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Causes of Sudden Onset of Deafness
• Drug toxicity (ototoxicity) results from administration or application of a drug or chemical which either directly or indirectly destroys cochlear hair cells, resulting in hearing loss or even total deafness. Many of the same agents are vestibulotoxic, disrupting the sense of balance so that the animal has a head tilt and may walk in circles. The most common cause of ototoxicity is the group of drugs known as the aminoglycoside antibiotics, which includes gentamicin, kanamycin, neomycin, tobramycin, and others. At times these drugs are the only treatment for a life-threatening infection, but they must be used with caution. Also, no-longer-available ear cleaning solutions containing chlorhexidine and a variety of less common chemicals can cause deafness (see G.M. Strain: "Aetiology, prevalence and diagnosis of deafness in dogs and cats." British Veterinary Journal 152:17-36, 1996). Concurrent conditions, such as an on-going infection or topical steroids, may increase penetration of a topical agent into the inner ear, increasing its toxicity. Recovery after hearing loss does not occur.
• General anesthesia may cause bilateral deafness from unknown causes. In rare cases, animals awaken from anesthesia deaf in both ears, often following ear cleaning or teeth cleaning. It may be the case that the body shunts blood away from the cochlea during anesthesia to protect other critical organs, or that pressure or jaw positioning compresses the arterial supply to the cochlea. A similar outcome is noted in humans with ischemia of the vertebrobasilar artery. The inner ear is especially vulnerable to reduced blood flow because it has no collateral circulation and the tissues have very high energy metabolism. None of the cases I have seen or had reported to me have recovered hearing except for two cases that may have been conductive deafness. This reinforces the need for a good otic examination to rule out ear canal obstruction or middle ear infections. It is not known if unrecognized cases of unilateral deafness result from anesthesia, since this condition is usually not recognized by owners. |
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