Does electrical stimulation of deaf cochleae prevent spiral ganglion degeneration?

Thirty-six drug deafened guinea pigs were studied to determine how electrical stimulation of the cochlea affects spiral ganglion cell (SGC) survival. Animals were divided into two groups, extracochlear and intracochlear stimulation, and each group was further divided into four stimulus subgroups: no stimulation (implanted controls), the inferior colliculus electrically evoked potential (ICEEP) threshold-2 dB, ICEEP threshold+2 dB, and ICEEP threshold+6 dB. Stimuli consisted of 200 micros/phase charge balanced biphasic current pulses presented at 100 pulses per second using monopolar stimulation. Animals were stimulated 5 h/day, 5 days per week, for 8 weeks. The animals were then perfused and the cochleae serially sectioned at 4 microm saving every 8th section. We counted the number of intact SGCs, those containing a nucleus with chromatin, in each 20% segment of the cochlea and also measured SGC densities (number of neurons per mm2 of Rosenthal's canal). The number of surviving spiral ganglion neurons was not significantly different (P > 0.05) between the implanted and the unimplanted ears in any of the experimental groups. However, the spiral ganglion neuron densities were significantly elevated in the electrically stimulated ears (P < 0.001) but not in the implanted but not chronically stimulated ears (P > 0.05). We measured the volume of Rosenthal's canal in one subgroup (ICEEP threshold+2 dB) and found a decrease in this volume in the stimulated ear compared to the unstimulated ear (P < 0.01). These findings support the hypothesis that chronic monopolar electrical intracochlear or extracochlear stimulation is not a neurotrophic factor, increasing spiral ganglion neuron survival, but instead causes a narrowing of Rosenthal's canal that accounts for the increased spiral ganglion neuronal densities seen in the stimulated cochleae.

Management of late extrusions of cochlear implants.

OBJECTIVES: The study aimed to identify the important principles in repairing and preventing delayed cochlear implant extrusions. STUDY DESIGN: The study design was a retrospective, international database review and three case presentations from the authors' local database. SETTING: The study was conducted at a tertiary referral center. PATIENTS: Cochlear Corporation's database of 8,665 implantees in North and South America and Israel was studied. A subset of this database, consisting of 74 patients implanted through the Louisiana State University/Eye, Ear, Nose and Throat (LSU/EENT) project, is analyzed separately and provides the case reports. INTERVENTION: Surgical repair of skin breakdown over the edge of three cochlear implants using a two-layer, pericranial and scalp rotation flap technique was performed. MAIN OUTCOME MEASURES: A functioning cochlear implant with a healthy skin covering was measured. RESULTS: The three LSU/EENT cases reported here used a pericranial flap to repair the capsule of the implant and a large scalp rotation flap to repair the skin defect. A novel pericranial tuck-under technique is especially useful for the Nucleus Mini-22 implant. With this technique, the authors have had a 100% success rate (3 of 3). The basic principles used in the repair and in the initial implant surgery are discussed. CONCLUSIONS: The following conclusions were reached: 1) avoid skin closure lines parallel to the cochlear implant edge that are closer than 1.5 cm from the implant edge; 2) excise enough skin and scar around the dehiscence to achieve principle 1; 3) design a large, well-vascularized anterior- or posterior-based scalp rotation flap to cover this defect; and 4) close the implant capsule defect with a well-vascularized pericranial flap.

Effects of stapedial arch fractures on conductive hearing loss and stapedial reflexes.

OBJECTIVE: It has been stated in the clinical literature that stapedial fractures could produce a significant conductive hearing loss while leaving the contralateral stapedial reflex intact. The objective of this study is to test this hypothesis in an animal model. DESIGN: Nine hooded rats underwent acoustic-stapedial reflex (ASR) and cochlear microphonic (CM) threshold determinations before and after middle ear surgery. An argon laser was used to divide the tensor tympani tendon, the anterior stapedial curs, and the posterior stapedial crus, sequentially. The initial functional measures were repeated after each laser procedure to document its effect. RESULTS: Dividing the tensor tympani tendon and the anterior stapedial crus had variable and small effects on ASR and CM thresholds. Division of the second (posterior) crus eliminated stapedial reflexes and produced a significant hearing loss. CONCLUSIONS: This study refutes the hypothesis that fracture of one (anterior) stapedial crus will significantly alter ossicular sound transmission, but it supports the hypothesis that measured stapedial reflexes would not be significantly altered by a single crus fracture. Therefore, the surgeon exploring an ear for a conductive hearing loss with an intact contralateral stapedial reflex should look carefully for another source of the hearing loss if finding only a single stapedial arch fracture.

Calcium oxalate crystal deposition in necrotizing otomycosis caused by Aspergillus niger.

Numerous calcium oxalate crystals were present within fruiting heads of Aspergillus niger and among necrotic debris in a case of bilateral invasive otomycosis occurring in a diabetic female with end stage renal disease. This is the first report of in vivo calcium oxalate crystal deposition associated with Aspergillus niger at this anatomic site. The presence of localized oxalate crystals within necrotic tissue from the external auditory canal is presumptive of otomycosis caused by Aspergillus niger, and may serve as a diagnostic clue to the etiologic agent before histologic demonstration of hyphae or growth in culture.

Measurement of sound.

The measurement of sound involves the analysis of frequency, intensity, and temporal dimensions of acoustic signals. Each dimension of sound can be related directly to clinically observed phenomena. Frequency information, measured in Hz, can be extracted from pure-tone and complex stimuli. Intensity represents the physical energy of a signal and is described by using the decibel scale--a logarithmic scale of ratios. Temporal characteristics of sound include duration, phase, and repetition rate. In the analysis of human hearing sensitivity, the middle ear system and its impedance characteristics also must be considered. In this article, we have reviewed some major principles of sound and have presented a series of practical clinical applications. Such principles as these help to predict and explain frequency of laryngeal tones, middle ear mechanics, ear canal resonance, real-ear measurements of hearing aids, the Articulation Index, hearing loss, understanding of speech in quiet and in noise, and the relation between hearing and speech.

Temporal response patterns of auditory nerve fibers to electrical stimulation in deafened squirrel monkeys.

Electroneural response patterns of single auditory-nerve neurons were studied in aminoglycoside-deafened squirrel monkeys. The electrical stimuli were delivered through bipolar electrodes implanted in the scala tympani. The effects of pulse width, shape, frequency, and intensity on neural adaptation, phase locking, and spectral content were evaluated. Our results did not demonstrate the characteristic adaptation seen in auditory-nerve neurons in response to acoustic stimulation. Phase locking to a broad stimulus pulse (3200 microseconds/phase) was found to a very restricted phase angle of the electrical stimulus which was broader for square wave than for sine wave stimulation. The latency of the phase locked response varied inversely with stimulus intensity with greater variation for square wave stimulation than for sine wave stimulation. Auditory neurons were capable of a very high degree of phase locking to a 200-microseconds/phase pulse presented at 156 pulses per second (PPS) and to the first pulse of a 2500-Hz pulse burst. Phase locking was much poorer for the subsequent 200-microseconds/phase pulses comprising the 2500-Hz pulse burst where the neuron's response was determined by its relative recovery status. These findings can be explained by an interaction between the neuron's relative refractory status and its integration of charge over the stimulatory half cycle of the electrical stimulus. These two factors also appear to determine the interspike interval of the neural response. This interval decreased monotonically with increasing stimulus intensity. The neural spike rate (150-500 Hz) producing this interval increased with intensity and may be a source of periodicity information which the central auditory nervous system could interpret as pitch. This may account for the proportional relationship between pitch and stimulus intensity seen in some cochlear implant patients. Our study demonstrates that auditory-nerve neurons comply with basic neurophysiological principles in their responses to electrical stimulation. These principles should be incorporated into the cochlear prosthesis stimulator if more normal neural response patterns are desired in the cochlear prosthesis patient.

Atypical invasive external otitis from Aspergillus.

We report a rare case of invasive external otitis caused by aspergillosis in an elderly nondiabetic patient. Amphotericin B therapy was curative. Atypical features of the presentation delayed diagnosis. Early use of tissue biopsy and culture to guide prompt initiation of therapy is recommended. The clinical spectrum and microbiology of invasive aspergillosis are also reviewed.

A model of electrical excitation of the mammalian auditory-nerve neuron.

A model of the mammalian auditory-nerve neuron has been developed based on the classical work of Frankenhauser and Huxley [(1964) J. Physiol. 171, 302-315], modified by McNeal [(1976) IEEE Trans. Biomed. Eng. BME-23, 329-336], and Reilly et al. [(1985) IEEE Trans. Biomed. Eng. BME-32, 1001-1011], and fine tuned to represent physiological data obtained from single auditory-nerve fiber experiments in squirrel monkeys. The model is capable of reproducing neural action potential waveforms due to electrical stimulation, and can reliably predict action potential thresholds and strength-duration curves. This paper explains the derivation of the mathematical model and the effects of varying certain independent parameters including fiber diameter, length of the nodes of Ranvier, internodal length, and myelin thickness. The model parameters were selected according to the anatomical findings of Liberman and Oliver [(1984) J. Comp. Neurol. 223, 163-176], and Liberman (Pers. Commun.). The length of the unmyelinated termination of the auditory-nerve that survives after aminoglycoside damage to the inner ear has not been experimentally determined. Therefore, it was investigated as an independent variable in the model. An unmyelinated terminal length of 10.0 micron was found to most accurately describe the experimental neural strength-duration curves obtained from aminoglycoside-deafened squirrel monkeys. The parameter that had the next most significant effect on the model was fiber diameter which affects all conduction pathways, across the membrane and through the fiber. Finally the results of the model are compared with behavioral data obtained from patients and monkeys implanted with cochlear prostheses. In the companion paper [(1987) Hear. Res. 31, 267-286] predictions of the model are quantitatively compared with single-neuron data from squirrel monkeys.

Auditory-nerve single-neuron thresholds to electrical stimulation from scala tympani electrodes.

Single auditory-nerve neuron thresholds were studied in sensory-deafened squirrel monkeys to determine the effects of electrical stimulus shape and frequency on single-neuron thresholds. Frequency was separated into its components, pulse width and pulse rate, which were analyzed separately. Square and sinusoidal pulse shapes were compared. There were no or questionably significant threshold differences in charge per phase between sinusoidal and square pulses of the same pulse width. There was a small (less than 0.5 dB) but significant threshold advantage for 200 microseconds/phase pulses delivered at low pulse rates (156 pps) compared to higher pulse rates (625 pps and 2500 pps). Pulse width was demonstrated to be the prime determinant of single-neuron threshold, resulting in strength-duration curves similar to other mammalian myelinated neurons, but with longer chronaxies. The most efficient electrical stimulus pulse width to use for cochlear implant stimulation was determined to be 100 microseconds/phase. This pulse width delivers the lowest charge/phase at threshold. The single-neuron strength-duration curves were compared to strength-duration curves of a computer model based on the specific anatomy of auditory-nerve neurons. The membrane capacitance and resulting chronaxie of the model can be varied by altering the length of the unmyelinated termination of the neuron, representing the unmyelinated portion of the neuron between the habenula perforata and the hair cell. This unmyelinated segment of the auditory-nerve neuron may be subject to aminoglycoside damage. Simulating a 10 micron unmyelinated termination for this model neuron produces a strength-duration curve that closely fits the single-neuron data obtained from aminoglycoside deafened animals. Both the model and the single-neuron strength-duration curves differ significantly from behavioral threshold data obtained from monkeys and humans with cochlear implants. This discrepancy can best be explained by the involvement of higher level neurologic processes in the behavioral responses. These findings suggest that the basic principles of neural membrane function must be considered in developing or analyzing electrical stimulation strategies for cochlear prostheses if the appropriate stimulation of frequency specific populations of auditory-nerve neurons is the objective.

Histiocytosis X: otologic presentations.

Histiocytosis X is a relatively rare disease presenting as 3 clinical syndromes: eosinophilic granuloma, Hand-Schuller-Christian disease, and Abt-Letterer-Siwe disease. Although the clinical expressions of the disease differ, the underlying histopathology appears to be the same. Since the disease frequently involves and not infrequently presents in the head and neck region, it is particularly relevant to the otolaryngologist. We have recently treated two young children presenting with bilateral otorrhea. Although histiocytosis X was entertained early in the evaluation of these patients, the diagnosis was difficult and prolonged in both cases. One case ultimately required electron microscopic study to confirm the diagnosis. Important aspects of the diagnosis are reviewed. Treatment of histiocytosis X includes surgery, radiation therapy and chemotherapy. These treatment strategies are discussed, with particular emphasis on the management of the ear involvement.

The bionic ear: principles and current status of cochlear prostheses.

The field of cochlear prostheses is reviewed, emphasizing the neurophysiological principles necessary for the development of a successful cochlear prosthesis. Pertinent auditory physiology is reviewed, and four conditions are proposed which, if met, should result in speech recognition by the patient with an implant. These conditions are: The surviving neural population must be adequate over the frequency distribution required to deliver the necessary information for speech discrimination. The central neural processing pattern for decoding and recognizing speech must have been established and still persist. The processing of the acoustic signal by the auditory system up to the level of the prosthesis interface must be understood and predictable from the acoustic stimulus. The relationship between the response patterns of the neural elements and the electrical stimulus must be well documented and controllable. The degree to which these conditions can presently be met is discussed, with some suggestions for future development. The function of the cochlear prosthesis is separated into a signal processing section and an electrical stimulus section. Two signal processing strategies are analog processing of the acoustic signal and speech feature extraction from the acoustic signal. Four possible electrical stimulation strategies are direct analog stimulation from the signal processor, pulse amplitude modulation, pulse width modulation, and stimulation designed to optimize the neural responses to electrical stimulation. Some of the present generation cochlear prostheses, including five approved by the FDA as investigational devices, are discussed according to this classification. The clinical results of testing these devices as aids to lip reading and as stand-alone speech reception aids are compared. All are a potential aid to lip reading. Some prosthesis designs have been implanted in patients who then showed exceptional open set speech discrimination. These exceptional patients have used both single channel and multichannel devices and devices with both analog processing and speech feature extraction strategies. These results are encouraging. More consistent and better speech reception is anticipated for the future as this field develops.

Mucocele of the petrous apex.

The first case of a primary mucocele of the petrous apex is presented and the differential diagnosis is briefly discussed. Since this area is unavailable for direct examination, a thorough radiographic evaluation is essential. A mucocele should be suspected when a lytic lesion has a multiloculated appearance and when the contralateral petrous apex is highly pneumatized. Even then, a biopsy may still be needed to make a definitive diagnosis. The appropriate treatment for these cystic lesions is fistulization into a radical mastoid cavity or an exteriorized sphenoid sinus.