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Author: Dr Mark Duffill, Dermatologist, Hamilton, New Zealand, 2008.
Ecthyma gangrenosum is a cutaneous infection most commonly associated with Pseudomonas bacteraemia. Ecthyma gangrenosum usually occurs in patients who are critically ill and immunocompromised. The characteristic lesions of ecthyma gangrenosum are haemorrhagic (bloody) pustules that evolve into necrotic (black) ulcers.
The initial lesions of ecthyma gangrenosum appear as painless, round, red patches in the skin which rapidly become pustular with surrounding redness. A haemorrhagic focus appears in the centre, forming a blister. As the haemorrhagic blister spreads peripherally, it evolves into a gangrenous ulcer with a black/gray scab surrounded by a red halo. An early lesion may transform into a necrotic ulcer in as little as 12 hours.
Ecthyma gangrenosum may appear at any site but mainly affects the anogenital area and armpits. The arms and legs, trunk and face are less often involved.
Impaired immunity leads to increased susceptibility to infections with Pseudomonas or other pathogens. Breakdown of mechanical defensive barriers such as skin and mucosa allow entry of infectious organisms.
The lesions of ecthyma gangrenosum result from bacterial invasion of the walls of arteries and veins in the skin and subcutaneous tissue. The bacterial invasion may come from inside the vessel in the case of septicaemia, or by direct inoculation of bacteria through the skin. Damage to the vessel walls causes interruption of the local blood supply with initially redness, oedema, pustule formation and haemorrhage then necrosis of the skin with scab and ulcer formation.
The following tests are performed to identify the exact cause of the infection.
A Gram stain of fluid from the central haemorrhagic pustule or blister can rapidly indicate the diagnosis. If no fluid is present, the scab should be lifted and a swab taken from underneath.
Two specimens are normally taken prior to starting antibiotic therapy. The optimum time for collection is during a temperature spike. Sensitivity studies are done on any isolated organisms.
A skin biopsy is taken for routine histology and special stains may also be done to to rule out other organisms that may cause ecthyma gangrenosum-like lesions.
Histopathology of ecthyma gangrenosum lesions shows vascular necrosis with few inflammatory cells but many surrounding bacteria. In sections stained with Gram stain, gram-negative rods are numerous surrounding necrotic vessels. Haemorrhage, oedema and necrosis are seen in and around the involved vessels.
A second skin biopsy is usually sent for tissue culture for bacteria, fungi, yeasts and mycobacteria. Sensitivity tests are done on any isolated organisms.
Ecthyma gangrenosum requires prompt diagnosis and treatment with appropriate antibiotics for the underlying cause. The presence of ecthyma gangrenosum should alert the physician to the likelihood of an accompanying Pseudomonas septicaemia.
Antibiotics which may be used include
While awaiting culture results, piperacillin is usually given in combination with an aminoglycoside. Antibiotic choice may require adjustment once antibiotic sensitivity results are known.
If a blood stream infection with fungi is suspected, systemic antifungal therapy should be considered to include cover against Aspergillus, Candida and Mucor species, with azoles such as itraconazole or fluconazole, and/or amphotericin B if appropriate.
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