Electrocochleography
Encyclopedia
Electrocochleography is a technique of recording stimulus-related responses or electrical potentials of the inner ear and auditory nerve. Components potentials of the human electrocochleogram are: cochlear microphonics (CM), summating potentials (SM), and action potential
(AP). These potentials can either be recorded independently or in various combinations. CM and SP are generated in the organ of corti
and are a type of receptor potential
. AP is the neural potentials generated by the cochlear nerve
.
(BM) and the hair cells function as a sharply tuned frequency analyzing mechanism. Once acoustic energy reaches the tympanic membrane (TM), it is converted into mechanical energy and carried on to the inner ear through the middle ear bones. As the stapes pushes the oval window, pressure wave in the perilymph
inside cochlea
causes the BM to vibrate. Place of maximum vibration amplitude depends on the sound frequency. The area of maximum displacement in the BM will cause the hair cells and supporting structures to move. During the upward displacement of the BM and the hair cells, the stereocilia
and the kinocilia of the hair cells move against the tectorial membrane. When the stereocilia are bent towards the modiolus, mechanically gated ion channels open and potassium (K+) and calcium (Ca2+) ions enter. An AC current flows through the hair bearing surface and this increase in the flow (or decrease in the resistance has the same frequency as the BM movement, and hence, the acoustic stimulus frequency. This measurable AC voltage is called the cochlear microphonic (CM), which mimics the stimulus. The hair cells function as a transducer, converting the mechanical movement of the BM into electrical voltage. Adenosine tri-phosphate (ATP) from the stria vascularis provides energy for the conversion. This potential derived from the CM initiate chemical processes in hair cells that lead to the release of neurotransmitters in the synaptic cleft between the hair cells and the spiral ganglion neurons. The neurotransmitters rapidly diffuse and combine with the receptor cells in their specific locations. This results in a built up of postsynaptic potential
or generator potential in the unmyelinated nerve endings. When a certain threshold value is reached, the generator potential depolarizes the first neuron. The axon stimulation by depolarization
results in AP.
Cochlear microphonic (CM) is an alternating current
(AC) voltage that mirrors the waveform of the acoustic stimulus. It stems from the outer hair cells of the organ of corti. The magnitude of the recording is dependent on the proximity of the recording electrodes to the hair cells. The CM is proportional to the displacement of the basilar membrane. Usually the recordings are generated primarily from the basal end of the cochlea which is nearest to the promontory or the electrodes. Summating potential (SP), first described by Tasaki et al. in 1954, is the DC response of the hair cells as they move in conjunction with the BM 1954. SP is the stimulus-related potential of the cochlea. Although historically least studied, renewed interest has surfaced due to its changes reported in cases of endolymphatic hydrops or meniere’s disease. Auditory nerve action potential is the most widely studied component in ECochG. AP represents the summed response of the synchronous firing of the nerve fibers. It also appears as an AC voltage. The first, and the largest wave is named N1 that is identical to wave I of auditory brainstem response
(ABR). Subsequent to N1 is N2, which is identical to wave II. The AP magnitude is the reflection of the number of fibers that are firing and latency is the time between the onset and the formation of the peak N1.
Electrodes can be either invasive of non-invasive. Invasive electrodes like transtympanic needle give clearer, more robust electrical response (larger amplitudes) since the electrodes are very close to the generators. The needle is placed on the promontory wall of the middle ear and the round window. Non-invasive electrodes or the extratympanic electrodes can be used atraumatically without causing pain or discomfort to the patient. Unlike the invasive electrodes, there is no need for sedation, anesthetics and medical supervision to use non-invasive electrodes. The responses, however, are smaller in magnitude. Some of the commercially available ear canal electrodes are foam earplug, eartrode and tiptrode.
or alternating
. The primary (non-inverted) recording site is the ear canal, TM or the promontory. The reference electrodes (inverting) can be contralateral earlobe, mastoid, or ear canal. Analysis time of 5-10 ms allows signal averaging and preamplifier amplification factor may be as high as 50,000 -100,000 times for ET and 5,000-25,000 times for TT is used depending on the level of background, electrical, myogenic
, and encephalographic noise. High pass filter at 1 Hz and low pass at 3 kHz is frequently used. Repetition rate f 5-11/sec is used at higher intensity level (85-90 dB HL).
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
(AP). These potentials can either be recorded independently or in various combinations. CM and SP are generated in the organ of corti
Organ of Corti
The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...
and are a type of receptor potential
Receptor potential
Receptor potential, a type of graded potential, is the transmembrane potential difference of a sensory receptor.A receptor potential is often produced by sensory transduction. It is generally a depolarizing event resulting from inward current flow...
. AP is the neural potentials generated by the cochlear nerve
Cochlear nerve
The cochlear nerve is a nerve in the head that carries signals from the cochlea of the inner ear to the brain...
.
History
CM was first discovered in 1930 by Wever and Bray in cats. Wever and Bray mistakenly concluded that this recording was generated by the auditory nerve [2]. They named the discovery the “Wever-Bray effect.” Hallowell Davis, AJ Derbyshire from Harvard replicated the study and concluded that the waves were in fact cochlear origin and not from the auditory nerve. Fromm et al. were the first investigators to employ the ECochG technique in humans by inserting a wire electrode through tympanic membrane and recording the CM from the niche of the round window/promontory. In 1935, they discovered CM in humans. They also found the responses following the CM, which consisted of N1, N2, and N3, but it was Tasaki who identified those waves were the AP waves attributed to the CNVIII. Fisch and Ruben were the first ones to provide evidence of round window recordings of the compound action potentials from both the round window and the eighth cranial nerve (CNVIII) in cats and mice. Ruben was the first person to use CM and AP clinically. Summating potential, a stimulus-related hair cell potential was first described by Tasaki and colleagues in 1954. Dr. Ernest J. Moore was the first investigator to record the CM from surface electrodes. In 1971, Dr. Moore conducted 5 experiments where he recorded CM and AP from 38 human subjects using surface electrodes. The purpose of the experiment was to establish the validity of the responses and to develop an artifact-free earphone system. Unfortunately, bulk of his work was never published.Cochlear Physiology
Basilar membraneBasilar membrane
The basilar membrane within the cochlea of the inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani .-Function:...
(BM) and the hair cells function as a sharply tuned frequency analyzing mechanism. Once acoustic energy reaches the tympanic membrane (TM), it is converted into mechanical energy and carried on to the inner ear through the middle ear bones. As the stapes pushes the oval window, pressure wave in the perilymph
Perilymph
Perilymph is an extracellular fluid located within the cochlea in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid...
inside cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
causes the BM to vibrate. Place of maximum vibration amplitude depends on the sound frequency. The area of maximum displacement in the BM will cause the hair cells and supporting structures to move. During the upward displacement of the BM and the hair cells, the stereocilia
Stereocilia
In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...
and the kinocilia of the hair cells move against the tectorial membrane. When the stereocilia are bent towards the modiolus, mechanically gated ion channels open and potassium (K+) and calcium (Ca2+) ions enter. An AC current flows through the hair bearing surface and this increase in the flow (or decrease in the resistance has the same frequency as the BM movement, and hence, the acoustic stimulus frequency. This measurable AC voltage is called the cochlear microphonic (CM), which mimics the stimulus. The hair cells function as a transducer, converting the mechanical movement of the BM into electrical voltage. Adenosine tri-phosphate (ATP) from the stria vascularis provides energy for the conversion. This potential derived from the CM initiate chemical processes in hair cells that lead to the release of neurotransmitters in the synaptic cleft between the hair cells and the spiral ganglion neurons. The neurotransmitters rapidly diffuse and combine with the receptor cells in their specific locations. This results in a built up of postsynaptic potential
Postsynaptic potential
Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials...
or generator potential in the unmyelinated nerve endings. When a certain threshold value is reached, the generator potential depolarizes the first neuron. The axon stimulation by depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
results in AP.
Cochlear potentials
Resting endolymphatic potential of + 80 mV is present in a normal cochlea. There are at least 3 other potentials generated upon cochlear stimulation:- Cochlear microphonic (CM)
- Summating Potential (SP)
- Action PotentialAction potentialIn physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
(AP)
Cochlear microphonic (CM) is an alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) voltage that mirrors the waveform of the acoustic stimulus. It stems from the outer hair cells of the organ of corti. The magnitude of the recording is dependent on the proximity of the recording electrodes to the hair cells. The CM is proportional to the displacement of the basilar membrane. Usually the recordings are generated primarily from the basal end of the cochlea which is nearest to the promontory or the electrodes. Summating potential (SP), first described by Tasaki et al. in 1954, is the DC response of the hair cells as they move in conjunction with the BM 1954. SP is the stimulus-related potential of the cochlea. Although historically least studied, renewed interest has surfaced due to its changes reported in cases of endolymphatic hydrops or meniere’s disease. Auditory nerve action potential is the most widely studied component in ECochG. AP represents the summed response of the synchronous firing of the nerve fibers. It also appears as an AC voltage. The first, and the largest wave is named N1 that is identical to wave I of auditory brainstem response
Auditory Brainstem Response
The auditory brainstem response is an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The resulting recording is a series of vertex positive waves of which I through V are evaluated...
(ABR). Subsequent to N1 is N2, which is identical to wave II. The AP magnitude is the reflection of the number of fibers that are firing and latency is the time between the onset and the formation of the peak N1.
Electrodes can be either invasive of non-invasive. Invasive electrodes like transtympanic needle give clearer, more robust electrical response (larger amplitudes) since the electrodes are very close to the generators. The needle is placed on the promontory wall of the middle ear and the round window. Non-invasive electrodes or the extratympanic electrodes can be used atraumatically without causing pain or discomfort to the patient. Unlike the invasive electrodes, there is no need for sedation, anesthetics and medical supervision to use non-invasive electrodes. The responses, however, are smaller in magnitude. Some of the commercially available ear canal electrodes are foam earplug, eartrode and tiptrode.
Recording Parameters
Broadband clicks with duration of 100microsec electrical pulse are used. The polarity of the stimuli can be rarefraction, condensationCondensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
or alternating
Alternating
Alternating may refer to:In mathematics:*alternating form*alternating group*alternating series*alternating knot*alternating mapIn electronics:*alternating current...
. The primary (non-inverted) recording site is the ear canal, TM or the promontory. The reference electrodes (inverting) can be contralateral earlobe, mastoid, or ear canal. Analysis time of 5-10 ms allows signal averaging and preamplifier amplification factor may be as high as 50,000 -100,000 times for ET and 5,000-25,000 times for TT is used depending on the level of background, electrical, myogenic
Myogenic
Myogenic contraction refers to a contraction initiated by the myocyte cell itself instead of an outside occurrence or stimulus such as nerve innervation.- Unstable Membrane Potentials :...
, and encephalographic noise. High pass filter at 1 Hz and low pass at 3 kHz is frequently used. Repetition rate f 5-11/sec is used at higher intensity level (85-90 dB HL).
Clinical Applications of ECochG
Most common clinical applications include:- Objective identification and monitoring of meniere’s disease and endolymphatic hydrops (EH): Enlarged SP reflects pathologic condition of EH. SP-AP amplitude ratio is more consistent feature to determine the presence of meniere’s disease. On average, SP-AP ratio of 0.45 or greater is considered abnormal.
- Intraoperative monitoring: Surgical operation on brainstem or cerebellum may bear risk of damage to the auditory system. ECochG recording during surgery assess the functional integrity of the peripheral and brainstem pathway directly during operation.
- ECochG also helps in identification or enhancement of wave I for subsequent measurement of interwave interval essential for ABR use for diagnostic purpose.
- ECochG recording can help diagnose auditory neuropathyAuditory neuropathyAuditory neuropathy is a variety of hearing loss in which the outer hair cells within the cochlea are present and functional, but sound information is not faithfully transmitted to the auditory nerve and brain properly....
.