5-12 Although such an approach may be used to obtain a psychoacoustic audiogram in which the frequency of the sound is plotted against the minimum intensity required to elicit a behavioral response, results are influenced by the dog’s mental state, rate of adaptation to the sound stimulus, and sensitivity to visual and vibratory stimuli. 5-12 As a result, proper training of both the dog and the human tester is required before reliable results can be obtained. These factors make the use of behavioral responses a poor choice for inexperienced testers, young puppies, and clinical patients. 7

Brainstem auditory-evoked response in dogs

Otoscopic evaluation of the external ear canal and tympanic membrane is the first diagnostic procedure that should be performed in animals presented with otitis externa. It is important to become familiar with the normal structures of the ear to identify any abnormalities. The video otoscope greatly enhances visualization of these structures as well as aiding in retrieval of foreign bodies, biopsy of masses, and performance of a myringotomy. In some cases, a complete otoscopic examination is not possible on the first visit, and the animal may require anti-inflammatory medications or otic flushing to visualize the entire ear canal as well as the tympanic membrane. The purpose of the otoscopic examination is to address the otitis by evaluating the condition of the ear, looking for any masses or foreign bodies; noting the presence, consistency, and color of any exudate; and evaluating the patency of the tympanic membrane. Once a complete otic evaluation has been performed, additional diagnostics as well as a treatment plan can be formulated.

Otoscopic evaluation of the ear canal.

The canine ear consists of the pinna, external ear canal, middle ear and inner ear. The external ear is composed of auricular and annular cartilage. The auricular cartilage of the pinna becomes funnel shaped at the opening of the external ear canal. The vertical ear canal runs for about 1 inch, then forms the horizontal ear canal, which is composed of auricular and annular cartilage. The middle ear consists of an air-filled tympanic cavity, three auditory ossicles, and tympanic membrane. The tympanic membrane is a semitransparent membrane divided into the pars flaccida and pars tensa. The tympanic cavity consists of a small epitympanic recess, a large ventral bulla and the tympanic bulla proper. On the medial wall of the tympanic cavity is the promontory, which houses the cochlea. The cochlear (round) window is located in the caudolateral portion of the promontory, covered by a thin membrane. The vestibular (oval) window is located on the dorsolateral surface of the promontory, covered by a thin diaphragm over which the footplate of the stapes is attached. The auditory tube is a short canal that extends from the nasopharynx to the rostral portion of the tympanic cavity proper. The auditory ossicles are the bones that transmit and amplify air vibrations from the tympanic membrane to the inner ear. The inner ear is housed in a bony labyrinth in the petrous portion of the temporal bone. The bony labyrinth contains the membranous labyrinth with its sensory organs responsible for hearing and balance.

Anatomy and physiology of the canine ear

Otitis externa represents one of the most common reasons that pets are presented for veterinary attention. They are also some of the more frustrating entities practitioners are called on to deal with. Successful management depends on an understanding of the primary, predisposing, and perpetuating factors involved in the pathogenesis of otitis externa. Emphasis is then placed on establishing a diagnosis through physical examination, cytologic examination, culture and sensitivity testing, radiographs and biopsies. The general goals of management are to resolve, remove, or control primary factors, clean and dry the ear, reduce inflammation and resolve secondary infections. These goals are achieved through various cleaning techniques, topical, and systemic therapies. Therapeutic alternatives are given for otitis involving bacteria, Malassezia, otodectes, demodex, allergies, ceruminous disorders, the idiopathic inflammatory/hyperplastic otitis externa of cocker spaniels, "Swimmer's Ear", contact/irritant reactions, overtreatment, and concurrent otitis media.

Management of Otitis Externa

Ear cleaning helps maintain the normal otic environment and is important in the treatment of otitis. Over cleaning, however, may trigger otitis through maceration of the epidermal lining. Simple manual cleaning is useful for routine cleansing but doesn’t remove tightly adherent debris. Bulb syringes are more vigorous but may damage the ear in inexperienced hands. Devices using mains water pressure or dental machines are also available. Thorough cleaning of the ear canals and middle ear cavity can only be achieved by retrograde flushing using specially adapted catheters, feeding tubes or video otoscopes under anaesthesia. Myringotomy, inspection and cleaning of the middle should be performed if the tympanic membrane appears abnormal. There are a wide variety of cleaning fluids available. Ceruminolytics soften and dissolve cerumen to facilitate cleaning. Surfactants emulsify debris, breaking it up and keeping it in solution. Astringents dry the ear canal surface, preventing maceration. Maintaining a low pH and incorporating antimicrobial agents can inhibit microbial proliferation and glucocorticoids can be used to reduce inflammation. Adverse effects and contraindications following ear cleaning can include maceration, contact reactions, otitis media, ear canal avulsion, vestibular syndrome, Horner's syndrome, facial nerve paralysis and deafness. Care should be exercised in selecting cleaning fluids if the tympanic membranes are ruptured.

D. P. Storrs

Papers

Healing of experimentally perforated tympanic membranes demonstrated by electrodiagnostic testing and histopathology.

Alterations in the brain stem auditory evoked response threshold and latency-intensity curve associated with conductive hearing loss in dogs.