Hearing loss – it’s not all about hair cells
Hearing loss is commonly caused by direct damage to the sound-sensing cells in the inner ear (or the nerves that connect these cells to the brain). But this isn’t the whole story. Damage to a specific structure in the inner ear, called the stria vascularis, can also cause hearing loss – sometimes called ‘strial’ or ‘metabolic’ hearing loss.
The stria vascularis is crucial to our ability to hear as it ‘powers’ the hair cells to send signals about sounds, repeatedly, to the brain. It does this by increasing the amount of potassium in the fluid outside the hair cells (called the endolymph) relative to the amount of potassium inside them. This difference is called the ‘endocochlear potential’ and it basically works like a battery, storing energy until it’s needed. Without this battery, we lose our hearing.
When a sound enters the inner ear, tiny channels in the hair cell surface open. The potassium outside rushes into the cell, equalising the levels inside and out. This change in potassium level is then used by the hair cell to generate an electrical signal which it passes to the auditory nerve and onwards to the brain.
But once the hair cell has sent this signal, it needs to be able to do so over and over again. The stria vascularis pumps potassium back into the endolymph, restoring the difference in potassium levels, and ‘recharging’ the battery. The hair cell can now send more signals to the brain but this whole process takes a lot of energy. This is largely obtained from the blood supply to the inner ear – and if that supply gets damaged, then our hearing is at risk.
Understanding more about the inner ear’s blood supply
Normally, blood vessels supply fluid, oxygen and energy to different parts of the body, allowing them to work properly across our lifetime. This is as true of the ear as any other organ. Healthy blood vessels can dilate (relax and widen) or constrict (tighten and narrow) to change how much blood reaches a particular part of the body. This allows us to divert more blood to parts of the body that need it the most – usually those using the most energy (such as leg muscles when we run).
As the inner ear is constantly using energy, blood supply to the ear is particularly important. Controlling this blood supply relies on how well individual cells in a blood vessel can contract and relax to change its width, as this determines how much blood reaches the inner ear. There is evidence that conditions such as diabetes, and, more generally, ageing, damage blood vessels and disrupt the control of blood flow to the ear, which may lead to hearing loss.
Despite this, we don’t know much about how blood supply to the inner ear is controlled, or how it’s affected by factors like diabetes or ageing. As a result, there are no clinically-available treatments that specifically affect blood supply to the inner ear.
That’s why we’re funding Dan Jagger, at the UCL Ear Institute, to study this in more detail. He’ll be working with Richard Rainbow, at the University of Liverpool, to investigate the different types of cells found in the blood vessels of the inner ear and find out how they work normally. The researchers will then test the effects of drugs that are known to affect blood supply in other organs of the body, looking for possible treatments to prevent hearing loss caused by problems with blood supply to the inner ear. Finally, they will study how diseases like diabetes, which damage blood vessels and blood supply throughout the body, might contribute to hearing loss.
What could this mean?
Their work will give us a better understanding of how the blood supply to the inner ear works, and how changes to this may affect our hearing. Even better, it could also identify potential drugs to better control the inner ear’s blood supply when it’s failing – and so prevent hearing loss.