DISCUSSION
SSNHL is mostly defined as a sensorineural hearing loss of at least 30 decibels (dB) at three consecutive frequencies, occurring within a timeframe of 72 hours. It can affect any age group, but mostly occurs in patients aged 40-60 years. In the majority of cases, adequate investigation does not reveal any underlying cause (e.g. vestibular schwannoma, stroke, malignancy, exposure to noise or ototoxic medication), so that the SSNHL is considered as idiopathic.(9,10) However, multiple etiologies have been proposed.
First, SSNHL has often been associated to viral infection. Serological techniques such as PCR and immunoglobulin detection have previously been used to confirm viral load in patients with SSNHL. However, a direct causality between acute viral infection and SSNHL remains uncertain since in vivo sampling of inner ear fluid is potentially harmful. Second, vascular impairment is suggested as a cause of SSNHL. Both cardiovascular risk factors and virally induced hypercoagulability or inflammatory edema can theoretically lead to cochlear ischemia with subsequent hearing loss.(10)
Thus, how about the specific SARS-CoV-2 infection and occurrence of hearing loss? Interestingly, the audiological profile of patients with asymptomatic SARS-CoV-2 infection was examined, showing significantly worse high frequency pure-tone threshold amplitudes and transient evoked oto-acoustic emissions in these patients compared to non-infected subjects. This finding suggests a damaging effect on cochlear outer hair cell function.(11)
It is known that SARS-CoV-2 affects cells through the angiotensin converting enzyme 2 (ACE2) receptor. This receptor is expressed in various cell types, such as lung respiratory epithelium and the sustentacular cells of the olfactory neuroepithelium. On the one hand, viral infection of the auditory nervous system could induce symptoms in a similar way as infection of the olfactory nervous system is believed to contribute to the pathophysiology of COVID-anosmia.(1,12) On the other hand, coagulopathy with high incidence of thrombotic events is well-described in COVID-19 patients.(13) ACE2 receptors are expressed in endothelial cells in various organs as well. The important findings of Varga et al. taught us that SARS-CoV-2 promotes the induction of endotheliitis. The team detected SARS-CoV-2 elements in endothelial cells, with evidence for the induction of endothelial dysfunction and cell apoptosis.(14) Hence, as hypothesized by Harenberg et al., hearing loss in COVID-19 patients could be a result of endothelial cell dysfunction with micro-thrombosis at the level of the auditory center in the temporal lobe, the auditory nerve or the cochlea.(12) In the case of Degen et al. however, the sudden hearing loss is described as a neurological complication of viral meningitis.(4)
COVID-19 is postulated to be the cause of SSNHL in our case. The risk of a false association, in particular because of the unclarified pathophysiology of SSNHL, should however be stated. Therefore, we agree with and applied the criteria of Satar et al. before proposing the relationship between SARS-CoV-2 infection and occurrence of SSNHL in our case.(15) First, the infection was laboratory-confirmed. Second, pathological ipsilateral Video Head Impulse Testing assigns for vestibular involvement, although our patient did not complain of associated symptoms. Next, other causes of SSNHL have been excluded anamnestically, serologically and by magnetic resonance imaging. Last, the temporal concordance between SARS-CoV-2 infection and onset of SSNHL remains unresolved. Four out of the nine cases did not report the onset of SSNHL. All five cases that did mention the onset of hearing loss, describe occurrence between at least 12 and 47 days after the first COVID-19 symptoms. We could therefore hypothesize that SSNHL in SARS-CoV-2 infection is more likely to appear in the downward phase of the infection, however, three patients were only able to notice their hearing loss when released from the Intensive Care Unit.
CONCLUSION This case report adds new well-founded evidence to the association between COVID-19 and hearing loss. We hope it will contribute to the prompt recognition of both SSNHL and SARS-CoV-2 infection. Based on this case and literature review, we advise to include PCR testing in the diagnostic work-up of patients presenting with SSNHL during the ongoing pandemic. Whether the sudden hearing loss is the result of endothelial cell dysfunction with micro-thrombosis in the cochlea or central auditory pathways remains unclear.