Medicine chapters
Psoriasis and lichen planus
The clinical types, signs, triggers and step-wise treatment of psoriasis, and how to recognise lichen planus and the other papulosquamous rashes that PGET questions compare with it.
- Psoriasis affects about 2% of people: well-defined red plaques with silvery scale on extensors (elbows, knees), scalp and lower back, with Köbner and Auspitz signs, nail pitting and arthritis.
- Treat in steps: topical (vitamin D analogue, steroid, tar) → phototherapy (narrowband UVB, PUVA) → systemic (methotrexate, ciclosporin, acitretin) → biologics; avoid systemic steroids, which can trigger pustular flares.
- Lichen planus: itchy, purple, polygonal, flat-topped papules on the flexor wrists with Wickham striae and white lacy oral lesions; treat with potent topical steroids.
On this page
Overview
Psoriasis and lichen planus are the two classic papulosquamous diseases (raised lesions with scale). Both show the Köbner phenomenon and both can affect nails, scalp and mucosa, so exam questions often place them side by side.
- Psoriasis is common, chronic, and linked to arthritis, metabolic syndrome and heart disease.
- Lichen planus is less common, very itchy and usually settles within 1–2 years.
Psoriasis
Epidemiology
- Affects about 2% of the population; men and women equally.
- Two peaks of onset: 16–22 years (commoner, often with a family history) and 55–60 years.
- Strong genetic component: concordance is about 73% in identical twins versus about 20% in non-identical twins. The main susceptibility locus is PSORS1 (the region that includes HLA-Cw6).
Pathogenesis (exam-level)
- T cells (especially the IL-23 / Th17 axis) and dendritic cells drive keratinocytes to divide far too fast.
- Keratinocytes reach the surface in days instead of about a month, so they keep their nuclei (parakeratosis) and pile up as thick scale.
- Dilated, tortuous capillaries in the dermal papillae make plaques red and explain bleeding when scale is removed.
Histology:
- Parakeratosis (nuclei retained in the stratum corneum).
- Loss of the granular layer.
- Acanthosis with regular elongation of rete ridges.
- Munro microabscesses (neutrophils in the stratum corneum).
- Thin epidermis above dilated dermal papillary vessels.
Triggers
- Streptococcal throat infection (guttate psoriasis).
- Drugs: lithium, beta-blockers, antimalarials (chloroquine, hydroxychloroquine), NSAIDs, ACE inhibitors, and withdrawal of systemic or potent topical steroids (pustular flare).
- Trauma (Köbner), stress, alcohol, smoking (palmoplantar pustulosis), obesity.
- HIV infection (severe or sudden psoriasis).
Clinical types
| Type | Features |
|---|---|
| Chronic plaque (commonest) | Well-defined, salmon-pink to red plaques with silvery-white scale on extensor surfaces (elbows, knees), scalp, lower back, natal cleft, ears |
| Guttate | Showers of small "raindrop" plaques on the trunk in children and young adults, appearing over days after a streptococcal sore throat; often clears; some go on to plaque psoriasis |
| Flexural (inverse) | Shiny, red, well-defined plaques with little scale in groin, axillae, under breasts; no satellite lesions (unlike candida) |
| Scalp | Thick scale that extends just beyond the hairline; hair usually not lost |
| Palmoplantar pustulosis | Sterile yellow-brown pustules on palms and soles; strong link with smoking |
| Generalised pustular (von Zumbusch) | Sheets of sterile pustules on red skin, fever, malaise; can follow steroid withdrawal; an emergency |
| Erythrodermic | Red, scaly skin over 90% or more of the body; risk of heat and fluid loss, infection and heart failure; an emergency |
Signs
- Auspitz sign: scraping the scale reveals pinpoint bleeding points (from the dilated capillaries just under a thin epidermis).
- Köbner phenomenon: plaques appear in scratches, scars and surgical wounds.
- Candle-wax sign: scratching the surface produces silvery scale like candle wax.
- Woronoff ring: a pale halo around a plaque during treatment.
A young adult with well-defined red plaques with silvery scale on the scalp, elbows and knees and pinpoint bleeding when scale is lifted (Auspitz sign) has chronic plaque psoriasis.
Psoriasis is pink and silvery on the outside (extensors); eczema is red and itchy on the inside (flexures).
Nail changes (in up to about half of patients)
- Pitting (commonest; small depressions in the nail plate).
- Onycholysis (separation of the nail from its bed, starting at the tip).
- Oil-drop or salmon patch discoloration.
- Subungual hyperkeratosis.
- Nail changes are especially common in patients with psoriatic arthritis.
Nail pitting is also seen in alopecia areata and eczema. In a stem with a scaly rash on the extensors, pitting points to psoriasis.
Psoriatic arthritis and comorbidities
- Psoriatic arthritis affects up to about 40% of people with moderate–severe disease; it is a seronegative spondyloarthritis. Patterns: distal interphalangeal joints (with nail disease), asymmetrical oligoarthritis, symmetrical polyarthritis, spondylitis, and arthritis mutilans. Dactylitis and enthesitis are typical.
- Metabolic syndrome, obesity, diabetes, fatty liver and cardiovascular disease are more common.
- Depression and poor body image.
Diagnosis
Mostly clinical. Biopsy if unsure. Throat swab or ASO titre in guttate psoriasis. Severity is scored with the PASI (Psoriasis Area and Severity Index) and quality of life with the DLQI.
Management
Psoriasis is controlled, not cured. Choose treatment by extent, site, arthritis and the patient's wishes.
1. Topical (mild, limited disease)
- Emollients to soften scale.
- Vitamin D analogues (calcipotriol, calcitriol): first-line, often combined with a potent steroid (e.g. calcipotriol with betamethasone). Limit calcipotriol to under 100 g per week to avoid hypercalcaemia.
- Topical corticosteroids: moderate–potent on the body; mild on the face and flexures.
- Coal tar and dithranol: effective but messy and staining; dithranol burns normal skin.
- Salicylic acid to remove thick scale, especially on the scalp, followed by tar shampoo.
- Tazarotene (topical retinoid).
2. Phototherapy (extensive disease or failed topical treatment)
- Narrowband UVB: first choice; also for guttate psoriasis.
- PUVA (psoralen + UVA): for thick plaques and palmoplantar disease; raises long-term skin cancer risk.
3. Systemic (severe, extensive, unstable, or with arthritis)
| Drug | Key points |
|---|---|
| Methotrexate | Given once weekly (up to about 25 mg) orally or by injection, with folic acid on another day; treats skin and joints; risks are bone marrow suppression, liver fibrosis, lung toxicity and teratogenicity; monitor blood counts and liver; NSAIDs and trimethoprim increase toxicity |
| Ciclosporin | Fast-acting; good for erythrodermic or unstable psoriasis; causes nephrotoxicity and hypertension |
| Acitretin | Oral retinoid; not immunosuppressive; useful with phototherapy and in pustular disease; teratogenic with very long-lived metabolites, so avoid pregnancy for 3 years after stopping |
| Apremilast, dimethyl fumarate | Newer oral agents |
4. Biologics (severe disease, e.g. PASI over 10, when other systemic treatment fails or is unsuitable)
- Anti-TNF: adalimumab, etanercept, infliximab.
- Anti-IL-12/23: ustekinumab. Anti-IL-17: secukinumab, ixekizumab. Anti-IL-23: guselkumab.
- Screen for latent TB and hepatitis B before starting.
Methotrexate is the classic answer to "one drug for both psoriatic skin and joint disease". Biologics such as anti-TNF agents also treat both.
Systemic steroids are avoided in psoriasis. They clear it briefly, then withdrawal can trigger generalised pustular psoriasis or erythroderma.
Emergency (erythrodermic or generalised pustular psoriasis): admit, keep warm, monitor fluids, temperature and electrolytes, apply bland emollients, treat infection, and start ciclosporin, acitretin, methotrexate or a biologic under a dermatologist.
Lichen planus
What it is
- A chronic inflammatory disease of skin, mucosa, hair and nails, caused by cytotoxic T cells attacking basal keratinocytes.
- Usually middle-aged adults. Most cases have no cause, but it is linked with hepatitis C (and hepatitis B).
- Lichenoid drug eruptions look similar: antimalarials, gold, thiazides, beta-blockers, ACE inhibitors, NSAIDs, anti-TB drugs.
Clinical features: the "6 Ps"
Purple (violaceous), Pruritic, Polygonal, Planar (flat-topped), shiny Papules and Plaques.
- Wickham striae: fine white lacy lines on the surface of papules.
- Sites: flexor wrists and forearms, shins, lower back, ankles and genitals (often annular on the penis).
- Köbner phenomenon in scratches.
- Hypertrophic LP: thick warty plaques on the shins; persists for years.
- Heals with marked post-inflammatory hyperpigmentation, especially in brown skin.
Mucosal involvement (about half of patients, and may occur alone):
- Oral: white, lacy (reticular) streaks on the buccal mucosa, usually painless; erosive oral LP is painful and has a small risk of squamous cell carcinoma.
- Genital: erosive vulvovaginal disease in women.
Nails: longitudinal ridging, thinning and pterygium (scar fusing nail fold to nail bed); can destroy the nail.
Scalp: lichen planopilaris causes scarring alopecia.
Histology
- Band-like (lichenoid) lymphocytic infiltrate hugging the dermo-epidermal junction.
- Saw-tooth rete ridges.
- Thickened granular layer (hypergranulosis), which causes Wickham striae.
- Civatte (colloid) bodies: dead basal keratinocytes.
Psoriasis loses the granular layer; lichen planus thickens it.
Management
- Potent or very potent topical steroids for skin; topical tacrolimus or steroid gels for the mouth.
- Widespread disease: short course of oral prednisolone, phototherapy, acitretin, or other immunosuppressants.
- Stop any culprit drug; check for hepatitis C.
Oral lichen planus (white lacy streaks on the buccal mucosa) is a classic clue to hepatitis C; the erosive form needs follow-up for SCC.
- Monitor chronic erosive oral or genital disease for SCC.
Prognosis: skin disease usually clears within 1–2 years; mucosal and hypertrophic disease last longer.
Psoriasis versus lichen planus
| Feature | Psoriasis | Lichen planus |
|---|---|---|
| Colour | Salmon-pink/red, silvery scale | Violaceous, shiny, little scale |
| Site | Extensors, scalp, back | Flexor wrists, shins, mouth |
| Itch | Mild to moderate | Severe |
| Surface sign | Auspitz sign | Wickham striae |
| Mouth | Rare | Common (lacy white) |
| Nails | Pitting, onycholysis | Ridging, pterygium |
| Granular layer | Lost | Thickened |
| Course | Lifelong, relapsing | Usually clears in 1–2 years |
Other papulosquamous rashes
Pityriasis rosea
- Acute, self-limiting rash of older children and young adults, possibly due to HHV-6 and HHV-7.
- Starts with a single larger oval herald patch, followed days later by many smaller oval pink macules with a fine collarette of scale.
- Lesions follow skin lines on the back in a "Christmas tree" pattern; mainly trunk and upper limbs.
- Mild itch; clears in 4–8 weeks without scarring. Reassure; emollient or mild steroid for itch.
A widespread papulosquamous rash that also involves the palms and soles in a sexually active adult is secondary syphilis until proved otherwise. Check VDRL/RPR.
Lichen sclerosus
Shiny ivory-white, itchy, atrophic patches on the vulva (often in a figure-of-eight around vulva and anus) or glans and foreskin; causes scarring, phimosis and a raised risk of SCC. Treat with a very potent topical steroid.
Expect: psoriasis lesion and sites, Auspitz and Köbner signs, nail pitting, guttate psoriasis after strep throat, drugs that worsen psoriasis (lithium, beta-blockers, antimalarials, steroid withdrawal), histology (parakeratosis, Munro microabscesses, lost granular layer), methotrexate for skin plus joints, acitretin and pregnancy, calcipotriol and hypercalcaemia, the 6 Ps and Wickham striae of lichen planus, oral lacy lesions, the hepatitis C link, and pityriasis rosea with herald patch.
Quick revision
- Psoriasis: 2%; silvery plaques on extensors; Köbner; Auspitz.
- Guttate after strep throat; pustular after steroid withdrawal.
- Worsened by lithium, beta-blockers, antimalarials.
- Nail pitting and onycholysis; arthritis up to about 40% of moderate–severe cases.
- Topical vitamin D ± steroid → narrowband UVB/PUVA → methotrexate, ciclosporin, acitretin → biologics.
- Methotrexate weekly with folic acid; treats skin and joints.
- Lichen planus: purple polygonal papules, flexor wrists, Wickham striae, lacy oral lesions, hepatitis C.
- LP histology: band-like lymphocytes, saw-tooth rete, hypergranulosis.
- Pityriasis rosea: herald patch, Christmas tree, clears in 4–8 weeks.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine).
FAQ
What is the Auspitz sign?
Pinpoint bleeding when the scale of a psoriatic plaque is scraped off, caused by dilated capillaries lying close to the surface.
Why are systemic steroids avoided in psoriasis?
They give only short-term benefit, and stopping them can trigger severe generalised pustular or erythrodermic psoriasis.
Which drug treats both psoriatic skin disease and arthritis?
Methotrexate, given once a week with folic acid; biologic agents such as anti-TNF drugs also treat both.
What are Wickham striae?
Fine white lacy lines on the surface of lichen planus papules and on the oral mucosa, due to a thickened granular layer.
How long does lichen planus last?
Skin lesions usually clear within one to two years, though pigmentation may persist and mucosal disease can last much longer.