SARS-CoV-2 the virus that keeps on giving is now believed to affect our hearing. However, so far the overall percentage of Covid-19 patients who have experienced ear problems is not known yet.
Many Covid-19 patients have reported symptoms affecting their ears, including but not limited to hearing loss and tinnitus. Other patients have also reported dizziness and balance problems, suggesting that the virus may also be able to infect the inner ear. A new study by researchers from MIT and Massachusetts Eye and Ear showed that the virus can actually infect inner ear cells, including hair cells, which when damaged can affect both hearing and balance. Their findings were published on October 29th in the journal Communications Medicine.
New cellular models of the human inner ear
To prove their point the researchers used new cellular models of the human inner ear that they developed themselves, as well as tissues from the human inner ear. “Having the models is the first step, and this work now opens up an opportunity to work not only with SARS-CoV-2 but also with other viruses that affect hearing,” notes Lee Gehrke, who co-led the study. Prior to the pandemic, the researchers began working on a project to develop cellular models to study inner ear infections in humans. Viruses such as cytomegalovirus, hepatitis virus, and mumps virus can all cause deafness. So far it is not well understood how the viruses do this.
These models allowed researchers to study the effects of SARS-CoV-2 on the ears of infected patients that complained of hearing loss, dizziness, and tinnitus. In human inner ear samples and cell-based stem cell models, the researchers found that certain types of cells-in this case the hair cells and the Schwann cells both expressed the proteins needed for SARS-CoV-2 to enter the cells. These proteins include the ACE2 receptor, which is found on the cell surface, and also two enzymes (furin and type 2 transmembrane serine protease). These two enzymes help the virus fuse with the host cell.
More attention to hearing problems after COVID infection needed
In their study, the researchers were indeed able to show that the virus can infect the inner ear, particularly the hair cells and, to a lesser extent, the Schwann cells. This specifically affects the vestibular hair cells, which are involved in sensing head movements and maintaining balance. Cochlear hair cells, which are involved in hearing, are much more difficult to obtain or generate in a cellular model. However, researchers have shown in mouse models that they also contain proteins such as the ACE2 receptor which allow SARS-CoV-2 to enter.
While this study suggests that Covid-19 may cause hearing and balance problems, the overall percentage of patients who experienced hearing problems is still unknown. “Initially, this was due to the fact that screening tests were not readily available to patients who received a positive diagnosis, and when patients had more life-threatening complications, they also paid little attention to their hearing,” concluded Konstantina Stankovic, who co-lead the study. “We don’t yet know the frequency, but our findings really call for increased attention to audio-vestibular symptoms in people exposed to the virus.
The virus can enter the ears from different places
The authors speculate that possible pathways for the virus to enter the ears include the Eustachian tube, which connects the nose to the middle ear. The virus can also escape from the nose through small openings around the olfactory nerves,” says Konstantina Stankovic. This will allow it to enter the brain and infect cranial nerves, including the one that connects to the inner ear.” For this reason, it is extremely important to get vaccinated to at least give a headstart to the immune system so that in case of infection it would be less likely to end up with long-term sequelae.