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A recently published study has discovered that COVID-19 may infect the inner ear. The study provides a promising explanation for why some COVID-19 patients have hearing loss post-infection. 

SARS-CoV-2 may target the cochlea

The study, published in Nature’s Communications Medicine journal, studied why some people experience hearing loss, ringing of the ears – or “tinnitus”, or dizziness during or post-COVID-19 infection.1 They examined ten people with COVID-19 and hearing loss-related symptoms. Six of the ten completed a hearing test that confirms the presence of tiny hair cells that line the organ of Corti – the organ for hearing – located in a structure called the cochlea. All six patients showed evidence of loss of function in these hair cells, suggesting that these cells may be affected by COVID-19 infection. 

Inner ear cells have enzymes that allow COVID-19 entry

Researchers analyzed a human-derived laboratory model to learn how the virus might infect the inner ear. They took inner ear tissue samples from non-COVID-19 patients. These samples are normally very hard to obtain clinically and were only obtained because the individual was having ear surgery.1 Researchers also created their own inner ear models by converting skin cells into stem cells that later developed into the different types of cells found in the inner ear. 

Both cochlea hair cells and Schwann cells – cells that surround nerves and produce a protective nerve coating called myelin – produce proteins that facilitate SARS-CoV-2 viral entry into human cells. A key protein found in these cells was angiotensin-converting enzyme two – ACE2.1 This protein receptor is found on the surface of cells. 

Other proteins found in these hair and Schwann cells included furin cofactors and transmembrane protease serine two (TMPRSS2). Both proteins help the COVID-19 virus merge with the cell membrane of human cells to facilitate entry into human cells.2 

The presence of ACE2, furin, and TMPRSS2 provides a possible explanation for the mechanical entry of SARS-CoV-2 into ear cells. These proteins help identify the link between COVID-19 infection and hearing loss.

How can SARS-CoV-2 enter the ear?

According to co-author of the study Konstantina Stankovic, the virus may enter the inner ear through small spaces in the nose that lead to nerves innervating the inner ear.2 The virus may also enter via a tube connecting the outer and inner ear called the Eustachian tube. Konstantina is the Chair of the Department of Otolaryngology – Head and Neck Surgery at Stanford University. 

Unfortunately, treatments for COVID-19-related manifestations still remain unknown and not clinically significant. Researchers are hopeful that this study will provide a strong base for developing models to analyze SARS-CoV-2 viral entry into human cells and developing treatments for COVID-19 infection and hearing loss.1 

References

  1. Jeong, M. et al. (2021). Direct SARS-CoV-2 infection of the human inner ear may underlie COVID-19-associated audiovestibular dysfunction. Communications Medicine Nature; 1(44). Doi: 10.1038/s43856-021-00044-w.
  2. McDonnell, S. (2021). Study finds the SARS-CoV-2 virus can infect the inner ear. EurekAlert! Accessed on Nov. 4, 2021. Retrieved from https://www.eurekalert.org/news-releases/933025.
  3. Image by Anemone123 from Pixabay 
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