What is the role of cultures in the evaluation of malignant otitis externa (MEO)?

Updated: Apr 13, 2020
  • Author: Brian Nussenbaum, MD, FACS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

Culture from the ear drainage should be performed ideally before antimicrobial therapy is initiated. The most common causative organism is P aeruginosa (95%). This organism is an aerobic, gram-negative rod. Pseudomonas species have a mucoid coating that deters phagocytosis. Exotoxins (ie, exotoxin A, collagenase, elastase) can cause tissue necrosis, and some strains produce a neurotoxin that may be partially responsible for cranial neuropathies.

Less common organisms identified include Aspergillus and Proteus species, Candida species, Staphylococcus aureus, and S epidermidis.

A study by Hobson et al concluded that vigilance for nonpseudomonal organisms is necessary in treating patients with malignant external otitis, especially those without diabetes, since methicillin-resistant S aureus (MRSA) is increasingly being diagnosed as the causative organism, and not always in the presence of diabetes. Among 20 patients with malignant external otitis, the investigators found 9 patients with P aeruginosa and 3 with MRSA. They also determined that although diabetes was present in all patients with P aeruginosa, it existed in only 1 of the patients with MRSA and in only 55% of all patients with nonpseudomonal infections. [12]

A study by Gruber et al indicated that in patients with refractory, culture-negative malignant external otitis, a fungal infection should be suspected. The study involved a cohort of 19 patients with malignant external otitis, three of whom had refractory, culture-negative disease. Polymerase chain reaction (PCR) assay revealed a fungal pathogen in all three, with empirical targeted therapy being used to successfully treat their condition. [13]


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