Purple, Flat, and Itchy: Reading Lichen Planus

Three rashes, three very different treatments

Lichen planus, eczema, and psoriasis are unrelated conditions that all cause chronic, itchy, inflamed patches of skin, which is exactly why they get confused so often. Lichen planus is thought to be driven by the immune system attacking the skin's own cells. Eczema, more precisely atopic dermatitis, involves a broken skin barrier combined with an overactive immune response, and often runs alongside allergies or asthma. Psoriasis is an immune-driven condition that speeds up skin-cell turnover far beyond normal, producing thick, scaly plaques.

Because the three mechanisms are different, so are the treatments, and a rash that resists a treatment aimed at the wrong one is one of the more common reasons a skin condition drags on for months. The sections below walk through what each one actually looks like.

What does lichen planus look like?

Lichen planus classically produces the six Ps: small, purple, polygonal, planar (flat-topped), pruritic (itchy) papules — bumps with distinct, sometimes angular edges rather than the rounder, blurrier borders of eczema or psoriasis. Looking closely, many lesions show fine, lacy white lines across their surface called Wickham striae, a detail that's fairly specific to lichen planus and rarely seen in the other two.

It favors the wrists, forearms, ankles, and lower back, though it can appear almost anywhere, including the scalp, nails, and mouth, where it often looks like a white, lacy web across the inner cheeks rather than a raised bump. The itch can be intense, and lesions sometimes appear in a line where the skin has been scratched or scraped, a pattern called the Koebner phenomenon that psoriasis can also show, though eczema typically does not.

Lichen planus is not contagious, and for many people it eventually resolves, though the pace and the path it takes vary from person to person.

What does eczema look like?

Eczema (atopic dermatitis) makes ill-defined, red or brownish patches with blurred rather than sharp edges, often dry, cracked, or thickened from scratching, and sometimes weepy or crusted during a flare. The National Institute of Arthritis and Musculoskeletal and Skin Diseases describes it as a chronic inflammatory skin disease that typically begins in childhood, follows a course of flares and remissions, and results from a combination of genetics, immune dysregulation, and environmental triggers 1Ref 1National Institute of Arthritis and Musculoskeletal and Skin Diseases (2024).Atopic Dermatitis (Eczema).That atopic dermatitis is a chronic inflammatory itchy skin disease that typically begins in childhood, follows a course of flares and remissions, and results from genetics, immune dysregulation, and environmental factors..

Distribution is one of the more useful clues: eczema favors the creases — the inside of the elbows, behind the knees, the neck, the wrists — rather than the flat-topped, symmetric plaques on extensor surfaces (the outside of elbows and knees) that psoriasis favors. It's also closely tied to the broader atopic march: many people with eczema also have or later develop asthma, hay fever, or food allergies, reflecting shared immune dysregulation and skin-barrier dysfunction 2Ref 2National Institute of Allergy and Infectious Diseases (2024).Eczema (Atopic Dermatitis).That atopic dermatitis involves immune dysregulation and skin-barrier dysfunction, and is associated with the atopic march of food allergy, asthma, and allergic rhinitis..

Unlike lichen planus, eczema rarely shows a sharply defined, flat-topped bump, and it doesn't produce the lacy white lines of Wickham striae; the skin looks more inflamed and less structured.

What does psoriasis look like?

Psoriasis produces thick, well-demarcated plaques covered in a distinctive silvery-white scale, usually symmetric, and classically on the outside of the elbows and knees, the scalp, and the lower back, though it can appear anywhere. Unlike the blurred edges of eczema, a psoriasis plaque typically has a crisp, almost drawn-on border where affected skin meets normal skin. Removing the scale can reveal small points of bleeding, a sign sometimes called the Auspitz sign that isn't seen with lichen planus or eczema.

Nail changes are common and distinctive: small pits across the nail surface, separation of the nail from the nail bed, or a yellowish-brown discoloration under the nail sometimes called an oil-drop sign. In a meaningful minority of people, psoriasis is accompanied by joint pain and stiffness — psoriatic arthritis — which neither lichen planus nor eczema typically causes.

Because psoriasis, eczema, and lichen planus can all produce scaly, itchy patches, distribution and scale texture together are usually more reliable than any single feature on its own.

A few features that reliably tell them apart

No single row settles a diagnosis on its own — overlap happens, especially early in a rash's course — but color, border sharpness, and scale texture together usually point strongly toward one of the three.

Could it be something else?

A fourth look-alike is worth ruling out before settling on any of the three: ringworm, or tinea, a fungal infection that produces a red, scaly, often ring-shaped patch with a clearer center, spread by contact with an infected person, animal, or surface 3Ref 3Centers for Disease Control and Prevention (2024).Ringworm Basics.That ringworm (tinea) is a contagious dermatophyte skin infection presenting as a circular, scaly rash, spread by contact with infected people, animals, or surfaces — used as a fourth look-alike to rule out.. Unlike lichen planus, eczema, or psoriasis, ringworm is contagious and typically responds to an antifungal rather than an anti-inflammatory treatment — applying a steroid to ringworm can make it look temporarily better while it actually spreads, which is one of the more common diagnostic traps in this group.

A rash that has a genuinely round, ring-like shape with central clearing, especially after contact with a pet or another person with a similar rash, is worth considering separately from the three chronic inflammatory conditions covered here.

Why getting the diagnosis right matters

Each of these three conditions has its own treatment ladder, and applying the wrong one doesn't just fail to help — it can waste months. Lichen planus generally starts with a topical steroid, following its own lichen planus treatment options, and escalates toward severe lichen planus treatment — a systemic medication — for widespread or resistant disease. Eczema's treatment moves from moisturizers and topical steroids 4Ref 4Sidbury R, Alikhan A, Bercovitch L, et al. (2023).Guidelines of care for the management of atopic dermatitis in adults with topical therapies.That first-line eczema treatment in adults centers on moisturizers, bathing practices, and topical corticosteroids or calcineurin inhibitors. toward newer targeted systemic therapies for more severe cases 5Ref 5Sidbury R, Davis DM, Alikhan A, et al. (2024).Guidelines of care for the management of atopic dermatitis in adults with phototherapy and systemic therapies.That moderate-to-severe eczema that doesn't respond to topical treatment escalates to phototherapy or systemic therapies such as biologics and JAK inhibitors.. Psoriasis has a separate systemic and biologic pathway entirely. None of these steps transfers cleanly to one of the other two conditions.

Access to a dermatologist who can make this call quickly isn't universal: dermatologists are unevenly distributed and concentrated in metropolitan areas, which creates real waits for an in-person diagnosis in many parts of the country 6Ref 6Feng H, Berk-Krauss J, Feng PW, Stein JA (2018).Comparison of Dermatologist Density Between Urban and Rural Counties in the United States.That dermatologists are concentrated in metropolitan areas, creating access disparities and longer waits for an in-person diagnosis outside major metro areas.. When lichen planus specifically is suspected, getting it right matters even more depending on where it shows up — nail lichen planus can scar the nail permanently if treatment is delayed, lichen planopilaris on the scalp destroys the hair follicles it reaches, and lichen planus in the genital skin or lichen planus inside the mouth each need their own tailored approach — which is why a rash that doesn't fit neatly into 'probably eczema' is worth a dedicated look rather than a guess.