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Optimizing the clinical fit of auditory brainstem implants
LONG, C.J., NIMMO-SMITH, I., Baguely, D.M., O'Driscoll, M., Ramsden, R., Otto, S.R., Axon, P.R. & CARLYON, R.P.
Proceedings of the Fourth International Conference on Vestibular Schwannoma and Other CPA Lesions. July 13-17, 2003
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Summary; An important goal when fitting an Auditory Brainstem Implant (ABI) is to obtain an appropriate rank ordering of electrodes, such that higher-frequency bands of speech are encoded on electrodes that evoke progressively higher pitch percepts. Unfortunately, the clinician does not know in advance what the correct ordering should be. As more than 3 million orderings are possible when 10 or more electrodes are active, this presents the clinician and patient with something of a challenge. We are investigating methods of helping the clinician to find the appropriate ordering quickly and accurately. Our proposed procedure is designed to determine the proper ordering of an initially unordered set of items in a highly efficient manner. Our tests of normal-hearing subjects and computer simulations of the responses of ABI users indicate that this procedure requires fewer patient responses and produces at least the same degree of accuracy as procedures presently used in clinical practice. The proposed fitting procedure has been successfully applied in the clinic with two ABI users to date. Objectives; To implement a new audiological fitting procedure for ABIs based on a mathematically efficient algorithm (Steinhaus, 1950). To evaluate the efficacy of this procedure relative to procedures presently used in clinical practice (Nevison 2002, Otto et al. 2002). Study design; The different procedures were first compared using computer models and simulations with normal-hearing subjects. This allows for an analysis of the accuracy of the procedures in a way that is not possible when testing ABI users. The root-mean-square error between the order estimated by the procedure and the true order was calculated. In addition, ABI users were tested with the new procedure to see if it could be successfully applied in clinic. The degree of variability of their results across runs and sessions was analyzed. The proposed procedure can best be described by a simple example: n Compare pitch of two electrodes, e1 and e2. n If e1 is higher, then order is [e2 e1]. n Later after additional comparisons, say the order has become [e2 e1 e4 e5 e3]. n Compare pitch of e6 to e4 (middle electrode). Say, e6 is higher. n Compare pitch of e6 to e5 (mid-low electrode of subset [e5 e3]). Say, e6 is lower. n Update map: [e2 e1 e4 e6 e5 e3]. Continue adding new electrodes by comparing to middle of set (and subsets). If there is no true middle, compare to electrode immediately below middle. Results; The tests of the normal-hearing subjects showed that our proposed procedure required fewer trials (22 on average) than procedures presently used in clinic (with 75 and 234 trials on average for the two procedures) to produce the same degree of accuracy. The computer modeling showed a similar advantage. Additional testing showed this advantage was maintained under a variety of conditions relevant to the clinic. Also, combining information across trials increased the accuracy of the results. The two patients tested were able to use this procedure with success. As expected, these ABI users were poor at discriminating pitch of electrodes. The patients showed results consistent with having a total of 4 discriminable groups of electrodes (d' = 1) with the 14 to 15 electrodes tested. Conclusion; The proposed procedure requires fewer trials to produce a clinically useful result and is well tolerated in the clinic. An additional advantage is that it allows testing to be broken down into several “blocks,” each containing a small number of trials. If the variability between blocks is small, information can be combined across blocks to increase the accuracy of the result. If the variability is large, perhaps between blocks on different days, this may reflect a significant change in the percepts generated by the implant and signal to the clinician that a significant alteration in the fitting is required. References; Otto SR, Brackmann DE, Hitselberger WE, Shannon RV, Kuchta J (2002). "Multichannel auditory brainstem implant: update on performance in 61 patients," J. Neurosurg 96, 1063-1071. Nevison B (2002). "Pitch Ranking" in Programming Guide for the ABI24M, Auditory Brainstem Implant. p15-16. Cochlear Limited. Steinhaus H (1950). Mathematical Snapshots. Oxford University Press. Ack / Grants This work was supported by the Leverhulme Trust.