pathway is still under debate, the treatment is known: steroids or immune-suppression .
A new therapy for lichen planus is baricitinib, a JAK inhibitor. The Janus kinase inhibitor interrupts the signaling pathway of interferon gamma, preventing plaque formation.
FUN FACT
Interleukins are extremely complex. For example, interleukin-17 is a signaling molecule(cytokine) found in Lyme arthritis. It also plays a protective role in preventing Lyme arthritis by clearing the bacteria from the body.
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Yan M, Ouyang Y, Xiao L, et al. correlations between gut microbiota and lichen planus: a two-sample Mendelian randomization study. Front Immunol. 2023 Sep 12;14:1235982.
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Hwang A, Kechter J, Do T, et al. Oral baricitinib in the treatment of cutaneous lichen planus. medRxiv(preprint) 2924 Jan 11:2024.o1.09.24300946.
Khurana A, Tandon S, Marfatia Y, et al. Genital lichen planus: an underrecognized entity. Indian J Sex Transm Dis AIDS 2019 Jul-Dec;40(2):105-112,