Autoimmune-caused Sensorineuronal Hearing Loss

People usually relate hearing loss to a disability that occurs due to earwax buildup, aging, or damage in the inner ear. However, do you know that hearing loss can also occur due to diseases like autoimmune? 

The World Health Organization stated that hearing loss can be diagnosed if someone is unable to hear something with a threshold of 20 dB or above with both ears. Hearing loss can be mild, moderate, severe, or profound— affecting either one or both sides of the ears, leading to difficulties in hearing sounds or voices. Hearing loss can occur due to several factors, such as age, damage to the eardrums, earwax or fluid buildup, loud noises, tinnitus, ear infections, and many more.

There are four types of hearing loss: conductive hearing loss, sensorineuronal hearing loss, mixed hearing loss, and an auditory neuropathy spectrum disorder. Conductive hearing loss can occur due to the blockage of sounds in the outer or middle ear, which can be treated with surgery or medicine. On the other hand, sensorineuronal hearing loss is a condition when the inner ear or the hearing nerve fails to work correctly. Mixed hearing loss is a condition that includes conductive and sensorineuronal hearing loss. Lastly, auditory neuropathy spectrum disorder is caused by damage to the inner ear or hearing nerve, which leads to the irregular input of sound waves. 

Sensorineuronal hearing loss can be caused by a condition called autoimmune inner ear disease (AIED): an inflammation of the inner ear due to the immune system misidentifying healthy cells in the inner ear as pathogens, triggering a false immune attack. This could result in damage to the inner ear components such as the cochlea, labyrinth, and vestibula, which play a crucial role in both hearing and balance.

For a long time, the inner ear has been thought of as immune-privileged, in other words, it is protected from inflammation against pathogens. The belief stems from the presence of the blood-labyrinthine barrier in the inner ear, which is structurally similar to the one located on the brain. However, in the case of AIED, circulating antibodies misrecognized the internal ear antigens and evoked unwanted damage through antibody-dependent cell-mediated cytotoxicity or direct destruction by cytotoxic T-cells. In other cases, immune-complex deposition hinders blood flow and reduces oxygen levels, causing impairment to hair cells and spiral ganglions, which are naturally responsible for translating sound vibration. 

The damages consequently lead to abnormalities in hearing function, frequently followed by body balance problems. A progressive bilateral asymmetric sensorineuronal hearing loss develops over weeks to months and causes symptoms such as dizziness, tinnitus, and aural fullness in 50% of patients. Moreover, it might be accompanied by dizziness, fatigue, achy muscles, swelling and redness, and a low-grade fever. AIED patients are treated with injections of steroids (mainly prednisone) into the middle ear to reduce inflammation and prevent further damage. Additionally, immunosuppressants like azathioprine and cyclophosphamide assist in decreasing the activity of the immune system. In contrast, some patients might still be helped by hearing aids, and a cochlear implant may be considered for others who are unresponsive to drug therapy and have persistent hearing loss. Despite all of the available ones, further development of advanced treatments is still hindered since the exact etiology and pathophysiology of the disease remain unknown.

References

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