The present protocol describes the measurement of the electrically evoked stapedius reflex (eSR) via cochlear implant (CI). Two applications are discussed: intraoperative detection of eSR for verification of the coupling between the cochlear implant and the auditory nerve and postoperative measurement of eSR thresholds (eSRT) for CI fitting.
Measuring the electrically evoked stapedius reflex during the fitting of cochlear implants (CIs) provides a reliable estimation of maximum comfort levels, resulting in the programming of the CI with high hearing comfort and good speech understanding. Detection of the stapedius reflex and the required stimulation level on each implant channel is already being performed during surgery, whereby intraoperative stapedius reflexes are observed through the surgical microscope. Intraoperative stapedius reflex detection is both an indicator that the auditory nerve is responding to electrical stimulation up to the brainstem and a test for the ability to perform postoperative stapedius reflex measurements. Postoperative stapedius reflex thresholds can be used to estimate upper stimulation levels in the CI fitting process. In particular, in children or patients unable to provide feedback on loudness perception, this method avoids inadequate stimulation with the CI, which can result in poor hearing performance. In addition, overstimulation can be avoided, which could even lead to refusal to use the device.
Electrically evoked stapedius reflex (eSRT) measurements are a known tool for verifying the coupling between a cochlear implant (CI) and the auditory nerve during surgery and postoperatively for adjusting the stimulation levels of CI programming. Whereas intraoperative measurements of the electrically evoked stapedius reflex (eSR) are quite common in clinical routine, postoperative measurements of the eSRT are still scarcely used in CI fitting because of the additional need for an acoustic impedance measurement device. However, the performance in speech perception of patients fitted with the eSRT method appears to be as good as with the method of loudness scaling1 In the latter procedure, the lower and upper limits of electrical stimulation with a CI are estimated by subjective scaling of the perceived loudness in response to stimulation on each single channel of the CI. Subjective loudness scaling requires the cooperation of the patients, who need to give feedback on their perception of electrical stimulation. Particularly young children, who today can be supplied with a CI at the age of less than one year, are not able to provide feedback on electrical stimulation. In this case, the objective eSR method is superior, and corresponding CI fitting in children allows for proper speech acquisition and cognitive development of the child at an early stage2.
The eSR method dates to the pioneering works of Stephan et al.3 who found a relation between the subjective loudness perception and the stapedius reflex threshold elicited through electrical stimulation. In these studies, a procedure was invented to measure the stapedius reflex in response to electrical stimulation in the ear canal using an acoustic impedance meter4. The correlation between the eSRT and subjective loudness ratings found originally by Stephan et al. was confirmed by other studies over the years2,4,5,6,7. Overall, programming of CIs based on eSRT results in good speech recognition scores in most patients supplied with CIs.
Today, the measurement of the eSR is a reliable tool to verify the coupling of a CI to the auditory nerve during surgery as well as a reliable estimator for upper stimulation levels in the postoperative fitting process of CIs. This paper presents both: first, a description of the eSR method during surgery for intraoperative verification that stimulation of the auditory nerve via CI is possible and processed up to the brainstem, and second, the application of the eSRT method for fitting of CIs in children and adults.
For the postoperative application of eSR testing, an acoustic impedance meter triggered by the programming interface of the CI is used to measure the eSRT during the fitting session. The entire process of CI fitting in clinical routine using the eSRT method is described. Issues to be considered such as distortions due to breathing noises or artifacts due to patient movements, which may result in distortions of the impedance measurement, are discussed. The protocol described is particularly suitable for CI fitting in children and in multiply handicapped patients.
The eSR measurement in hearing rehabilitation with CIs has two applications: first, to verify the coupling of stimulation electrodes to the auditory nerve during implantation, and second, to estimate upper stimulation levels in the postoperative programming of the audio processor.
Intraoperative measurements have the purpose of detecting the eSR as an indicator for a neural response and the processing of the electrical stimulation up to the brainstem. The contraction of the stapes tendon in response to electrical stimulation is observed visually, which requires a good view of the stapes and normal anatomical structures.
These values cannot be used for adjusting the stimulation parameters of the implant in the postoperative fitting due to the low correlation of intraoperative eSRT with postoperative loudness perception5. Intraoperative eSR measurements may further be influenced by general anesthesia13 as well as other physiological parameters (e.g., bleeding, malformation, or scar tissue).
For the detection of the eSR, an acoustic impedance measurement device is used to continuously monitor the change in acoustic impedance of the ear during electrical stimulation. To allow easy and reliable detection of the eSR, the impedance measurement should have a trigger function that is activated when the stimulus is delivered via the CI. This allows synchronous recording of eSR in response to the electrical stimulus. Alternatively, the impedance meter can be operated in continuous mode, but this appears less suitable and convenient for the application of the method. The impedance meter and the associated ear probe are typically calibrated according to audiometric standards, with the probe tone frequency of 226 Hz and sound level of 85 dB SPL. Alternatively higher frequencies can be used as probe tones as well.
The first postoperative eSRT measurement is preferentially performed 1 month after the activation of the CI. The activation session is usually scheduled 4 weeks after cochlear implantation, whereby the primary focus of the CI fitting is to make the patient accustomed to electrical stimulation at moderate stimulation levels and to motivate the patient to accept wearing the audio processor for a whole day of listening. In the next update session, which is normally scheduled 7-10 days later, the stimulation is further increased but not up to maximum comfort levels. From the third fitting session onward, the eSRT method is applicable. The perceived loudness at the eSRT is described as loud or very loud by most patients. Hence, stimulation at uncomfortable loudness levels must be avoided. Therefore, the patient needs to be carefully observed during the eSRT measurement, as eSRT may eventually be elicited but not detected. If no eSRT can be detected in both ears, the measurement must be aborted.
The method of eSRT-based programming of CIs is applicable in children as well as in adults. However, since children are typically unable to give feedback on the sound of electrical stimulation through the CI, the objective eSRT method is preferred to psychoacoustic methods, which rely on subjective feedback from the patient. It is well established by several authors that eSRT is a good estimator for comfort levels needed for programming the audio processor. This channel-specific profile created by the eSRT measurement is unique in each patient and may change over time of implant use. Therefore, the fitting of the CI must be updated at regular intervals.
The eSRT method for fitting CI certainly has one important limitation, which is the requirement of an intact middle ear. Surgical details particularly about the ossicles must be retrieved from the patient's records. Thus, postoperative eSRT measurements require preservation of the middle ear structures during surgery. In particular, during complex surgeries in case of middle ear malformations or other middle ear pathologies, the ossicular chain could be damaged. In this case, no postoperative eSRT measurement is possible on the operated ear. However, as an alternative, the ear probe can be placed on the contralateral ear and the eSRT measurement performed. In contrast to its detection, the threshold itself at which the stapedius reflex is elicited by electric stimulation does not depend significantly on the ear on which the acoustic impedance measurement is performed.
Typically, the impedance probe is placed ipsilaterally, with stimulation via the CI and reflex detection taking place in the same ear. On the ipsilateral side, the majority of the patients with CIs do not have acoustic hearing, so the probe tone of 226 Hz presented at a sound level of 85 dB SPL is not perceived. As the eSRT measurement on all channels of the CI can take a while, it is more comfortable not to hear the probe tone during the fitting procedure. If patients have acoustic hearing in the contralateral ear, the measurement may be somewhat inconvenient due to the increased listening effort during the presentation of the probe tone.
Prior to any eSRT measurement session, the current middle ear status must be checked. One limitation of eSRT-based CI fitting in children is the frequent occurrence of negative pressure in the tympanic cavity due to tube ventilation dysfunction. In this case, the acoustic impedance measurement for eSR detection is more complex. In these children, the application of nasal drops is often sufficient to relieve the pressure in the tympanic cavity to a sufficient degree that enables a successful impedance measurement in CI fitting.
The measurement of eSRT levels on all electrodes of a CI may take a while. With the equipment used in this work, a complete measurement series on 12 electrodes takes approximately 10-30 min. During this time, the acoustic impedance measurement must be stable. To ensure this condition for the entire duration of the measurement, proper positioning of the ear probe and passive cooperation by the patient are crucial.
The acoustic impedance measurement may continuously be influenced by noise from breathing and must be carefully observed as this effect may cause artifacts in ESR traces. The electrical stimulation bursts for eliciting the stapedius reflex should be initiated during phases where such distortions are small. Furthermore, the heart rate of a patient may cause a periodic change of the acoustic impedance, which can potentially be misinterpreted as a stapedius reflex.
The protocol described has been used successfully in numerous CI fitting sessions in our department and should encourage other colleagues to consider CI fitting based on eSRT in children and in multiply handicapped patients who cannot give reliable feedback on auditory perception.
The authors have nothing to disclose.
The authors thank the cochlear implant team for pediatric patients and the team of pediatric speech therapists of our department for their support in realizing this work.
Audiointerface | any | Audiointerface with Stereo Line Input is required to Record Acoustic Signal in the Ear Canal for Deriving the Ear Drum Impedance Pattern | |
Cable Clips | any | Cable Clips for fixation of the Probe Cable at the Head Band | |
Desktop Computer or Notebook | any | Required for Cochlear Implant Programming Software | |
Ear Probe | Bio-Logic | Ear Probe for Presenting and Recording Acoustic Signals in the Ear Canal | |
Head Band | any | Head Band for fixation of the Ear Probe | |
Maestro Software | Med-El | Programming Software for Med-El Cochlear Implants | |
MAX Coil | Med-El | Cable connecting Interfacebox and Cochlear Implant via Transcutaneous Inductive Coupling for Telemetry Measurement | |
MAX Interface Box | Med-El | Interfacebox to access Cochlear Implant during the Fitting Session | |
MAX Programming Cable | Med-El | Cable connecting Interfacebox and Audioprocessor for Adjusting Electrical Stimulation Levels | |
Single use Ear Tips | Sanibel Supply | Ear Tips for Ear Probe, Size depending on the Patients Ear | |
Sonnet 2 Audioprocessor | Med-El | External Part of the Implant System. Has to be worn by the Patient to Perceive Auditory Sensations |
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