• 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.
• Noise trauma, depending on the loudness, can produce temporary or permanent hearing loss. Tiny muscles in the middle ear reflexly contract to reduce sound transmission into the inner ear in response to loud sounds (and prior to vocalization), which helps in sustained or continuous noise. However, percussive noise, such as occurs with gun fire and explosions, occurs too rapidly for the reflex to provide protection. Shoulder-supported rifles and shotguns used for hunting and target shooting produce a peak sound pressure level in excess of 140 dB, and fireworks and other explosives can be even louder. The noise actually disrupts the hair cells and their support cells. Since noise-induced trama has limited recovery, repeated exposure produces cumulative hearing loss, a serious problem for hunting dogs where the gun is fired over the dog's head. If there is a ringing sensation in the owners's ears after loud noise (gun fire, loud music, etc), damage is occurring in both the owner's ears and his/her pet's ears. Click here to go to a site at the web page of the National Institute on Deafness and Other Communication Disorders of the NIH on noise-induced hearing loss, and here for the National Institute for Occupational Safety and Health Noise and Hearing Loss Prevention web page.
• Otitis. Infections of the middle ear (otitis media) or inner ear (otitis interna) can produce transient or permanent deafness. Otitis media may leave behind "crud" after the infection ends that blocks sound transmission to the inner ear (conduction deafness). It may take several weeks, but the body will eventually clear this out and the hearing will gradually improve in parallel. Otitis interna, if not quickly treated, will produce permanent nerve deafness.
• The animal's hearing may in fact have been diminishing over time, but the owner may have been unaware because the animal compensated until it reached a point where it could no longer hear adequately to get by. To the owner the onset appears sudden, but it has been progressive. This is common in older animals with presbycusis (age-related hearing loss), which is progressive with time and cannot be prevented or reversed. Click here to go to a site at the web page of the National Institute on Deafness and Other Communication Disorders of the NIH on presbycusis.
• One ear may have been deaf since birth (usually in animals with some white fur) but the owner was unaware because the only deficit of unilateral deafness is difficulty localizing a sound's source. Animals often quickly adapt to this deficit, so there is no overt behavioral manifestation. Any impairment of hearing in the opposite ear then is expressed as total deafness.
• Sudden sensorineural hearing loss (SSHL) in humans can be caused by over 100 mechanisms, but the cause is seldom identified. Click here to go to a site at the web page of the National Institute on Deafness and Other Communication Disorders of the NIH on SSHL.
June 7, 2006