Etiology
Acute otitis externa (AOE) is most commonly caused by bacterial infections. In North America, 98% of AOE is caused by bacteria.[1] It is often polymicrobial, but the most common pathogens are Pseudomonas aeruginosa (20% to 60% prevalence) and Staphylococcus aureus (10% to 70% prevalence).[1] Other etiologies are idiopathic, trauma (from scratching, aggressive cleaning), chemical irritants, allergy (most commonly to antibiotic ear drops such as neomycin), high-humidity conditions, swimming, or skin disease (seborrheic dermatitis, allergic dermatitis, atopic dermatitis, psoriasis).[2] Fungal etiology is uncommon in primary AOE, but may be more common in chronic otitis externa, after treatment of AOE with antibiotics, particularly topical antibiotics, in tropical countries, in humid areas, in people with diabetes, or in people who are immunocompromised. The most common fungal pathogens are Aspergillus species (60% to 90%) and Candida species (10% to 40%).[1]
Pathophysiology
The pathogenesis is multifactorial. Several risk factors can predispose to infection or initiate inflammation and subsequently the infectious process. Intact ear canal skin and cerumen production have a protective effect against infections. This is secondary to the fact that cerumen produces a pH in the ear canal that is slightly acidic.[1] On the other hand, breakdown of skin integrity, insufficient cerumen production, or blockage of the ear canal with cerumen (which promotes water retention) can predispose to infection. Skin integrity can be injured by direct trauma, heat, and moisture or persistent water in the ear canal. Such damage is believed to initiate the inflammatory process.[3][13] Subsequently, edema may result, followed by bacterial inoculation and overgrowth.
Classification
Scott-Brown[2]
No official classification system has been published, and different authors have classified otitis externa differently. Perhaps the most detailed classification system is as follows:
Localized otitis externa (furunculosis): localized infection in the hair follicles in the cartilaginous portion of the external auditory canal.[3]
Diffuse otitis externa: infection is limited to the skin of the external auditory canal and concha, and possibly the tympanic membrane.
Part of a generalized skin condition: patients have other skin conditions such as seborrheic dermatitis, allergic dermatitis, atopic dermatitis, and psoriasis.
Invasive (granulomatous/necrotizing) otitis externa: necrosis of adjacent cartilage or bone of the external auditory canal.
Others (keratosis obturans): hyperkeratosis of the external auditory canal skin, leading to corrosion of the canal bone.
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