Stimulation and Encoding Strategies for Cochlear Prostheses

The piezoelectric cochlear implant: concept, feasibility, challenges, and issues

A better understanding of the fundamental phenomena occurring in both the healthy and the artificially stimulated cochlea will greatly aid in the engineering of more effective cochlear implant devices and will, in general, enhance mankind's knowledge of inner ear function. This study was initiated to probe the feasibility of use of artificial piezoelectric transducer devices, both for the understanding of cochlear phenomena and as a possible cochlear implant. Aspects of feasibility of such an implant, the issues involved, the materials science challenges that need to be overcome to fabricate such a device, and results from initial in vivo experiments are discussed.

New approach for cochlear implantation: cochleostomy through the middle fossa

The middle fossa approach was used in 11 patients with profound bilateral hearing loss for insertion of a cochlear implant. Fibroadhesive otitis media (n = 1), bilateral cavity radical mastoidectomy (n = 1), autoimmune inner ear disease (n = 2), previous cranial trauma (n = 1), genetic prelingual deafness (n = 5), and otosclerosis (n = 1) were the causes of deafness. A cochleostomy was performed on the most superficial part of the basal turn, and the electrode array was inserted up to the cochlear apex. Speech perception tests (1-9 months after cochlear implant activation) yielded better results in these patients compared with a homogeneous group of postlingually deaf patients operated on through the traditional transmastoid route. Insertion of the implant through the middle fossa cochleostomy furnishes the possibility of stimulating areas of the cochlea (ie, the middle and apical turns) where a greater survival rate of spiral ganglion cells is known to occur, with improvement of information regarding the formants relevant for speech perception.

Improved auditory performance of cochlear implant patients using the middle fossa approach

The preliminary results of insertion of a cochlear implant via the middle fossa in nine patients with profound bilateral hearing loss are described. Aetiologies included a bilateral radical mastoidectomy cavity, adhesive otitis media, autoimmune inner ear disease, previous cranial trauma, genetic pre-lingual deafness, and otosclerosis. A classic middle fossa approach was adopted. A small cochleostomy measuring 1.5 mm in diameter was performed on the most superficial part of the basal turn. A Nucleus 24M cochlear implant system (Cochlear Corporation) was inserted in four patients, a Lauraflex implant (Philips Hearing Implants) was used in three patients and a Combi 40+ (Med-el) with a double electrode array in two. Single electrode arrays were inserted from the cochleostomy to the cochlear apex and occupied a portion of the basal turn, as well as the middle and apical turns. Double electrode arrays were inserted, one towards the apex and one into the basal turn of the cochlea towards the round window. The receiver stimulator was positioned in a bone well previously drilled in the temporal squama and the electrode carrier was inserted in the fenestrated cochlea. The activity of the inserted electrodes was tested by means of telemetry and intraoperative recording of electrically evoked auditory brainstem responses (EABR). Speech recognition tests, performed over a period of time ranging from one to six months after cochlear implant activation, yielded better results in these patients compared with those obtained in postlingually deaf patients operated on via the traditional transmastoid route. Cochlear implant insertion via the middle fossa approach is a technique which is suitable for the implantation of patients with bilateral radical mastoidectomy cavities, chronic middle ear disease, middle ear malformations, or with partial obliteration of the cochlea in the basal turn. However, the main advantage of inserting the implant through the middle fossa cochleostomy consists in the possibility of stimulating, with the single array, areas of the cochlea, i.e. part of the basal, middle and apical turns, where a greater survival rate of spiral ganglion cells is known to occur. In addition, with the double array total occupation of the cochlea is possible, providing the possibility of replicating the tonotopic organization of the cochlea. This new approach has led to major improvements in speech recognition in all patients compared with patients operated on via the transmastoid approach and, given the present state of the art, may be the elective approach for optimal implantation outcomes.

Additive noise can enhance temporal coding in a computational model of analogue cochlear implant stimulation

Conventional analogue multichannel cochlear implants are unlikely to convey formant information by the fine time structure of evoked discharges. Theoretically, however, the addition of noise to the channel outputs could enhance the representation of formants by time coding. In this study, the potential benefit of noise in analogue coding schemes was investigated using a computer model of cochlear implant stimulation. The cochlear nerve was modelled by the Frankenhauser-Huxley equations. For all five vowels investigated, the optimal addition of noise to the first channel of the simulated implant (200-671 Hz) caused enhancement of the first formant representation (as seen in amplitude spectra of the simulated discharges). For vowels with a low-frequency second formant, clear enhancement of the second formant resulted from the optimal addition of noise to the third channel (1200-2116 Hz). On the basis of the present computational study, additive noise would be expected to enhance the coding of temporal information by the discharges of a single nerve fiber.

Comparative results with different cochlear implants

The cochlear implant program in Budapest began in 1985, since when 60 operations have been performed, 14 of them on children (51 primary procedures and 9 reimplantations). Different devices and also different techniques have been used: extracochlear promontory, extracochlear round window and intracochlear implants. Various speech processors were applied; at first a digital pulsatile sound-encoding system, later on analog processors were used, while processors operating on the basis of the CIS strategy are preferred nowadays. The operations were performed on both pre- and postlingual patients. In several cases contralateral hearing improvement was observed 6 months after the operation. Considerable experience has been gained of unique cases, such as a deaf-blind prelingual child and prelingual twins. Evaluating cochlear implant performance, in addition to the usual audiological tests measuring postoperative speech understanding, warble tone sound field thresholds were also established.

Computer-aided three-dimensional reconstruction in human cochlear maps: measurement of the lengths of organ of Corti, outer wall, inner wall, and Rosenthal's canal

This paper describes the application of computer-aided three-dimensional reconstruction to measurements of the length of the organ of Corti (mean +/- SD, 35.58 +/- 1.41 mm), scala tympani outer wall (40.81 +/- 1.97 mm), scala tympani inner wall (18.29 +/- 1.47 mm), and Rosenthal's canal center (15.98 +/- 1.33 mm) in eight adult male cochleas. The Rosenthal's canal center ranged between 1 3/4 and 2 turns, did not appear to be linearly related to the organ of Corti, and lay near the basal end of the latter. The length of the organ of Corti measured with three-dimensional reconstruction differed by 7.6% +/- 3.2% ("cutting angle difference") from that derived from traditional two-dimensional reconstruction on the plane perpendicular to the plane of section, and by 2.6% +/- 1.7% ("geometric difference") from that derived from two-dimensional reconstruction on the axial plane at right angles to the modiolar axis.

Temporal bone computed tomography in the preoperative evaluation for cochlear implantation

High resolution computed tomography of the temporal bone is performed routinely in the preoperative evaluation for cochlear implantation. A retrospective review was performed to determine the effect of these studies. The clinical significance was determined through consultation with the referring otolaryngologist and review of the operative reports. Twelve examinations were reviewed. Abnormalities were identified in 12 of the 24 ears examined (eight of 12 patients studied). In two patients the CT examination strongly influenced the selection of the ear to be implanted. In four additional cases information felt to be useful in preoperative planning was provided. Useful information related to potential complications with the mechanical insertion of the electrode apparatus. We conclude that temporal bone abnormalities are common in the population undergoing cochlear implantation and that high resolution temporal bone CT is essential in preoperative evaluation. The CT scan should be interpreted with adequate knowledge of the operative procedure to evaluate any possible barriers to the mechanical insertion of the internal components of the system.

Computer-generated three-dimensional reconstruction of the cochlea

Computer-generated three-dimensional reconstructions of the nerve fibers from the organ of Corti to the spiral ganglion were used to determine the optimum maximal length of the cochlear implant electrode. The spiral ganglion within the modiolus is much shorter than the organ of Corti. The spiral ganglion has 1 3/4 turns and reaches no higher than the middle of the second turn of the organ of Corti, which has 2 3/4 turns. The spiral ganglion is concentric and basal with respect to the organ of Corti. The spiral ganglion dendrites within the osseous spiral lamina of the basal turn project radially, nearly perpendicular to the central axis of the modiolus. Upon entering the modiolus, they turn basally at an angle of approximately 120 degrees. The projection of dendrites within the osseous spiral lamina became increasingly oblique as the ganglion extended apically. The organization of the cochlear nerve results from the spiraling of the ganglion. These findings are in agreement with previous reports. Implications of these findings and their possible relevance to the optimum length of the cochlear implant electrode are discussed with reference to cochlear damage resulting from longer electrodes.