Other viral infections — including rubella, measles, and cytomegalovirus — are known to cause hearing problems, but research into possible links between COVID-19 and hearing issues remains at an early stage.
The authors emphasize that these figures may overestimate the true scale of the problem.
This is because the reports that they reviewed do not always make it clear whether the symptoms were new or whether they were preexisting symptoms that had temporarily become worse.
In addition, most of the studies were based on medical records or questionnaires in which patients reported their own symptoms, rather than the appropriate hearing tests.
“There is an urgent need for a carefully conducted clinical and diagnostic study to understand the long-term effects of COVID-19 on the auditory system,” says senior author Kevin Munro, professor of audiology at ManCAD.
“Though this review provides further evidence for an association, the studies we looked at were of varying quality, so more work needs to be done,” he adds.
Prof. Munro is leading a year-long study in the U.K. that will compare control patients with patients who recovered after being hospitalized with COVID-19.
He and his colleagues hope to get a more accurate picture of the incidence and severity of COVID-related hearing issues, as well as which parts of the auditory system are affected.
They will also investigate possible associations between audiovestibular problems and other factors, such as lifestyle, other medical conditions, and the treatments the patients received in intensive care.
Prof. Munro explains:
“Over the last few months, I have received numerous emails from people who reported a change in their hearing, or tinnitus after having COVID-19. While this is alarming, caution is required, as it is unclear if changes to hearing are directly attributed to COVID-19 or to other factors, such as treatments to deliver urgent care.”
The inner ear contains the sound-sensing spiral cavity of the cochlea, and the fluid-filled semicircular canals, which are involved in balance.
In their paper, the researchers note some of the proposed explanations for damage to the inner ear that occurred when people had COVID-19.
direct viral infections of the inner ear or the nerve that carries sensory signals from it
autoimmune attack by antibodies or immune cells, or damage caused by excessive production of cytokines, which are immune signaling molecules that cause inflammation
blood clots that block the blood supply to the cochlea or semicircular canals, depriving them of oxygen
The authors conclude their paper by urging caution when interpreting their estimates of the prevalence of hearing problems associated with COVID-19.
They write that some of the problems may have been preexisting, pointing out that around 11% of adult population in the U.K. already have some hearing disability and that 17% have tinnitus.
Moreover, they say that in around half of the studies reviewed, they were unable to determine whether the authors were reporting a new symptom or a worsening of an existing one.
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