by Jessica Dixon
figures by MacKenzie Mauger

Do you have difficulty hearing conversations in noisy spaces? Turns out, you’re not alone! According to the World Health Organization, almost 20% of the global population has hearing loss, with this number increasing to 30% in people over the age of 60. Hearing loss can lead to social isolation and difficulty with daily tasks. Even mild hearing loss can substantially increase the risk for dementia. Thus, it is important to identify hearing loss to support patients by decreasing the negative impacts from loss of hearing on their lives.

Currently, the standard test for identifying hearing loss is called an audiogram. During an audiogram, the patient wears headphones in a quiet environment. For each side of the headphones, pure sound tones of varying frequencies, which are sensed as pitch, are played at increasing volumes. The patient indicates the lowest volume, or threshold, at which they can hear each frequency through each ear.

Despite the importance of identifying hearing loss, one type of hearing loss, aptly named Hidden Hearing Loss (HHL), is missed by audiograms. HHL was historically referred to as psychogenic hearing loss, implying that this condition is psychological in origin, while the true biological basis of this disorder eluded the auditory research community for decades. Due to the difficulty of diagnosing HHL, HHL is under-treated despite the fact that it greatly hinders one’s ability to understand speech in environments with background noise, like those present in many social spaces, such as cafes and bars. As of yet, HHL is irreversible, so it is crucial to identify it early to slow down further progression. To better understand why HHL is missed by standard audiograms, let’s start by looking at the structure in your inner ear responsible for sensing sound: the cochlea.

The Inner Ear

In your inner ear, there is a structure known as the cochlea. Within the cochlea, there are cells that have protrusions, called stereocilia, that move in response to sound waves. The bundles of stereocilia on these cells cause them to appear as if they have “hair”, thus these sound sensors are referred to as hair cells. These “hairs” are important because they are pushed by sound waves in a frequency-dependent manner along the length of the cochlea, allowing for the detection of each frequency. There are two types of hair cells: inner hair cells (IHCs) and outer hair cells (OHCs). IHCs send signals to neurons via connections called synapses. These neurons, supported by a type of cell called Schwann cells, rapidly transmit sound information to the brain, allowing for the perception of sound. In contrast, OHCs are thought to act as mechanical amplifiers of sound waves that increase the response of lHCs, allowing for detection of sound at lower volumes. 

Biological basis of Hidden Hearing Loss

Standard hearing tests are adept at identifying damage or loss of OHCs due to their measurement of the quietest volume that a patient can hear a frequency. However, individuals that suffer from HHL have intact OHCs, making HHL undetectable with standard methods. Unlike standard hearing loss, HHL is not an issue of sound amplification, but rather is caused by neurodegeneration that affects the sound information that the brain can receive.

One cause of neurodegeneration that can lead to HHL is from sustained exposure to sound volumes equivalent to the sound level produced by a motorcycle or hand dryer. This level of sound exposure has been shown to drive the loss of synapses between IHCs and neurons in animal models. Loss of these synapses reduces the amount of sound information that is transmitted from the ear to the brain, without necessarily damaging the IHCs themselves. This means that the sound sensors can be intact, but unable to send all of the sound information they detect to the brain. When the neurons that lose connections with the IHCs stop receiving auditory information, they begin to degenerate over time. This decrease in transmission to the brain causes people to struggle to make out specific sounds in loud areas, as their brains are receiving less “sound data” to process. 

HHL can also occur when the connections between the IHCs and the neurons remain, but Schwann cells are lost. To understand why this is, we need to understand more about the function of Schwann cells with neurons. Neurons send information to other neurons by sending data down their axons, which can be thought of as neural “wires”. To increase the speed of this signal transmission, the axons carrying sound information are insulated by a “myelin” sheath made by Schwann cells, in a process called myelination. In fact, sound is the fastest of the five senses to be processed by the brain, in part due to this insulation! Thus, Schwann cells support the neuron’s ability to carry sound information to the brain in a timely manner; when Schwann cells are lost and myelination is compromised, the processing of the timing of sound is disrupted. Our sensing of the timing of sound waves allows us to distinguish between speech and other noise. Therefore, impaired processing of sound timing information can cause individuals with decreased or disrupted myelin in their inner ear to struggle to comprehend speech. In animal models, even after myelination recovers following loss, long-term HHL remains.

HHL Diagnosis, Treatment, and Future Directions

The auditory research field has made great strides to further understand HHL in the last decade. The first biological basis of HHL, loss of synapses, was only first discovered in 2009, and loss of myelin as a cause of HHL was identified in 2017. An important first step in addressing HHL is to improve our ability to identify it. While there is currently no standard diagnostic protocol for HHL, new tests are being developed for the purpose of detecting it. One test involves playing words, instead of simple tones, at a threshold high enough that an individual should be able to hear them with their given audiogram results. If individuals understand fewer words than predicted, this suggests neurodegeneration in the inner ear. It is also possible to measure neural activity in response to sound, which involves playing tones or clicks and recording the neural response to these stimuli via electrodes placed on the scalp. Some of these measurements have been suggested to be predictive of HHL.

While there are currently no treatments that can reverse the neurodegeneration present in HHL, there is cause for hope that treatments may be developed to rescue hearing after the initiation of HHL. For example, neurons survive a long time after they lose their synapses with IHCs before they eventually are lost themselves, so ongoing research is underway to determine whether/how new synapses can be regenerated between the remaining neurons and IHCs to attempt to reverse HHL. In the meantime, we can work together to better the lives of people with HHL by encouraging those with potential symptoms to talk to an audiologist, as well as making efforts to include people with HHL in social situations by providing a quiet environment for a listening ear.

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Jessica Dixon is a second-year PhD candidate in the Neuroscience Program at Harvard University. She studies the development of the cochlea. You can find her on Twitter at @JessDixonNeuro.

MacKenzie Mauger is a Ph.D. student in the Biological and Biomedical Sciences program at Harvard Medical School, where she is studying cell type-specific gene repression.

Cover image by PublicDomainPictures from pixabay. Figures created with BioRender.com.

For more information:

• Read here for a more detailed description of the current understanding of causes, underlying mechanisms, and methods for diagnosis of Hidden Hearing Loss.
• To learn about Hidden Hearing Loss from the scientist, Dr. M. Charles Liberman, whose lab discovered that a loss of synapses can cause Hidden Hearing Loss, click here.
• To read about how hearing loss is linked to health risks and the myths that prevent people from using hearing aids to help, click here.
• Elevated sound levels can cause hearing loss. For a guide on safe sound levels and the sound levels of objects and situations you may encounter, read here.