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Vestibular Schwannoma
Published in R James A England, Eamon Shamil, Rajeev Mathew, Manohar Bance, Pavol Surda, Jemy Jose, Omar Hilmi, Adam J Donne, Scott-Brown's Essential Otorhinolaryngology, 2022
Mathieu Trudel, Simon K. W. Lloyd
Optimising hearing rehabilitation in NF2 is critical as bilateral hearing loss has a very significant impact on quality of life. In those who develop profound hearing loss in an ear with a stable tumour (whether through its natural behaviour or through treatment with radiotherapy/bevacizumab) there is now good evidence for the effectiveness of cochlear implantation. Similarly, there is good evidence that, in selected cases, tumour removal with preservation of the cochlear nerve together with cochlear implantation offers reasonable hearing outcomes. Outcomes are, however, less good in NF2 than the average non-NF2 cochlear implant user, with untreated ears having the best outcome and those having had radiotherapy or nerve-preserving surgery having poorer outcomes. For most patients having surgery, cochlear nerve preservation is not possible and the only way to provide some audition is through auditory brainstem implantation (ABI). These provide environmental sound awareness and act as an aid to lipreading in most cases. Only 10% of recipients achieve open-set speech discrimination. Both cochlear implantation and ABI have a non-user rate of around 20% in NF2.
Central Nervous System (Brain, Brainstem, Spinal Cord), Ears, Ocular Toxicity
Published in Tiziana Rancati, Claudio Fiorino, Modelling Radiotherapy Side Effects, 2019
Federica Palorini, Anna Cavallo, Letizia Ferella, Ester Orlandi
In a series of 53 patients treated with SRS, Brown et al. (2011) found a strong correlation between the mean percentage of cochlear volume receiving >5.3 Gy and a ΔBCT >20 dB (p = 0.02). A higher tumor coverage would also adversely affect hearing outcome, maybe due to the higher integral dose to the cochlear nerve segment within the internal auditory canal. But the cochlear nerve is hardly recognizable as a discreet structure on MRI scans. Therefore, both Brown et al. and Jacob et al. strongly recommended that tumor marginal dose should not be reduced to spare cochlear structures, as it could decrease long-term tumor control.
Electrophysiology and Monitoring
Published in John C Watkinson, Raymond W Clarke, Louise Jayne Clark, Adam J Donne, R James A England, Hisham M Mehanna, Gerald William McGarry, Sean Carrie, Basic Sciences Endocrine Surgery Rhinology, 2018
Patrick R. Axon, Bruno M.R. Kenway
The aim of monitoring the status of the auditory pathway during cerebellopontine angle (CPA) surgery is the prevention of avoidable post-operative hearing deficit. The achievement of this laudable aim is fraught with difficulty and hearing preservation rates in comparison with facial nerve outcomes remain poor.35 The cochlear nerve is sensitive to mechanical manipulation and easily damaged, as the intra-cranial section of the nerve is sheathed in central myelin and has no perineurium.36 Additionally, the cochlear nerve is intimately involved with pathologies, such as vestibular schwannoma, and hence at very considerable risk during the surgical removal of such lesions, even when every care is taken to preserve the nerve anatomically. The basic principle of intra-operative monitoring is that changes in recordable neuroelectric potentials occur whilst the injury is still reversible and before permanent deficits result.37 Recent research38 has demonstrated that this principle holds for changes in auditory brainstem responses (ABR) wave V amplitude (and to a lesser extent latency) in rat auditory nerves manipulated in a fashion analogous to that undergone in humans during vestibular schwannoma removal. It is therefore theoretically feasible that monitoring auditory function may inform the surgeon of reversible injury to the cochlear nerve.
Short-term overstimulation affects peripheral but not central excitability in an animal model of cochlear implantation
Published in Cochlear Implants International, 2023
Susanne Schwitzer, Moritz Gröschel, Horst Hessel, Arne Ernst, Dietmar Basta
It is remarkable, that the excitability was not affected by an over-threshold stimulation of approximately 40 CL. The small increase of the threshold level of the CON and the LOS group could be possibly induced by the insertion trauma. This could temporarily impair cochlear nerve responses (Pfingst et al., 2015). This was also observable in the MOS group after four hours of stimulation. The value of the HOS group is impressively high compared to clinical practice of CI fitting where the C-level is normally set close to the eCAP-threshold (as in the ‘LOS’-group). Higher levels could only be reached by ‘behavioral’ adjustments during the fitting procedure due to neuronal adaptation. A human study showed that the stimulation intensity induced loudness adaptation in electrical hearing (Tang et al., 2006). Other studies have also demonstrated that in carefully fitted implantees, no change could be observed for eCAP-thresholds over time (Coco et al., 2007 (study with cats); Telmesani and Said, 2016 (study with children)).
Sound localisation of low- and high-frequency sounds in cochlear implant users with single-sided deafness
Published in International Journal of Audiology, 2023
J. Seebacher, A. Franke-Trieger, V. Weichbold, O. Galvan, J. Schmutzhard, P. Zorowka, K. Stephan
The reason for the improvement in sound localisation when patients with SSD are given a CI in their deaf ear is not yet fully understood. The acoustic information is delivered to the auditory system via two distinctly different pathways: in one ear via normal (acoustic) hearing and on the other side via direct electrical stimulation of the cochlear nerve. Two different inputs (acoustic and electric signals) have to be combined along the auditory pathway in order to evaluate ITDs and ILDs of incident sound. Coding of these cues relevant for localisation of sounds is not primarily considered in today’s CI coding strategies. Originally, the primary focus of development was to improve speech perception, followed by the sound quality. Typically, signal processing in CI is optimised to encode the envelope of the acoustic signal. Fundamental observations were made by Shannon and colleagues, who found that envelope coding of a few tonotopically arrayed frequency bands was already sufficient for speech recognition in patients with CI (Dillon et al. 2016). Current processing strategies further attempt to encode the low-frequency acoustic fine structure information of sound, which could also be used for localisation issues.
Chameleons, red herrings, and false localizing signs in neurocritical care
Published in British Journal of Neurosurgery, 2022
Boyi Li, Tolga Sursal, Christian Bowers, Chad Cole, Chirag Gandhi, Meic Schmidt, Stephan Mayer, Fawaz Al-Mufti
Damage to the vestibulo-cochlear nerve can cause auditory neuropathy, as the propagation of auditory information downstream of the mechanical and electrical conduction is disrupted.36 Notably, this damage leading to hearing loss can be caused by temporal bone fracture, trauma even in the absence of fracture, transient demyelination, and infarction due to stroke.37–39 As a FLS, hearing loss, in addition to the other otologic symptoms of pulsatile tinnitus and aural fullness, has been reported as a complication of IIH.3,40 The hearing loss is low-frequency and may also present with vertigo.40 About 1/3 of patients with IIH demonstrate prolonged interpeak latencies in auditory brainstem-evoked response, suggesting that the pathophysiology of this hearing loss is the ICP causing stretching and/or compression of the vestibulocochlear nerve and brainstem.40 Detection of this type of low-frequency sensorineural hearing loss may be especially helpful in diagnosing IIH, as it can be detected even when other symptoms are absent.41 Thus, audiological evaluations play an important role in the diagnosis of IIH, which should always be suspected in the presence of neurotological symptoms such as tinnitus, vertigo, aural fullness, and hearing disturbances.41