Chinese researchers investigated how different SARS-CoV-2 variants affect the inner ear and whether they can cause hearing loss. The researchers conducted a mouse experiment comparing four viral variants: the original strain, Delta, Omicron BA.1, and BA.2.
All four SARS-CoV-2 variants were capable of invading the inner ear and damaging its structures. The most pronounced pathological changes affected the spiral ganglion neurons – cells that transmit auditory signals from the cochlea to the auditory nerve. Infection with the original strain and the Delta variant caused significantly more severe damage than infection with the Omicron variants.
The coronavirus can invade the inner ear via the central nervous system. SARS-CoV-2 may spread from the olfactory bulb to the brain and subsequently reach the auditory nerve and spiral ganglion neurons.
Spiral ganglion neuron death is associated not so much with inflammation as with the direct action of the SARS-CoV-2 spike protein. The spike protein suppresses the mTOR signaling pathway – one of the key regulators of protein synthesis and autophagy.
Due to the suppression of autophagy, neurons are less efficient at clearing stress granules – transient aggregates of RNA and proteins that form during viral infection and other damaging stimuli. As a result, stress granules begin to accumulate, impair neuronal function, and ultimately lead to neuronal death.
These findings help explain why standard treatment for COVID-19-associated hearing loss is often insufficiently effective. Currently, such disorders are typically treated with glucocorticoids, which suppress inflammation in the inner ear. However, the primary cause of hearing loss in COVID-19 is direct viral damage to auditory neurons, rather than the inflammatory response alone.
Reference
SARS-CoV-2 directly infects the inner ear and causes hearing dysfunction
