Doctors explain how Coronavirus causes hearing loss
Many patients infected with the coronavirus have complained of hearing loss, ringing in the ears, dizziness and balance problems. These are all indications that SARS-CoV-2 can infect the inner ear.
Coronavirus and hearing loss
It is no secret that viral infections are a frequent cause of hearing loss as well as dizziness and balance problems, i.e. vestibular dysfunction. Herpes viruses, paramyxoviruses, polio, hepatitis, HIV, rubella, and influenza can lead to these problems: they damage the inner ear, including the coronary organ and the vestibular nerve, causing immune-mediated damage and inflammation, including neuroinflammation. In addition, viruses can infect the middle ear and provoke conductive hearing loss due to the accumulation of effusion-exudative fluid. Although coronaviruses are also thought to cause middle ear infections, their role in this has not yet been systematically studied.
Among the main symptoms and effects of coronavirus are loss of sense of smell (anosmia) and loss of taste sensitivity (aguesia). More recently, patients have also mentioned hearing loss, but such reports were based on their own observations. The authors of the new study – otolaryngologists from Stanford University School of Medicine in California and Harvard Medical School in Boston (USA) – presented the most comprehensive description to date of hearing and vestibular disorders caused by infection with SARS-CoV-2 coronavirus, both at the time of the disease and two to four months later. Their work is published in the journal Communications Medicine.
“To find out whether such symptoms can be explained by direct infection of the auditory system and vestibular apparatus, we examined gene and protein expression associated with SARS-CoV-2 penetration into human and mouse inner ear tissue. And to understand the pathogenesis of Covid-19 audiovestibular symptoms, we extracted otic prosensory cells (precursors of hair cells, receptors of the auditory system and vestibular apparatus in vertebrates. – Ed. note) and Schwann cell precursors (are closely related to axons. – Ed. note),” the scientists wrote. They managed to collect a total of six samples of fresh inner ear tissue obtained during labyrinthectomy and translabyrinth resection of vestibular schwannoma.
Hairy and schwannoma cells were found to express proteins necessary for SARS-CoV-2 entry, including the ACE2 receptor that lines the cell surface, as well as furin and the transmembrane serine protease 2 (TMPRSS2), which help the pathogen bind to the host cell. It was hair cells and, to a lesser extent, Schwann cells that were susceptible to infection. Other types were not susceptible to the virus.
The results of the experiments with cell models and tissues were then compared with observations of six men and four women, aged 22-72 years, who developed hearing problems, tinnitus or ringing in the ears and persistent dizziness within three weeks of a Covid-19 diagnosis confirmed by PCR test or antibody analysis. Most patients had profound (over 90 decibels) or severe (71-90 decibels) hearing loss, except for two individuals (56-70 decibels and 24-35 decibels).
The symptoms were treated with prednisone or a combination of prednisone and dexamethasone, and after such therapy, patients had complete or moderate hearing recovery. None of these people’s vestibular dysfunction due to Covid-19 became chronic. According to scientists, the virus enters the ears through the Eustachian tube, the canal that connects the middle ear cavity to the nasopharynx. It can also “exit” through small holes surrounding the olfactory nerves: eventually, SARS-CoV-2 enters the brain space and infects the cranial nerves.
Although the new study confirms that coronavirus disease can lead to hearing and vestibular problems, it is not yet clear how common such symptoms are. The scientists plan to use the cellular models they have developed to test possible treatment strategies for inner ear infections triggered by Covid-19 and other viruses.