Animals that are deaf tend to sleep soundly and do not respond promptly to calls for them. Sometimes, they will continue sleeping until they are awakened with a touch. Unilaterally deaf animals may sleep through calls to them if they are lying on their good (hearing) ear.

Hearing is important for pets because they depend on auditory cues for commands as well as alerting to dangers in the environment. However, deaf animals can make great pets once the deafness is recognized.

Causes of deafness can be divided into either conduction disturbances or sensorineural disturbances. Conduction deafness is caused by abnormalities of the pinna (external ear), ear canal, tympanic membrane (eardrum), auditory ossicles or middle ear. Sensorineural deafness is caused by abnormalities of the inner ear, auditory nerve, or in the brain itself. Some of the more common causes of deafness are:

Old age

Congenital or inherited deafness

Otitis externa, which is inflammation of the external ear

Otitis media, or inflammation of the middle ear

Otitis interna, or inflammation of the inner ear

Loud noises such as gunfire

Hypothyroidism

Tumors in the ear or in the brain

Diuretics such as ethacrynic acid, furosemide and bumetanide

Ototoxic drugs, which are drugs that have a deleterious effect on the ear or the otic nerve, such as gentamycin, neomycin, streptomycin, amikacin, polymyxin B, minocycline, erythromycin and chloramphenicol

Otic cleaning agents such as ethanol, iodine, chlorhexidine and benzalkonium that are infused into the middle ear through a perforated ear drum

Other drugs such as salicylates, cetrimide and cisplatin that can cause temporary and permanent hearing loss

Deafness can be unilateral (affecting one ear) or bilateral (affecting both ears). Partial deafness is difficult to recognize but in some animals, partial hearing loss can be observed by some owners. Unilateral and partial deafness can be tested using a brainstem auditory evoked response (BAER) test. The BAER test requires specialized equipment that may only be available at veterinary referral centers. The BAER test is the only reliable and accurate way to assess deafness, particularly unilateral deafness. Animals as young as 5 weeks old can be tested for deafness using the BAER test. The BAER is absent in affected animals.

Cats born deaf usually inherit this problem from their parents. Inherited deafness is sensorineural deafness due to degeneration of the inner ear structures. You will typically notice the deafness at a young age. White, merle or piebald coats increases the chances that an animal has inherited deafness. Deafness has been linked to certain breed characteristics such as heterochromia iridis (incomplete iris pigmentation) although definite links have not been proven.

In animals that have inherited deafness, all the breeding animals should be tested using the BAER test. If they are found to be deaf in one or both ears, they should be removed from the breeding programs. Inherited deafness cannot be treated with hearing aids or surgical treatment.