HEAR, HEAR… WHY ARE DOGS DEAF?

By Shelley Greggs

What causes a dog to lose it’s hearing? It turns out that many of the same things that cause hearing loss in humans cause hearing loss in dogs. Genetic defects, congenital deafness (born deaf), ear infections, injury to the ear, exposure to loud noises or certain drugs may be at fault. Some dogs experience gradual hearing loss due to old age. Information from a large scale pet owner’s survey in 2010 reported that there are roughly 35,000 completely deaf dogs and 120,000 partially deaf dogs in the United States, with approximately two births per every 1,000 dogs born are deaf.

Recent research now tells us that deafness in Cavalier King Charles Spaniels can be congenital or progressive. There are several possible causes of deafness in dogs and specifically in Cavalier King Charles Spaniels. Two documented hereditary causes are congenital deafness and primary secretory otitis media (PSOM). Another possible hereditary cause of deafness is progressive hearing loss and there are two non-hereditary causes as well and noise-induced hearing loss.

Dr. Lynette Cole of Ohio State University’s Veterinary Dermatology and Otology Service has studied hearing loss in Cavalier King Charles Spaniels. She reports that, “In the dog breed Cavalier King Charles Spaniel (CKCS), hearing disorders may be due to conductive hearing loss, which may occur with primary secretory otitis media (PSOM), or due to sensorineural hearing loss, which may occur when there is damage or an abnormality of the sensory cells in the cochlea or the auditory nerve.”

Cavaliers appear to carry the piebald gene, and are potentially subject to pigment-associated congenital sensorineural deafness, (which should be evident in the puppy age span) and thus are predisposed to this form of congenital deafness. Congenital deafness is present at birth due to a lack of formation or early degeneration of receptors in the inner ear. If there is un-pigmented skin in the inner ear, the nerve endings atrophy and die off in the first few weeks of the puppy’s life, resulting in deafness. (Please note that you cannot tell the color of hairs in the inner ear by looking at the visible color of the dog’s ears. Although many dogs with white ears will be deaf, many deaf dogs have colored ears as well).

                  

“Primary secretory otitis media (PSOM), also called "glue ear", which can produce a conductive hearing loss, has become more frequently diagnosed in Cavalier King Charles Spaniels. It consists of a highly viscous mucus plug that fills the dog's middle ear and may cause the tympanic membrane to bulge. PSOM has been reported almost exclusively in cavaliers.

Because the pain and other sensations in the head and neck areas, resulting from PSOM, are similar to some symptoms caused by Syringomyelia (SM), some examining veterinarians may have mis-diagnosed SM in Cavaliers that actually have PSOM and not SM.

The cause of PSOM is unknown. It is suspected to be due to a dysfunction of the middle ear or the Eustachian (auditory) tube: either (a) the increased production of mucus in the middle ear, or (b) decreased drainage of the middle ear through the auditory tube, or (c) both.

The principal symptoms are moderate to severe pain in the head or neck, holding the neck in a guarded position, and tilting the head. Other signs may include scratching at the ears, itchy ears, head tilt, excessive yawning, crying out in pain, ataxia, drooping ear or lip, inability to blink an eye, rapid eyeball movement, facial paralysis or nerve palsy, Vestibular disease, some loss of hearing, seizures, and fatigue. These symptoms, in many cases, are very similar to those of Syringomyelia and, to some extent, to those of progressive hereditary deafness. Therefore, the examining veterinarian should take care to consider these other possible causes of the dog's symptomatic behaviors”. (CavalierHealth.org)

Dr. Michael Podell, pictured above, suggested based on his research, that Cavaliers may develop a progressive hereditary hearing loss, which usually begins during puppyhood and worsens, or progresses, until the dog is completely deaf, usually between the ages of three and five years. The progressive nature of this form of deafness in CKCSs was believed to be due to degeneration of the hearing nerve, rather than the lack of formation or early degeneration of the inner ear receptors.

Noise-induced hearing loss (NIHL) is the result of loud noises, such as gunfire and explosions. The NIHL effect is cumulative and irreversible. It shows up in military and police dogs and also in dogs kept in shelters or kennels where noise levels are high. Loud sounds can damage the ear drums (tympanic membrane) or the bones of the inner ear or the hair cells that line the inner ear and transmit the noise to the brain. Loud noises can also induce a very loud ringing in the ears (tinnitus) that may subside over time. Sound levels above 120-140 dB can cause hearing loss.

CKCSs may be tested as early as eight weeks of age for congenital deafness, once the puppies’ ear canals are completely open. However, the condition of progressive hearing loss – due to degeneration of the hearing nerve – has a later onset and cannot be detected in young puppies. If you suspect your dog is experiencing hearing loss, you can test your dog’s hearing by clapping loudly or rattling a can of coins and noting his response. Partial hearing loss or deafness in one ear only may be can be difficult to identify. Try testing with softer sounds like snapping your fingers close to one ear or the other to look for a response.

At the veterinary clinic, the veterinarian will conduct a history and physical examination to measure hearing loss and determine any possible causes. Hearing tests may be used to diagnose hearing loss. Examination of the ear canal will detect wax accumulation, hair overgrowth, any foreign object blockage, infection, inflammation or injury and ear drum state. If the veterinarian suspects an ear infection, ear swabs and cultures may be done to diagnose the infecting agent and determine the proper mode of treatment. In some instances, a brainstem auditory evoked response (BAER) test will be conducted to measure the brain’s response to auditory stimuli. Radiographs may be used to determine possible causes of deafness.

Having a deaf Cavalier does not mean that you will not have a long and meaningful relationship with your dog. There are many ways to communicate with a deal dog. In the next few columns we will look at several of the most popular methods of training and talking with a deaf Cavalier.

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