What are the clinical and histopathological differences between scalp psoriasis and scalp pemphigus foliaceus?

Differential Diagnosis: Scalp Psoriasis vs. Scalp Pemphigus Foliaceus

Clinical Differentiation

Scalp psoriasis presents with well-demarcated erythematous plaques covered by thick, silvery-white scales, while pemphigus foliaceus manifests as superficial crusted erosions in a seborrheic distribution without the thick adherent scales characteristic of psoriasis. 1, 2

Scalp Psoriasis Clinical Features:

• Plaque morphology: Well-demarcated erythematous plaques with thick, silvery-white scales that can be removed to reveal pinpoint bleeding (Auspitz sign) 1
• Distribution patterns: Seven distinct clinical presentations including classic plaque psoriasis (most common), thin scales variant, sebopsoriasis, psoriatic cap, pityriasis amiantacea, cicatricial alopecia, and pustular forms 1
• Scale characteristics: Adherent silvery-white scales that accumulate in thick layers; in pityriasis amiantacea variant, yellowish adherent scales wrap around hair shafts 1
• Mucous membrane involvement: Absent—psoriasis does not affect oral mucosa 3

Pemphigus Foliaceus Clinical Features:

• Lesion morphology: Superficial crusted erosions without intact blisters (blisters rupture immediately due to their superficial nature) 2
• Distribution: Seborrheic areas including scalp, face, upper trunk; can progress to exfoliative erythroderma 4, 2
• Scale characteristics: Thin, loose crusts rather than thick adherent scales 2
• Mucous membrane involvement: Characteristically spared (unlike pemphigus vulgaris) 5, 2

Trichoscopy Findings (Scalp Psoriasis Only):

• Vascular patterns: Twisted red loops, red dots and globules, signet ring vessels, bushy capillaries depending on clinical subtype 1
• Scale patterns: Silvery-white scales visible on dermoscopy 1
• Pustular variant: "Flower shape" pustular lesions with simple red loop capillaries 1

Histopathological Differentiation

The definitive histopathological distinction is that psoriasis shows regular acanthosis with neutrophilic collections in mounds of parakeratosis and spongiform pustules of Kogoj in the mid-epidermis, while pemphigus foliaceus demonstrates subcorneal/granular layer acantholysis with minimal epidermal hyperplasia. 6, 2

Scalp Psoriasis Histopathology:

• Epidermal changes: Regular acanthosis with clubbed and evenly elongated rete ridges 6
• Parakeratosis pattern: Mounds of parakeratosis containing neutrophils (highly specific) 6
• Neutrophilic collections: Spongiform micropustules of Kogoj in the mid-to-upper epidermis and Munro microabscesses in the stratum corneum 6, 7
• Hypogranulosis: Thinning or absence of the granular layer beneath parakeratotic areas 7
• Dermal changes: Dilated tortuous capillaries in elongated dermal papillae with perivascular lymphocytic infiltrate 6
• Mitotic activity: Increased mitotic figures (≥6 per high-powered field) 6

Pemphigus Foliaceus Histopathology:

• Acantholysis location: Subcorneal or granular layer cleavage with loss of intercellular connections 2, 5
• Epidermal architecture: Minimal to no acanthosis; epidermis typically not hyperplastic 4
• Inflammatory pattern: Eosinophilic spongiosis may be present in early lesions; neutrophilic spongiosis can occur but is less common 5
• Absence of psoriasiform features: No regular acanthosis, no clubbed rete ridges, no spongiform pustules of Kogoj 4
• Dermal infiltrate: Superficial perivascular infiltrate without the prominent papillary capillary changes of psoriasis 2

Critical Diagnostic Pitfall:

Pemphigus foliaceus can rarely present with psoriasiform epidermal hyperplasia, mimicking psoriasis histologically—this "psoriasiform pemphigus foliaceus" variant requires direct immunofluorescence for definitive diagnosis. 4 Additionally, early pemphigus lesions may show neutrophilic spongiosis without obvious acantholysis, potentially mimicking pustular psoriasis 5

Confirmatory Testing

Direct Immunofluorescence (Essential for Pemphigus):

• Pemphigus foliaceus: Intercellular IgG and C3 deposition in a "chicken-wire" pattern throughout the epidermis (gold standard for diagnosis) 3, 2
• Psoriasis: Negative for intercellular immunoglobulin deposition 6

Serology for Pemphigus:

• ELISA testing: Detects anti-desmoglein 1 antibodies (specific for pemphigus foliaceus) 3, 2
• Indirect immunofluorescence: Shows intercellular antibodies but less sensitive than direct immunofluorescence 3

When to Biopsy:

• Multiple biopsies recommended: When histopathological findings are non-definitive, obtaining additional skin biopsies from different sites increases diagnostic accuracy 7, 8
• Biopsy site selection: For pemphigus, take perilesional intact skin for direct immunofluorescence and lesional skin for histology 3

Algorithmic Approach to Diagnosis

1. Clinical assessment: Thick silvery scales with well-demarcated plaques → psoriasis; superficial crusted erosions without thick scales → pemphigus foliaceus 1, 2

2. Histology review: Regular acanthosis + neutrophilic mounds + Kogoj pustules → psoriasis; subcorneal acantholysis + minimal hyperplasia → pemphigus foliaceus 6, 2

3. If histology shows focal acantholysis with psoriasiform features: Perform direct immunofluorescence immediately—intercellular IgG confirms pemphigus foliaceus 4

4. If neutrophilic spongiosis without clear acantholysis: Direct immunofluorescence is mandatory to exclude pemphigus 5

5. If diagnosis remains uncertain: Obtain multiple biopsies from different lesions and perform anti-desmoglein 1 ELISA 7, 3