Geriatric Otology
by
Steven A Melman, VMD, DermaPet and the Animal Dermatology Clinic
Ear diseases in older dogs and cats are frequently chronic in nature. Their
treatment and development often results in diminished hearing capacity. Management
of chronic ear diseases requires definitive diagnosis and control of the numerous
underlying causes and perpetuating factors. Cleaning the external ear canal
and middle ear cavity along with the use of oral antimicrobials, oral corticosteroids,
topical antimicrobials, and topical corticosteroids may be necessary in the
treatment of ear diseases (see Table 14-1). Otitis externa involves inflammation
and/or infection of the outer ear where the tympanic membrane is present. Otitis
media involves inflammation and infection of the middle near where frequently
the tympanic membrane is ruptured. In many cases, lifelong ear cleaning and
drying agents may be required to prevent relapse of ear diseases.
Diseases of Pinna
Lesions of the pinna may be extension of more generalized skin diseases and
can be bacterial, fungal, parasitic, immune-mediated, or vascular (drug-induced)
in origin (Roth, 1988). Bacterial and yeast folliculitis of the pinna with focal
areas of alopecia occurs in dogs, and dermatophytosis of the pinna with focal
alopecia and extensive crust formation can be found in cats. Parasitic pruritus
and crusting of the margins of the pinna can be caused by Sarcoptes mites in
dogs and Notoedres mites in cats.
Trauma
Trauma to the pinna can occur at any age. A tear in the pinna is usually the
result of a fight with another dog or cat. The resulting bleeding may be impressive,
and the placement of several sutures is often curative. A small, fresh tear
(up to 1 cm in length) can be sutured after cleaning and removal of some of
the hair. The skin on the concave side of the pinna is apposed and sutured with
interrupted sutures, starting at the edge of the pinna. Then the skin on the
convex side is apposed and sutured in the same way. The cartilage is not included
in these sutures. Bleeding stops during suturing but a resistant artery may
have to be ligated separately. In all cases, the wound should be considered
contaminated, and a short period of systemic antimicrobial treatment is indicated.
When the wound is not fresh and inflammation is apparent, the surgical correction
is postponed and the inflammation is treated first. Surgical correction then
begins with refreshening of the wound edges.
Aural hematoma
Aural hematomas occur at all ages. The bleeding occurs between the cartilaginous
layers of the pinna and usually results from trauma, such as shaking the head
or scratching the ear as a result of hypersensitivity. Other causes include
immune mediated, clotting defects and hormonal imbalances. Surgical intervention
is necessary, because without treatment, the pinna will shrivel, and subsequent
ossification of the cartilage will cause continuous irritation. In addition,
shriveling of the pinna may cause obstruction of the external ear canal and
thus induce chronic otitis externa. Surgery allows for removal the blood clots
and presses the layers of the pinna together long enough to effectuate reunion
of the layers. A reliable surgical method consists of suturing through all layers
of the pinna, placing sutures over the entire surface of the pinna.
Abscesses
In cats, abscesses of the pinna are usually caused by a penetrating wound
inflicted by the claw of another cat. The skin over the abscess should be opened
and the suppurative material removed by gentle compression, followed by flushing
with copious amounts of sterile saline solution. A systemic antimicrobial agent
should be administered for 10 to 14 days. In some older cats, both pinnae are
shriveled because of multiple abscesses. The shriveled pinna can cause occlusion
of the external ear canal and continuous otitis externa may be the result. Ossification
of the pinna usually causes continuous irritation to the cat and amputation
of the pinna results in remarkable relief.
Abscesses of the pinna are uncommon in dogs but they are treated in the same
way as in cats. The healing of the pinna can be slow and painful, and hence
an analgesic agent should be given in addition to antimicrobial therapy. In
addition, in the dog a shriveled pinna can cause considerable irritation and
chronic otitis externa. When there are also complications in the external ear
canal, such as ossification of the cartilage or chronic proliferation of the
skin, removal of the pinna and the ear canal in one procedure can be the preferred
long-term solution (Venker-van Haagen, 1989). This surgery may seem radical
but there is no reason to exclude older dogs from radical surgery when the alternative
is continuous distress and pain.
Tumors of pinna
Tumors of the pinna occur at all ages in dogs and cats. Tumors in dogs may
include squamous cell carcinomas, basal cell tumors, lymphomas, histiocytomas,
mast cell tumors, sebaceous gland adenomas, fibromas, and hemangiopericytoma
(Van der Gaag, 1986). Squamous cell tumors are most commonly seen in white dogs
and cats. Dog breeds most frequently represented in histiocytomas are retrievers
and boxers, mast cell tumors are boxers and Rottweilers, and sebaceous gland
adenomas are cocker spaniels. Squamous cell and histiocytomas can be removed
by partial resection of the pinna. Amputation of the pinna may be necessary
to prevent further extension of the tumor and potential metastasis. Mast cell
tumors should be excised with a wide margin of surrounding tissue. The pinna
and the external ear canal may have to be removed together.
Squamous cell carcinoma is the most common tumor affecting the pinna in the
cat (Roth, 1990). Older white cats seem to be especially susceptible to the
development of squamous cell carcinoma, but it does occur in any colored cat.
Squamous cell carcinomas first appear as a nonhealing granulomatous inflammation
at the edge of the pinna and are often misdiagnosed as an inflammatory lesion.
As the squamous cell carcinoma grows, hemorrhage from the lesion at the edge
of the pinna becomes a frequent nuisance. Cytologic examination of material
collected by fine needle aspiration biopsy or histopathologic examination of
surgical biopsy material will confirm the diagnosis. Unilateral or bilateral
amputation of the pinna is an effective therapy, especially since metastasis
seldom occurs in an early stage. Unilateral or bilateral pinna amputation will
change the appearance of the animal considerably so the owner needs to be informed
of such changes prior to surgery.
The surgical removal of the pinna in the cat and dog are similar, but along
the base of the dog's pinna on the convex side the skin is incised and the individual
arteries and veins are freed and ligated separately for effective hemorrhage
control. After hemorrhage is controlled, the pinna is then removed by cutting
the cartilage and the skin on the concave side and the skin edges are closed
using standard suturing techniques.
Diseases of External and Middle Ear
Dogs and cats with recurrent otitis externa should always be evaluated for co-existing
otitis media. This often requires sedation or general anesthesia to properly
evaluate the external ear canal and middle ear cavity. The optimal indicator
of otitis media is a ruptured tympanum. In cases where the TM is intact, a myringotomy
is required. Magnetic resonance imaging or computer tomography may also be used
to diagnose otitis media (Cole et al, 2000). Sudden onset of peripheral vestibular
syndrome or deafness may occur after any otic flushing or dental cleaning procedure
in older dogs and cats, especially when ototoxic drugs are utilized. Hearing
return may take longer, if ever, when using ototoxic agents such as chlorhexidine
or be more transient as when using alcohol. (Mansfield, )
Ear Evaluation
The preferred method for flushing the external ear canal and middle ear cavity
when the tympanic membrane is ruptured should utilize fiberoptic video-enhanced
endoscopy (Cole et al, 2000). This method ensures good visualization of the
ear structures and safe flushing procedure being the more delicate structures
are located in the middle and dorsal areas of the middle ear cavity. Use of
the myringotomy allows the examiner to collect samples directly from the middle
ear cavity and, therefore, is the practical method of diagnosing an otitis media
where the tympanic membrane is intact in the presence of chronic ear problems.
After the myringotomy, the middle ear cavity is flushed with about 200 to 300
ml of tepid sterile saline solution followed by a solution of tris-EDTA (TrizEDTA,
DermaPet, Potomac, Maryland) then packed with 22-100 mg of flouroquinolones
(Baytril, Bayer, St Josephs, Missouri) and 2-6 mg of dexamethasone sodium phosphate.
A serous, slightly sanguineous discharge may be observed for a few days after
myringotomy. Postoperative analgesic agents can be used for at least 2 to 3
days.. Insidious damage to the facial nerve can develop because of the existing
otitis media or when cleaning, medicating, or performing diagnostic procedures
in an ear with a ruptured tympanic membrane.
The author does not do standard culture and sensiitivity testing unless there
is a treatment failure. Some experts believe a primary indication for performing
standard bacterial culture and sensitivity testing is in dogs and cats diagnosed
with severe proliferative otitis with bacterial rods where systemic antimicrobial
therapy is indicated and management of otitis media is needed (Cole et al, 1998).
Cytologic examination in these cases usually shows numerous leukocytes in stained
otic smears. Because multiple potentially pathogenic organisms may be cultured,
it is important to combine the cytologic examination with bacterial culture
results. The bacteria that appear most numerous may be determined and it allows
a more appropriate antimicrobial regimen to be chosen. Laboratories that report
antimicrobial sensitivities with minimum inhibitory concentration information
rather than the standard Kirby-Bauer susceptibility test allow for better determination
of systemic antimicrobial dosages (Cole et al, 1998). For example, Pseudomonas
organisms may show intermediate or sensitive patterns to enrofloxacin by Kirby-Bauer
susceptibility tests but be resistant on the minimum inhibitory concentration
tests.
Otitis Externa
Otitis externa has both primary and secondary causes (Logas, 1994). Primary
causes of otitis externa in older dogs other than bacterial, fungal, and yeast
infections are hypersensitivity such as atopic dermatitis, food allergy and
contact hypersensitivity, and Otodectes infestation. Otitis externa may be the
only presenting sign in atopic dogs. Often, dogs with diagnosed food allergy
will show their allergic disease with only an otitis externa.
Food allergy should be the primary differential consideration in any older dog
with an acute onset of otitis externa without previous history of otic disease.
Flea allergy does not usually cause just as an otitis externa, but causes total
body skin disease. Prophylactic treatment for ear mites is essential to be certain
that mites are not a component of otitis externa. Systemic therapy for ear mites
is safer and easier with the use of selamectin or various ivermectin products.
Secondary causes of otitis externa include topical irritant reactions and Pseudomonas
or Malassezia infection. Topical irritant reactions should be considered any
time a case of otitis externa fails to respond or worsens with topical therapy.
Therapeutic ingredients in ear medications such as propylene glycol (>10%)
may induce inflammation in the previously damaged aural epidermis. Treatment
of topical irritant reactions consists of discontinuation of topical therapies
and limiting topical products to normal saline solution or aqueous-based products.
Systemic therapy for topical irritant reactions is preferred.
Most Pseudomonas otitis externa are sensitive to polymyxin B, ticarcillin, or
enrofloxacin. Polymyxin B is inactivated by purulent debris and must be applied
only in clean external ear canals.
Malassezia otitis externa is best treated with a non-ototoxic combination of
2% acetic and 2% boric acid (DermaPet Ear/Skin Cleanser aka MalAcetic Otic,
DermaPet, Potomac, Maryland) aqueous solution by flushing daily for 7 days.
Symptoms including erythema, pain and discharge as well as cytology were reversed
in 17 dogs (Gotthelf and Young, 1997). Malassezia otitis media occurs infrequently
and is treated with systemic antifungal agents such as ketoconazole (10 mg/kg
orally once daily) or itraconazole (10 mg/kg orally once daily).
Veterinarians previously have compounded enrofloxacin with a normal saline solution
(4 ml of 22.7 mg/ml enrofloxacin with 8 ml to 12 ml of normal saline solution),
acetic/boric acid cleanser or trisEDTA (TrizEDTA). Topical enrofloxacin may
be effective even when sensitivity testing indicates a resistance because of
the increased concentration achieved when applied topically. Silver sulfadiazine
(2%) solution is also effective against Pseudomonas.. A formulation of enrofloxacin
(5 mg/ml) combined with 1% silver sulfadiazine (Baytril Otic, Bayer) is effective
for bacterial and, to a much lesser extent, Malassezia Otitis externa; it has
not been proven to be safe or effective in cases where there is a ruptured tympanum..
Acetic acid (2%) is effective against Pseudomonas organisms after 1 minute of
contact time. However, 2% acetic/2% boric acids are proven to be synergistic
against Pseudomonas.(Benson).
Ethylenediaminetetraacetic acid (EDTA) has a direct bactericidal action against
Pseudomonas aeruginosa by chelating metal ions important for the integrity of
its cell wall, inactivating the efflux pump in gram negative bacteria and inactivating
enzymes secreted by Pseudomonas that would otherwise cause ulceration and tissue
necrosis. Buffer solutions containing tromethamine (Tris) are used to enhance
the effects of EDTA on the Pseudomonas aeruginosa and other gram negative organisms.
The EDTA binds to metal ions, which compete with aminoglycosides for cell-wall
receptors. (Ashworth). Thus, the combination of aminoglycoside and Tris-EDTA
is effective against bacteria causing an otitis externa/media infection, including
Staphylococcus intermedius, Proteus mirabilis, Escherichia coli, and Pseudomonas
aeruginosa. The Tris-EDTA buffer solution (TrizEDTA, DermaPet, Potomac, Maryland)
is less effective at inhibiting gram-positive bacteria than gram-negative bacteria.
The optimal pH for the activity of aminoglycosides and Tris-EDTA buffer solution
is approximately 8.0 (Wooley); many antibiotics are enhanced at alkaline pH
and have diminished efficacy at an acidic pH. It is most effective as it approaches
body temperature when it has been demonstrated to irreversibly bind enzymes
that would otherwise cause ulceration. TrizEDTA is not considered to be ototoxic.
It is not recommended for yeast infections and, therefore, is not likely to
be effective against Malassezia.
Tris-EDTA buffer solutions are beneficial as long-term therapy for preventing
the recurrence of Pseudomonas infection, using them as either part of a regular
ear-cleaning regimen or as a presoak before instilling antimicrobial solutions
into the external ear canal. Such solutions are typically applied two to three
times a week as needed for preventative purposes.
Otitis Media
Most dogs with chronic otitis externa have concurrent otitis media and accompanied
frequently by an intact tympanic membrane (Little et al, 1991). Evidence of
inflammation to the tissues surrounding the middle ear cavity, or even the inner
ear, indicates that otitis media may be present. The entire tympanic membrane
in most older dogs and cats is difficult to completely visualize by using the
standard otoscopic evaluation procedure. Even with an intact tympanic membrane,
various abnormalities of the middle ear cavity in dogs and cats may be visualized
by using fiberoptic video-enhanced endoscopy (Cole et al, 2000). The affected
tympanic membrane is nontransparent and usually red or ruptured (Little et al,
1991). The discharge from the middle ear cavity may be mucopurulent in acute
inflammation and more dry and white in chronic inflammation. If the tympanic
membrane is intact and otitis media is suspected, a myringotomy may be necessary
to confirm the otitis media. The tympanic membrane should close within 4 weeks
when middle ear disease is cured.
Tumors of External Ear Canal
Tumors in the external ear canal more often occur in older cats than in older
dogs. The tumors in the external ear canal of older dogs may include ceruminal
gland adenomas, papillomas, ceruminal gland carcinomas, and squamous cell carcinomas
(Van der Gaag, 1986). The tumors in the external ear canal in older cats may
include ceruminal gland adenomas, ceruminal gland carcinomas, sebaceous gland
adenomas (Fig. 14-2), papillomas, mast cell tumors, and rhabdomyosarcoma (Rogers,
1988). The unilateral or bilateral tumor can be diagnosed by otoscopic examination.
Since each tumor damages the external ear canal and causes pain and continuous
secondary inflammation, wide surgical excision or application of laser surgery
is always indicated. Malignant tumors are especially threatening because they
will eventually invade the tissues around the ear canal. Total ablation of the
external ear canal is always indicated when a malignant tumor is confirmed and
as long as the tumor is still confined within the cartilaginous wall.
Loss of Vestibular Function
The loss of vestibular function may be caused by middle ear disease, trauma,
or ototoxicity in dogs and cats of all ages. Idiopathic vestibular disease commonly
occurs in dogs with an average age of onset of 12.5 years (Schunk and Averill,
1983). In cats with signs of peripheral vestibular dysfunction, the seemingly
idiopathic vestibular syndrome is most often associated with middle ear disease
(Burke et al, 1985).
The case history of idiopathic vestibular disease in dogs is characterized by
a peracute onset (within hours) of head tilt, incoordination, and nystagmus.
The incoordination may be so severe that the dog cannot effectively rise and
walk. Vomiting that is secondary to vertigo may be seen during the first 24
to 48 hours. The neurologic examination shows a head tilt, falling and/or rolling
toward the head tilt, and horizontal to rotary nystagmus. There may be spontaneous
nystagmus and the fast phase of the nystagmus occurring away from the head tilt.
The type of nystagmus typically does not change. There often is generalized
incoordination and a base wide stance. No cranial nerve deficits will be noted,
and there is no weakness or postural test deficits. If these signs are seen,
then a brain stem lesion involving the central vestibular system is most likely.
If the nystagmus type is vertical, a brain stem lesion is most likely to be
present.
On the initial day of presentation, the incoordination and disorientation is
often too severe to allow for adequate assessment of strength and postural test
reactions. It may take 48 hours before any assessment can be used to help separate
central vestibular disease from peripheral vestibular disease. Once vestibular
signs have been localized to the peripheral vestibular system, the primary differentials
are otitis interna and head trauma with resultant fracture of the petrous temporal
bone. If facial nerve paralysis and Horner's syndrome are present on the same
side of the head tilt, this usually indicates otitis media/interna co-exists.
Treatment is supportive care since the peripheral vestibular problem will resolve
on its own. The resolution timetable is usually consistent from case to case
and if the dog does not follow the timetable, there should be concern that the
initial diagnosis was wrong. The nystagmus should disappear within 4 days, the
dog should be able to rise and walk fairly well within 7 days, and the dog's
gait should be normal by 3 weeks. A few dogs may have a residual head tilt and/or
some incoordination after complete recovery when performing quick movements
that require agility. Treatments that may be helpful during the first few days
include: fluid therapy for maintaining normal hydration, diazepam (5-15 mg three
times daily) for sedation if disorientation is severe, meclizine (25 mg orally
once daily) for vertigo, and diphenhydramine (4-8 mg/kg orally). These treatments
are usually unnecessary after 72 to 96 hours after onset of peripheral vestibular
signs.
Primary Middle Ear Tumors
Primary middle ear tumors are uncommon in dogs and cats (Fiorito, 1986; Indrieri
and Taylor, 1984). Tumors are suspected when the external ear canal is narrowed
near the osseous external meatus and the otic discharge is hemorrhagic. Clinical
signs include pain on the affected side and vestibular dysfunction. Survey radiographs
may show densities in and around the tympanic bulla, but these findings do not
differentiate between chronic inflammatory disease and tumor. Computer tomography
is helpful to detect lesions in the tympanic bulla and the petrosal bone. The
destruction caused by the tumor at the time of diagnosis precludes any attempt
at surgery.
Systemic Antimicrobial Therapy
Systemic antimicrobial therapy is indicated for management of severe otitis
externa/media, topical irritant reactions, poor response to topical otic therapy,
and when severe proliferative otitis externa is present and the owner cannot
administer regularly scheduled topical treatments. Antimicrobial agents that
are known to penetrate into bone, concentrate within inflammatory cells, and
have an excellent success in the treatment of otitis media should be selected
and given at doses that are at the high end of the recommended dosage range.
Antimicrobial and antifungal agents should be administered for at least 14 days
after a complete clinical cure is obtained, often up to 6 tp 12 weeks of therapy
are necessary. Examples of systemic antimicrobial agents that are useful for
the management of otitis media and proliferative otitis externa are clindamycin
7-10 mg/kg twice daily, sulfadimethoxine-ormetoprim 55 mg/kg for first day and
25 mg/kg once daily on subsequent days, enrofloxacin 5-20 mg/kg once daily,
marbofloxin 2.5-5 mg/kg and orbifloxacin 2.5-12.5 mg/kg once daily. In general,
fluoroquinolones in dogs are needed at higher doses for Pseudomonas aeruginosa
infections such as 10-20 mg/kg enrofloxacin once daily, 5-12.5 mg/kg orbifloxacin
once daily, or 5 mg/kg marbofloxacin once daily.
Intralesional Therapy
Stenosis of the vertical portion of the external ear canal occasionally may
be nonresponsive to the administration of systemic corticosteroids. In these
cases, intralesional triamcinolone acetonide (4 mg/ml) may be useful in widening
the lumen of the vertical portion of the external ear canal. After effective
cleaning of the external ear canal, multiple injections of triamcinolone acetonide
at 0.1 ml per injection site are made as deep as possible into the proliferative
tissue lining in a ring like fashion around the external ear canal.
Bacterial Otitis Protocol
The following is a protocol for the treatment of bacterial otitis media.
1. Start the antibiotic which cultures sensitive systemically for at least
30-60 days. The author currently prefers fluoroquinolones.
2. Clean with TrizEDTA twice daily. Do not refrigerate as one mechanism of action
is to reverse the activity of elastase enzymes which are bound irreversibly
at body temperature.
3. Instill topical antibiotics (the author currently prefers Baytril) in either
a TrizEDTA gemish (12 ml of TrizEDTA and 50-100 ml of Baytril injectable) or
diluted with saline. For first three weeks add 6mg dexamethasone SP (sodium
phosphate).
4. For the first 14 days, I would use systemic prednisone at 1-2 mg/kg if OM
is diagnosed. The rationale is to decrease the
inflammation, mucous secretion from the mucoperiosteum, pain and viscosity of
the exudate in the tympanic cavity.
5. If OM is diagnosed, I would anesthetize and with visualization flush the
ear with TrizEDTA and directly instill Baytril and NaDex
phosphate into the tympanum. If the tympanum is present and OM is suspected
then I would do a MYRINGOTOMY. The
location of the myringotomy should be ventral. Maintenance therapy would include
biweekly TrizEDTA cleansing long-term.
Surgical Intervention for Ear Diseases
In dogs with recurrent otitis, surgical intervention may be indicated. Such
instances may include:
Loss of Hearing
Loss of hearing in older dogs and cats is well recognized. The cause of hearing
loss may be related to a loss of spiral ganglion cells in the cochleas and secondary
to hair cell loss (Knowles at al, 1989; Schuknecht et al, 1965). Hearing in
older dogs and cats can be tested most effectively by using brain stem auditory
evoked responses (Sims, 1988; Venker-van Haagen et al, 1989). The case histories
provided by owners of older dogs and cats examined for hearing deficits usually
suggest a progressive elevation of the hearing threshold rather than acute total
hearing loss. However, hearing loss may be associated with existing otitis media,
sudden onset immune-mediated otitis media/interna or ototoxicity.
References and Supplemental Reading
Ashworth C. Topics in Drug Therapy. JAVMA 197:1513-1514, 1990
Benson CE: Efficacy of Acetic and Boric Acids against selected microbial flora seen in Otitis externa. Proc Amer Ac Vet Derm, 1999
Burke EE, Moise NS, de Lahunta A, et al: Review of idiopathic feline vestibular syndrome in 75 cats. J Am Vet Med Assoc 187:941, 1985.
Cole LK, Kwochka KW, Kowalski JJ, et al: Microbial flora and antimicrobial sensitivity testing of isolated pathogens from the horizontal ear canal and middle ear of dogs with otitis media. J Am Vet Med Assoc 212:534, 1998.
Cole LK, Kwochka KW, Podell M, et al: Radiography, otoscopy, pneumotoscopy, impedance audiometry, and endoscopy for the diagnosis of otitis media in the dog. Proc Fourth World Congress of Veterinary Dermatology, San Francisco, CA, 4:3, 2000.
Fiorito DA: Oral and peripheral vestibular signs in a cat with squamous cell carcinoma. J Am Vet Med Assoc 188:71, 1986.
Gotthelf LN, Young SE: New treatment of Malassezia otitis externa in dogs. Vet Forum 14:47, 1997.
Gotthelf LN: Small Animal Ear Diseases. Saunders, 2001.
Indrieri RJ, Taylor RF: Vestibular dysfunction caused by squamous cell carcinoma involving the middle ear and inner ear in two cats. J Am Vet Med Assoc 184:471, 1984.
Knowles K, Blauch B, Leipold H, et al: Reduction of spiral ganglion neurons in the aged canine with hearing loss. J Vet Med Assoc 36:188, 1889.
Little CJL, Lane JG, Gibbs C, et al: Inflammatory middle ear disease of the dog: The pathology of otitis media. Vet Rec 128:293, 1991.
Logas DB: Diseases of the ear canal. Vet Clin North Am 24:905, 1994.
Mansfield, S. Ototoxicity. Ch in Gotthelf LN: Small Animal Ear Diseases. Saunders, 2001
Melman SA: New Approach to Pruritic Otitis (Chapter) in Gotthelf LN: Small Animal Ear Diseases. Saunders, 2001.
Rogers KS: Tumors of the ear canal. Vet Clin North Am 18:860, 1988.
Roth L: Pathologic changes in otitis externa. Vet Clin North Am 18:755, 1988.
Schuknecht HF, Igarashi M, Gacek RR: The pathological types of cochleo-saccular degeneration. Acta Otolaryng 59:154, 1965.
Schunk KL, Averill DR: Peripheral vestibular syndrome in the dog: A review of 83 cases. J Am Vet Med Assoc 182:1354, 1983.
Sims MH: Electrodiagnostic evaluation of auditory function. Vet Clin North Am 18:913, 1988.
Van der Gaag I: The pathology of external ear canal in dogs and cats. Vet Q 8:307, 1986.
Venker-van Haagen AJ: Disease and surgery of the ear. In Sherding RG (ed): The Cat. Diseases and Clinical Management. New York, Churchill Livingstone, 1989, p 1631.
Venker-van Haagen AJ: Diseases and surgery of the ear. In Venker-van Haagen AJ, Siemelink RJG, Smoorenburg GF: Auditory brain stem responses in the normal beagle. Vet Q 11:129, 1989.
Wooley, R. In-vitro effect of combinations of antimicorbial agents and tris-edta
on Pseudomonas. JAVMA, 1521-1524, 1983.
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