Electrocochleography is a procedure to record the potentials generated by the cochlea and the auditory nerve. There are three classes of potentials that can be recorded from the cochlea and auditory nerve in response to a sound stimuli. They are compound action potential of auditory nerve (AP), Summating potential (SP), and cochlear potential (CP) also known as cochlear microphonic.
This is an objective way of estimating hearing threshold of a patient. When these recordings are made from electrodes placed close to the source of impulse the recorded potentials are quite large. If the recording is made from electrodes placed in far field, the potentials recorded are small in nature. It is always advisable to place the recording electrodes as close to the source of impulse as possible i.e. over the promontory.
Types of electrodes: Three types of electrodes are used to record ECog. They are:
Transtympanic membrane electrodes: These are needle electrodes, which are placed over the promontory by penetrating the ear drum. The ear drum must be anesthetised before placing the electorde. The needle passes through posterior inferior quadrant of ear drum. These electrodes are 22 gauge hypodermic needles.
Intrameatal electrodes: These electrodes are placed in the external auditory canal.
Surface electrodes: These are attached outside the external canal. These electrodes are clipped to the earlobe.
Figure showing placement of electrode for Electrocochleography recording
Advantages and disadvantages of transtympanic electrode:
Recordings from the promontory reveal evoked response, whose amplitudes are larger than that obtained by placing electrodes in the far field, like external canal or pinna. This is the advantage of using a transtympanic electrode in recording ECog. Infact this is the only advantage of using this electrode. The whole procedure is invasive, with the attendent complications of anesthetic medication.
Advantages and disadvantages of intrameatal and surface electrodes: Since recordings from these electrodes are far field, the impulses recorded are of low amplitudes, and need to be augmented. The main advantage of these electrodes is that they are non invasive, and patient need not be anesthetised before placing the electrode. These electrodes can be used in screening procedure in neonates.
Type of acoustic signal used used to stimulate Ecog: The appropriate signal used to stimulate ECog should be abrupt so that a large number of acoustic nerve fibers are stimulated. Click signals are the most appropriate stimuli. Tone pips can also be used. The sound pressure levels used should be between 80 - 85 dB.
Action potential: This is an A.C. potential representing the summed potential of synchronous firing of thousands of auditory nerve fibers. Action potentials are dominated by contributions from high frequency nerve fibers as a response to transient stimuli of rapid onset. It occurs at the onset of stimulus. It can be produced by clicks or tone pips. It is characterised by predominantly negative peaks i.e. N1 and N2. These waves are independent of stimulus phase and duration.
Figure showing action potential
The most useful features of action potential curve are its magnitude and latency. Out of them the magnitude has a direct relationship to the number of nerve fibers firing. Latency represents the time interval between the onset of stimulus and the peak of N1. The presence of action potential is dependent on the proper functioning of acoustic nerve fibers. Wave N1 is absent in patients with retrocochlear lesions.
Cochlear microphonics: These are generated predominently by the outer hair cells of cochlea. This is an A.C. potential. The basal turn of cochlea play an important role in generation of cochlear microphonics. This is more so when the recording is made from electrodes close to the round window. Its phase and duration are mostly stimulus dependent. It is diminised when stimulus is present with alternating polarity. It helps in differentiating cochlear from nerve deafness.
Summating potential: This is direct current. Outer hair cells of cochlea are responsible for this potential. The direction of this potential is dependent on complex interaction between the stimulus parameters and the location of the recording electrode.
Electrocochleography is predominently used in the diagnosis, assessment and monitoring Menier's disease, and endolymphatic hydrops. In Meneier's disease the summation potential is enlarged. It is also used for intra operative monitoring of peripheral auditory system. It is also useful in objectively assessing auditory threholds.