How do you diagnose pseudopelade of Brocq (PPB) in a middle‑aged woman with slowly progressive, well‑defined, smooth, ivory‑white scarring alopecia patches lacking erythema, scaling, itching, or pain, and differentiate it from lichen planopilaris (LPP) and discoid lupus erythematosus (DLE)?

Diagnosis of Pseudopelade of Brocq

Pseudopelade of Brocq (PPB) is diagnosed by exclusion after ruling out lichen planopilaris and discoid lupus erythematosus through combined clinical assessment, histopathology, and direct immunofluorescence (DIF), with the diagnosis requiring characteristic smooth, ivory-white atrophic patches lacking inflammation, negative DIF, and histology showing minimal lymphocytic infiltrate without follicular plugging. 1, 2

Clinical Diagnostic Criteria

PPB presents with distinctive clinical features that guide initial diagnosis:

• Smooth, ivory-white or porcelain-white atrophic patches with complete absence of follicular ostia, creating a "footprints in the snow" appearance 1, 2
• Absence of active inflammation - no erythema, scaling, itching, or pain distinguishes PPB from active LPP and DLE 1
• Slowly progressive course over months to years, eventually becoming stationary after several years 1
• Patches coalesce into larger irregular plaques with polycyclic borders, typically affecting vertex and parietal scalp 3, 2
• Middle-aged women are most commonly affected, though rare cases occur in children 3

Differentiation from Lichen Planopilaris (LPP)

LPP shows active perifollicular inflammation that PPB lacks:

• Perifollicular erythema and scaling are prominent in active LPP but absent in PPB 2
• Symptoms of pruritus, burning, or tenderness occur in LPP but not in PPB 1
• Histopathology shows dense lichenoid lymphocytic infiltrate at the dermal-epidermal junction with prominent follicular plugging in LPP, versus minimal infiltrate and absent/minimal plugging in PPB 1, 2
• DIF demonstrates shaggy fibrinogen deposits along the basement membrane zone in 18% of LPP cases, while PPB shows negative DIF or only occasional IgM 2, 4

Differentiation from Discoid Lupus Erythematosus (DLE)

DLE demonstrates inflammatory features and immunological deposits absent in PPB:

• Active erythematous plaques with adherent scaling and follicular plugging characterize DLE, contrasting with smooth atrophic patches in PPB 2
• Hyperpigmentation and hypopigmentation are prominent in DLE but minimal in PPB 2
• DIF shows characteristic lupus band test with granular IgG, IgM, and C3 deposits at the basement membrane zone in 21% of DLE cases, while PPB is negative 2, 4
• Histopathology reveals interface dermatitis with basement membrane thickening and dermal mucin in DLE, versus minimal inflammation in PPB 2

Essential Diagnostic Algorithm

Follow this stepwise approach to establish the diagnosis:

Step 1: Clinical Assessment

• Confirm characteristic smooth, ivory-white atrophic patches without inflammation 1
• Document absence of erythema, scaling, pruritus, or pain to exclude active LPP or DLE 1, 2
• Assess distribution pattern - vertex and parietal involvement is typical 3

Step 2: Scalp Biopsy from Active Margin

• Obtain two 4mm punch biopsies from the advancing edge of the most recent patch 2
• Bisect one specimen vertically - send half for histopathology in formalin and half for DIF in Michel's medium 2

Step 3: Histopathological Examination

PPB shows characteristic features:

• Little or only moderate lymphocytic infiltrate around upper and mid-follicle 1
• Absence of significant follicular plugging distinguishes from LPP 1
• Absence or marked decrease of sebaceous glands with follicular scarring 1
• No interface dermatitis or basement membrane thickening excludes DLE 2

Exclude LPP if present:

• Dense lichenoid lymphocytic infiltrate at dermal-epidermal junction 2
• Prominent follicular hyperkeratosis and plugging 2

Exclude DLE if present:

• Interface dermatitis with vacuolar changes 2
• Basement membrane thickening and dermal mucin 2

Step 4: Direct Immunofluorescence

• Negative DIF or only occasional IgM at basement membrane supports PPB diagnosis 1, 4
• Shaggy fibrinogen deposits indicate LPP 2, 4
• Granular IgG, IgM, and C3 deposits (lupus band) indicate DLE 2, 4
• DIF has high specificity and sensitivity for LE (both high), making it particularly valuable when DLE is suspected 4
• DIF has high specificity but low sensitivity for LP, so negative DIF does not exclude LPP 4

Step 5: Serum Autoantibody Testing

• Check ANA, anti-dsDNA, anti-ENA panel to exclude systemic lupus erythematosus 2
• All autoantibodies should be negative in primary PPB 2

Immunophenotypic Patterns

Research identifies two distinct patterns that help differentiate primary from secondary PPB:

• Secondary PPB (from LPP/DLE) shows lymphocyte-predominant infiltrate with CD3+ cells, high CD4+/CD8+ ratio, and fewer resident cells than lymphocytes 5
• Primary idiopathic PPB shows resident cell-predominant infiltrate with conspicuous macrophages, mast cells, and fibroblasts outnumbering lymphocytes 5
• Fibrogenic cytokines (IL-4, IL-6, bFGF, TGF-beta) are more strongly expressed in primary PPB, suggesting T-helper 2 and 3 cytokine involvement 5
• IFN-gamma expression is characteristic of LPP 5

Critical Diagnostic Pitfalls

Avoid these common errors:

• Diagnosing PPB without biopsy - clinical diagnosis alone misses 66.6% of cases that are actually end-stage LPP or DLE 2
• Biopsying only atrophic areas - take specimens from the active advancing margin where diagnostic features are present 2
• Relying on histopathology alone - DIF is essential when LE is in the differential, as it has high specificity and sensitivity for LE 4
• Accepting nonspecific histology as definitive PPB - 45.5% of cases with nonspecific histology may still represent end-stage LPP or DLE 2
• Failing to recognize that PPB may not be a distinct entity - in 66.6% of cases, it represents the end stage of LPP or DLE rather than primary disease 2

Final Diagnostic Classification

After completing the algorithm:

• Primary idiopathic PPB is diagnosed only when clinical features are characteristic, histology shows minimal inflammation without specific features of LPP or DLE, DIF is negative, and autoantibodies are negative 1, 2
• Secondary PPB is diagnosed when histology or DIF reveals features of LPP or DLE, even if clinical inflammation has resolved 2
• The distinction matters for prognosis - primary PPB becomes stationary after several years, while secondary forms may continue progressing 1