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Lichen planus
Lichen planus is a T-cell mediated inflammatory disease that attacks the basal layer of the epidermis, presenting with itchy, flat, purple papules with fine white lines (Wickham striae), and also on the oral mucosa, genitals, nails and scalp.
Why it matters in the exam
Exam stems show the "four Ps" — planar, purple, pruritic, polygonal papules — on the wrists and ankles, together with a white lacy network inside the cheeks. Asked are the link to hepatitis C, the Koebner phenomenon (new lesions along a scratch), drugs that cause lichenoid reactions (beta-blockers, ACE inhibitors, thiazides, antimalarials, PD-1 inhibitors), the SCC risk in erosive oral lichen planus, and lichen planopilaris of the scalp, which causes scarring alopecia. In the nails a pterygium forms — scarring that joins the skin to the nail and can cause permanent nail loss. Remember that skin lesions usually clear within one to two years and leave dark marks, whereas the oral form lasts for years and needs regular follow-up. For the exam, remember further variants as well: hypertrophic lichen planus on the shins, which itches intensely and can give rise to SCC; lichen planus pigmentosus in darker skin; and actinic lichen planus in sun-exposed areas. In children the disease is relatively rare, and in adults it appears mainly in middle age. The treatment decision follows the site involved and the severity of itch and pain, not only the extent of the lesions.
Key points
- Lichen planus is a T-cell mediated disease that attacks the basal layer and presents with purple, flat, itchy, polygonal papules.
- Wickham striae and the Koebner phenomenon are typical, and lesions appear mainly on the wrists, ankles and lower back.
- Oral involvement is common, and the erosive form carries a small SCC risk.
- Test for hepatitis C and look for drugs that may cause a lichenoid reaction.
- Treatment is potent topical steroids, and for widespread disease phototherapy, acitretin or short systemic steroids.
Summary table
| Site | Appearance | Significance | Treatment |
|---|---|---|---|
| Skin | Purple, flat, itchy papules with white lines (Wickham) on the wrists and ankles | Usually clears within one to two years; leaves hyperpigmentation | Potent topical steroids; NB-UVB or acitretin for widespread disease |
| Mouth | White lacy (reticular) network inside the cheeks; painful erosive form | The erosive form carries a small SCC risk | Topical steroid gel, tacrolimus; periodic follow-up |
| Genitals | Erosive lesions, adhesions; vulvovaginal-gingival syndrome | Scarring and SCC | Clobetasol; multidisciplinary care |
| Nails | Thinning, longitudinal ridges, pterygium | Permanent nail damage | Early intralesional or systemic steroids |
| Scalp — lichen planopilaris | Perifollicular redness and scale, scarring alopecia | Permanent hair loss | Potent steroids, hydroxychloroquine |
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Sources
- British Association of Dermatologists guideline on lichen planus, 2023 — Cutaneous, oral and genital disease
- European S1 guideline on the management of lichen planus (EADV), 2020 — Treatment
- AASLD guidance on hepatitis C, 2023 — Extrahepatic manifestations
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.