Deafness

33 Deafness



INTRODUCTION


Air conducts sound waves to the tympanic membrane. This energy is transmitted via the malleus, incus and stapes ossicle chain of the middle ear to the fluid of the cochlear duct and thus its hair cells. Failure of the transmission of sound by the external or middle ear is called conductive deafness.


The cochlear duct is found in an osseus spiral-shaped passageway within the petrous-temporal bone known as the cochlea. The bony cochlea along with the vestibule and semicircular canals containing the vestibular apparatus comprise the inner ear. Movement of the hair cells creates action potentials which travel via the cochlear portion of CN VIII into the medulla oblongata. The auditory pathway decussates multiple times as it ascends to the auditory cortex. Failure of the generation of action potentials or failure of their transmission to the auditory cortex is called sensorineural deafness.


Deafness in an otherwise normal animal is a result of a peripheral lesion (external, middle or inner ear, or cochlear nerve peripheral to the brainstem) rather than a central (brainstem pathways or higher) one. To create complete bilateral deafness from a brainstem lesion would be incompatible with life (Fig. 33.1).


image

Figure 33.1 Diagram of the ear.



SIGNALMENT


A 2-year, 11-month-old female English springer spaniel.



CASE PRESENTING SIGNS


Not responding to commands.



CASE HISTORY


For the last month the dog had been running in the wrong direction when called and was thought to be deaf. When she shook, she fell down.



CLINICAL EXAMINATION


On the first examination, the dog had a normal gait and posture with normal postural reactions (hopping and proprioception) and cranial nerve examination. Wax obscured the left ear drum.



NEUROANATOMICAL DIAGNOSIS


The dog was suspected of being unilaterally deaf, given the poor ability to localize the direction of sound. Falling when shaking suggested a loss of balance and so vestibular involvement was suspected despite the lack of a head tilt or abnormal nystagmus.



DIFFERENTIAL DIAGNOSIS



Cause of deafness


Inherited sensorineural deafness occurs in cats and dogs, and is linked to genes controlling coat colour. Merle or dapple coats have dark, irregular blotches on a white background, e.g. dalmatians, and such breeds have a high incidence of deafness. White-coated animals with or without blue irises have a high risk of deafness.


Agenesis or degeneration of the hearing receptor is responsible and is present from birth or soon after in the majority of breeds. BAER testing is generally performed at 5–8 weeks of age before puppies from at-risk breeds are sold and re-homed. There is no treatment for inherited deafness. Hearing aids magnify sound and are of no benefit as the hearing receptor is destroyed.


Acquired sensorineural deafness occurs as an ageing change. Ototoxicity from aminoglycosides, diuretics, antiseptics and chemotherapy has been reported in animals.


Conditions which block the transmission of sound through the external or middle ear cause hearing loss of varying severity. Deafness is a complication of tympanic bulla surgery.





In this case

Otitis interna was considered most likely in a dog of this age.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Deafness

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