"The melting ear": How I deal with pseudomonas otitis (Proceedings)

On the inside: I run. And I wish I could refer them to someone else. And it does not help that we have little evidence-based studies to guide therapy for Pseudomonas otitis. Alas, on the outside, I have to be more adult and professional. Here's how the average case unfolds in our clinic and what I've keep in mind to help me get these cases under control.

I believe it is important to do cytology from each and every case of otitis. If on cytology I see an overgrowth of rods, especially if accompanied by neutrophils, I become very suspicious of a pseudomonas infection (P. aeruginosa). The knowledge specific to this bacteria and its implications on therapy have to be relayed to the owner.

           a) Pseudomonas has a number of virulence factors such as proteases, pyocyanin, and a slime layer that promote tissue invasion, suppress local host immune responses, and confer development of antibacterial resistance. Translation to owner: this is a stubborn organism that is can be difficult to cure, and if not treated aggressively it will become resistant to antibiotics quickly. We have to do cultures to know how to best treat the infection, instructions need to be followed and we have to be able to assess progress.

           b) This is an opportunistic organism. It is always associated with an underlying cause and is one of the more common bacteria isolated from chronic otitis cases. Translation to client: Your pet's ear(s) is / are on fire and we will deal with that. To prevent this from happening again, we need to do more than just treat the infection, we need to look for an underlying cause, starting with a dermatologic exam and history.Most cases of otitis are secondary to underlying allergies (atopy, food reactions).

Step 1: Initial Treatment

1) Prior to any cleaning or treatment, I submit a culture because of Pseudomonas' propensity for antibacterial resistance. In the rare cases where the ear canal is not exceptionally painful, otoscopy to evaluate the tympanic membrane is helpful to guide therapy (i.e., minimize risk of ototoxicity, assess need for systemic therapy). Unfortunately most cases I see are too painful and the canal too full of debris to evaluate well.

2) Anti-inflammatory Therapy

Decreasing the swelling of the pseudomonas infected canals is actually an important factor in successful treatment. Decreasing inflammation opens the canal, allowing better aeration, and makes for more effective cleaning. I start topical anti-inflammatory therapy, often in conjunction with initial topical antibiotic. I also start oral corticosteroids at anti-inflammatory doses (1 mg / kg / day). The duration of the oral steroids depends upon the severity / chronicity of the otitis, with the chronic cases needing longer courses (often weeks after an initial taper).

3) Antibacterial therapy pending culture results:

For acute, first-time cases of Pseudomonas otitis, a commercially prepared gentamicin or neomycin product is a good choice, if we are assured the TM is intact. Ointment-based products are contra-indicated when the TM is damaged.

If a topical enrofloxacin solution has not been used, I start this pending C & S results If topical enrofloxacin has been used, I usually wait to start topical therapy based on the culture results. I wait to start any systemic antibiotic, if indicated, until the culture results are back.

Comments on interpreting culture results: A study published in 2007 showed that marbofloxacin may be more effective in treating Pseudomonas than enrofloxacin, and that disk diffusion susceptibility testing tended to overestimate susceptibility to enrofloxacin compared to MIC testing.

Also, standard susceptibility testing and assessment is based on tissue levels based on systemic administration of antibiotics. Topically application achieves much higher concentrations of antibiotics and an antibiotic listed as "intermediate" or even resistant may be effective used topically.

4) Cleaning the ear canal at home

While I am waiting for culture results and for anti-inflammatories to kick in, I start frequent (daily) gentle cleaning. Often the ears are painful. I cannot expect owners to do much massaging of the canals. I often send home saline and have the owners instill 3-5 cc of saline into the affected ear(s) once a day followed by gentle massage until we recheck and do anesthesia and cleaning.

Another good option is using a Tris-EDTA product, Tris-EDTA disrupts the bacterial cell wall, allowing better penetration of topical antibiotic. Tris-EDTA is incorporated into a number of commercially available ear products. It is most effective if instilled into the ear canal 15-30 minute prior to antibiotic instillation. Even prior to starting a topical antibiotic, this can act as a mechanical ear cleanser.

Epi-otic Advanced ear cleanser (Virbac) also has effective anti-pseudomonas properties and if affected canals are not terribly ulcerated and TM is intact, daily application of this is helpful.

If the canals are not ulcerated (no rbc's on cytology), acetic acid (1:1 to 1:2 dilution of vinegar in water) is effective at killing pseuodomas and can be a mechanical cleanser. Some of my colleagues like using silver sulfadiazine. This is incorporated into Baytril otic solution but can also be prepared by mixing one part Silvadene Cream with 9 parts of water and instilled into the ear.

What about ototoxicity? The prevalence of ototoxicity secondary to the use of flushing solutions / medications when a ruptured tympanic membrane exists is unknown. While there are individual reports of ototoxicity associated with use of a number of topical agents, studies attempting to document ototoxicity after application of agents such as gentamicin and Tris-EDTA have failed to do so. I warn owners ototoxicity can happen unpredictably and we have to weigh potential risk vs. potential benefit.

5) Antimicrobial therapy

Comments on interpreting culture results: A study published in 2007 showed that marbofloxacin may be more effective in treating Pseudomonas than enrofloxacin, and that disk diffusion susceptibility testing tended to overestimate susceptibility to enrofloxacin compared to MIC testing. Susceptibility testing and assessment is based on tissue levels based on systemic administration of antibiotics. Topically application achieves much higher concentrations of antibiotics and an antibiotic listed as "intermediate" or even resistant may be effective used topically.

The acute cases of Pseudomonas otitis may not need systemic antibiotics and can be controlled with topical therapy only which is preferred. The chronic cases with thickened canals and higher likelihood of otitis media do warrant systemic antibiotics, though topical therapy is still the mainstay of treatment. Often, the only effective oral antibiotics are enrofloxacin or marbofloxacin. Enrofloxacin needs to be administered at 20 mg / kg / day for pseudomonas treatment; marbofoxacin at 5.5 mg/ kg/ day. It is not uncommon to receive culture results and find or orally administered antibiotics to be effective. In these cases, I often forego systemic antibiotics and focus on topical only, unless there is marked otitis media, severe ulceration, or the pet has an adverse reaction to topical therapy.

Step 2: Dilution is the solution to the pollution

A thorough cleaning of the ear is very important to therapy success. Many antibacterial agents are inactivated in purulent debris; copious debris also blocks appropriate penetration of antibacterial agents.

I plan for anesthesia and deep ear flushing after the pet has been on oral anti-inflammatory medication for 4-5 days and after the culture is complete. Anesthesia is necessary; sedation is rarely adequate for thorough cleaning. Prior to flushing is a good opportunity to do imaging of the middle ears and ear canals. If CT is available, this is a more sensitive assessment of middle ear disease.

Flushing techniques are as high-tech as use of video-otoscope with irrigation / suction equipment, or as basic as using a red rubber or tom cat catheter and syringe, visualizing the canal with a handheld otoscope. I start with a ceruminolytic agent such as squalene (KlearOtic, DermaPet) or DSS (dioctyl sodium succinate). I let the ceruminolytic agent sit in the canal for at least 2 minutes, preferably 5-10, before doing a flush. This contact time is needed to best break up the wax. I flush with warm saline or water until I have a clean ear canal. Absolutely avoid chlorhexidine-based cleansers in these cases because of potential for ototoxicity.

Assessing the status of the tympanic membrane can be a challenge because of debris, swelling, and stenosis. One trick is to watch to bubbles emanating from the area of the TM in the canal filled with saline. Also, if an awake pet swallows when fluid is introduced into the ear canal, this is a sign of an imperforate TM. If the TM is not intact or it appears fibrotic or there is fluid behind the TM, it is recommended to do a myringotomy to culture and flush the middle ear. When viewing the left ear canal, a safer place to puncture is at the 5:00 position; 7:00 for the right ear (to avoid the germinal center of TM). Once flushing is done, the canal should be dried as thoroughly as possible by suction.

Step 3: Care at home after flushing

The deep ear flushing is just the start of therapy. Cleanings and treatment must continue at home. The antibacterial / anti-inflammatory agent should be instilled at least 2x/day. Cleansing usually needs to be done every other day for the first week and can decrease to 2x/week thereafter if improving. This is a good time to use a non-ceruminolytic "cleanser" such as Tris-EDTA solution to potentiate the antibiotic effects. If there is still much waxy or purulent debris in the canal(s), a prepared ear cleanser (such as EpiOtic) is needed to remove that debris.

These dogs are still painful and need analgesia to help facilitate treatment. This may be your favorite NSAIDs ± tramadol along with topical anti-inflammatories. Sometimes the pain or memory of the pain is too great for owners to touch the ear and suturing a red rubber tube to the skin of the head, inserted into the ear canal for medication administration may be necessary.

Step 4: Follow up and determining the cause of otitis

Rechecks to assess response and client compliance is critical to avoid stopping therapy prematurely, leading to more resistance. For me, cytology and otoscopy are the best tools; some colleagues repeat cultures when no rods are seen on cytology to assure not residual pseudomonas lingering in the canaI. I recheck 2 weeks after the deep cleaning. I do not expect infection to be well controlled at that time; I want to make sure things are improving and that we are working on determining an underlying cause. A recheck 3-4 weeks later (6 weeks after the flushing) is when I expect to see good control of the otitis.

Most cases of chronic otitis are secondary to underlying allergies and thorough dermatologic history, food trial, and antihistamine therapy + fatty acids are all warranted, as is allergy testing / immunotherapy if the former efforts fail to help. I see rare cases of otitis that are unilateral, always the same ears, and the pet has no other signs suggestive of allergies. If foreign bodies and otic masses are ruled out, I suspect these patients have abnormal conformation of the ear canal that causes poor aeration and increase susceptibility to infection. They start with simpler infections such as yeast and staph, but as the canal disease progresses, pseudomonas takes over. These patients are best managed with surgery, either lateral wall resections or total ear canal ablation, depending upon the extent of damage.

If and when I achieve good control (I hesitate to say resolution) of the infection, the ear canal is "open and healthy," and we are addressing any underlying causes, I stop the topical antimicrobial / anti-inflammatory agents and continue cleanser only. Of course, we need another recheck in 3-4 weeks to make sure infection is not recurring.

If the ears canals are fibrosed and stenotic (not just swollen), controlling underlying allergy will not successfully prevent infection and the ears need either 1) chronic topical therapies to prevent infection and remove excessive ceruminous debris; 2) steroid injections into the wall of the ear canal; or 3) total ear canal ablation ± bulla osteotomy (TECA-BO). On fewer occasions, a lateral ear canal resection can alter canal aeration enough to control infection, but we must know that the bulla is free of infection and healthy before this procedure should be considered. Many patients that undergo lateral ear canal resections ultimately need TECA-BO.

Maintenance therapy of a chronically infected ear: I continue daily therapy until at least 1 week past controlled infection (no significant numbers of bacteria or wbc's seen on cytology, canal not painful). After that time, I decrease treatment with the antibiotic / topical steroid twice a week as well as weekly ear cleansing with a commercial cleanser. I require another recheck in 3-4 weeks to assure infection controlled on this regime.

Intraluminal steroid injection: I determine the patient's systemic dose of triamcinolone or methylprednisolone acetate. I give that dose or less into the wall of the ear canal using a 20 or 22 gauge needle, injected several places in the external canal wall. Not to sound like a broken CD, but follow up is critical to see if this helps enough. Generally, by the time I am doing this procedure, I am already talking to the client about TECA and we are doing this as a "Hail Mary."

When do I recommend TECA-BO? 1) Cockers with ceruminous hyperplasia. Do not recommend lateral ear canal resections or intraluminal steroids for these dogs; you can't fight genetics. They will need TECA-BO. 2) The non-cocker with such chronic otitis that there is marked stenosis of the canal. 3) If ear canals are calcified. 4) The patient that is impossible to treat, or when client compliance leads to recurrent and increasingly resistant infection.

Key points

When you find rods on an ear cytology, do aerobic bacterial culture and sensitivity; pseudomonas is a likely infection and is frequently resistant to antibiotics.

This infection in an opportunist and in addition to treating infection, we have to address predisposing factors such as underlying allergies or abnormal ear canal conformation.

Topical therapy (antimicrobial and cleaning) is essential to treat the infection; we cannot rely on systemic therapy only. Anti-inflammatory agents are also essential to decrease ear canal inflammation.

This is a resilient organism and it takes aggressive treatment on our part as well as exceptional client compliance to control this infection. You are not going to cure this infection in one visit. Make sure the client hears this message.

Topical Recipes

Selected References

Griffen CE Pseudomonas Otitis Therapy. In Kirk's Current Veterinary Therapy XIII. 2000 86-588.

Nuttal T, Cole L. Evidence-based veterinary dermatology: A systematic review of interventions for treatment of Pseudomonas otitis in dogs. The Authors. Journal compilation (c) 2007 ESVD and ACVD. 18 ; 69–77 2007.