The causes of otitis externa can be categorized into predisposing, primary and perpetuating factors. Predisposing factors
place a patient at risk of developing otitis while primary factors directly incite the problem. Perpetuating factors prevent
the resolution of a problem once it has been established. Predisposing factors include environmental conditions (high humidity
and ambient temperature), anatomic variations (stenotic canals, hairy canals), and systemic diseases (diabetes mellitus, hyperadrenocorticism).
Primary factors include parasites (Otodectes cynotis ), foreign bodies (plant material, tumors), hypersensitivities (atopy, food allergy), keratinization abnormalities (primary
seborrhea, hypothyroidism), autoimmune diseases (Pemphigus foliaceus), and juvenile cellulitis. Perpetuating factors include
infectious agents (bacteria and yeast), pathologic changes in the ear canal (proliferative tissue), and iatrogenic factors
Pseudomonas aeruginosa is one of the most difficult infectious agents to deal with in otitis cases. It is often resistant to many antibiotics, produces
large amounts of exudate, and can cause significant ulceration and pain in the ear canal. Successful resolution of Pseudomonas otitis will require thorough and often repeated cleaning of the ear canal, topical antibiotic or disinfectant treatment,
and many times systemic antibiotic therapy. Once the infection has been cleared, it is important to keep the environment within
the ear canal unfavorable for the growth of Pseudomonas. This requires control of inflammation (topical corticosteroids as
well as addressing any underlying diseases), keeping the ear canal clean and dry, and keeping the pH in the canal low.
Removing debris and purulent exudates from the ear canal is extremely important. Exudate can cause more inflammation, can
block contact of the topical antibiotic with the bacteria, and can inactivate some antibiotics. If the ears are extremely
swollen, ulcerated, or painful, thorough ear cleaning/flushing may need to be postponed for a week or longer while anti-inflammatory
and anti-infective therapy is begun. Many cases of Pseudomonas otitis externa also have otitis media due to rupture of the tympanic membrane. These dogs should have their external and
middle ears flushed while under general anesthesia to allow complete cleaning of the middle ear cavity. Cleaning solutions
useful for helping to remove purulent exudate include tris-EDTA, Otifoam® cleanser, Epi-otic®, chlorhexidine containing flushes,
and many others. Ear bulb syringes are useful for flushing out gross debris, then for removing debris from the deeper aspects
of the ear, I like red rubber feeding tubes attached to a syringe. Video otoscopy is particularly useful for cleaning and
visualizing the deeper parts of the horizontal canal, the tympanic membrane, and the middle ear cavity. Flushing with copious
amounts of water or saline to rinse out any residual ear cleaning solution may be helpful in preventing ototoxic reactions.
Potential adverse effects of deep ear cleanings include vestibular signs, hearing loss, and Horner's syndrome. Clients should
be advised of this possibility; however, they should also be advised that leaving the infection untreated can have the same
or more serious consequences. Once the initial cleaning has been done, clients should be taught how to do ear cleaning at
home as part of the therapeutic plan.
Culture and sensitivity testing is recommended to guide antibiotic selection for both systemic and topical therapy. One should
keep in mind that some topical antibiotics that are reported as resistant may still be effective due to the much higher concentrations
achieved with topical application. Topical antibiotics that may be effective against Pseudomonas include gentamicin, amikacin (dilute injectable 1:1 with saline, can add dexamethasone SP), neomycin (usually as the combination
product cortisporin- with polymyxin B and hydrocortisone), tobramycin (ophthalmic), enrofloxacin (injectable enrofloxacin
mixed with dex SP and saline), and polymyxin B. Silver sulfadiazine (dilute the cream 1:10 with water) and acetic acid (2%)
also have good efficacy against Pseudomonas. Use caution with acetic acid in an inflamed ear. Tris-edta has anti-Pseudomonas activity as well as rendering the bacteria more susceptible to the action of other antibiotics so can be useful as a pre-treatment
or on its own. In some resistant cases, ticarcillin may need to be used topically, but this must be reserved as a last resort.
The three gram bottle can be diluted with 40 ml of sterile water and kept refrigerated for a month. My normal dose is 0.5
– 1 ml in each ear 2-3 times a day.