Topical Treatment of Otitis Media
North American Veterinary Conference, 2005
Dr. Louis N. Gotthelf
Animal Hospital of Montgomery
Montgomery, AL
Otitis media is often not even considered in the treatment of otitis externa cases. Frequently, otitis externa resists topical treatment since infection extends through the tympanic membrane into the tympanic bulla. This secondary otitis media occurs in approximately 16% of acute otitis externa cases and as high as 50% of chronic cases. Primary otitis media, extension of infection from the nasopharynx through the auditory tube to the tympanic bulla, is rare in the dog, but may be seen in cats as a complication of upper respiratory disease.
Recent studies in dogs suggest that otitis media may be more common than previously recognized. Many dogs with otitis externa that have intact eardrums also had significant bacterial and yeast populations isolated from the middle ear. These dogs may have had a ruptured eardrum that healed, trapping bacteria and yeast in the tympanic bulla. So the presence of an eardrum does not rule out otitis media. When the eardrum is ruptured, exudates drain from the external ear canal into the tympanic bulla resulting in severe inflammation and disease.
The mucous membrane lining the bulla, the mucoperiosteum, becomes inflammed and can produce copious exudate, including mucus and pus, which complicates the continued treatment of otitis externa. Liquid medications that can be placed into the external ear canal cannot get into the bulla if it is already filled with mucus and pus. After suctioning exudate and flush solution from the external ear canal, the eardrum may not be visualized, but the bony tympanic bulla can be visualized beyond the annulus of the tympanic membrane.
Treatment of Otitis Media
Myringotomy
When otitis media is suspected, but the eardrum is intact, myringotomy can be
done to explore the bulla. Myringotomy is performed at the 5:00 or 7:00 position
in the ventral portion of the pars tensa at the ventralmost portion of the attachment
of the eardrum to the annulus. In an already diseased eardrum, applying pressure
to a 5 Fr polypropylene catheter and perforating the eardrum may perform a myringotomy.
The tip is then directed to the puncture site and is pushed through the eardrum.
Using these locations for myringotomy prevents disruption of the blood supply
to the eardrum and does not disrupt sound transmission from the malleus to the
cochlea.
Culture and Cytology
To obtain cytology or culture samples from the middle ear a new catheter is
introduced into the bulla if the eardrum is ruptured or it is inserted through
the myringotomy hole. Gentle suction is applied to the catheter to fill the
catheter lumen without suctioning up the contents into a syringe or vacuum bottle.
The contents of the catheter are then placed onto a culturette for bacterial
culture and sensitivity and onto clean cotton tipped applicator to be used for
cytological staining in the office. Although it is done, cytology often is unrewarding,
especially if there is a lot of mucus in the bulla because the stains will not
penetrate through mucus to allow identification of bacteria or yeasts.
Flush the Bulla
Irrigation and suction of the bulla can be done from the myringotomy site. A
very high flow of warmed saline or water is applied to the bulla creating a
vortex of flow in and around the middle ear structures. This helps hydrate and
loosen mucus and pus to make their removal easier. The bulla is then dried with
suction to allow the bulla to fill with air.
If there are rod bacteria identified in the middle ear, the bulla can be flushed with tris-EDTA (TrizEDTA, Dermapet) for 5 minutes prior to the instillation of antibiotics into the bulla. Obviously this can only be done at the time of treatment.
Bulla Infusion
After the bulla is flushed out and dried as much as possible, infusion of topical
medications directly into the bulla can be accomplished. Bulla infusions provide
extremely high levels of antibiotics, antifungals, or corticosteroids. The bulla
is a blind pouch with poor drainage, so medication can remain in the bulla for
4-5 days with each application. As the bulla produces mucus and pus, the medication
is displaced back into the horizontal ear canal or through the auditory tube
into the throat.
Because of ototoxicity issues (vestibular disease and deafness) associated with most topical otic products, there are a limited number of medicines that can be infused into the bulla. Enrofloxacin and tobramycin appear to be safe antibiotics when applied directly inside the bulla. Miconazole and any aqueous corticosteroid are also safe.
Initial bulla infusion is limited to the open space in the bulla, which recedes as the inflammation subsides. After a week the volume of the bulla increases and more medication can be infused.
Corticosteroids
The inflammation in the bulla has to be controlled with topical corticosteroids,
systemic corticosteroids, or both. The large amount of mucus produced is associated
with inflammation. In addition, corticosteroids tend to make the mucus less
viscous.
In dogs, since the bulla space is need for the topical antibiotics, systemic cortocosteroids are administered. Prednisone at a dose of 1mg/lb may be given orally. I prefer to use Dexamethasone as an intravenous bolus at a dose of 0.1mg/lb administered at the time of treatment. Since dexamethasone is more anti-inflammatory, it has a faster effect.
In cats, since an oral antibiotic is used, the corticosteroid is infused into the bulla. Dexamethasone sodium phosphate or a combination of fluocinoline and DMSO (Synotic, Ft. Dodge) is infused into the bulla to overflowing. In cats, there may be drainage into the throat through the auditory tube.
Systemic Antimicrobials
Infectious organisms sequestered in deep crypts of the hyperplastic epithelial
folds of the mucoperiosteum and in sclerotic tissue of the submucosal layer
may be controlled using systemic antibiotics or systemic antifungals. Systemic
drugs may not achieve adequate levels in the middle ear to kill some resistant
bacteria, especially pseudomonas.
In cats, the antibiotic Zithromax (Zithromax oral suspension, Pfizer) is used orally at a dose of 5mg/lb every other day for 2-3 doses to combat upper respiratory infection, caused by Staph., Strep., Bordetella, Chlamydia, and Mycoplasma. This new intracellular antibiotic has a long half life and has an affinity for respiratory epithelium and inflammatory cells, making it an ideal antibiotic for cats with URI.
Recheck
Weekly rechecks are needed in the topical treatment of otitis media due to the
mucus production. Since topical antibiotics do not diffuse through mucus, it
needs to be removed periodically to effect topical treatment. In my practice,
I usually re-treat the bulla 2-3 times on average before resolution. Eighty
percent (80%) of otitis media cases will resolve using weekly bulla infusions.
If resolution does not occur after 4-6 weeks of therapy, total ear canal ablation
and bulla osteotomy is indicated.
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