Otitis externa is an inflammation (usually infective) of the external auditory canal, which can spread to the pinna, periauricular soft tissues, or even the temporal bone. Usually the entire canal is involved, but a external auditory canal furuncle can be considered a localized form of otitis externa. Diffuse otitis externa is typically a bacterial infection, the most common pathogens being Pseudomonas, Staphylococci, and Proteus, but fungi may also be involved (most commonly Aspergillus and Candida).
Pathogenesis is thought to be related to canal skin maceration from scratching with an instrument and/or loss of the protective lipid layer in the presence of humidity. Bacteria ingress through tiny epithelial tears, proliferate, and result in swelling of the canal lining. The patient typically complains of intense otalgia and may also have otorrhea, fullness, or hearing loss. Without treatment, infection can spread to the pinna and periauricular soft tissues (complicated OE) or persist in a chronic form. In an immunocompromised patient, spread to the temporal bone results in Necrotizing (previously "Malignant") Otitis Externa.