Coexistence of Erythrodermic Psoriasis and Bullous Pemphigoid: A Case Report.

BACKGROUND: Erythrodermic psoriasis (EP) represents a severe inflammatory subtype of psoriasis. Bullous pemphigoid (BP), the most common autoimmune subepidermal blistering disorder in clinical practice, predominantly affects elderly populations. Comorbidity of these two diseases is uncommon, and BP secondary to EP in young-adult patients is particularly rare. CASE PRESENTATION: A 33-year-old male with a 14-year relapsing-remitting history of psoriasis was admitted for erythrodermic psoriasis flare. Tense soybean-sized bullae with clear fluid appeared on bilateral lower extremities on the 5th hospital day. Skin biopsy demonstrated subepidermal blister formation with psoriasiform epidermal alterations. Direct immunofluorescence revealed basement-membrane-zone (BMZ) IgG (±) and C3 (+). Serum serology revealed markedly elevated levels of both anti-BP180 and anti-BP230 antibodies, whereas anti-desmoglein-1 and anti-desmoglein-3 antibodies were negative, supporting the diagnosis of coexisting EP and BP. The patient was treated with oral acitretin combined with high-potency topical halometasone cream, no new bullae occurred subsequently. Both psoriatic erythrodermic lesions and bullous lesions achieved marked improvement over the 6-month follow-up period. CONCLUSION: This rare case of BP secondary to EP in a young adult underscores that clinicians should maintain high suspicion for BP when tense bullae develop de novo in patients with EP. Timely performance of histopathology, perilesional direct immunofluorescence, and pemphigoid-targeted serological testing is critical for establishing a definitive diagnosis.