Electrocochleography and Cochlear Implantation

Introduction

Electrocochleography is a method of recording electrical potentials generated in the cochlea in response to acoustic stimuli. For purposes of the following discussion, they are listed below:

Name                                                               Principle Site of Origin                      Potential Type

1. Summating Potential (SP)                       Outer hair cells                           DC

2. Action Potential (AP, CAP)               Auditory nerve axons                 DC

3. Cochlear Microphonic (CM)               Outer hair cells                            AC

4. Auditory nerve neurophonic (ANN)       Auditory nerve dendrites           AC

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Clinicians are familiar with extracochlear recording of SP and AP in the diagnosis of Meniere’s disease and CM in the diagnosis of auditory neuropathy spectrum disorder. However, intracochlear recording of ECochG with CI electrodes is a more recent development. Calloway et al.¹ demonstrated that CI recipients with severe to profound hearing loss routinely have measurable ECochG responses to acoustic stimuli when recording with an intracochlear CI electrode.

Over the past ten years or so, Craig Buchman, Oliver Adunka and their colleagues, as well as other teams, have been investigating the intraoperative use of ECochG to monitor electrode insertion trauma during cochlear implantation.^1-6 In 2010, Adunka et al.² demonstrated in gerbils that CM and AP can be used as physiologic markers of electrode contact with the basilar membrane and may be able to provide forewarning to the surgeon to avoid permanent loss of neural function or histological damage.

What’s New

Recently Koka et al.⁶ reported monitoring ECochG in human subjects during CI electrode insertion to estimate electrode position and conservation of residual hearing. ECochG, specifically the CM, was recorded from the apical electrode (closest to low frequency place) in response to 110 dB SPL tone bursts. As interpreted by a pragmatic algorithm based on amplitude and phase, changes in ECochG correctly identified electrode array position (S. tympani vs. vestibuli) in 26 (82%) of 32 human subjects while 6 (18%) electrodes were wrongly identified as translocated (sensitivity = 100%, specificity = 77%; positive predictive value = 54%, and negative predictive value = 100%). ECochG changes could be seen when an electrode first contacted the basilar membrane providing advanced warning of translocation from S. tympani to vestibuli.

Intracochlear ECochGs have also been recorded from implanted guinea pigs which were then studied histologically by Helmstaedter et al.⁷ The authors suggest that the SP can be used to identify the pitch place of individual electrode contact and the AP was generally correlated with the degree of electrode insertion trauma. For further information on intracochlear measurements of ECochG potentials, Bester et al.⁸ have provided a detailed characterization.

Finally, Koka et al.⁹ explored the relationship between ECochG thresholds and behavioral thresholds in a group of CI recipients tested on the same postoperative day. These studies showed strong correlation (r=0.87) with a mean difference of -3.2 dB (+/- 9dB) suggesting that ECochG may accurately predict audiometric threshold.

Take Home

ECochG may have several important applications in cochlear implantation:

• Assess electrode array location (S. tympani/ vestibuli)
• Asses the pitch place of individual contacts and the cochleotopic boundary of residual low frequency hearing
• Identify real-time onset of one type of electrode insertion trauma (interaction with the basilar membrane prior to permanent damage)
• Determine audiometric thresholds postoperatively
• Help develop minimally-traumatic electrodes (MTEs) and insertion techniques
• Assist in surgical training of neural conservation techniques.

Like the facial nerve monitor, ECochG may provide adjunctive forewarning of intra-cochlear neural trauma. The use of ECochG has the potential to improve CI outcomes by conserving functional neural structures, residual hearing, and balance.

References

1. Calloway NH, Fitzpatrick DC, Campbell AP, Iseli C, Pulver S, Buchman CA, Adunka OF. Intracochlear electrocochleography during cochlear implantation. Otology and Neurotology 2014; 35: 1451-1457.
   
   
2. Adunka OF, Mlot S, Suberman TA, Campbell AP, Surowitz J, Buchman CA, Fitzpatrick DC. Intracochlear Recordings of Electrophysiological Parameters Indicating Cochlear Damage. Otology & Neurotology. 2010; 31:1233-1241.
   
   
3. Harris MS, Riggs WJ, Giardina CK, O'Connell BP, Holder JT, Dwyer RT, Koka K, Labadie RF, Fitzpatrick DC, Adunka OF. Patterns Seen During Electrode Insertion Using Intracochlear Electrocochleography Obtained Directly Through a Cochlear Implant. Otol Neurotol. 2017 Dec;38(10):1415-1420.
   
   
4. Riggs WJ, Roche JP, Giardina CK, Harris MS, Bastian ZJ, Fontenot TE, Buchman CA, Brown KD, Adunka OF, Fitzpatrick DC. Intraoperative Electrocochleographic Characteristics of Auditory Neuropathy Spectrum Disorder in Cochlear Implant Subjects. Front Neurosci. 2017 11:416-422.
   
   
5. Harris MS, Riggs WJ, Koka K, Litvak LM, Malhotra P, Moberly AC, O'Connell BP, Holder J, Di Lella FA, Boccio CM, Wanna GB, Labadie RF, Adunka OF. Real-Time Intracochlear Electrocochleography Obtained Directly Through a Cochlear Implant. Otol Neurotol. 2017 Jul;38(6):e107-e113.
   
   
6. Koka K, Riggs WJ, Dwyer R, Holder JT, Noble JH, Dawant BM, Ortmann A, Valenzuela CV4, Mattingly JK, Harris MM, O'Connell BP, Litvak LM, Adunka OF, Buchman CA, Labadie RF. Intra-Cochlear Electrocochleography During Cochear Implant Electrode Insertion Is Predictive of Final Scalar Location. Otol Neurotol. 2018 Sep;39(8):e654-e659.
   
   
7. Helmstaedter V, Lenarz T, Erfurt P1, Kral A, Baumhoff P. The Summating Potential Is a Reliable Marker of Electrode Position in Electrocochleography: Cochlear Implant as a Theragnostic Probe. Ear Hear. 2018 Jul/Aug;39(4):687-700.
   
   
8. Bester CW, Campbell L, Dragovic A, Collins A, O'Leary SJ. Characterizing Electrocochleography in Cochlear Implant Recipients with Residual Low-Frequency Hearing. Front Neurosci. 2017 Mar 23;11:141.
   
   
9. Koka K, Saoji AA, Litvak LM. Electrocochleography in Cochlear Implant Recipients with Residual Hearing: Comparison with Audiometric Thresholds. Ear Hear. 2017 May/Jun;38(3):e161-e167.

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