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[This corrects the article DOI: 10.3389/fnins.2022.850245.].
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Neuroplasticity following deafness has been widely demonstrated in both humans and animals, but the anatomical substrate of these changes is not yet clear in human brain. However, it is of high importance since hearing loss is a growing problem due to aging population. Moreover, knowing these brain changes could help to understand some disappointing results with cochlear implant, and therefore could improve hearing rehabilitation. A systematic review and a coordinate-based meta-analysis were realized about the morphological brain changes highlighted by MRI in severe to profound hearing loss, congenital and acquired before or after language onset. 25 papers were included in our review, concerning more than 400 deaf subjects, most of them presenting prelingual deafness. The most consistent finding is a volumetric decrease in gray matter around bilateral auditory cortex. This change was confirmed by the coordinate-based meta-analysis which shows three converging clusters in this region. The visual areas of deaf children is also significantly impacted, with a decrease of the volume of both gray and white matters. Finally, deafness is responsible of a gray matter increase within the cerebellum, especially at the right side. These results are largely discussed and compared with those from deaf animal models and blind humans, which demonstrate for example a much more consistent gray matter decrease along their respective primary sensory pathway. In human deafness, a lot of other factors than deafness could interact on the brain plasticity. One of the most important is the use of sign language and its age of acquisition, which induce among others changes within the hand motor region and the visual cortex. But other confounding factors exist which have been too little considered in the current literature, such as the etiology of the hearing impairment, the speech-reading ability, the hearing aid use, the frequent associated vestibular dysfunction or neurocognitive impairment. Another important weakness highlighted by this review concern the lack of papers about postlingual deafness, whereas it represents most of the deaf population. Further studies are needed to better understand these issues, and finally try to improve deafness rehabilitation.
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Auditory neuropathy spectrum disorder (ANSD) refers to a range of hearing impairments characterized by deteriorated speech perception, despite relatively preserved pure-tone detection thresholds. Affected individuals usually present with abnormal auditory brainstem responses (ABRs), but normal otoacoustic emissions (OAEs). These electrophysiological characteristics have led to the hypothesis that ANSD may be caused by various dysfunctions at the cochlear inner hair cell (IHC) and spiral ganglion neuron (SGN) levels, while the activity of outer hair cells (OHCs) is preserved, resulting in discrepancies between pure-tone and speech comprehension thresholds. The exact prevalence of ANSD remains unknown; clinical findings show a large variability among subjects with hearing impairment ranging from mild to profound hearing loss. A wide range of prenatal and postnatal etiologies have been proposed. The study of genetics and of the implicated sites of lesion correlated with clinical findings have also led to a better understanding of the molecular mechanisms underlying the various forms of ANSD, and may guide clinicians in better screening, assessment and treatment of ANSD patients. Besides OAEs and ABRs, audiological assessment includes stapedial reflex measurements, supraliminal psychoacoustic tests, electrocochleography (ECochG), auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs). Hearing aids are indicated in the treatment of ANSD with mild to moderate hearing loss, whereas cochlear implantation is the first choice of treatment in case of profound hearing loss, especially in case of IHC presynaptic disorders, or in case of poor auditory outcomes with conventional hearing aids.
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OBJECTIVE: After 20 years of experience with different types of middle ear implants, we analyzed our database about the Vibrant Soundbridge (VSB) to know the rate of complications, the effect on the residual hearing and the audiometric gain in our center. METHOD: The study was retrospective and included all VSB implants bound to the long process of the incus in our tertiary medical center between january 1999 and february 2015. We observed the effect of surgery on residual hearing by comparing bone and air conduction thresholds before and after implantation. The functional results of the implant were quantified by measuring, at several post-operative intervals, the thresholds with the VSB in pure tone audiometry and speech audiometry, in quiet and in noise. RESULTS: 53 VSB were implanted in 46 patients aged between 22 and 81 years old (average 53.9). 48 patients (90%) suffered from a sensorineural hearing loss, and 5 patients from a mixed hearing loss due to an otosclerosis (but only 3 of them have undergone stapedotomy). There were no major complications (e.g. facial palsy, dead ear or postoperative infection). The placement of the implant created an insignificant deterioration of the air conduction thresholds (5,6 dB HL) and bone conduction thresholds (2.2 dB HL) at 6 weeks post-implantation. The bone conduction thresholds increased by 4.7 dB HL 2.5 years after surgery in comparison with the preoperative results, which is also considered clinically insignificant. With the implant turned on, the pure tone audiometry thresholds in open field, in quiet, were significantly improved (gain of 13.9 dB on average on frequencies from 250 to 8000 Hz), particularly at frequencies of 1000, 2000 and 4000 Hz as the average gain on these frequencies amounted to 19.4 dB. The speech intelligibility in a cocktail party noise was also improved by 18.3% on average at 6, 52 and 104 weeks post-implantation. CONCLUSION: The Vibrant Soundbridge with the electromagnetic vibrator fixed to the long process of the incus is a safe active middle ear implant with no major complications; it has no significant impact on the residual hearing. The VSB is particularly suitable for patients who are unable to wear conventional hearing aids due to anatomical or infectious problems in the external ear canal, or in case of poor audiometric results with conventional hearing aids. The VSB brings significant hearing gain, as it is particularly efficient in frequencies for the speech range and higher, resulting especially in better speech intelligibility in noisy environments.
