NRT should correctly stand for Neural Response Telemetry. Basically it uses the possibility to use the electrodes to stimulate electrically the nerve and to measure at the same time its response to the stimulus.
As far as I know it is exactly the test they usually do after implantation (when still in the surgery room also).
There is a kind of correlation between the profile got from NRT and the T and C levels of the map. It is especially useful for small kids, whose active collaboration is usually not so much expected and not so reliable.
Anyway it remains an objective measure, avoinding any bias due to personal reactions to the stimulus. The difficult part of using NRT for mapping seems to be the function that correlate the profile you can get from it and the actual T and C levels. There is a personal variability thatmakes things difficult.
There are several studies on that subject, here below two examples from PubMed data bank:
Ear Hear. 2005 Aug;26(4 Suppl):38S-44S.
A different approach to using neural response telemetry for automated cochlear implant processor programming.
McKay CM, Fewster L, Dawson P.
ABSTRACT:
OBJECTIVE: This study explores the theoretical relation between the psychophysically measured current levels required for sound processor fitting in cochlear implants and the objectively measured compound action potential threshold (as measured by Neural Response Telemetry, NRT). The objective was to gain understanding of the variability across implantees in this relation and determine possible ways (using objective measures) of improving the predictability of NRT thresholds for behavioral levels needed for mapping. DESIGN: A model of how rate of stimulation affects loudness is presented. The model can be used to understand differences among implantees in the way that rate affects loudness and hence explain the disappointing correlation between NRT and psychophysical measures. Suggestions are made, based on the model, for additional information that may improve the usefulness of NRT measurements. One such option (measuring the effect of interphase gap on NRT amplitude) was experimentally explored in eight subjects (26 electrodes). It was hypothesized that the current change required to maintain equal NRT amplitude when interphase gap was changed from 8 to 45 musec would be correlated with the offset between behavioral and NRT thresholds. RESULTS: The above hypothesis was not supported by the data, and several possible reasons for this outcome are discussed. CONCLUSIONS: The loudness model provides useful insights into why NRT thresholds are not good predictors of the behavioral levels needed for mapping and how NRT might be made more useful by additional objective information. These insights should be investigated in further experimental studies.
Zhonghua Er Bi Yan Hou Ke Za Zhi. 2002 Dec;37(6):435-9.Links
Comparison of neural response telemetry thresholds with behavioral T/C levels
[Article in Chinese, english translated abstract]
Chen X, Han D, Zhao X, Wang S, Kong Y, Liu S, Mo L, Liu B, Wu Y.
ABSTRACT:
OBJECTIVE: To provide an objective method to estimate T-levels and C-levels using neural response telemetry (NRT) thresholds for children, from whom we can not get accurate responses because they are very young or have other disabilities, by comparing NRT thresholds and T/C levels. METHODS: Seventy patients implanted with the nucleus CI24M multiple cochlear implant system participated in this study. 329 electrodes were tested. The software used in this study was NRT 2.04. Monopolar stimulation mode was used during NRT threshold measurement. No. 3, 5, 10, 15, 20 electrodes were tested for each patient. The T-levels and C-levels were obtained at the same visit. RESULTS: 92.7% of all the tested electrodes recorded NRT responses. There were variations in the amplitudes and thresholds of NRT responses across subjects and electrodes. NRT thresholds fell within the different points of the MAP dynamic range. The mean of NRT thresholds was shown to fall between the mean of T-levels and C-levels. The NRT thresholds and T/C levels reduced from basal to apical ends of the cochlear. CONCLUSIONS: Considerable variability across subjects was noted. So it is difficult to estimate T-levels and C-levels accurately by testing only NRT thresholds only. NRT technology provides a new objective method for estimating T-levels and C-levels for children who are unable to cooperate with audiologists during mapping after operation. The new NRT technology should be improved in the future.
