ecochG.pptx

ECochG is a variant of brainstem audio evoked response (ABR) where the recording electrode is placed as close as practical to the cochlea. We will use the abbreviation ECOG and ECochG interchangeably below. ECOG is preferable to us as it is shorter.

ECOG is intended to diagnose Meniere's disease, and particular, hydrops (swelling of the inner ear). ECOG may also be abnormal in perilymph fistula, and in superior canal dehiscence. The common feature connecting these illnesses is an imbalance in pressure between the endolymphatic and perilymphatic compartment of the inner ear.

ECOG can also be used to show that the cochlea is normal, in persons who are deaf. The cochlear microphonic of ECOG may be normal in auditory neuropathy (Santarelli and Arslan 2002) as well as other disorders in which the cochlea is preserved but the auditory nerve is damaged (Yokoyama, Nishida et al. 1999).

Finally, ECOG's have also been used to as a indicator of the temporary threshold shift that may follow noise injury (Nam et al, 2004).

1. Seminar on Recent advancements in Electrocochleography (ECochG) Mukesh Sharma Ph.D. Scholar (ASLP ) Session (2022-23)

2. Electrocochleography (ECochG) involves the measurement of stimulus-evoked potentials generated by cochlear hair cells and auditory nerve fibers ( Eggermont , 2017 ).  HISTORY: (Surgical & Non-surgical recording period ) The CM was first discovered in 1930 by Ernest Wever and Charles Bray in cats. Fromm et al. (1935),were the first to employ the ECochG technique in human by inserting electrode through the TM and recorded the CM from the round window & cochlear promontory. Introduction:

3. Lempert et al. (1947) coined the term “ Cochleogram ”. The SM, a stimulus-related hair cell potential, was first described by Davis et al . (1950) & later Tasaki et al . (1954). Ruben (1961), was the first to record CAP. Non-surgical period started in 1967. Moore (1971), recorded CM & AP using surface electrode. ECochG was used in the diagnosis of Meniere’s disease in late 1984. In 1990s, things picked up again in the following 12 years with improving the use of ECochG in the diagnosis of Meniere’s disease. Number of articles, usefulness of ECochG was improved by 2010.

4. Principles of ECochG Both Inner Hair Cells (IHC) and Outer Hair Cells (OHC) generate receptor potentials in response to sound (Russell & Sellick , 1978; Dallos et al., 1982 ). Compound responses from the cochlea reflecting these hair cell potentials can be recorded at round window, TM or even from the scalp and can be used clinically . These responses are called as Cochlear Microphonics (CM) and Summating Potential (SP).

5. Electrophysiology ECochG measures electrophysiological responses from the cochlea and the auditory nerve ( Bakhos et al., 2017) .   During ECochG, a brief duration of acoustic stimuli (clicks or acoustic tone bursts) of alternating polarity is used to elicit electrophysiological responses that can be measured using skin electrodes, extra tympanic electrodes, transtympanic electrodes, or intracochlear CI electrodes (Gibson WP, 2017). An acoustic tone burst generates electrophysiological responses from a localized region in the cochlea or the auditory nerve, whereas clicks are known to elicit responses across a broader frequency range .

6. ECochG components: CM- AC- OHC. ANN- phase-locked signal - AN SP and CAP are direct currents. SP – OHC & IHC CAP - AN

7. Cochlear M icrophonic:- (CM) The CM is produced almost exclusively from OHC receptor currents and when recorded from the round window, dominated by the responses of OHCs in the basal turn (Russell, 2008 ). Alternating current (AC) voltage that reflects the instantaneous displacement of the basilar membrane along some distance defined by the effective site of the recording within the cochlea, the method of recording and the stimulus conditions. The popularity of CM in the laboratory derives from, Its link to cochlear transduction. Its sensitivity to the health of the cochlear partition. The relative ease with which it can be recorded within the fluid spaces of the cochlea.

8. CM mimics the stimulus, so it is difficult to separate it from stimulus artifact. CM is sensitive to changes in middle ear transmission characteristics. CM follows polarity of click stimulus, summing or averaging the response to stimuli of alternating polarity will generally cancel out the CM ( Yoshie , 1981). Reductions in CM magnitude have been reported for various disorders such as MD & vestibular schwannomas (Gibson & Beagleus 1976)

9. Summating P otential (SP): It is a complex response made up of several components. It is direct current shift receptor potential reflecting cochlear electrical activity in response to acoustic stimulation. It follows envelops of the stimulus and clearly influenced by the stimulus duration but not markedly influenced by stimulus frequency. The SP manifests itself as a shift in the CM baseline, the direction of which is dictated by an interactive effect between stimulus parameters (frequency and intensity) and the location of recording electrode. When recorded Extra tympanic means the SP is often seen as a downward deflection that persists for the duration of the acoustic stimulus.

10. Some of the components of SP represent inherent nonlinearities associated with the transduction process in the cochlea. Its sensitive to mechanical or electrical biasing. ( Durrant & Dallos 1974) . SP is enlarged in MD/ELH. (Schmidt et al., 1974 )

11. Compound Action Potential (CAP): It is a summed response of numerous auditory nerve fibers firing synchronously . It is an AC voltage, characterized by a series of brief predominantly negative peaks. AP involves comparing its magnitude to that of SP in patients suspected of having MD/ELH. (SP/AP magnitude ratio). Enlarged magnitude ratio to click stimulus is a positive finding for ELH.( Eggermont , 1976; Coats, 1981)

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

13. TT ECochG is an invasive procedure that involves passing a needle electrode through the TM to rest on the cochlear promontory. During surgeries that expose the middle ear space, TT recordings can also be made with a ball electrode on the round window via the surgical field. Most audiologists who perform ECochG in the clinic prefer ET approaches, wherein recordings are performed with an electrode resting against the skin of the ear canal or surface of the TM. For the latter recording site, the procedure is also referred to as “Tympanic or TM ECochG ” ( Ferraro & Ferguson 2000), even though this approach is still considered to be ET.

14. ECochG Recording Parameter: Parameter Selection Electrode placement   Non-inverting Ear canal or TM or Promontary or RW  Inverting Contralateral mastoid  Ground Fpz (low forehead ) or earlobe Signal Averaging Setting Time Base 5-10 ms Amplification 50000-1,00,000 (ET), 5000 – 25000 (TT) Bandpass Filter 5Hz – 3K Hz Repetition 1000-1500 (ET), 100-200 (TT) Stimuli Type Broadband Clicks or Tone Bursts Polarity Alternating Repetition Rate 11.3/seconds Level 85-95 dB HL Inter electrode impedance Within + 2kohms

15. Electrode Types & Placement: 1. Trans-tympanic electrodes (TT): Placement of needle on the promontory/ Round window. Advantage: “Near field”, requiring relatively little signal averaging. Larger Amplitude Enhanced reliability Confidence in waveform analysis and in diagnosis of Meniere’s disease. Limitations: Invasiveness Assistance of physician and are therefore limited to medical setting. Local anesthetics should be used (Beyond et al., 1998 ).

16. 2 . Tiptrode Electrode :- It is inserted deeply in to the ear canal . Any discomfort to the patient should be monitored

17. 3. Tympanic Membrane Electrodes: This is designed to maximize response amplitudes while minimizing clinical preparation cleansing and insertion requirements . The electrode consists of a small gauge silver wire enclosed within a flexible silastic tube and connected to a soft foam sponge at the tip, which will be filled with conductive gel. The electrode tip is made to contact with the TM.

18. 4. Subdermal Needle electrodes: During intraoperative neurophysiologic monitoring, sub dermal needle electrodes are often used. It has the advantage of secure attachment to the patient during surgical procedure and lower impedance. Two main health risks posed by needle electrodes are: Spread of disease by an unsterilized needle. Needle breakage. Should be done by appropriate medical professional.

19. Comparative ecochg findings with different electrode type and location

20. ECochG waveform analysis & interpretation : Three general ECochG analysis strategies A comparison of relative amplitude for the SP & AP components and the SP/AP ratio. A comparison of the latency difference for the AP component evoked with signals of rarefaction vs condensation polarity. Calculation of the duration of the SP and AP complex and calculation of area under SP and AP components.

21. 1. Absolute latency: Absolute latency of components is a fundamental analysis strategy. Identification of reliable AP component is the initial step in waveform analysis of ECochG. In normal ears, latency is slightly shorter for AP components evoked with rarefaction vs condensation signals. 2. Amplitude analysis: Amplitude can be measured from a single point or a baseline. Typically the peak in SP and AP amplitude in microvolt is calculated from a baseline. This values used to calculate SP/AP magnitude ratio for click stimuli, which is helpful to diagnose and monitor MD/ELH. SP/AP magnitude ratio for click stimuli ranges 0.16 to 0.31, despite the use of different recording approaches in normal's.

22. SP to tone burst persists as long as stimulus. AP in turn seen at onset. SP magnitude is measured at the midpoint of the wave form with reference to baseline magnitude. Thus the polarity of SP depends on whether the voltage at midpoint is above (positive SP) or below (negative SP) the baseline voltage. For both the recording TM and TT the polarities of the SPs at 500 and 8000 Hz are slightly positive where as negative SPs are seen at 1,2 and 4 KHz. This feature tends to vary across frequencies, recording approaches and across subjects.

23. MD the SP amplitude is enlarged . Rationale is that an increase in endolymph volume creates mechanical /electrical biasing of vibration of the organ of corti to which the SP is sensitive. Other factors such as biochemical and or vascular changes may also be responsible for an enlarged SP in MD/ELH

24. 3. Duration analysis: Duration or width of the SP and AP components in combination, is a measurement parameter for the diagnosis of MD ( Ge & Shea 2002). The width of the SP & AP portion of the wave form, in ms, is defined from the onset to the point where the wave form returns to the baseline. The suspected mechanism underlying the prolongation in the SP/AP width at least in MD is, slowed velocity of the traveling wave within the cochlear secondary to restricted basilar membrane movement with increased loading in ELH (Ferraro & Tibbils 1999)

25. Clinical Application: Assessment of peripheral hearing loss. Enhancement of ABR Wave I. Diagnosis of MD Diagnosis of Auditory Neuropathy. Intraoperative monitoring Recent Advancement in ECochG Nowadays, ET ECochG is most often used as an additional measurement in the diagnosis of patients with Meniere’s Disease (MD). It may also be a promising tool for the diagnosis and evaluation of hearing loss in children with an auditory neuropathy/ dyssynchrony spectrum disorder (ANSD) and potential candidates for a cochlear implant ( McMahon et al. , 2009).

26. ECochG is an important tool for the diagnosis and evolution of ELH, with an existing preference for the ET electrode positioning ( Lamounier et al., 2014 ). Increased SPs were observed with ET ECochG in patients with MD (Kumar & Peepal 2012). ECochG found its main application in the diagnostic evaluation of Meniere's disease (MD). However, in the last decade, the focus has shifted towards cochlear implantation (CI). In patients with residual hearing after CI, combined electric and acoustic stimulation has resulted in improved hearing and speech outcomes (Carlson et al.,   2021).

27. During implantation, real-time ECochG offers an opportunity to measure frequency specific CMs elicited from a localized region in the cochlea as the surgeon inserts the electrode array. In extracochlear ECochG recordings, the recording electrode can be placed on the promontory, the stapes, or the tympanic membrane. Intracochlear ECochG can be performed by inserting a recording electrode into the cochlea or by using one of the CI electrodes as the recording electrode. The loss of intraoperative ECochG signal may indicate cochlear trauma from electrode insertion. The ability to monitor cochlear trauma during CI electrode placement holds promise to improve hearing preservation outcomes, modify surgical techniques, and change electrode design ( Carlson et al.,  2021 ).  

28. Reference: Bakhos , D., Marx, M., Villeneuve, A., Lescanne , E., Kim, S., & Robier , A. (2017). Electrophysiological exploration of hearing.  European annals of otorhinolaryngology , head and neck diseases ,  134 (5), 325-331. Gibson, W. P. (2017). The clinical uses of electrocochleography .  Frontiers in neuroscience ,  11 , 274. Harris, M. S., Riggs, W. J., Giardina , C. K., O’Connell, B. P., Holder, J. T., Dwyer, R. T ., & Adunka , O. F. (2017). Patterns seen during electrode insertion using intracochlear electrocochleography obtained directly through a cochlear implant.  Otology & neurotology : official publication of the American Otological Society, American Neurotology Society [and] European Academy of Otology and Neurotology ,  38 (10), 1415 . Choudhury , B., Fitzpatrick, D. C., Buchman, C. A., Wei, B. P., Dillon, M. T., He, S., & Adunka , O. F. (2012). Intraoperative round window recordings to acoustic stimuli from cochlear implant patients.  Otology & neurotology : official publication of the American Otological Society, American Neurotology Society [and] European Academy of Otology and Neurotology ,  33 (9), 1507 .

29. Pappa , A. K., Hutson , K. A., Scott, W. C., Wilson, J. D., Fox, K. E., Masood , M. M., & Fitzpatrick, D. C. (2019). Hair cell and neural contributions to the cochlear summating potential.  Journal of neurophysiology ,  121 (6), 2163-2180. Minaya , C., & Atcherson , S. R. (2015). Simultaneous extratympanic electrocochleography and auditory brainstem responses revisited.  Audiology research ,  5 (1), 105. Tasaki, I., Davis, H., & Eldredge , D. H. (1954). Exploration of cochlear potentials in guinea pig with a microelectrode.  The Journal of the Acoustical Society of America ,  26 (5), 765-773. . Wever , E. G., & Bray, C. W. (1930). Auditory nerve impulses.  Science ,  71 (1834), 215-215 . Fromm, B., Nylen , C. O., & Zotterman , Y. (1935). Studies in the mechanism of the Wever and Bray effect.  Acta oto-laryngologica ,  22 (3), 477-486 . Moore, E. J. (1971).  Human cochlear microphonics and auditory nerve action potentials from surface electrodes . The University of Wisconsin-Madison.