Basic Concepts of Electrical Response Audiometry
Electrical response audiometry (ERA) is a description used for an assortment of procedures in which electrical potentials are recorded while being evoked by a sound stimulus. The presence of the response or the response characteristic allows us to surmise the subjects’ hearing capability or the performance of their auditory pathways. ERAs are considered an “objective” evaluation because the subject is not required to actively participate in the assessment in many of the tests, unlike various other tests. The short-latency automatic components are favored for threshold estimation, as they are modestly affected by the brain state of the subject. The long-latency components are generally used to surmise the cognitive processing capacity of the brain and are often called event-related potentials (ERPs)1. ERA and auditory-evoked potentials (AEP) are used interchangeably.
Types of ERA
Electrical response audiology is a common testing method performed in a clinical setting and in many areas of research because of its objectivity. This chapter will emphasize the ERAs that are most widely used in clinical applications. There is a great amount of literature available for ERA and the specific response or potentials. This is not an in-depth review of all available ERA.
Below is a list of electrical response testing available. However, it must be stated that not all are widely performed or available in clinical settings and there are those that are used primarily in a research capacity.
Electrocochleography (ECoG or ECochG)
Auditory brain stem response (ABR), brain stem-evoked response audiometry (BERA), brain stem auditory-evoked response audiometry (BAER).
Cortical electric response audiometry (CER or CERA), N1-P2 response
Auditory steady-state response (ASSR), auditory steady-state evoked potential (ASSEP)
Middle-latency response (MLR)
Cervical vestibular-evoked myogenic potentials (cVEMP)
Occular vestibular-evoked myogenic potientials (oVEMP)
Somatosensory-evoked potential (SSEP)
Neural response telemetry (NRT)
Classification of AEPs By Latency
Burkard et al1 stated that the classification of AEPs are primarily based on peak response latency that distinguishes between short-latency, middle-latency response (MLR), and long-latency (auditory late responses—ALR) AEPs.
ABR peaks are indicated by roman numerals:
Waves I, II, III, IV, and V
The most reliable are waves I, III, and V
Po, Na, Pa, Nb, and Pb
P1, N1, P2, and N2.
Generators of Auditory-Evoked Responses
There is ongoing debate over the generation sites of a number of evoked responses and it is commonly accepted that there is more than one neural origin involved in creating each response2. This is currently the subject of much research. However, below you will find the presently recognized generator sites of the AEPs.
Cochlea, eighth nerve, and brain stem:
Wave I = distal end of the eighth nerve, cochlear
Wave II = proximal end of the eighth nerve, cochlear
Wave III = caudal (lower) brain stem near trapezoid body and superior olivary complex
Wave IV = superior olivary complex
Wave V = lateral lemniscus as it enters the inferior colliculus
Waves VI and VII = inferior colliculus
Na = possibly thalamus
Pa = Primary auditory cortex (measured over temporal lobe)
Pa = Subcortical generator (measured with a midline electrode)
P2 = primary or secondary auditory complex
P3 = auditory regions of hippocampus in medial temporal lobe
Mismatched negativity response (MMN):
Subcortical and primary cortical auditory regions
Electrocochleography (ECoG) has an array of clinical applications and is beneficial in the evaluation of the inner ear and auditory nerve function1. This is a method that is used to record the potentials produced from the cochlea and the auditory nerve. Knowledge of the electrophysiology of the cochlea and the electrical potentials in the cochlea is needed to fully comprehend the measurements of the ECoG. When performing an ECoG, we are analyzing the electrical potentials that occur with sound stimuli. These include the summating potential, action potential, and the cochlear microphonic. Detailed descriptions of these events are numerous; however, for the scope of this chapter, this section will assess the key features related to clinical ECoG application.
Summating potential (SP):
Outer hair cells
Organ of Corti
Inner hair cells (> 50%)
Compound action potential (CAP):
Distal eighth cranial nerve afferent fibers
Cochlear microphonics (CM):
Outer hair cells
The CM is an alternating current (AC) voltage primarily occurring from the outer hair cells and the organ of Corti. The CM exactly echoes the acoustic stimulus at low to moderate levels which causes difficulty in distinguishing between the CM response and stimulus artifacts in clinical settings using noninvasive techniques. Alternating stimuli for phase cancellation of the CMs are used for ECoG tests. Recently, however, there has been a greater focus on the use of CMs for the evaluation of site of lesion via ECoG for diagnosing auditory neuropathy, along with otoacoustic emissions. While evaluating CMs in auditory neuropathy population, they have been found to be pronounced and less susceptible to mild middle ear pathologies. For this reason, there has been increasing focus in distinguishing CMs for use in clinical settings and research settings.1
The SP is seen as a direct current (DC) voltage that reflects the time-displacement pattern of the cochlear partition in response to the stimulus envelope. Depending on the interaction between the location of the recording electrodes and the stimulus parameters, a positive or negative shift in the CM baseline occurs, causing the DC shift. Some components of the SP are believed to reflect the nonlinear distortion in the transduction product when DC voltage reacts to AC voltage.
Since the SP is generated by hair cells, it being reduced or absent indicates a lesion or dysfunction at the level of the hair cell and can be affected in sensorineural hearing loss. This means that the ECoG can also be used to evaluate for a normal cochlea versus one with sensorineural hearing loss or neural and retrocochlear lesions.1
When the SP is decidedly large in amplitude, this is agreed upon as an indication of endolymphatic hydrops/Ménière disease. Due to this finding the primary clinical use for the ECoG has now been shifted into evaluating for Ménière disease.
The compound action potential (CAP/AP) is a transient response that occurs at the onset of a click stimulus. It is a postsynaptic response and reflects the synchronous firing of a number of auditory nerve fibers. The CAP is an AC voltage that primarily appears as negative deflections called N1 and N2 that are synonymous with waves I and II, respectively, of an ABR.1
There are currently three methods of recording an ECoG, including both invasive (transtympanic) and noninvasive (extratympanic) techniques. The distance of the electrode site from the source of the impulse, in this case the cochlea, affects the amplitudes and the reliability of the ECoG. It is also important to note that the normative data are altered by the electrode site when analyzing the results.
Transtympanic electrode: A needle electrode is used to penetrate the tympanic membrane at the inferior portion and is placed over the cochlear promontory. This is an invasive technique that requires the tympanic membrane to be anesthetized prior to placement. This technique produces ECoG recording with optimal quality and amplitudes.
TIPtrode or intrameatal electrode: An insert earphone that is covered in gold foil is inserted into the external auditory canal making contact with the canal walls. This far-field placement produces low amplitudes that require significantly more signal averaging.
Tymptrode electrode: The electrode is placed in direct contact with the tympanic membrane without penetrating. This method yields better amplitudes than the TIPtrode method, because of the fact that the electrode is closer to the cochlea.
Ménière disease and endolymphatic hydrops
Diagnosis, assessment, and monitoring through the measurement of the SP/AP ratio resulting in that about 65% to 70% of the Ménière ears will be detected as abnormal with a false-positive rate around 5%.1 The SP/AP ratio percentages differ dependent on the electrode used for the test.2
TIPtrode: > 50% = abnormal
Tymptrode: > 35% = abnormal
Transtympanic: > 30% = abnormal
Enhancing the AP amplitude in individuals whose wave I may be absent or difficult to identify
Intraoperative monitoring of the peripheral auditory system
Objective assessment of audiometric thresholds:
However, the ABR has become more widely used than the ECoG for threshold evaluation.
Acoustic neuromas: The ABR has replaced the ECoG as the standard because it is a more accurate test in this application.
Auditory neuropathy (AN):
Diagnosis of AN by comparing an ABR tracing to an ECoG tracing:
Absent neural function (ie, abnormal ABR) in the presence of normal cochlear (ie, normal CM) function
Auditory Brain stem Response Audiometry
An ABR is an objective test that elicits brain stem potentials in response to click or tone burst/tone pip stimuli. A computer system filters and averages the response of the auditory pathway to the auditory stimuli, resulting in a waveform with peaks that represent generator sites; waves I, II, III, IV, and V, as stated earlier in this chapter. ABRs can be performed via air conduction using earphones/insert earphone or via bone conduction.
It is generally agreed that ABRs can be affected by the subjects’ sex, age, body temperature, and degree of hearing loss, but are not acutely affected by most sedatives anesthesia, drugs, or state of arousal. The ABR should be used in conjunction with other audiologic procedures.