A 68-year-old man presents to the dermatology clinic with a 3-week history of tense, fluid-filled blisters on the flexural aspects of both forearms, lower abdomen, and inner thighs. He denies oral involvement. On examination, the blisters do not rupture easily and the surrounding skin appears erythematous but not erosive. Nikolsky sign is negative. Serum IgG and IgA are normal. Direct immunofluorescence (DIF) shows linear IgG and C3 deposits along the basement membrane zone. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Bullous Pemphigoid ### Key Clinical Features **Key Point:** Bullous pemphigoid presents with tense, intact blisters on flexural surfaces (forearms, abdomen, inner thighs, axillae) in elderly patients, with negative Nikolsky sign and NO oral involvement. ### Immunological Hallmark **High-Yield:** Direct immunofluorescence shows **linear IgG and C3 deposits at the basement membrane zone (BMZ)** — this is the gold standard diagnostic finding. ### Pathophysiology 1. Autoantibodies (IgG) against BP180 (collagen XVII) and BP230 hemidesmosomal antigens 2. Complement activation → subepidermal blister formation 3. Blisters form **below the epidermis** (subepidermal) → remain tense and intact ### Distinguishing Features from Other Bullous Disorders | Feature | Bullous Pemphigoid | Pemphigus Vulgaris | Linear IgA Disease | |---------|-------------------|-------------------|--------------------| | **Blister location** | Subepidermal (tense) | Intraepidermal (flaccid) | Subepidermal (tense) | | **Nikolsky sign** | Negative | Positive | Negative | | **Oral involvement** | Rare | Common (90%) | Possible | | **DIF pattern** | Linear IgG + C3 at BMZ | Intercellular IgG | Linear IgA at BMZ | | **Age of onset** | Elderly (>60 yrs) | Any age | Any age | | **Rupture tendency** | Resistant (tense) | Easy (flaccid) | Resistant (tense) | ### Clinical Pearl **Clinical Pearl:** The **negative Nikolsky sign** and **absence of oral erosions** are crucial discriminators against pemphigus vulgaris, which typically presents with flaccid blisters and intraoral involvement. ### Why DIF is Diagnostic - **Linear IgG + C3 at BMZ** = Bullous pemphigoid or Linear IgA disease - **Intercellular IgG** = Pemphigus vulgaris - **Granular IgA at dermal-epidermal junction** = Dermatitis herpetiformis **High-Yield:** In this case, the linear pattern at the BMZ (not granular) excludes dermatitis herpetiformis. The absence of oral involvement and negative Nikolsky sign exclude pemphigus vulgaris. Linear IgA disease is less common in the elderly and typically presents with more grouped vesicles ("string of pearls" pattern). [cite:Robbins 10e Ch 25]