5.2 Lichen Planus, Lichenoid Eruptions & Other Papulosquamous Disorders

5.2 Lichen Planus, Lichenoid Eruptions & Other Papulosquamous Disorders

Papulosquamous disorders present as scaly papules and plaques. Many look alike clinically, and the examination often uses a clinical image plus a histology clue to separate them. This section covers the main conditions other than psoriasis.

Lichen Planus (LP)

LP is a T-cell-mediated interface dermatitis affecting roughly 0.5–1% of adults, with oral involvement in many patients. The classic lesion follows the "6 Ps": planar (flat-topped), purple, polygonal, pruritic papules and plaques. Fine white lines on the surface are Wickham striae. Common sites are the flexor wrists, forearms, shins, lower back, and genitals. The Koebner phenomenon is frequent.

Associations: hepatitis C virus (the association varies by region and is strongest in Southern Europe and Japan), and other autoimmune disease.

Histology: compact orthohyperkeratosis, wedge-shaped hypergranulosis, saw-tooth rete ridges, vacuolar change of the basal layer, a band-like lymphocytic infiltrate hugging the dermo-epidermal junction, Civatte (colloid) bodies, and sometimes clefts (Max-Joseph spaces). Direct immunofluorescence shows shaggy fibrinogen along the basement membrane zone and IgM on colloid bodies.

Oral LP and cancer: oral LP carries a small risk of oral squamous cell carcinoma, about 1% in meta-analyses, higher in erosive disease, smokers, and tongue lesions. Persistent ulcers or thickened areas need biopsy, and patients need long-term review.

Treatment: potent or very potent topical corticosteroids for skin lesions; topical steroids or tacrolimus for oral disease; and dental hygiene, including removal of irritant restorations where a lichenoid reaction to amalgam is suspected. Extensive or refractory disease may need a short course of oral prednisolone, acitretin, narrowband UVB, methotrexate, or other immunosuppressants. Hydroxychloroquine is used for lichen planopilaris.

Lichenoid Drug Eruption

Drugs can cause an LP-like eruption, typically weeks to months after starting. Culprits include ACE inhibitors, beta-blockers, thiazides, antimalarials, gold, and immune checkpoint inhibitors. Clues are photodistribution, a more eczematous appearance, and absence of Wickham striae. Histology may show parakeratosis, eosinophils, and a deeper perivascular infiltrate. Resolution after stopping the drug can take months, often with pigmentation.

Lichen Nitidus and Lichen Striatus

• Lichen nitidus: tiny (1–2 mm) shiny skin-coloured papules, often in children. Histology shows a small infiltrate held by rete ridges like a "ball in claw".
• Lichen striatus: a self-limiting linear band of papules along Blaschko lines in children. It clears within about a year, sometimes leaving pigment change.

Pityriasis Rubra Pilaris (PRP)

PRP is rare. The Griffiths classification has six types: type I (classic adult, the most common, usually resolving within about 3 years), type II (atypical adult), types III–V (juvenile forms; familial type V is linked to CARD14), and type VI (HIV-associated).

• Clinical: follicular hyperkeratotic papules (often starting on the dorsal fingers), spreading orange-red erythroderma with well-defined islands of sparing, and an orange, waxy palmoplantar keratoderma ("sandal").
• Histology: alternating orthokeratosis and parakeratosis in vertical and horizontal directions (checkerboard), "shoulder" parakeratosis around follicles, and follicular plugging.
• Treatment: oral retinoids (acitretin or isotretinoin), methotrexate, and, in case series, TNF, IL-17, and IL-23 inhibitors.

Pityriasis Rosea

• Clinical: a single larger herald patch, followed after days to 2 weeks by smaller oval pink patches with a collarette of scale. They follow skin cleavage lines on the trunk in a "Christmas-tree" pattern. It mainly affects adolescents and young adults.
• Cause: reactivation of HHV-6 and HHV-7 is the most accepted explanation.
• Course: self-limiting over about 6–8 weeks; treat itch with emollients and mild steroids.
• Pregnancy: pityriasis rosea in early pregnancy has been associated with miscarriage, so affected pregnant women should be told about this and followed up.
• Key differential: secondary syphilis. Palm and sole lesions, lymphadenopathy, mucosal lesions, or no herald patch should prompt syphilis serology. Also consider drug-induced pityriasis rosea-like eruptions and tinea corporis.

Seborrhoeic Dermatitis

A common inflammatory response linked to Malassezia yeasts in sebum-rich areas: scalp (dandruff), eyebrows, nasolabial folds, ears, and presternal skin.

• Infants: "cradle cap" and flexural disease in the first months, usually self-limiting.
• Adults: chronic and relapsing. It is more severe in HIV infection and Parkinson disease and other neurological conditions.
• Treatment: ketoconazole shampoo or cream, other antifungals, short courses of low-potency corticosteroids, and topical calcineurin inhibitors for the face.

Parapsoriasis and Pityriasis Lichenoides