Dr. Supreet Singh Nayyar, AFMC For more presentations, visit Electrocochleography

Headings Introduction Principles Recording techniques Interpretation Clinical applications

Introduction Electrocochleography (ECochG) Method for recording the electrical potentials of the cochlea Stimulus-related Often includes measurement of the whole nerve or compound action potential (AP) of the auditory nerve

Principles CM An alternating current voltage that mirrors the waveform of the acoustic stimulus at low-moderate levels of stimulation Generated predominantly by the outer hair cells Phase and duration are stimulus dependent

Principles SP A Direct Current generated predominantly by the outer hair cells Seen as a DC shift in the CM baseline Duration is dependent on the duration of the acoustic stimulus Independent of stimulus phase

Principles AP AC potential that represents the summed response of the synchronous firing of several thousand auditory nerve fibers N1 and N2 are virtually the same components as waves I and II of the ABR Independent of stimulus phase and duration

Principles The normal SP duration is b/w 0.30 and 0.45 ms, and the absolute amplitude of the SP can range from 0.1mV to 1.0 mV AP has a latency of 1.5 ms(1.3 ms to 1.7 ms) duration is normally between 0.80 ms and 1.25 ms, absolute amplitude can vary greatly, ranging from 0.6 mV to 3.0 mV

Principles Normal SP/AP ratios generally range from 0.1 to 0.4 across subjects with a mean value of Graphic Angle Measurement (GAM) Technique

Recording techniques There are two, general approaches for recording ECochG Transtympanic (TT) Extratympanic (ET )

Recording techniques Recording parameters

Recording techniques The following suggestions are offered to help to reduce stimulus artifact ▫ test subjects in sound booth with the examiner and AEP unit located outside of the booth; the windows of the booth should be shielded with a copper screen; ▫ plug the AEP unit into an isolated electrical socket equipped with a true-earth ground; ▫ use a grounded cable for the primary electrode (such cables are commercially available); ▫ turn off lights in the testing room and unplug unnecessary electronic equipment (it also may be necessary to turn off the lights in the examiner room); ▫ consider encasing the transducer in grounded Mu metal shielding

Interpretation Normal electrocochleogram from the tympanic membrane to clicks presented in alternating polarity at 80 dB HL

Interpretation Normal electrocochleogram from the tympanic membrane to a 2,000 Hz toneburst presented in alternating polarity at 80 dB HL

Clinical applications Menieres’ disease/ Endolymphatic hydro The reported incidence of an enlarged SP and SP/AP amplitude ratio in the general Meniere’s population is only approximately 60%-65%

Clinical applications Enhancement of wave I Wave I is absent in the conventional ABR tracings but recordable with the ECochG

Clinical applications Intraoperative monitoring of inner ear and auditory nerve status during surgeries identification of anatomic landmarks (such as the endolymphatic sac)

Clinical applications Other applications Estimation of hearing thresholds

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