Abstract
Objective
To analyse the characteristics of electrically evoked middle latency responses (EMLR) in patients with profound sensorineural hearing loss (SNHL) and tinnitus.
Methods
Thirty adults with profound SNHL and tinnitus who underwent cochlear implantation were selected as the tinnitus group, and 15 patients with profound SNHL but without tinnitus served as the control group. Preoperative data were retrospectively analysed. Electrically evoked middle latency responses were recorded intraoperatively, and the electrophysiological characteristics were compared between groups. Changes in tinnitus were assessed using the Tinnitus Handicap Inventory (THI), and correlations between EMLR parameters and THI scores were analysed.
Results
No postoperative complications occurred in any of the patients. Electroauditory responses were successfully elicited in all 45 patients. Intraoperative EMLR showed increased Na–Pa amplitude and delayed Pa latency, with significant differences observed between the tinnitus and control groups (P < 0.05). One year after cochlear implantation, the average comfort level (C value) and the EMLR threshold showed a significant linear correlation (r = 0.915, P < 0.01). Postoperative THI scores showed a significant decrease (P < 0.05) compared with preoperative scores. A significant correlation was found between the intraoperative EMLR Na–Pa wave amplitude and THI change at 1 year (r = 0.815, P < 0.05), with an area under the ROC curve of 0.673 (P = 0.002).
Conclusion
The intraoperative EMLR waveform of patients with profound SNHL and tinnitus is significantly different from that of patients without tinnitus. Cochlear stimulation effectively inhibits tinnitus, and EMLR provides an objective tool for evaluating tinnitus and optimising its management. However, further studies with larger sample sizes are needed to validate these preliminary findings.
Keywords: Tinnitus, Cochlear implant, Electrically evoked middle latency responses, Tinnitus handicap inventory, Sensorineural hearing loss
Introduction
Tinnitus is the subjective perception of sound when there is no external sound. It affects over 740 million adults worldwide, with around 120 million reporting a significant impact on daily life [1]. Tinnitus can lead to insomnia, memory loss and even suicidal thoughts, in addition to imposing a substantial economic burden [2, 3].
Early studies suggested that tinnitus originates in the cochlea, is recognised and processed through the auditory pathway and is ultimately perceived as tinnitus in the subcortical centres [4]. More recent studies have shifted the focus to the thalamus. The thalamic evaluation system is primarily composed of the ventromedial prefrontal cortex, nucleus accumbens and limbic structures such as the thalamus, amygdala and hippocampus [5]. This system processes information, assesses its emotional value and determines which information is transmitted to the cerebral cortex [6]. In chronic tinnitus, dysfunction within this system may lead to abnormal transmission of tinnitus signals to the higher auditory cortex [7, 8].
The auditory middle latency response (AMLR) reflects how auditory information is integrated from the brainstem to the thalamus and cortex. The Na wave (a negative wave at approximately 20–25 ms) originates from midbrain and thalamic structures, whereas the Pa wave (a positive wave at approximately 25–35 ms) is associated with the primary auditory cortex and the medial geniculate nucleus of the thalamus. The Nb–Pb components may arise from the reticular activation system [9]. Early studies indicated that the appearance of the Na and Pb waves in AMLR is age-related, with around 20% of children displaying Pa waves before they are 1 year old, 65% showing Na waves and adult levels being reached at 10–12 years of age [10]. Martin-Lagos et al. [11] found that all AMLR components (Pa, Pb, Na, Nb) in patients with tinnitus exhibited delayed latencies, with a significant increase in the Pa wave amplitude, suggesting that AMLR could serve as a marker of tinnitus severity. Gerken et al. [12] compared the auditory brainstem responses (ABRs) and MLRs of elderly patients with tinnitus, both with normal hearing and with hearing loss, and found that the Pa–Nb amplitude was markedly higher in the tinnitus group, further supporting a link between tinnitus and middle latency evoked potentials.
Although traditional objective methods, such as otoacoustic emissions (OAEs) with contralateral suppression, have been used to assess tinnitus [13], these methods are not applicable in patients with profound sensorineural hearing loss (SNHL) who are candidates for cochlear implantation. Previous studies have shown that patients with tinnitus exhibit defective medial olivocochlear bundle function when assessed through contralateral suppression of OAEs [13, 14]. However, electrically evoked middle latency response (EMLR) offers a unique advantage in evaluating the central auditory pathway in cochlear implant candidates, where peripheral measures are not feasible. EMLR directly stimulates the auditory nerve through cochlear implant electrodes, bypassing damaged cochlear structures. Unlike acoustic methods requiring functioning hair cells, EMLR can objectively assess the central auditory pathway from brainstem to cortex in patients with profound hearing loss, providing critical measurements for understanding tinnitus mechanisms.
Many studies have reported secondary improvement or even complete resolution of tinnitus symptoms after cochlear implantation, with improvement rates ranging from 34% to 92% [15]. A meta-analysis of 17 studies confirmed the benefits of cochlear implants on quality of life and tinnitus symptoms, with Levy et al. noting that tinnitus symptoms improved in approximately 75% of patients with implants and completely resolved in 15% of patients [16]. The exact therapeutic mechanism is unknown, but one hypothesis is that enhanced auditory nerve input from the implant may initiate neuroplastic recombination of the central auditory pathway and associated brain regions [17]. Recent studies have shown that electrical stimulation parameters, such as pulse rate, electrode placement and stimulation level, can influence tinnitus suppression [18, 19]. Both low-rate stimulation (< 300 pulses per second [pps]) and high-rate stimulation (≥ 900 pps) are reported to suppress tinnitus, though individual responses vary considerably [20]. Borges et al. [21] conducted a systematic retrospective study on 51 patients with post-lingual deafness and severe tinnitus. They suggested that short-term tinnitus reduction after cochlear implantation may result from sound masking and direct electrical stimulation of the auditory nerve, whereas long-term reduction (around 6 months post-implant) may be due to brain plasticity and reorganisation. However, the exact mechanisms by which cochlear implants influence central auditory processing and tinnitus perception are still to be clarified.
Multi-channel cochlear implantation has been performed in our hospital since 1995, and EMLR testing began in 2010 [22]. To date, more than 100 patients have undergone EMLR testing.
In this study, we collected data from adults with profound SNHL and tinnitus undergoing cochlear implant surgery to explore the characteristics of EMLR in patients with tinnitus and the inhibitory effect of cochlear implantation on tinnitus. We aimed to establish an objective and reliable method to evaluate the condition of patients with tinnitus and develop a personalised treatment plan.
Materials and methods
Study population
This retrospective study included 30 patients with profound SNHL and unilateral or bilateral tinnitus who underwent cochlear implant surgery in our hospital between December 2014 and December 2015. This tinnitus group included 18 men and 12 women, aged 17–69 years, with a median age of 40.8 years. All patients were adults with post-lingual deafness.
The inclusion criteria were unilateral or bilateral tinnitus (non-pulsatile) lasting more than 3 months; preoperative hearing tests (pure tone audiometry, ABR, auditory steady-state response [ASSR]) indicating severe or profound SNHL (pure tone hearing threshold > 80 dB HL); and normal middle ear, cochlea and internal auditory canal structure on imaging, with or without large vestibular aqueduct syndrome (LVAS). The exclusion criteria were age < 18 years or > 70 years; systemic or psychiatric conditions such as cardiovascular or cerebrovascular disease, asthma, epilepsy or depression; and tinnitus not associated with hearing loss. Originally, 32 patients were enrolled, but 1 patient with depression and 1 patient with low literacy who failed to complete the follow-up questionnaire were excluded, leaving 30 patients in the tinnitus group. A further 15 patients (9 men and 6 women) with profound SNHL without tinnitus were selected for a control group. Ages ranged from 16 to 70 years, with a median of 42.3 years. All patients had post-lingual deafness and normal cochlear development on imaging.
Preoperative objective audiometry met the implantation criteria.Among the 30 patients in the tinnitus group, 28 underwent multi-channel unilateral cochlear implantation and 2 underwent bilateral implantation. All 15 patients in the control group underwent multi-channel unilateral cochlear implantation. All the implants were activated approximately 1 month post-surgery, followed by regular programming and rehabilitation training.
Although patients were followed for 2 to 4 years total, the formal assessments reported in this study were conducted preoperatively and at 1 year post-implantation. The extended follow-up period ensured stable device use and allowed for observation of any late complications, though these additional data are not included in the current analysis.
Methods
Preoperative assessment
Retrospective analysis of preoperative audiology data (pure tone audiometry, ABR, ASSR) and imaging data (temporal bone thin-slice CT and brain MRI) from 45 patients in two groups showed that the cochlear and internal auditory canal structures were all normal, including 7 patients with LVAS. Hearing tests were conducted according to the Guidelines for Cochlear Implantation (2013 edition) published by the Chinese Medical Association [23]. Deafness duration in the implanted ear was defined as sudden or slowly progressive hearing loss >80 dB HL. In the tinnitus group, twenty-eight patients had multi-channel unilateral cochlear implants, and two had bilateral implants. All 15 patients in the control group received multi-channel unilateral cochlear implants. Details of the tinnitus group, including tinnitus laterality (unilateral vs. bilateral), tinnitus frequency matching and duration, are shown in Table 1.
Table 1.
Information of tinnitus group (n = 30)
| Patients | Gender | Age | Cause of deafness | Duration of deafness (years) | Tinnitus duration (years) | Implanted ear | Tinnitus Frequency HZ | Model |
|---|---|---|---|---|---|---|---|---|
| 1 | female | 56 | sudden deafness | 7 | 7 | Left | 4 K | CA |
| 2 | male | 26 | Unknown cause | 13 | 13 | Right | 8 K | CA |
| 3 | male | 17 | LVAS | 11 | 11 | Right | 4 K | CA |
| 4 | male | 28 | Unknown cause | 20 | 5 | Left | 4 K | CI422 |
| 5 | female | 54 | neurogenic deafness | 5 | 5 | Right | 8 K | F-CA |
| 6 | male | 39 | head trauma | 1.5 | 1.5 | Left | 8 K | F-CA |
| 7 | male | 45 | neurogenic deafness | 9 | 9 | Right | 8 K | CA |
| 8 | female | 62 | neurogenic deafness | 8 | 8 | Right | 4 K | CA |
| 9 | male | 45 | sudden deafness | 7mon | 7mon | Right | 8 K | ST |
| 10 | male | 64 | sudden deafness | 6mon | 6mon | Left | 4 K | ST |
| 11 | female | 20 | LVAS | 8 | 5 | Double | 8 K | 512 |
| 12 | male | 47 | neurogenic deafness | 18 | 18 | Right | 8 K | F-CA |
| 13 | male | 39 | sudden deafness | 5mon | 5mon | Right | 4 K | CA |
| 14 | female | 27 | Unknown cause | 10 | 10 | Double | 4 K | CA |
| 15 | male | 43 | neurogenic deafness | 6 | 6 | Right | 4 K | F-ST |
| 16 | female | 21 | LVAS | 1 | 1 | Right | 8 K | 422 |
| 17 | female | 50 | sudden deafness | 6mon | 20 | Left | 8 K | F-ST |
| 18 | female | 29 | Unknown cause | 13 | 20 | Left | 8 K | 512 |
| 19 | male | 26 | Unknown cause | 9 | 14 | Right | 4 K | 512 |
| 20 | female | 28 | sudden deafness | 6mon | 10 | Right | 4 K | F-ST |
| 21 | female | 28 | LVAS | 16 | 16 | Left | 8 K | F-ST |
| 22 | male | 25 | LVAS | 12 | 12 | Right | 8 K | 422 |
| 23 | male | 47 | otitis media | 35 | 35 | Left | 8 K | F-ST |
| 24 | male | 24 | LVAS | 5mon | 9 | Right | 4 K | 512 |
| 25 | female | 69 | neurogenic deafness | 8 | 8 | Left | 8 K | NERO |
| 26 | male | 20 | LVAS | 2 | 2 | Left | 4 K | F-ST |
| 27 | male | 45 | neurogenic deafness | 1 | 4 | Left | 8 K | F-CA |
| 28 | female | 29 | Unknown cause | 20 | 20 | Right | 8 K | 422 |
| 29 | male | 40 | sudden deafness | 2 | 2 | Right | 4 K | 422 |
| 30 | male | 23 | Unknown cause | 5 | 5 | Left | 4 K | F-CA |
LVAS Large Vestibular Aqueduct Syndrome
Intraoperative EMLR testing
All 45 patients from both groups underwent intraoperative EMLR testing. A Bio-logic Navigator Pro AEP (version 7.0.0) auditory evoked potentiometer (Natus Medical Incorporated, Pleasanton, CA, USA)was used to collect the data together with a custom-built electrical stimulator, compliant with human safety standards (IEC 60601-1 medical electrical equipment standards). The stimulator generates biphasic current pulses with adjustable pulse width (25–400 µs) and amplitude (0–2000 current levels [CLs]); cochlear implant systems typically use 0–255 CLs to represent logarithmic steps of current amplitude). The device includes safety features such as current limiting and electrical isolation.
Recording procedure
Under general anaesthesia, the recording electrodes were positioned according to the international 10–20 system at the vertex (Cz, active), contralateral mastoid (reference) and forehead (ground). A test electrode was placed in the round window niche via posterior tympanotomy. After confirming proper electrode placement, the external processor coil was coupled with the implanted receiver-stimulator coil.
Initial stimulation parameters were set at a pulse width of 50 µs, stimulation rate of 35 Hz and stimulation intensity of 200 CLs. Stimulation was monopolar, with the active electrode in the cochlea and the reference electrode being the extracochlear ground electrode. Responses were averaged over 500–1000 sweeps within a 0–80 ms post-stimulus window.
If typical evoked potential waveforms were observed, cochlear implantation proceeded. The electrode array was implanted through the round window or cochleostomy via the facial recess approach. Following full insertion, EMLR data were collected from multiple electrode positions (apical, middle, and basal), and waveform parameters, including Na–Pa amplitude and Pa latency, were measured.
Device activation and programming
All 45 patients underwent device activation 1 month after surgery. The ACE (advanced combination encoder) strategy was used with the following parameters: a stimulation rate of 900 Hz and 22 channels with 8 maxima selected per stimulation cycle. The ACE strategy combines the high stimulation rate of the continuous interleaved sampling strategy with the spectral peak selection method.
At activation, neural response telemetry (NRT) was used to record the electrically evoked compound action potential of the auditory nerve. Pure tone audiometry was performed with the cochlear implant activated, and the average hearing threshold of the implanted ear was determined. The behavioural threshold (T level) and maximum comfort level (C level) were established from patient feedback, and the dynamic range was defined as the difference between C and T levels. Both C and T levels were measured in CLs, and data from electrode 12 (mid-array position) were used for analysis, as this position typically provides stable responses.
Assessment of tinnitus
The Tinnitus Handicap Inventory (THI) was used to evaluate the effect of cochlear implantation on tinnitus in the 30 patients of the tinnitus group at two time points: before surgery and 1 year after surgery. The THI, originally developed by Newman in 1996 [24], consists of 25 items grouped into 3 subscales. First is the functional evaluation (11 items), which assesses mental, social/occupational and physical functioning. Next, the emotional evaluation (9 items) assesses the patient’s affective responses to tinnitus. Finally, the catastrophic subscale (5 items) measures the most severe reactions to tinnitus.
Patients were instructed to answer all questions in order without skipping any, and responses were scored as follows: 4 points for ‘yes’, 2 points for ‘sometimes’, and 0 points for ‘no’. The maximum possible score was 100.
According to the classification system determined by the British Association of Otorhinolaryngology and Head and Neck surgery, the THI score was divided into five grades: Grade I (0–16, slight), Grade II (18–36, mild), Grade III (38–56, moderate), Grade IV (58–76, severe), Grade V (78–100, catastrophic). The higher the score, the more severe the tinnitus. An improvement of ≥ 20 points compared with the preoperative score was considered an effective response. The change in THI score from before surgery to 1 year was denoted as ΔTHI.
Statistical analysis
The results were expressed as mean ± standard deviation. SPSS 26.0 statistical analysis software was used to conduct independent sample t-tests for EMLR indicators and THI results in the tinnitus and control groups. In the tinnitus group, paired-sample t-tests were used to compare total THI scores before surgery and 1 year after surgery. Linear regression and correlation analyses were used to examine the relationship between the intraoperative EMLR thresholds and the postoperative C values. Receiver operating characteristic (ROC) curve analysis was performed to compare intraoperative EMLR amplitude and ΔTHI. A significance level of α = 0.05 was used, with p < 0.05 considered statistically significant.
Results
Intraoperative EMLR detection
Well-differentiated Po–Na–Pa–Nb–Pb waveforms were successfully evoked in all 45 patients (both tinnitus and control groups) during intraoperative EMLR testing, with a 100% elicitation rate (Figs. 1 and 2). As shown in Table 2, patients in the tinnitus group exhibited significantly larger Na–Pa amplitudes and prolonged Pa latencies compared with the non-tinnitus group (P < 0.05). Patients with tinnitus exhibited the characteristic ‘large Na–Pa wave’.
Fig. 1.
In patients with sensorineural hearing loss and tinnitus, well-differentiated Po-Na-Pa-Nb-Pb waveforms can be seen. The amplitude of Na-Pa is large and the latency of Pa wave is delayed
Fig. 2.
Well-differentiated Po-Na-Pa-Nb-Pb waveforms can be seen in patients with sensorineural hearing loss without tinnitus. The amplitude of Na-Pa was low and the latency of Pa wave was normal
Table 2.
Parameters of Na-Pa wave detected by EMLR during operation in two groups of patients
| Group | Number | Na-Pa amplitude * | Pa latency * |
|---|---|---|---|
| Tinnitus group | 30 | 1.12 ± 0.17 | 30.37 ± 2.92 |
| Control group | 15 | 0.46 ± 0.07 | 21.07 ± 1.79 |
| t | 18.525 | 11.288 | |
| P | 0.000 | 0.000 |
*Indicates a statistically significant difference
Device activation testing: NRT and C value collection
Following surgery, all 45 patients obtained good electroauditory responses, with no complications such as facial nerve stimulation or vertigo. Neural response telemetry was tested at device activation (Fig. 3), with a detection rate of 76.67%. Among these, two patients had no detectable NRT response, five patients had normal waveform differentiation and the remaining patients showed good waveform differentiation.
Fig. 3.
NRT diagram at post-operation device activation showing N1-P1 waveform
The T and C levels stabilised after six months of device use. At 1 year post-activation, the C values from electrode 12 (a stable mid-array site) were recorded for all patients, showing an average C value of 165.82 ± 11.14 CL. The intraoperative EMLR threshold was also collected in both groups, and the mean value was 158.36 ± 12.93 CL.
Linear correlation analysis was performed between the EMLR thresholds and the C values of the two groups of patients (Table 3; Fig. 4). The regression equation was.
 |
Table 3.
Correlation analysis of EMLR value and C value during operation between the two groups
| Group | Number | EMLR Threshold (CL) | Switch-up C value (CL) | r value |
|---|---|---|---|---|
| Experimental group | 45 | 158.36 ± 12.93 | 165.82 ± 11.14 | 0.915 |
Fig. 4.
Pearson correlation analysis of EMLR threshold and C value at device activation
where Y = EMLR threshold and X = C value. The correlation was statistically significant (P = 0.001 < 0.05), with the Pearson correlation coefficient r = 0.915 and coefficient of determination r²=0.833, indicating a strong positive correlation between EMLR thresholds and C values.
Postoperative follow-up
In this study, 30 adults with post-lingual deafness were followed for more than 2 years after cochlear implantation. With unilateral or bilateral cochlear implants, the patients’ hearing improved considerably within 1–3 months, enabling them to gradually understand familiar voices in a quiet environment. By 6 months post-surgery, they were able to communicate with acquaintances in daily conversations.
The THI assessment performed at two time points (Table 4) showed that before cochlear implantation, 3 patients (10%) had catastrophic tinnitus (Grade V, 78–100 points), 15 patients (50%) had severe tinnitus (Grade IV, 58–76 points), 9 patients (30%) had moderate tinnitus (Grade III, 38–56 points), 3 patients (10%) had mild tinnitus (Grade II, 18–36 points) and 0 patients had slight tinnitus (Grade I, 0–16 points).
Table 4.
THI results of tinnitus group (n = 30) patients at different time points
| Group | Pre-operation | 1 year after operation* | 1 year after operation ΔTHI | t |
|---|---|---|---|---|
| Tinnitus group | 68.20 ± 10.42 | 23.37 ± 28.18 | 42.6 ± 24.97 | 10.464 |
* P < 0.05 was statistically significant
At the 1-year follow-up, 15 patients (50%) reported that their tinnitus had disappeared, and 9 patients (30%) reported substantially weakened tinnitus (THI score decrease ≥ 20 points). Three patients (10%) showed limited improvement (< 20-point reduction), 2 patients (6.7%) experienced no change and 1 patient (3.3%) reported aggravated tinnitus.
The overall effectiveness rate was 80%. Paired-sample t-test analysis confirmed a significant reduction in THI scores at 1 year after cochlear implantation compared with the preoperative scores (P = 0.000 < 0.05).
Linear correlation analysis of intraoperative EMLR wave Na–Pa amplitude and ΔTHI at 1 year after surgery (Table 5; Fig. 5) gave the following regression equation:
 |
Table 5.
Correlation analysis of Na-Pa amplitude of EMLR and ΔTHI 1 year after operation in tinnitus group
| Group | Number | Na-Pa amplitude | ΔTHI | r |
|---|---|---|---|---|
| Tinnitus group | 30 | 1.12 ± 0.17 | 42.6 ± 24.97 | 0.815 |
Fig. 5.
Correlation analysis of Na-Pa amplitude of EMLR and ΔTHI 1 year after operation in tinnitus group
where Y = Na–Pa amplitude and X = ΔTHI. The Pearson correlation coefficient r = 0.815 and coefficient of determination R²=0.652 demonstrated a statistically significant correlation (P = 0.000 < 0.05), indicating a positive correlation between Na–Pa amplitude and ΔTHI at 1 year after surgery.
Na–Pa amplitudes detected by EMLR and postoperative tinnitus outcomes were analysed using ROC curve analysis. The area under the ROC curve was statistically significant at 0.673 (P = 0.002), indicating that Na–Pa amplitude has significant predictive value for tinnitus treatment outcomes (Fig. 6).
Fig. 6.
ROC analysis of Na-Pa amplitude and postoperative tinnitus treatment effect
Discussion
Due to the lack of universally accepted objective evaluation methods for tinnitus, the prevalence rate of tinnitus in the past relied on patient questionnaires [25]. As tinnitus is a highly subjective experience, questionnaires cannot fully assess its true impact on patients’ lives. In recent years, numerous studies have been conducted on tinnitus audiology detection indicators [26]. Although acoustic stimulation ABR, MLR examination and OAEs with contralateral suppression can be used as objective methods to study tinnitus [14, 15], these techniques have limitations in patients with profound hearing loss. In contrast, EMLR offers a unique advantage to study tinnitus by combining cochlear implantation with electrophysiological assessment. This study evaluated the inhibitory effect of cochlear implantation on tinnitus by observing the EMLR parameters of patients with cochlear implants during surgery, providing theoretical support for the treatment of tinnitus with electrical stimulation.
Tinnitus and intraoperative EMLR
The thalamic evaluation system serves as a regulatory mechanism in tinnitus perception. When functioning normally, chronic tinnitus signals are suppressed and not transmitted to the higher auditory cortex. When dysfunction occurs, abnormal signals are transmitted, resulting in chronic tinnitus perception. Studies have shown that AMLRs in patients with tinnitus are substantially different from those in patients without tinnitus, suggesting that AMLRs can serve as objective indicators of tinnitus status. Manta et al. [27] concluded that establishing objective and reliable methods to detect and classify tinnitus will help personalise treatment, reduce the number of ENT outpatient visits, reduce healthcare costs, and enhance patient confidence and satisfaction.
In this study, 30 patients with tinnitus showed significantly larger Na–Pa amplitudes during intraoperative EMLR compared with the control group. These findings are consistent with reports of acoustically stimulated MLR changes in patients with tinnitus, supporting the view that EMLR can reflect thalamic dysfunction within the auditory pathway. Thus, intraoperative EMLR not only identifies tinnitus-related abnormalities but also reflects the electrophysiological characteristics of impaired thalamic function in patients with tinnitus, consistent with Manta et al.’s results. Future research should further explore EMLR parameters in tinnitus.
Assessment of tinnitus before and after cochlear implantation
In the process of treating profound SNHL with tinnitus, cochlear implants have been found to have an inhibitory effect on tinnitus, which suggests a new treatment option. Most patients with post-lingual deafness also experience tinnitus of varying severity. In this study, the duration of tinnitus in some patients was longer than the duration of deafness. Quaranta et al. [28] reported reductions in tinnitus severity ranging from 20% to 86% following cochlear implantation. A meta-analysis by Yang Shiming et al. [29] on studies published in the last 10 years indicated that 33%–65.8% of patients experienced complete resolution of tinnitus, 25%–39% experienced alleviation and 5%–8% experienced worsening tinnitus after cochlear implantation. Similarly, Ramakers et al. [30] reported that the inhibitory effect of cochlear implantation on tinnitus was bilateral in most patients, caused by electrical stimulation of the cochlea.
Recent studies have investigated the optimal stimulation parameters for tinnitus suppression. Perreau et al. [31] explored electrical stimulation with varying frequencies (low rate: 100–300 pps; high rate: ≥900 pps), different electrode numbers and various stimulation levels to help suppress tinnitus. They found that individual responses varied considerably, with some patients responding better to low-rate stimulation, whereas others benefited from high-rate stimulation [18, 19]. Zeng et al. [20] reported that low-rate stimulation (< 100 Hz) delivered to the apical cochlea could completely suppress tinnitus in some patients, suggesting that patient-specific stimulation strategies may be needed for optimal tinnitus management, distinct from those used for speech processing.
In addition, tinnitus improvement after cochlear implantation surgery can partially improve or completely disappear both when the device is on and when it is off [32]. The THI scale can assess the severity of tinnitus and the psychological state of patients from multiple angles. One year after surgery, 80% of the 30 patients with tinnitus reported disappearance or reduction of tinnitus, three patients had no obvious improvement, and one patient reported aggravated tinnitus.
Van et al. [33] suggested that high-frequency stimulation (HFS) can disrupt the abnormal neuronal auditory pathway activity related to tinnitus, leading to inhibition. In the tinnitus group in our study, electrical stimulation of auditory cells through the cochlear implant may have acted through two mechanisms: (1) stimulating spiral ganglion cells to produce sound perception and increasing the sound signals incoming to the brain stem and auditory cortex, thereby reducing the excitability of auditory cortex remodelling and feedback regulating neurons and weakening or resolving tinnitus, and (2) locally altering cochlear electric fields and suppressing abnormal excitation of residual auditory cells, ultimately leading to the suppression of tinnitus. It should be noted that in three patients (9,10 and 13) with tinnitus duration < 1 year, the postoperative improvement did not rule out the possibility of self-healing. Further studies are needed to clarify such points.
Intraoperative EMLR and postoperative adjustment and tinnitus evaluation
In this study, the intraoperative EMLR detection rate (100%) in 45 patients with cochlear implantation was significantly higher than that of NRT (76.67%) at device activation. Notably, patients without detectable NRT values all had varying degrees of cochlear malformation, but electroauditory responses were obtained after cochlear implantation, indicating that EMLR can evaluate the auditory pathway more accurately than NRT.
A significant correlation was observed between the C value at device activation and the intraoperative EMLR threshold, suggesting that the EMLR threshold can be used to guide postoperative programming, especially for young patients with cochlear implants who are unable to provide reliable behavioural responses. Programming adjustments can be made based on intraoperative EMLR thresholds, thus avoiding excessive electrical stimulation during initial device activation, which could potentially damage residual hearing.
Di Nardo et al. [15] proposed that cochlear implant electrodes stimulate the spiral ganglion cells of the cochlea, transmitting sound information to the central auditory system, which may cause functional changes and even structural reorganisation of the nervous system, thus easing tinnitus. After cochlear implantation, the patient can better hear external sounds, which can mask the perception of tinnitus and positively affect the patient’s mood.
In this study, the THI correlation coefficient between the intraoperative Na–Pa amplitude of EMLR and the degree of tinnitus improvement 1 year after surgery was high in all 30 patients in the tinnitus group. The area under the ROC curve was 0.673, indicating that intraoperative Na–Pa amplitude reflects central electrophysiological alterations associated with tinnitus, likely linked to thalamic dysfunction. These findings suggest that intraoperative EMLR waveforms not only provide objective insights into tinnitus severity but may also serve as a predictor of postoperative tinnitus suppression through cochlear electrical stimulation.
Zeng et al. [34] explored the possible mechanisms and prospects of tinnitus inhibition through electrical stimulation. This study demonstrated that certain intraoperative EMLR parameters are correlated with tinnitus improvement after cochlear implantation, supporting the therapeutic potential of electrical stimulation in tinnitus management. It should be noted that these EMLR parameters are electrophysiological measurements, distinct from cochlear implant stimulation parameters such as pulse rate or stimulation level.Intraoperative EMLR provides an objective evaluation method. However, this study has several limitations. First, it was a retrospective study with a relatively small sample size. Second, the mechanism by which electrical stimulation inhibits tinnitus remains unclear. Third, the study did not systematically investigate different electrical stimulation parameters for tinnitus treatment, which could be explored in future studies. Finally, the optimal electrode sites and stimulation strategies for tinnitus suppression require further investigation. Further studies with larger sample sizes, prospective designs and systematic evaluation of stimulation parameters are needed to address these issues.
Conclusion
This study demonstrates the potential of intraoperative EMLR as an objective method for evaluating tinnitus in patients with cochlear implants. The significant differences in EMLR waveforms between the groups with and without tinnitus suggest that EMLR can reflect the location and extent of auditory pathway dysfunction in patients with tinnitus. Furthermore, cochlear implantation proved effective in reducing tinnitus symptoms, with a correlation found between intraoperative EMLR thresholds and postoperative programming parameters, highlighting EMLR’s clinical utility in optimising tinnitus management. Future research should focus on developing specific electrical stimulation protocols for tinnitus suppression and validating these findings in larger, prospective studies.
Acknowledgements
Not applicable.
Conflict of interest
All of the authors had no any personal, financial, commercial, or academic conflicts of interest separately.
Authors’ contributions
BW, XHG conceived of the study, and KLC, CGW participated in its design and coordination and WL, ZQG, SJW helped to draft the manuscript. All authors read and approved the final manuscript.
Funding
1. General Program of Beijing Natural Science Foundation (7252103) 2, Clinical Research Project of Central High-level Hospital (2022-PUMCH-A-031).
Data availability
All data generated or analysed during this study are included in this published article.
Declarations
Ethics approval and consent to participate
This study was conducted in accordance with the Declaration of Helsinki and approved by the ethics Committee of the Peking union medical college hospital, P. R. China.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Bin Wang and Xiaohui Guo contributed equally to this work and share first authorship.
References
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Data Availability Statement
All data generated or analysed during this study are included in this published article.