Electrically Evoked Stapedius Reflex: Relation Between its Threshold and Psychometric Quantities

Background:

The physiological stapedius reflex can be evoked electrically (ESR) by the cochlear implant (CI) and observed visually through the microscope in the operation situs just following implantation. For the determination of its threshold, the ESRT routine is implemented in the fitting software of several manufacturers, e.g. Cochlear CustomSound EP or MED-EL MAESTRO. The use of reflex threshold for processor fitting has already been widely investigated. In this work, special attention is directed to the profile (channel dependence) of the reflex threshold in relation to the profiles of psychometric sensation levels.

Objective:

To facilitate processor fitting in subjects with limited cooperation.

Method:

Intraoperative ESRT (electrical stapedius reflex threshold) was determined in 86 patients equipped with MED-EL implants (SONATA, CONCERTO and SYNCHRONY). Stimuli of varying intensity were delivered to all electrodes sequentially while the surgeon observed the stapes fibre across the microscope.

Postoperative most comfortable stimulation level (MCL) was determined psychometrically at the instant of first fit.

The correlations between the profiles of ESRT and MCL were examined statistically.

Results:

The correlation between the profiles of ESRT and MCL are significant in only 54% of cases. This may be attributed at least partially to the different stimulation paradigms used in ESRT procedure and fitting. Especially if fine structure programming is involved, the thresholds will result much lower in CSSS-channels due to the higher stimulation rate. But even if this is effect taken into account, the percentage of significantly correlating profiles is not higher.

The difference between ESRT and MCL is positive in the overwhelming number of cases. Very high ESRT values may occur especially in the basal cochlear region.

Discussion:

From the absence of a convincing correlation between their respective profiles, the use of ESRT profile as a guide for MCL in individual fitting appears to be questionable. This may be different if impedance based reflex thresholds are considered.