The use of reverse transfer function (RTF) in the fitting procedure of implantable hearing devices

Abstract

Group II ( p > 0.001), whereas only wave V latency and amplitude were improved in Group 1 ( p < 0.01). These results (see Figure 2) show that the changes due to auditory deprivation are greatly reversible under electrical stimulation. DiscussionEABRs have been obtained in several multichannel CI devices (Pelizzone et al., 1989; Shallop et al., 1990; Brown et al., 1994; Gallego et al., 1996; Brown et al., 1999; Gordon et al., 2002), mainly in adult patients and/or after several months of electrical stimulation. All patients of our study were children and most of them were implanted before the age of 5 years. This could explain the differences that we observed such as longer latencies in the intraoperative EABR recorded in congenitally deafened children (Group II, near 4.5 ms instead of 3.8-4 ms as in most studies). These findings could be due to a reduced number of neurons and/or an impaired myelination in the central auditory pathways as recently suggested by animal studies of immature auditory pathways (Hardie and Shepherd, 1999). Electrical stimulation induced different changes in early congenitally deafened children (Group II) and in progressively deafened children (Group I). This could indicate that more profound changes were induced by cochlear implantation in congenitally deafened children (maturation or plastic modifications of the neural circuits) than in progressively deafened children. These data are consistent with those found by others in cortical potentials and support the concept of early implantation in congenitally deafened children (Ponton et al., 1999).

The benefit of the reverse transfer function in the fitting process of the Vibrant Soundbridge middle ear implant

“…Winter et al [4] were the first to introduce a totally objective means to evaluate the implant's performance, by using a cadaver model and comparing RTF values to laservibrometry-measured anterograde movement of the stapes and also by correlating postoperative VSB gain with RTF values in middle-ear implanted patients. Both CHAs and MEIs should rest upon real-ear measurements during fitting sessions.…”

“…In the case of CHAs, fitting procedures ideally depend on the realear unaided response, resonance properties of the ear canal, loudness discomfort level, real-ear insertion gain, real-ear-to-coupler difference (RECD), and other measurements [7]. Similarly, the optimal fitting of MEIs would require in situ measurements in the ear canal while taking into account resonance properties of the ear canal, anterograde vibration of the stapes toward the inner ear, and retrograde vibration of the malleus handle and tympanic membrane toward the ear canal (RTF) [4]. In the case of CHAs, real-ear performance can be converted from dB HL to dB SPL by adding RECD to the acoustic response of the hearing instrument measured in a 2 ml coupler [7].…”

“…However, a small part of the mechanical energy is transmitted to the external acoustic canal by retrograde vibration of the malleus handle and tympanic membrane. This retrograde energy has been labeled reverse transfer function (RTF) by Winter et al [4]. In the case of conventional hearing aids (CHAs), there are a variety of realear measurements with a probe tube microphone that allow an objective assessment of the realear performance of the hearing aid [5].…”