Lichen planus is an inflammatory condition in which immune cells act against the boundary between the two upper layers of the skin. Because that boundary exists wherever there is lining tissue, the same process can appear on the skin, inside the mouth, on the genitals, in the nails and on the scalp.
What is actually happening in the skin?
Between the epidermis and the dermis lies a boundary, a thin structured interface that anchors one to the other. Most conditions happen above it or below it. Lichen planus happens at it, and that single fact explains almost everything else about the condition.
In lichen planus, immune cells gather along that interface and act against the cells sitting on it. The damage is concentrated in a narrow band rather than spread through a layer, which is why the appearance is so distinctive and why a pathologist can recognise the pattern with unusual confidence.
The consequences of lichen planus follow from that location. Pigment released from damaged cells drops into the dermis below, where it clears slowly, and that is why marks persist long after the active phase has passed. The surface above changes too, producing the shine that clinicians describe.
Where else it appears is the part people find surprising. That same interface exists in the lining of the mouth and the genitals, in the nail unit and in the scalp, so lichen planus can arrive in any of them, sometimes without any skin involvement at all. What causes the immune system to behave this way is not established.
How would a dermatologist describe it?
Interface dermatitis is the technical description of the process, and it names the location rather than the cause. Anything acting at that boundary belongs to the same family, which is why the term appears in reports about several conditions.
Clinicians describe the skin lesions of lichen planus as flat-topped, shiny and violet-toned, and those three words together are more specific than any one of them alone. Wickham striae is the term for the fine pale network sometimes visible across the surface, and a dermatoscope makes it far easier to see.
Oral lichen planus is named separately because it behaves differently, is monitored differently, and often persists longer. Lichen planopilaris describes involvement of hair follicles on the scalp, where the concern is different again.
The Koebner phenomenon appears here as it does in psoriasis: new lesions arising at sites of injury to the skin. Clinicians ask about scratches, tattoos and surgery for that reason.
What usually happens at a consultation?
The history covers when it started, where, whether it itches, and what has already been applied. Expect direct questions about the mouth, the genitals, the nails and the scalp even if you came about your arms, because involvement elsewhere changes the assessment.
The examination frequently extends to the mouth, and being asked to open it on a skin appointment is routine rather than a sign that something has been found. A dermatoscope is often used on the skin lesions themselves.
A biopsy is proposed more often in lichen planus than in most conditions, because the microscopic pattern at that interface is characteristic. Being offered one is a step towards an answer rather than an indication that anything alarming is suspected.
Questions about medicines you take come up because some drug reactions imitate this appearance closely. That line of questioning is standard and is not an accusation about anything you are taking.
What is worth asking?
Ask which sites the clinician has checked and which they want to keep an eye on. Lichen planus is a condition of several territories, and knowing which ones are in play tells you what the follow-up is actually for.
Ask whether what you are seeing is active or a mark left behind. That distinction governs everything downstream, and people frequently spend months treating discolouration that had already finished.
Ask what would be worth reporting between appointments, particularly regarding the mouth. Named changes are easier to watch for than a general instruction to keep an eye on things.
If a biopsy is suggested, ask what question it is expected to answer. The reply explains the reasoning better than any general account of the procedure.
What this page does not do
Nothing here tells you whether a rash is lichen planus. Several conditions produce flat violet-toned lesions, several produce white patches in the mouth, and separating them is what an examination and sometimes a sample are for. No treatment is recommended and no medicine, dose or duration appears anywhere on this page.
Common questions
Why does it affect the mouth as well as the skin?
Because the process targets the boundary between surface tissue and what lies beneath, and that boundary exists in lining tissue as well as in skin. The mouth is affected often enough that clinicians ask about it directly rather than waiting for it to be mentioned.
Is it contagious?
No. Nothing is passed between people. It is an immune process rather than an infection, which is also why it does not respond to anything aimed at organisms.
Why do the marks stay after it settles?
Inflammation at that boundary can leave pigment behind in the layer below, where it clears far more slowly than colour held higher up. A clinician will distinguish that lasting discolouration from active disease, and the two need quite different conversations.
Why is a biopsy sometimes suggested?
Because the appearance can resemble other conditions, and what settles the question is how the tissue looks under a microscope at that boundary. What a biopsy involves practically is described elsewhere on this site.
Does it always itch?
Itch is common on the skin and can be intense, while involvement in the mouth more often produces burning or soreness than itch. Describing which sensation you actually have is more useful than reaching for the word that seems expected.
Sources
- Lichen planus — NHS (United Kingdom) · accessed
- Patient information on skin conditions — British Association of Dermatologists (United Kingdom) · accessed
Last updated . How this page was written.