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Electrocochleography (ECochG) is a technique of recording stimulus-related responses in terms of electrical potentials of the inner ear and auditory nerve. Component potentials of the human electrocochleogram are: cochlear microphonics (CM), summating potentials (SP), and action potentials (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 are the neural potentials generated by the cochlear nerve.

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 originated 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 membrane (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. Amplitude of maximum vibration 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 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 initiates 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 buildup 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 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 Potential (AP)

Cochlear microphonic (CM) is an alternating current (AC) voltage that mirrors the waveform of the acoustic stimulus. It is dominated by 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.

Recording Parameters
Broadband clicks with duration of 100 microseconds electrical pulse are used. The polarity of the stimuli can be rarefaction, condensation 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/s 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 diagnostic purpose on ABR.


 * ECochG recording can help diagnose auditory neuropathy.