Cochlear Involvement In Asymptomatic SARS-Cov-2 Infection

Original Article: M.W.M. Mustafa,  Audiological profile of asymptomatic Covid-19 PCR-positive cases. Am J Otolaryngol.

Author of summary: Silvia Dell’Anna; Reviewer: Giulia Poggi

SARS-CoV-2 infection might impair cochlear function even in asymptomatic cases; audiometric and otoacoustic findings could be relevant in preventing this potential damage to go undetected.

The ability to induce hearing loss (HL) has been recognized for several viruses: they typically cause sensorineural HL, either through direct inner ear or brainstem damage, or through immunopathological mechanisms; however, conductive and mixed HL are also possible. Measles can directly damage various cochlear components, such as hair cells in the organ of Corti, and has also been linked to the development of otosclerosis; HIV can lead to conductive HL through increased susceptibility to bacterial and fungal infections, as a result of the virus- induced immunosuppression. However, if SARS-CoV-2 is capable of affecting auditory function has not been established yet.

This study recruited 20 SARS-CoV-2- PCR- positive cases showing no symptoms of COVID-19; additional criteria were: age range between 20 and 50 years, to rule out any age- related hearing issues, and no previous history of HL or of known causes of HL. Additional 20 healthy subjects with normal hearing and with no history of know causes of HL formed the control group.

All study participants underwent: thorough history taking; otological examination; tympanometry (impedance audiometry) to exclude impaired middle ear function; pure- tone audiometric testing (PTA), consisting of measurement of air conduction and bone conduction audiometric   thresholds (over the 250-8000 Hz range of  frequencies for air conduction and 250-4000 Hz range for bone conduction), and recording of  TEOAEs (Transient- evoked otoacoustic emissions).

Although no statistically significant difference (p > 0.05, paired t-test) was found between the two groups in pure-tone audiometry for all frequencies and mid frequencies (250, 500, 750, 1000, 1500, 2000 and 3000 Hz), audiometric thresholds were significantly higher (p< 0.05, paired t-test) in the test group relative to the control group when measured at high octaves (frequencies of 4000, 6000 and 8000 Hz).  Moreover, TEOAEs had a significantly lower average amplitude in the study group relative to controls (p< 0.001, paired t-test).

TEOAEs, acoustic emissions elicited by transient stimuli delivered to the external auditory canal, are a phenomenon ascribed to the amplification function of outer hair cells; hence their reduced amplitude, combined with the presence of deterioration of audiometric thresholds at high frequencies, can be attributed to the occurrence of a virus-mediated damage to these specialized cells.

Audiological testing and otoacoustic emissions could therefore be useful in asymptomatic COVID-19 cases for  the detection of subtle cochlear function compromise,  that would otherwise be clinically unrecognized, Further studies are needed to determine the mechanisms of SARS-CoV-2- induced impairment of inner ear function.

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