Discussion
Some viral infections are known to cause hearing loss, most of them typically damaging inner ear cells [21]; however, some viruses can also affect the auditory brainstem [22].
Coronavirus-related otoneurological symptoms, such as tinnitus and balance disorders, have been described so far [10, 23-29], as neurotrophic and neuroinvasive capabilities are typical of some coronaviruses [30]. Previous studies on other coronavirus infections showed a brain involvement, focusing attention on the possible neuro-auditory impairment following this infection [31, 32]. The effects of SARS-CoV-2 on the neuronal tissue could be due to a direct infection of the central nervous system or related to a vascular damage caused by vasculitis or vasculopathy, similarly to the mechanism described for varicella zoster virus (VZV) and human immunodeficiency virus (HIV) [31, 32]. The latter could be supported by the evidence that COVID-19 patients present direct signs of hypercoagulability [33].
See more: Covid 19 equilibrium
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Hearing alterations and balance disorders can be dependent on vascular damage because the inner ear structures are particularly susceptible to ischemia due to their characteristics of terminal vasculature and high-energy requirement. Both primary and secondary vasculitis are commonly associated to audiovestibular symptoms, and primary cardiovascular disease can presents episodes of dizziness [34, 35]. Several viral infections, such as hepatitis B and hepatitis C, can be associated with vasculitis and published evidence indicates that in COVID-19 vasculitis is one of the clinical manifestation [36]. Additionally, benign paroxysmal positional vertigo has been clinically reported in COVID-19 patients, although data have not been published yet. It could be hypothesized that prolonged hospitalization and bed rest may be responsible for otolith detachment.
Despite the growing amount of scientific literature on COVID-19, studies that correlate audiovestibular symptoms to SARS-CoV-2 infection are still limited and further investigation is necessary for a better estimate of their incidence [10, 23-29]. Furthermore, the incidence of audiovestibular symptoms among patients infected with other types of coronavirus (i.e., MERS and SARS) is debated [28].
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The reported prevalence of audiovestibular symptoms in COVID-19 patients is very low. It is still unclear if this indicates that these symptoms are rare or if the attention of the researchers was more focused on potentially life-threatening symptoms. In the medical literature, tinnitus and balance disorder in COVID-19 patients have been reported in a few studies without identifying common clinical characteristics. Moreover, these reports do not describe in details the relationship with COVID-19 therapies, particularly with chloroquine or hydroxychloroquine, that are well-known causes of inner ear damage [16-18, 37]. Our results, although preliminary, suggest that subjective otoneurological symptoms such as tinnitus and balance disorders may be present in a significant percentage of COVID-19 patients, and should, therefore, be further investigated. It should be taken into account the role of the central nervous system in the onset of investigated symptoms, and especially for equilibrium disorders for which it is impossible to discriminate through our questionnaire the peripheral or central origin.
Limits of this study
This preliminary study presents several limitations that should be considered. The first is the absence of COVID-19 severity evaluation among patients included in the study. Therefore, the severity of the disease cannot be correlated to the presence of the investigated symptoms, as well as the effects of oxygen therapy on inner ear circulation that may be present in patients that underwent this type of support during treatment. The second is the absence of clinical evaluation of these patients (otoscopy, audiovestibular examination); however, the study investigates specifically the newly onset of these symptoms and exclude patients with pre-existing audiovestibular conditions. The third is that the study has been designed as an online reporting tool of subjective symptoms; such reporting may have been influenced by other factors that have not been investigated such as the psychological status of the patient.
