What is the cause and treatment of otitis media externa?

Otitis Externa: Causes and Treatment

Otitis externa is a bacterial infection of the external auditory canal caused primarily by Pseudomonas aeruginosa (20-60% of cases) and Staphylococcus aureus (10-70% of cases), and should be treated with topical antimicrobial therapy, NOT oral antibiotics. 1

Causative Organisms

Bacterial Pathogens (98% of cases)

• Primary pathogens: Pseudomonas aeruginosa and Staphylococcus aureus account for the vast majority of infections, often occurring as polymicrobial infections 1
• Other bacteria: Gram-negative organisms (excluding P. aeruginosa) cause no more than 2-3% of cases individually 1

Fungal Pathogens (Uncommon in Primary Infection)

• Species: Aspergillus and Candida species are distinctly uncommon in primary acute otitis externa 1
• Risk factors: Fungal involvement is more common in chronic otitis externa or after treatment with topical/systemic antibiotics 1
• High-risk populations: Patients with diabetes are more susceptible to fungal infections (otomycosis) 2

Predisposing Factors

Environmental and Mechanical Triggers

• Moisture exposure: Swimming converts the normal acidic environment of the external auditory canal to alkaline, promoting bacterial growth 1
• Trauma: Manipulation of the ear canal (cotton swabs, hearing aids) disrupts natural defenses 3

Patient-Specific Risk Factors

• Diabetes mellitus: Increases risk for necrotizing otitis externa and fungal infections 1, 2
• Immunosuppression: Elevates risk for severe, progressive infection 1
• Prior radiotherapy: Modifies tissue response and healing 2

First-Line Treatment Approach

Topical Antimicrobial Therapy (Mainstay)

• Superiority over oral antibiotics: Topical therapy delivers 100-1000 times higher antibiotic concentrations directly to the infection site compared to systemic therapy 2, 4
• Clinical efficacy: 65-90% of patients achieve clinical resolution within 7-10 days with topical therapy alone, regardless of specific agent used 2
• Comparative outcomes: Randomized controlled trials demonstrate topical antibiotics achieve clinical cure rates of 77-96% versus only 30-67% for oral antibiotics 2

Specific Topical Agents

For intact tympanic membranes:

• Any topical antimicrobial preparation effective against P. aeruginosa and S. aureus 1
• Corticosteroid-containing drops may provide faster pain relief 4

For perforated tympanic membranes or tympanostomy tubes:

• ONLY non-ototoxic fluoroquinolones: Ciprofloxacin 0.2% or ofloxacin 0.3% 2, 4, 5, 6
• Avoid: Neomycin/polymyxin B preparations due to ototoxicity risk 2, 4

Essential Adjunctive Measures

Aural toilet (canal cleaning):

• Perform gentle suction, dry mopping, or removal of obstructing debris BEFORE administering drops to ensure medication reaches infected tissues 7, 2
• Avoid: Flushing/irrigation when possible 3

Pain management:

• Assess pain severity and prescribe appropriate analgesics (acetaminophen, ibuprofen, or stronger agents based on intensity) 1, 2, 4
• Pain typically improves within 48-72 hours of starting treatment 2, 4

When Oral Antibiotics Are Indicated

Specific Circumstances ONLY

Oral antibiotics should NOT be used as initial therapy for uncomplicated otitis externa but are reserved for: 1, 2, 4

1. Extension of infection beyond the ear canal (cellulitis of pinna, periauricular tissues) 1, 2, 4
2. Diabetes mellitus (risk for necrotizing otitis externa) 1, 2, 4
3. Immunocompromised status 1, 2, 4
4. Topical therapy cannot reach infected area (severe canal obstruction) 2, 4
5. Treatment failure with topical therapy 2, 4

Recommended Oral Antibiotic

• Fluoroquinolones (e.g., ciprofloxacin): Provide coverage against both P. aeruginosa and S. aureus 2, 4

Critical Pitfalls to Avoid

Inappropriate Antibiotic Use

• Overuse of oral antibiotics: Approximately 20-40% of patients inappropriately receive oral antibiotics, which are often inactive against P. aeruginosa and increase antibiotic resistance without clinical benefit 2, 4
• Declining susceptibility: Bacterial resistance to neomycin and polymyxin B has increased markedly, with mean MICs rising above susceptibility breakpoints 8

Ototoxicity Risk

• Never use aminoglycosides or polymyxin B in patients with perforated tympanic membranes or tympanostomy tubes 2, 4

Missing High-Risk Conditions

• Necrotizing otitis externa: Life-threatening condition in diabetic/immunocompromised patients that can progress to skull base osteomyelitis, cranial nerve deficits, meningitis, and death 1
• Fungal infections: Especially in diabetic patients or those failing antibacterial therapy 2

Inadequate Treatment Delivery

• Failure to remove debris: Obstructing cerumen or discharge prevents medication from reaching infected tissues 7, 2
• Poor patient adherence: Patients tend to overadminister drops when pain is greatest and underadminister as symptoms resolve 2

Expected Clinical Course and Reassessment

Timeline for Improvement

• Symptoms should improve within 48-72 hours of initiating appropriate therapy 2, 4
• Complete resolution: Expected within 7-10 days 2

Reasons for Treatment Failure

If no improvement at 48-72 hours, consider: 2, 4

• Inadequate drug delivery due to canal obstruction
• Poor adherence to therapy
• Allergic contact dermatitis from topical agents (especially neomycin)
• Fungal co-infection
• Incorrect diagnosis (may be acute otitis media with tympanic membrane erythema mimicking otitis externa)

Prevention Strategies

For Recurrent Infections

• Acidification: Use 2% acetic acid drops with hydrocortisone after water exposure 3
• Drying: Use hair dryer on low setting after swimming 4, 3
• Avoid trauma: No cotton swabs or objects inserted into ear canal 4, 3
• Ear plugs: During swimming 4
• Regular cerumen removal: For patients with excessive buildup 4