ecthyma gangrenosum

Ecthyma gangrenosum results from bacterial invasion of blood vessel walls (vasculitis), leading to thrombosis, ischemia, and necrosis of the skin. Pseudomonas aeruginosa is the most common causative organism, invading the media and adventitia of vessels via virulence factors such as exotoxin A and elastase, though other organisms (e.g., Aeromonas, Klebsiella, Escherichia coli, fungi like Candida or Aspergillus) can rarely cause similar lesions. It most commonly occurs in neutropenic patients (e.g., those with hematologic malignancies, on chemotherapy), patients with severe burns, or those who are otherwise immunocompromised (e.g., HIV, diabetes). Lesions begin as painless erythematous macules that evolve into hemorrhagic vesicles or pustules, which then ulcerate centrally, forming a necrotic black eschar surrounded by an erythematous halo, often on the gluteal or perineal region but can appear anywhere. It can occur with or without bacteremia, though its presence in a febrile, immunocompromised patient strongly suggests underlying Pseudomonas sepsis and warrants immediate blood cultures and empiric antipseudomonal antibiotics (e.g., piperacillin-tazobactam, cefepime, or a carbapenem, often combined with an aminoglycoside). Rapid diagnosis and treatment are critical, as mortality is high if bacteremia is untreated. Skin biopsy with histopathology and culture can confirm the diagnosis, showing vascular invasion by bacteria with minimal inflammatory infiltrate due to neutropenia.