AOE: Predisposing factors and pathogens

Everyone, whether a child or adult, is at risk for otitis externa. The most common form is bacterial, and it is likely that all individuals will experience an episode of otitis externa at some point in life. Often, it is a mild episode that manifests itself with itching and a sensation of fullness in the ear. This condition usually resolves on its own without any sequelae, but sometimes it becomes proliferative and causes significant discomfort.

Frequently, acute otitis externa (Figure) begins with an itching sensation, but may progress to pain with associated sensation of pressure and decreased hearing. Occasionally, a patient may have a low-grade fever.

The two main underlying causes of otitis externa are pH alteration of the external auditory canal and trauma to the ear canal. Under normal circumstances, the pH of the external auditory canal is about 6.1, which is a slightly acidic environment. A change in pH from acidic toward basic condition results in increased proliferation of bacteria and formation of external otitis. Trauma to the ear canal can be mild. Simply scratching the ear canal with a fingernail or any kind of device that can cause small lacerations in the ear canal can become an inciting event for development of AOE.

Predisposing factors for AOE

Exposure to water in the ear predisposes children to develop AOE, especially when they swim in heavily chlorinated pools. Chlorine is a basic compound that changes the pH level of the external ear. Children who live in humid climates, such as in the southern portion of the United States, the Caribbean and Latin America, have a slightly higher incidence of external otitis due to the accumulation of moisture and consequent alteration of pH levels in the outer ear canal.¹

Conditions such as congenital narrowing of the external auditory canal also predispose a child to external otitis. In those children, impaction of debris and cerumen in the ear canal can lead to associated discomfort, manual irritation, small lacerations, and then development of external otitis.

A child may be predisposed to AOE because of impacted cerumen alone. Moisture may become entrapped behind the impacted cerumen, which can lead to infection. Furthermore, a patient with impacted cerumen tends to scratch the ear canal, which also can lead to the development of infection. At the same time, lack of cerumen can cause AOE. An ear canal devoid of cerumen may lead to breakdown of the thin skin layer, causing small lacerations and development of infection.

Some dermatologic conditions also affect the ear canal. Seborrheic keratitis, psoriasis and eczema can lead to significant skin dryness, formation of small cracks in the skin and infection. Atopic dermatitis essentially produces inflammation of the skin, formation of dry, flaky skin and small abrasions. The bony medial portion of the ear canal only has a thin skin layer that covers the bone without the subcutaneous skin layers that contain a rich blood supply, so any small abrasions can easily lead to the development of infection.

Other predisposing factors for AOE, although rare, are diabetes and immunocompromised state, such as in patients undergoing chemotherapy.

Finally, insertion of devices used in the ear canal, such as hearing aids or earphones, can cause slight lacerations, increase the degree of cerumen impaction and decrease the ability of the ear canal to clear the cerumen on its own, which can lead to infection.

AOE Pathogens

In a multicenter study on pathogenesis of otitis externa, published in Laryngoscope in 2002, Roland et al showed that 52.9% of pathogenic bacteria in otitis externa were gram negative, whereas 45.3% were gram positive, and approximately 1.7% represented fungi and yeast species.¹ Across the United States, 101 investigators took cultures from ear canals of 2,039 patients. Investigators found that the major offender in the gram-negative group was Pseudomonas aeruginosa, representing 37.7% of the pathogenic bacteria identified. The second major offender was Staphylococcus species, representing 27.4% of pathogenic bacteria identified. These two bacterial species are responsible for approximately 65% of ear infections, so any treatment directed toward eradicating the infection should be directed toward eradicating these pathogens.

In the everyday clinical experience, most physicians do not find a need for culturing external otitis. However, if conventional treatment does not produce improvement and the infection persists, then obtaining a culture is necessary.

Infection with fungi and yeast is generally uncommon. Most fungi and yeast are normal residents of the external ear canal; they may proliferate, however, after prolonged treatment with oral antibiotics for other conditions. Fungal infection is also more prevalent in immunocompromised patients and in patients receiving immunosuppressive medications.

Recommendations

Pain is a symptom of AOE, but it is often difficult for children to verbalize the severity of their pain. To assess pain in children, physicians must watch how children respond to them and how they interact with their parents. Guarding is a clear indication of pain in the ear. The child will often try to prevent the physician from touching or manipulating the ear. Physicians must realize that the abnormal visual appearance of the ear canal most likely indicates that the child is in pain.

As an otolaryngologist, I do not recommend irrigation of the ear canal. Using a pressurized stream of water can damage the ear canal and tympanic membrane. If the patient’s tympanic membrane has an opening from a tube or perforation caused by infection, then irrigation may significantly damage the middle ear. For clearing the ear canal, I prefer either careful curetting or suctioning of the debris with direct visualization of the ear canal, either with an operating otoscope or a microscope. Dry mopping can also be used in patients with otitis externa after the ear canal is completely cleared of debris.

Using an ear wick is an excellent method for managing severe otitis externa. An ear wick is an indispensible tool when a patient’s ear is so swollen that medication drops are unable to enter the ear canal. The drops are placed directly on the wick, which holds the drops and then delivers the medicaton continuously to the surrounding tissue. Once the infection and inflammation settle, the wick can be easily removed.

Topical antibiotic ear drops

Several studies demonstrate that antibiotic drops, especially when used in combination with a steroid agent, achieve a clinical cure within one week.^{2, 3} Pain reduction begins within 12 hours of starting medication, and most patients report being pain-free within four days of starting therapy.

From a microbiologic perspective, the use of topical antibiotic agents is advantageous because the drops deliver high concentrations of antibiotic directly to the infected tissue.⁴ These high concentrations of antibiotic are 100 to 1,000 times greater than minimum inhibitory concentrations required to completely eradicate the pathogenic bacteria in the ear canal. The steroid component of the drops decreases inflammation and pain, and most likely allows better delivery of the antibiotic to the infected tissue. Using a topical antibiotic agent allows delivery of a potent, effective drug directly to the tissue with minimal systemic absorption, minimizing systemic effects.⁵ The use of topical agents prevents development of bacterial resistance because of almost complete eradication of pathogenic bacteria with high concentrations of the antibiotic.⁶

Safety is an important component in treating AOE, and the combination of ciprofloxacin/dexamethasone has a well established safety profile.⁷ As with any topical medication, however, development of contact dermatitis is possible, but rare. With this topical antibiotic and steroid combination, incidence of associated pruritus, pain and erythema is noted in less than 1.5% of patients.

The use of the potent antibiotic ciprofloxacin is effective against AOE-causing gram-negative and gram-positive pathogens Pseudomonas aeuruginosa and Staphylococcus aureus.⁷ These are two of the most frequently isolated pathogens in AOE.1

Preventing AOE

To prevent development of AOE, I advise against any kind of mechanical trauma to the ear canal. The use of cotton swabs, bobby pins or any kind of hard object in the ear is not recommended because small lacerations in the ear canal can lead to development of infection. In addition, using these kinds of devices may impact the wax in the ear canal and push it into the more medial portion of the canal, exacerbating the problem.

I also recommend that patients with chronic dermatitis of the external canal be monitored regularly because of predisposition to development of recurrent or chronic otitis externa.

Children who swim frequently or live in climates with high humidity and are predisposed to the development of AOE may benefit from maintaining an acidic environment in th ear canals after exposure to water. This can be achieved by instilling several drops of a mixture of equal parts of plain white vinegar and water in the ear canals. This mixture can be safely used only when the tympanic membrane is completely intact and there is no evidence of a perforation or pressure equalization tube in the ear.

References

1. Roland PS, Stroman DW. Microbiology of acute otitis externa. Laryngoscope. 2002;112(7 Pt 1):1166-1177.
2. Wall GM, Stroman DW, Roland PS, Dohar J. Ciprofloxacin 0.3%/dexamethasone 0.1% sterile otic suspension for the topical treatment of ear infections: a review of the literature. Pediatr Infect Dis J. 2009;28(2):141-144.
3. Morden NE, Berke EM. Topical fluoroquinolones for eye and ear. Am Fam Physician. 2000;62(8):1870-1876.
4. Roland PS. External otitis: a challenge in management. Curr Infect Dis Rep. 2000;2(2):160-167.
5. Spektor Z, Jasek MC, Jasheway D, et al. Pharmacokinetics of Ciprodex otic in pediatric and adolescent patients. Int J Pediatr Otorhinolaryngol. 2008;72(1):97-102.
6. Weber PC, Roland PS, Hannley M, et al. The development of antibiotic resistant organisms with the use of ototopical medications. Otolaryngol Head Neck Surg. 2004;130(3 Suppl):89-94.
7. Ciprofloxacin 0.3% and dexamethasone 0.1% Sterile Otic Suspension [package insert]. Fort Worth, Texas: Alcon Laboratories, Inc; 2003.

Dr. Zorik Spektor is director of the Center for Pediatric ENT – Head and Neck Surgery in Boynton Beach, Florida. He is an assistant clinical professor in the Department of Otolaryngology at the University of Miami Miller School of Medicine and an affiliate clinical assistant professor of biomedical science at Florida Atlantic University in Boca Raton, Florida.

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