Age-Related Hearing Loss Predicts Mental Decline in Dogs

Source: Courtesy of SC Psychological Enterprises Ltd.

Age is not friendly when it comes to cognitive abilities. It is well established that both humans and dogs become more susceptible to memory losses, reduced mental efficiency, and slower thinking as they grow older. In people, extreme cases of such mental declines are referred to as dementia and perhaps one of the most severe forms of dementia is Alzheimer's disease.

There is a canine version of this kind of dementia, and it is known as canine cognitive dysfunction (CCD). While many factors have been implicated as contributing to the risk of dementia, one that is not well known to the general public is hearing status. Decreased hearing ability is associated with a higher probability of showing signs of dementia in humans. A team of researchers headed by Gilad Fefer at North Carolina State University in Raleigh, North Carolina, wanted to know if older dogs with age-related hearing losses are also more likely to show symptoms of dementia and poorer mental efficiency.

The Link Between Diminished Hearing and Cognitive Ability

Age-related hearing loss (technically called presbycusis) is a common condition in older human populations. Research has shown that one-third of people over the age of 65 experience hearing loss. Diminished hearing ability in elderly individuals has serious consequences. It has been estimated that the rate of cognitive decline is approximately 30 to 40 percent faster in people with hearing loss.

Why should hearing be linked to mental abilities? It is been suggested that this association comes about because sound interpretation and a lot of cognitive processing occur in the same areas of the brain. It is the temporal lobe of the brain that processes both auditory information and short-term memory storage. It is also often the first area to be affected by Alzheimer's disease. It has been suggested that poor hearing can potentially lead to a progressive deterioration in this brain region. It works this way: A reduction in sound inputs to the brain results in the cortical cells being understimulated. Since the brain often works on a "use it or lose it" principle, areas of the brain that are not well utilized will tend to shrink, cortical cells will die, and the efficiency of this part of the brain will be reduced. This may be happening in the temporal lobe of the brains of individuals with hearing losses simply because, in the absence of auditory inputs, it no longer has to work to understand sound.

It certainly seems likely that the same mechanism would apply to older dogs since age-related hearing decline also occurs in canines. Typically, canine hearing begins to diminish at approximately 8 to 10 years of age, with the greatest losses in the high frequencies followed by the middle frequencies of sound.

Testing Hearing and Mental Status

In this recent investigation, 39 "senior" dogs were tested. The median age was about 13 years, and, to be included in the study, the dogs had to be otherwise healthy. The hearing of each dog was tested using the BAER method. This stands for brainstem auditory evoked response. It involves placing electrodes on the dog's head to measure electrical responses to sounds in the cochlear nerve and brain stem.

The dogs were divided into three groups based on their hearing ability. Those considered to be without hearing loss could hear tones at 50 dB, those with moderate hearing loss could hear tones at 70 dB, and the most severely hearing impaired were those who required 90 dB or more to hear (roughly the sound level of a jet plane at takeoff).

The dogs' owners filled out two questionnaires. The first evaluated the quality of life of the dog. Quality of life is often lower in individuals who have dementia. This inventory included questions on the dog's vitality (such as whether the dog showed a lack of energy or was still doing its favorite activities). It also asked questions about the quality of companionship (such as showing a normal amount of affection and enjoying being petted and touched).

The second questionnaire directly looked for symptoms of CCD or cognitive decline. Among other items, it asked whether the dog appeared to be disoriented in familiar environments, had difficulties recognizing familiar people and animals, or showed a reduced ability to remember and respond to commands that it had previously learned.

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The results were fairly straightforward. When the researchers compared the hearing scores with the quality-of-life scores, they found that vitality and companionship behaviors decline significantly as hearing ability deteriorates.

When the investigators looked at whether a dog's hearing ability was associated with the appearance of signs of CCD, the results were quite striking. First, you must remember that the dogs in this sample are already in their senior years (what would normally be the last 25 percent of their life span), so we would expect some of them to already be showing signs of dementia. If we look at the dogs with normal hearing, we find that 61 percent are symptom-free, while 31 percent show only mild symptoms of CCD, and only 5 percent show moderate cognitive decline. For the dogs with moderate hearing loss, the number of symptom-free dogs dropped to 25 percent, while 50 percent now showed moderate signs of dementia. Perhaps most striking is that among the group with the largest hearing losses, we find no dogs (0 percent) who are symptom-free, while 38 percent show moderate levels of dementia and half of this group shows severe levels of CCD.

Researchers who look at human age-related cognitive declines often conclude that hearing loss is one of the largest risk factors for dementia in people. This newest study seems to suggest that the same connection between hearing loss and dementia appears in aging dogs.

When it comes to people, we know that correcting age-associated hearing loss through some sort of hearing aid can delay or prevent dementias and help stave off severe cognitive decline. Extrapolating from the human research suggests that there is at least the potential to treat older dogs and alleviate some of their cognitive problems. This may not be as easy as inserting a hearing aid into the ear of an elderly human, since dogs do not tolerate that kind of apparatus very well. However, cochlear implants, which are becoming more routine and affordable in people, can improve hearing and might eventually provide a buffer against cognitive decline for our canine companions as well.

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