Electrocochleography

Procedures

Two general approaches to ECochG recording are employed: transtympanic (TT) and extratympanic (ET). TT ECochG provides more reliable recordings yet is also more invasive because it requires a needle electrode to pass through the tympanic membrane (TM) and be positioned directly on the cochlear promontory. Due to the invasive nature of this method, it has largely been superseded in the clinical context by extratympanic (or tympanic) ECochG, as patients much better tolerate this method.[5][6] ET ECochG utilizes a foam rubber or similar insert with an electrode to detect ECochG responses (see Image. Electrocochleography Tracing). 

Notably, with the advent of modern cochlear implantation techniques, TT ECochG has seen renewed interest, as it offers superior accuracy during intraoperative monitoring, allowing surgeons to ensure that the implantation is causing minimal trauma to the cochlea.[7]

While TT ECochG has significant accuracy benefits over ET approaches due to the proximity of the recording electrodes to the signal-generating anatomical structures, it requires physician monitoring and patient cooperation and is challenging to perform in most clinical settings.[1] TT ECochG remains useful in certain intraoperative situations and is generally considered a more accurate recording method than ET ECochG. Nevertheless, TT ECochG has certain limitations beyond patient tolerance. Firstly, the needle risks penetration of an abnormally positioned round window in cochlear malformations.[8] Secondly, the high impedance factor of a needle may exclude lower action potential frequencies. TT ECochG is particularly helpful in the intraoperative setting because it provides precise neuromonitoring without concern for patient compliance. Local or topical anesthetics may improve patient tolerance in clinical settings. Three primary methods of stimulus are presented: clicks, tone bursts, and chirps.[6] Click stimuli assess a large population of neurons with a single, brief stimulus, while tonal stimuli are more precisely aligned with audiometric thresholds, especially at 1,000 Hz, 2,000 Hz, and 4,000 Hz; chirp usage is less common. The application of each method remains controversial and is dependent on clinician preference.

ET ECochG monitoring may be performed non-invasively and typically painlessly through in-ear inserts. While not as precise as TT ECochG due to the distance of the electrode from the cochlea, ET ECochG provides generally accurate data while being much more easily tolerated by patients. Insert earphones may still be somewhat uncomfortable, particularly if there is close proximity to the TM. ET ECochG provides a much more easily tolerated screening examination for patients with suspected MD/ELH.

Indications

Historically, ECochG was largely utilized for diagnosing and monitoring MD/ELH. As briefly mentioned above, the primary measurements used for interpretation are the SP and the AP, which will be discussed more thoroughly below. Patients for whom ECochG should be considered as a potential diagnostic tool are mainly those with symptoms consistent with MD/ELH. Patients with severe vertigo, nausea/vomiting, tinnitus, disequilibrium, aural fullness, and similar related symptoms. Criteria for the diagnosis of MD were standardized in 1995 by the American Academy of Otolaryngology-Head and Neck Surgery; the Gibson 10-point score may also be used to assess the likelihood of MD/ELH and may be further utilized to determine the efficacy of ECochG.[9][1][10] ECochG is also employed to monitor patient progress in response to treatment.  

Other indications for ECochG include intraoperative nerve monitoring and identification of the first wave of the ABR in patients with extremely poor hearing or a diagnosis of “hidden hearing loss” (auditory neuropathy).[11] In cases of auditory neuropathy, the AP amplitude will be reduced, as will the ANN. While ABR may not be as precise as ECochG, it is more easily obtainable and reliable and does not require the same level of patient cooperation.[1] Hence, ECochG, while valuable in specific instances, may not be a patient first-choice assessment tool, depending on the clinical scenario. More recent data have also suggested that ECochG may help predict MD, diagnose superior semicircular canal dehiscence, and monitor cochlear function intraoperatively during its repair.[12]

Potential Diagnosis

The primary condition diagnosed and monitored by ECochG is MD/ELH. Intraoperative nerve monitoring also remains an important indication for ECochG, particularly in removing cerebellopontine angle tumors. In the context of cochlear implantation, ECochG provides valuable real-time feedback regarding the functional integrity of the cochlea, assisting surgeons in making critical intraoperative decisions. ECochG may also be used to estimate hearing thresholds, but more convenient testing methods, such as ABR, have largely been replaced in this indication. While the reproducibility, specificity, and sensitivity of ET recordings remain poor, an SP:AP ratio ≥0.45 is nevertheless a significant diagnostic indicator of MD.[11] As there is substantial variability with both recordings, particularly with the SP, the SP:AP ratio must be interpreted cautiously and in the appropriate clinical context.

ECochG monitoring via ET electrode may be used to assess whether the electrode has traversed the cochlea's basilar membrane during cochlear implant insertion and to assess residual hearing. However, correlations between ECochG and pure-tone audiograms have demonstrated significant variability concerning postoperative results.[13]

Normal and Critical Findings

Sound is transmitted from the tympanic membrane to the inner ear via ossicular chain movement. The vibration of the stapes footplate on the oval window creates a hydromechanical wave of perilymph in the cochlea, which generates vibration at different points on the basilar membrane, depending on the frequency of the sound stimulus. Basilar membrane movement causes the corresponding movement of hair cell stereocilia, thereby opening ion channels and converting mechanical energy into electrical voltage. This voltage elicits neurotransmitter release in the spiral ganglion and results in the perception of sound. The CM represents the alternating current voltage caused by the outer hair cells' potassium and calcium ion influx in response to an auditory stimulus.

The SP represents the sum of transduction processes in the cochlea, from the inner hair cells, outer hair cells, and auditory nerve. The SP magnitude corresponds to the amount of distortion in these processes. Consequently, the SP may be enlarged in situations with increased distortion, such as in ELH, where pressure is increased relative to patients with normal inner ear and temporal bone anatomy. The AP, specifically its N1 peak, has 2 main characteristics that are clinically relevant: latency and magnitude. Latency describes the time duration from stimulus to N1 peak, while magnitude correlates with the number of firing nerve fibers. Thus, while the SP magnitude increases with distortion in the transduction process, the AP will remain relatively constant. This results in an increased SP:AP ratio, which is pathognomonic for ELH.[1] The AP also has a second peak (N2), which corresponds to ABR Wave II, although there is currently no known clinical significance associated with the N2 peak of the AP.[1]

SP and AP amplitudes depend on the stimulus's intensity, whether click or tonal stimuli are used, and the method of stimulus presentation. Measurements may be recorded using baseline reference or "peak-to-peak" amplitudes, which may improve consistency due to variability in baseline levels from ET recordings. The SP:AP ratio is considered to be abnormal if it is greater than 0.45, although there is some variability and debate regarding the precise cutoff, particularly if using the ET method, as this typically results in a lower SP:AP ratio when compared to TT ECochG.[14][15][16]

Interfering Factors

Interfering factors in ECochG primarily include a lack of patient cooperation and stimulus artifacts. Artifact is most pronounced in ET recordings, given the greater distance of the stimulus from the cochlea and subsequently reduced amplitude of responses, which enables more minute electrical artifacts to affect the recording.[17] Additionally, light and other electrical equipment in testing rooms may contribute to artifacts due to interference. Grounding electrodes, reducing or removing unnecessary electrical equipment, using tubal insert transducers, placing electrodes on the TM, and purpose-built audiological testing facilities may all help reduce electrical interference.[17]

Complications

As TT ECochG may be uncomfortable and poorly tolerated for patients in the awake setting, it typically requires sedation or anesthesia. Thus, the inherent risks of sedation or anesthesia should be discussed with patients if this method is employed instead of ET ECochG. Of primary concern with TT ECochG is accidental puncturing of the round window, which may result in perilymph leakage into the middle ear and create a potential for hearing loss. ET ECochG is very well tolerated, with no commonly reported complications.[18]

Patient Safety and Education

ECochG is a safe and either non-invasive or minimally-invasive procedure that may diagnose MD/ELH, characterize its severity, or be utilized intraoperatively for neuromonitoring. Patients should be cautioned that the interpretation of results obtained is somewhat subjective and that the procedure may not provide conclusive diagnostic evidence, particularly in the setting of significant artifacts. Patients should also be informed of alternative testing methods, including ABR, which may be tolerated or interpreted more quickly and may be more appropriate for certain patients either instead of or in addition to ECochG.[18]

Clinical Significance

ECochG is a helpful tool for evaluating inner ear fluid distortion and increased pressure in the MD/ELH setting. It additionally may be used to help assess hearing thresholds and auditory neuropathy, as ECochG has been demonstrated to correlate with pure-tone thresholds, albeit with some variability. Recently, other testing methods, such as ABR, have largely replaced ECochG in specific clinical settings due to patient tolerance and ease of use. ECochG has become less commonly utilized in the evaluation of MD/ELH. However, with the growing number of cochlear implantations performed, the role of ECochG in intraoperative neuromonitoring has become increasingly significant. ECochG facilitates optimal electrode placement, potentially reducing postoperative complications and ensuring the best possible outcomes for the patient.[19]