Aetiology

• the unique structure of the external ear canal contributes to the development of this condition
• the sac-like structure is prone to collection of water, get moist and dampened, making it an excellent environment for bacterial and fungal growth leading to infection (1)

Causative agents of infective otitis externa include:

• infection:
  • bacteria
    • the vast majority of cases (98%) the cause is bacterial (2)
      • most common pathogens are Pseudomonas aeruginosa and Staphylococcus aureus
    • other possible organisms include proteus and E.coli
  • fungi - candida, aspergillus
• allergy - eczema; contact allergy to cosmetics, shampoos
• iatrogenic - frequent ear syringing, especially when it causes trauma

Otitis externa is more common in people with a narrowed or tortuous ear canal, particularly if the ear canal is continually wet, such as from swimming.

• there are many precipitants/risk factors for development of otitis externa (1)
  • moisture
    • swimming
    • bathing
    • perspiration
    • high humidity
    • water contaminated with bacteria
      • water exposure - typically from swimming, with freshwater being highest risk, but organisms responsible for acute otitis externa are also often found in hot tubs and water sources that comply with water quality standards (2)
    • more common in regions with warmer climates, increased humidity (1)
      
      
  • foreign objects in the ear canal
    • cotton buds
    • finger nails
    • hearing aids
      • use of hearing aids or ear plugs. These can introduce bacteria and cause trauma, and may cause the ear to sweat
  • trauma to the ear canal
    • vigorous cleaning
      • regular cleaning of the ear canal removes cerumen, which is an important barrier to moisture and infection (2)
        • cerumen creates a slightly acidic pH that inhibits infection (especially by P aeruginosa) but can be altered by water exposure, aggressive cleaning, soapy deposits, or alkaline eardrops
    • scratching
      
      
  • chronic skin diseases
    • eczema
    • psoriasis
      
      
  • other risk factors for development of otitis externa include:
    • diabetes, other causes of immunocompromise, or older age
    • previous ear surgery

Reference:

1. Rosenfeld RM, Schwartz SR, Cannon CR, et al. Clinical practice guideline: acute otitis externa. Otolaryngol Head Neck Surg2014;150(Suppl) :S1-24.doi: 10.1177/0194599813517083
2. Barry V et al. 10-Minute Consultation - Otitis externa. BMJ2021;372:n714http://dx.doi.org/10.1136/bmj.n714