The form and pattern of first-order and transsynaptic degeneration in the central auditory pathway was studied in monkeys following inner ear stimulation by a cochlear implant. Multielectrode, scala tympani, and modiolar systems were implanted; in some cases, neomycin was perfused into the cochlea to destroy the organ of Corti at the time of implantation. The monkeys were maintained chronically for 5 to 120 weeks, then the cochleas and brainstems were examined histologically. The extent of spiral ganglion cell loss across animals showed variability, reflecting the different procedures and devices used. The degree and distribution of spiral ganglion cell loss was related to the degree and distribution of neural degeneration seen in the cochlear nucleus in all cases. Peripheral damage progressed toward the cochlear apex as survival time increased, and this progression was reflected in the cochlear nucleus by a ventrolateral shift in the locus of degeneration over time. In addition, evidence for transneuronal degeneration was seen at the superior olive, the lateral lemniscus and the inferior colliculus. Our findings indicate that several factors inherent in the use of a cochlear prosthesis, i.e., insertion trauma, host reaction, and/or electrical stimulation, may be associated with a long-term, continuing process of central degeneration visible at several levels of the auditory system.

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Clinical experience with the tracheoesophageal anastomosis for intractable aspiration.

Clinical experience in six patients in whom the tracheoesophageal diversion procedure or a modification thereof has been performed is presented. Two of these patients have been reconstructed with resulting normal laryngeal and esophageal function. Three of the patients have successfully undergone tracheoesophageal anastomosis and are candidates for reconstruction. One patient in whom the tracheoesophageal anastomosis has been present for 2 1/2 years may never demonstrate neurological return sufficient for reversal of the procedure. On the basis of these six cases, we consider the tracheoesophageal anastomosis to be an effective, yet reversible, procedure for intractable aspiration.

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Operating ranges for cochlear implants.

The range of simple electrical stimuli that may be used for prosthetic stimulation via scala tympani implants was explored using psychophysical procedures in macaque monkeys. Biophysical considerations placed further limitations on the operating range. The operating range was dependent onstimulus waveform, electrode configuration, and the condition of the implanted cochlea and eighth nerve.

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Comparison of cochlear histopathology following two implant designs for use in scala tympani.

Two designs of intracochlear implants were evaluated for their histopathologic effects in the monkey ear. A molded electrode designed to fit the contour of the basal turn of the scala tympani tended to create basilar membrane fistulas and osseous spiral lamina fracture, along with relatively extensive loss of spiral ganglion cells in sites adjacent to the implant. A delicate, free-fit electrode induced local encapsulation but little or no mechanical damage and limited degeneration of spiral ganglion cells. Both electrode types occasionally induced changes in stria vascularis.

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Brainstem histopathology following chronic scala tympani implantation in monkeys.

Degeneration in brainstem auditory nuclei was studied in monkeys following chronic implantation with scala tympani multielectrode systems. Nauta and Fink/Heimer-stained material from animals with survival times to 16 months were studied. The density and distribution of degeneration in the cochlear nuclei were consistent with observed patterns of spiral ganglion cell degeneration. Transneuronal degeneration was seen up to the level of the inferior colliculus. The density and form of the degeneration material in animals of varying survival times was consistent with the interpretation that a process of continuing degeneration occurred in these implanted ears.

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Horseradish peroxidase studies in animals with neuromuscular transpositions.

Horseradish peroxidase (HRP) is used to trace axonal connections from the motor end-plate to the driving neuron. This technique has confirmed that the neurons activating the sternothyroid muscle are located in the cervical spinal cord, while those controlling the posterior cricoarytenoid (PCA) are found in the nucleus ambiguus ipsilaterally. Eight rabbits underwent a sternothyroid ansa pedicle implantation to the PCA at the time of sectioning the recurrent laryngeal nerve ipsilaterally. After two months, four of these animals received HRP injections into the previously implanted PCA. Brainstem staining HRP did not reveal any retrograde transport to the motor neurons that were known to control the sternothyroid. Possible for the failure of retrograde transport are discussed.

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Cochlear implant effects on the spiral ganglion.

To evaluate the effects of chronic intrascalar implants on spiral ganglion cells, we studied 15 monkey cochleas that had had implants for periods ranging from 1 to 28 months. All cases exhibited loss or ongoing degeneration of cells. Cell loss typically was greater in the basal turn, where the implants were located, than at more apical locations. Increasing apical damage was a function of postimplant survival time. Preimplant local treatment of the inner ear with neomycin did not influence the loss of spiral ganglion cells. Osteoneogenesis occurred in the majority of cases, appearing in the basal turn with occasional extension into middle and apical turns. Cell loss was inconsistently associated with new bone formation. Electrical stimulation had no obvious influence on cell survival.

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Cochlear implant effects on the spiral ganglion.

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Cochlea↗