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Evaluation of eighth nerve integrity by the electrically evoked middle latency response
R W Jyung1, J M Miller, S C Cannon
1Kresge Hearing Research Institute, University of Michigan, Ann Arbor 48109-0506.
Summary
Electrically-induced middle latency response (EMLR) shows promise as an objective test for assessing eighth nerve fiber loss, crucial for cochlear implant candidate selection. EMLR slope correlates with spiral ganglion cell density, indicating its utility in evaluating auditory nerve health.
Area of Science:
- Neuroscience
- Auditory Neuroscience
- Ototoxicity Research
Background:
- Cochlear implantation requires objective measures of surviving auditory nerve fibers.
- Existing methods for assessing eighth nerve degeneration are limited.
Purpose of the Study:
- To evaluate the electrically-induced middle latency response (EMLR) as a reliable objective test for estimating eighth nerve fiber survival.
- To correlate EMLR measures with histologic evidence of spiral ganglion cell loss.
Main Methods:
- Guinea pigs received kanamycin and ethacrynic acid to induce graded eighth nerve degeneration.
- Electrically-induced middle latency response (EMLR) was recorded at various post-drug intervals.
- Animals were euthanized for histologic preparation and spiral ganglion cell density determination.
Main Results:
- Significant, progressive spiral ganglion cell loss was observed starting at 4 weeks post-drug.
- EMLR threshold remained stable over time.
- The slope of the EMLR input/output function decreased significantly with increasing post-drug intervals.
- This decrease in EMLR slope directly correlated with reduced spiral ganglion cell density.
Conclusions:
- EMLR is a viable objective measure for assessing eighth nerve degeneration.
- The EMLR input/output function slope is a sensitive indicator of spiral ganglion cell loss.
- EMLR shows potential for selecting cochlear implant candidates based on auditory nerve integrity.