What is hearing loss reversible
Unnoticed Hearing Loss: Hidden Damage To Hearing
Compared to the 1980s, there are now much more sophisticated methods with which we can examine the molecular structure of the synapses involved. Antibodies coupled to fluorescent dyes mark proteins on both sides of the synaptic gap between the hair cell and nerve fiber. This allows us to easily count the synapses under the light microscope. According to our data, just a few days after exposure to noise, when the hearing threshold had already returned to normal, half of all synapses between hair cells and nerve fibers had disappeared; and these no longer regenerated. After a few months, the remaining parts of the damaged auditory neurons were also missing, namely their cell bodies and the long processes (axons) that pull into the brain stem. Two years later, only about half of the auditory neurons were left. With the loss of synaptic connections to the hair cells, the affected neurons had also lost their function; they no longer responded to tones of any intensity and thus became superfluous.
Is an epidemic of hearing loss rolling towards us?
In recent years we have been able to detect the loss of synapses caused by noise not only in mice, guinea pigs and chinchillas, but also in the tissues of deceased people. In all cases, the contact points begin to dwindle before the loss of hair cells increases the hearing threshold. It is now a widely accepted hypothesis that such damage to the auditory nerve initially causes hidden hearing impairment, which contributes significantly to the generally decreasing hearing ability with age. Hearing researchers and medical professionals are developing new test methods to investigate the extent of the problem and to clarify whether our noisy lifestyles could lead to an epidemic of hearing damage in people of all ages.
So far, the audio audiogram has been the gold standard for hearing tests. It essentially measures the hearing threshold at different frequencies and shows extremely sensitive damage to the hair cells of the cochlea. However, impairment of the auditory nerve fibers can hardly be detected with this method. According to our research results, the nerve damage caused by hidden hearing loss does not make it difficult to perceive sounds, but rather the understanding of language and the interpretation of other complex noise patterns. They could even be the main reason why many older people hear when others speak, but often fail to understand what they are saying.
People with similar audio audiogram scores perform very differently when it comes to recognizing speech against background noise - audiologists have known this for a long time. Such tests determine the number of words understood as the noise level increases. The experts used to explain the deviating results with individually different signal processing in the brain. However, our research suggests that it mainly depends on how many of the auditory nerve fibers are still alive.
Hidden hearing loss could also explain other common hearing disorders such as tinnitus and hyperacusis (excessive sensitivity to noise). People often have a normal audiogram. Therefore, researchers and doctors assumed for decades that the causes of these disorders also lay in the brain, while we again believe that the auditory nerve could be impaired.
This raises new questions about the risks of frequent high sound levels at concerts, in music clubs and when listening to music with headphones. Noise-related hearing loss is a well-known problem with professional musicians, including in classical music. In contrast, epidemiological studies of people who only occasionally attend concerts have so far not shown any relevant effect on the audiogram. The guidelines for minimizing noise-related harm in the workplace are based on the assumption that hearing will fully recover when the hearing threshold returns to normal after exposure to noise. But according to the new findings, this is not the case. The current noise protection regulations may therefore not be sufficient for many workplaces to reliably prevent damage to the auditory nerve and the resulting hearing loss.
There is therefore a need for improved examination methods that reveal such neuronal impairments at an early stage. Of course, counting the synapses in the inner ear of the deceased is not enough. A very promising approach is based on the measurement of electrical activity in auditory neurons, the so-called auditory brainstem reaction (abbreviation ABR, for auditory brainstem response). With this brainstem audiometry, electrodes attached to the scalp register the neural response to sound impulses of different frequencies and volumes, which also works for people who are sleeping. So far, the result has been interpreted according to the all-or-nothing principle: if a reaction occurred, the hearing was considered normal, otherwise it was considered damaged.
In animal experiments, however, we observed that the amplitude of the ABR at high sound levels provides further information. It grows in proportion to the number of auditory nerve fibers that have a functioning connection to the inner hair cells. A recent epidemiological study used a variant of the ABR test in a group of British students with normal audiograms. Result: Those who had already been exposed to the high sound levels of music clubs and concert events had lower ABR amplitudes.
Can the new findings be implemented in therapies for hidden hearing loss? We are currently trying to reverse the noise-induced degeneration of the auditory nerve fibers in laboratory experiments. To do this, we treat the surviving neurons with substances that allow new nerve fibers to sprout. These could then restore the lost connections to the inner hair cells. Because: While exposure to noise immediately destroys the synapses, the other parts of the neurons - cell bodies and axons - die more slowly. This makes us optimistic that normal function can be restored for many of those affected. The first animal experiments in which we introduced nerve growth-promoting substances called neurotrophins directly into the inner ear showed encouraging results.
So maybe the hidden hearing loss can soon be treated this way. A gel injected through the eardrum would gradually release growth factors that trigger the formation of new auditory nerve synapses over the course of months to years. Such treatment should, for example, be used immediately after a pop trauma, such as the bomb explosion at the finish line of the Boston marathon in 2013, where more than 100 spectators suffered severe hearing damage. One day, an otologist could even introduce such drugs directly into the cochlea in a minimally invasive way. This means that hearing damage caused by noise can be treated with as easy a treatment as ophthalmologists are already correcting myopia using laser surgery on the cornea.
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