How can I differentiate psoriasis, atopic dermatitis, lichen simplex chronicus, lichen sclerosus, and lichen planus?

Differentiating Psoriasis, Eczema (Atopic Dermatitis), Lichen Simplex Chronicus, Lichen Sclerosus, and Lichen Planus

The key to distinguishing these conditions lies in recognizing specific morphologic features, distribution patterns, and associated symptoms: psoriasis shows well-demarcated plaques with silvery scale on extensor surfaces; atopic dermatitis presents with poorly-demarcated eczematous patches in flexural areas with intense pruritus; lichen simplex chronicus displays single or few lichenified plaques from chronic scratching; lichen sclerosus shows porcelain-white atrophic patches primarily in anogenital regions; and lichen planus exhibits violaceous polygonal papules with Wickham striae, often on flexor surfaces and oral mucosa. 1, 2, 3

Primary Distinguishing Features

Psoriasis

• Morphology: Well-demarcated erythematous plaques with thick, silvery-white scale 1, 3
• Distribution: Extensor surfaces (elbows, knees), scalp, sacrum, and nails 1, 3
• Histology: Regular (uniform) acanthosis with elongated rete ridges of equal length, confluent parakeratosis with neutrophils in stratum corneum (Munro microabscesses) 3
• Key feature: Plaques are sharply demarcated with uniform thickness 3, 4

Atopic Dermatitis (Eczema)

• Morphology: Poorly-demarcated erythematous patches with xerosis, erosions, oozing, crusting, and lichenification in chronic cases 1
• Distribution: Age-specific patterns - infants have involvement of cheeks, neck, trunk, and extensor surfaces (sparing diaper area); children and adolescents show flexural involvement (antecubital and popliteal fossae) 1
• Key symptoms: Intense pruritus is the hallmark; chronic or relapsing course (>2 months in infants, >6 months in older children) 1
• Associated features: Personal or family history of atopy (allergic rhinitis, asthma), elevated IgE 1

Lichen Simplex Chronicus

• Morphology: Single or few well-demarcated lichenified plaques with accentuated skin markings, often with excoriations 5, 6, 4
• Distribution: Can occur anywhere but commonly on accessible areas (nape of neck, ankles, anogenital region) 5, 6
• Key feature: Results from chronic scratching and rubbing; represents an itch-scratch cycle rather than a primary disease 5, 6
• Important distinction: Often develops as a secondary condition superimposed on other dermatoses (candidiasis, psoriasis, lichen sclerosus) 5, 6
• Histology: Irregular acanthosis with hyperpigmentation in basal layer, more excoriation, and lichenoid papules around plaques 4

Lichen Sclerosus

• Morphology: Porcelain-white papules and plaques with areas of ecchymosis, follicular delling, fragile atrophic skin prone to fissures 1, 7
• Distribution: Predominantly anogenital (interlabial sulci, labia minora, clitoral hood in women; glans, foreskin, frenulum in men); perianal involvement in 30% of women but extremely rare in men 1, 7
• Key symptoms: Intractable pruritus (often worse at night), dyspareunia, dysuria from fissuring 1, 7
• Scarring complications: Loss of labia minora, clitoral hood sealing, introital stenosis in women; phimosis, meatal stenosis, urethral strictures in men 1, 7
• Histology: Typically shows epidermal atrophy with hyperkeratosis and homogenized collagen band beneath epidermis; however, complicated or early disease may show acanthosis 3, 8
• Critical pitfall: Carries 4-6% risk of squamous cell carcinoma, requiring long-term surveillance 7

Lichen Planus

• Morphology: Violaceous, polygonal, flat-topped papules with fine white lines (Wickham striae) on surface 2, 3
• Distribution: Flexor surfaces (wrists, ankles), oral mucosa commonly involved (unlike other conditions) 2
• Key feature: Wickham striae are pathognomonic - best visualized with tangential lighting or dermoscopy 2
• Histology: Lichenoid (band-like) lymphocytic infiltrate hugging the dermoepidermal junction is the hallmark; irregular acanthosis with parakeratosis 3
• Chronic lesions: Become hyperkeratotic and plaque-like 2

Diagnostic Algorithm

Step 1: Assess Distribution Pattern

• Extensor surfaces (elbows, knees, scalp) → Consider psoriasis 1, 3
• Flexor surfaces (antecubital/popliteal fossae in children; wrists/ankles for violaceous papules) → Consider atopic dermatitis or lichen planus 1, 2
• Anogenital region with white atrophic changes → Consider lichen sclerosus 1, 7
• Localized accessible areas with single lichenified plaque → Consider lichen simplex chronicus 5, 6

Step 2: Examine Morphology Closely

• Check for Wickham striae using tangential lighting - if present, diagnose lichen planus 2
• Assess plaque borders: Sharp demarcation suggests psoriasis; poorly-demarcated suggests atopic dermatitis 1, 3
• Look for porcelain-white color with ecchymosis in anogenital area - pathognomonic for lichen sclerosus 1, 7
• Evaluate scale quality: Thick silvery scale suggests psoriasis; fine scale with oozing/crusting suggests atopic dermatitis 1, 3

Step 3: Check Oral Mucosa

• Oral involvement is common in lichen planus (lacy white pattern) but not typical in other conditions 2
• Vaginal/cervical mucosa is always spared in lichen sclerosus (unlike lichen planus) 1

Step 4: Assess Sensation (Critical for Anogenital Lesions)

• Test sensation with light touch and pinprick in all facial or anogenital lesions 2
• Normal sensation with anogenital white plaques confirms lichen sclerosus (not leprosy) 2

Step 5: Consider Age and Chronicity

• Onset in infancy (60% by age 1 year, 90% by age 5) strongly suggests atopic dermatitis 1
• Bimodal age distribution (prepubertal and postmenopausal in women; young boys and adult men) suggests lichen sclerosus 1

Step 6: Biopsy When Uncertain

• Mandatory before treatment for lichen sclerosus and lichen planus 2
• Histology distinguishes psoriasis (regular acanthosis, Munro microabscesses) from lichen planus (lichenoid infiltrate at dermoepidermal junction) from lichen sclerosus (homogenized collagen band) 3, 8

Common Pitfalls to Avoid

• Do not confuse lichen simplex chronicus with primary disease - it is a secondary condition from chronic scratching that can overlay psoriasis, lichen sclerosus, or atopic dermatitis 5, 6
• Lichen sclerosus with acanthosis indicates complicated disease with higher malignancy risk and poorer treatment response 3, 8
• Striking ecchymosis in children with anogenital lichen sclerosus may mimic sexual abuse, but diagnosis of lichen sclerosus does not exclude abuse 1, 7
• "Psoriatic neurodermatitis" on elbows/knees bilaterally is likely psoriasis with superimposed lichen simplex chronicus from scratching 4
• Atypical lichen planus can mimic guttate psoriasis - look for Wickham striae and oral involvement to distinguish 9
• Early mycosis fungoides can show similar histology to lichen planus (irregular acanthosis, lichenoid infiltrate) - presence of atypical lymphocytes is the key distinguishing feature 3