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Researchers identify potential presbycusis contributor

Could age-related hearing loss someday become a thing of the past? That is the question some researchers at Johns Hopkins are asking themselves, and recent developments indicate that the answer just might be yes.

As one of the most common conditions affecting the aging population, age-related hearing loss, also known as presbycusis, affects approximately one in three people in the United States between the ages of 65 and 74  and nearly half of those older than 75.

Plastic model of ear
The inner ear contains the sensory cells
and the nerve cells. It is colored blue
in the above model of the ear. 
A complex disorder with many possible causes, presbycusis is typically progressive and is characterized by a bilateral loss of hearing, especially that of high frequencies, over a long period of time. Because people are living longer due to advances in health care, the number of people with presbycusis is rapidly increasing; the World Health Organization estimates the number of people living with hearing loss is expected to reach 900 million by the year 2025. But now exciting new research that identifies a potential contributor to presbycusis could help prevent it in the future.

By studying the hearing of mice, researchers at the Johns Hopkins School of Medicine have found that an increased number of connections between sensory cells and nerve cells may be a factor in presbycusis. The thought is that if these new connections could be prevented from forming, presbycusis could be prevented as well.

To understand the new research, we have to first understand what happens to the ear in response to sound: It all starts with two kinds of cells: the sensory cells and the nerve cells.

The sensory cells are essentially filament-like structures that act like antennae as they pick up sound waves. When sound happens, these sensory cells inside the inner ear take the sound waves and convert them to electrical signals.

Deep within the inner ear, there are two kinds of sensory cells, also called hair cells: an inner set and an outer set. The inner set of sensory cells lies closer to the brain, but it is the outer set that has always been thought to be most relevant to presbycusis. The outer set not only converts sound waves, but also serves as an amplifier that allows us to hear very quiet sounds. Due to causes such as certain medications, disease or just plain old genetics, as we age we also lose many of those outer hair cells; losing the outer hair cells means our ears have reduced capabilities of sound amplification.

Now we come to the nerve cells, and their connections with the sensory cells. Each nerve cell is assigned only one role: either incoming or outgoing. The job of the outgoing nerve cell connections is to bring signals from the brain to the cochlea, while the job of the incoming nerve cell connections is to send signals from the ear to the brain in order to convert the signals into recognizable sound. Most importantly, the incoming nerve cell connections serve to lower the amplification in response to noise.

Based on previous research which had already suggested an age-related decrease in the outgoing connections and an increase in the incoming connections, researchers decided to test the efficiency of the connections. The results were surprising.

When researchers looked at the electrical signals from within the inner hair cells of mice, they found that the incoming nerve cell connections were more active in the mice that had hearing loss. The higher activity of the nerve cell connections combined with the increase in numbers resulted in a significant dampening of the amplification in response to noise.

Whether the increase in incoming connections is just to fill the space vacated by the reduced outgoing connections isn’t yet known. What is known is that the increase in inbound connections serves to dampen sound significantly, and that is a problem when it comes to hearing.

It has long been thought that the death of sensory hair cells in the inner ear is almost entirely to blame for age related hearing loss. While some causes of presbycusis, such as excessive exposure to noise, can be avoided in order to reduce the risk of age-related hearing loss, other causes such as genetics cannot. This new information suggests that if doctors could prevent these new connections from forming, many cases of hearing loss could be avoided.

So, could the prevention of these new connections be like a fountain of youth for your ears? Quite possibly.

“If confirmed, our findings give us new ideas for how physicians may someday treat or prevent age-related hearing loss,” said Paul Fuchs, Ph.D., the John E. Bordley Professor of Otolaryngology-Head and Neck Surgery at the Johns Hopkins University School of Medicine.
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