Longitudinal Analysis of the Absence of Intraoperative Neural Response Telemetry in Children using Cochlear Implants

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Severe or profound sensorineural hearing loss causes several deficiencies that are not only sensorial but also social and emotional. Characterized by a lack of perception and speech coding, individuals with this degree of hearing loss might need special resources to minimize the consequences of such a loss.

With the advancement of medicine and technology, the cochlear implant (CI) represents an alternative that allows access to the sounds of speech in individuals with severe to profound hearing loss.1 Such improvements can also be observed in the objective methods used to verify the integrity of the cochlear device and the electrophysiologic response of users. Neural response telemetry (NRT), developed from the studies by Abbas et al,2 is a fast and easily applicable techniquethat assesses the response of the peripheral segment of the auditory nerve to electrical stimulation. The implant used for NRT elicits stimuli and records the electrically evoked compound action potential (ECAP). Given that this is a fast and easily applicable technique, NRT is also used in the intraoperative setting.

The major contribution of NRT is the confirmation of the physiologic integrity of the auditory nerve.3 It is also useful when establishing the electrodes that might be included in a given map, the best stimulation speeds, the speech coding strategies, and the estimation of the stimulation of T (minimumstimulation) and C (maximum stimulation) levels.4 Composed of a negative peak (N1) followed by a positive peak (P2), the ECAP is analyzed with regard to the amplitude of the response. The measurement between N1 and P2 yields the wave amplitude, which varies according to the increased stimulation intensity. The ECAP represents the synchrony of a group of neurons, and the amplitude of response is proportional to the number of neurons activated by a stimulus.

Consequently, the presence of the ECAP allows one to predict a satisfactory postoperative performance,5 which likely corresponds to better synaptic efficiency and synchronization of the neural response.6 Knowledge of the physiology and responses of the auditory nerve to electrical stimuli is important in establishing the current level used for stimulation and other programming adjustments.7 In some cases, the ECAP is not present during surgery,whichmight suggest a dysfunction of the cochlear nerve and existing neural structures. In some of these cases, continual stimulation predicts the appearance of neural response, which might be related to the synchronization of nerve fibers.8 Cafarelli Dees et al9 and van Dijk et al10 suggested the rates of postoperative presence of ECAP to be 96 and 90%, respectively, whereas Guedes et al reported a rate of 80%.3 Guedes et al found that the absence of responses was correlated with limited prognosis.11 The continued use of CIs has tended to
result in the emergence of the action potential after some months of stimulation. Even longitudinal studies on neural responses, however, have failed to mention the possibility of the appearance of a response or the average time required for the onset of such a response.

This study aimed at establishing whether the ECAP of the auditory nerve, when absent intraoperatively, can appear
after continual electrical stimulation. When NRT responses were absent during surgery, we analyzed the evolution of
these responses after at least 3 months of stimulation.