Automated objective surgical planning for lateral skull base tumors.

PURPOSE: Surgical removal of pathology at the lateral skull base is challenging because of the proximity of critical anatomical structures which can lead to significant morbidity when damaged or traversed. Pre-operative computed surgical approach planning has the potential to aid in selection of the optimal approach to remove pathology and minimize complications. METHODS: We propose an automated surgical approach planning algorithm to derive the optimal approach to vestibular schwannomas in the internal auditory canal for hearing preservation surgery. The algorithm selects between the middle cranial fossa and retrosigmoid approach by utilizing a unique segmentation of each patient's anatomy and a cost function to minimize potential surgical morbidity. RESULTS: Patients who underwent hearing preservation surgery for vestibular schwannoma resection (n = 9) were included in the cohort. Middle cranial fossa surgery was performed in 5 patients, and retrosigmoid surgery was performed in 4. The algorithm favored the performed surgical approach in 6 of 9 patients. CONCLUSION: We developed a method for computing morbidity costs of surgical paths to objectively analyze surgical approaches at the lateral skull base. Computed pre-operative planning may assist in surgical decision making, trainee education, and improving clinical outcomes.

Channel noise in neurons.

The probabilistic gating of voltage-dependent ion channels is a source of electrical 'channel noise' in neurons. This noise has long been implicated in limiting the reliability (repeatability) of neuronal responses to repeated presentations of identical stimuli. More recently, it has been shown to increase the range of spiking behaviors exhibited in some neural populations. Channel numbers are tied to metabolic efficiency and the stability of resting potential, and channel noise might be exploited by future cochlear implants in order to improve the temporal representation of sound.

How do cochlear prostheses work?

The past two decades have witnessed a revolution in the treatment of sensorineural hearing loss. Cochlear prostheses have evolved from laboratory experiment to a commercial technology that has benefited over 20,000 people. Paralleling this phenomenal development has been a substantial increase in our understanding of the biophysical, physiological and psychophysical mechanisms underlying the function of these devices.

Localization of cochlear implant electrodes in radiographs.

Multielectrode cochlear implantation is the most effective treatment for profound sensorineural hearing loss. In vivo three-dimensional 3-D localization of cochlear implant electrodes is important for modeling of the electrical field in the cochlea, design of electrode arrays, and may improve speech processor programming for better speech recognition. The prerequisite for 3-D localization of the electrodes is their 2-D localization in x-ray radiographs. In this paper, we develop a practical method to localize the electrodes with high efficiency, accuracy, and reproducibility. In this method, a priori knowledge of the electrodes and their approximate positions are utilized, an intelligent thresholding and segmentation mechanism is embedded, and the electrode center is computed as the weighted geometric center of segmented electrode pixels. Experiments with physical phantoms and human data demonstrate the feasibility and utility of this method. The PC-based program developed for this project is disseminated on the Web.

An empirically based model of the electrically evoked compound action potential.

The relationship between electrically evoked single-fiber action potentials and the electrically evoked compound action potential of the auditory nerve is of interest to those attempting to model such responses with computational techniques. It also relates to efforts to exploit the gross potentials that can now be recorded by some implantable cochlear prostheses. In this paper, we develop a computational model of the auditory nerve response to single, pulsatile, electrical stimuli based upon the response characteristics obtained from 230 single fibers of 13 cats. These fibers were stimulated by brief (39s) monophasic cathodic stimuli delivered by a monopolar intracochlear electrode. The data were pooled to obtain an estimate of the distribution of fiber thresholds. Post-stimulus time histograms were modeled using Poisson functions and adjusted to account for empirically determined latency and jitter characteristics. The probabilistic nature of single-fiber input-output functions (i.e. Verveen's (1961) 'relative spread') was also modelled. PST histograms from 5000 modelled fibers were then summed and convolved with an estimated 'unit potential' following the method of Goldstein and Kiang (1958). This convolution produced modelled compound action potentials, which were then compared with experimentally obtained data. Manipulations of model parameters affecting threshold, jitter, and relative spread suggest that the most important determinant of the shape of the EAP amplitude-level function is the threshold distribution. A model based solely on threshold distribution produces an EAP input-output function similar to one that accounts for probabilistic single-fiber input-output functions. Discrepancies between these two models do occur if the threshold distribution function is compressed significantly, as might be the case in pathological cochleae with altered distributions or numbers of nerve fibers.

The neuronal response to electrical constant-amplitude pulse train stimulation: evoked compound action potential recordings.

The purpose of this study was to gain a greater understanding of the electrically evoked compound action potential (EAP) responses to pulse train stimulation. Analysis of EAP amplitude responses suggested that an alternating pattern varied depending upon stimulus level, interpulse interval (IPI), stimulus waveform, and stimulus polarity. Stimulus level-dependent recovery was seen in the cat and the guinea pig: higher stimulus level tended to provide faster recovery. Both polarity-dependent recovery and polarity-dependent adaptation were observed in the cat and these stimulus polarity effects were less consistent in the guinea pig. The polarity-dependent recovery effect supports the hypothesis that anodal and cathodal stimuli excite different sites along auditory nerve fibers. Amplitude differences between the response to the second pulse and the steady-state response at the same IPI are significantly greater for anodal stimuli than for cathodal stimuli in all cats. These data suggest that there is a cumulative refractory effect in the auditory nerve of cats, especially in response to anodal stimuli.

The neuronal response to electrical constant-amplitude pulse train stimulation: additive Gaussian noise.

Experimental results from humans and animals show that electrically evoked compound action potential (EAP) responses to constant-amplitude pulse train stimulation can demonstrate an alternating pattern, due to the combined effects of highly synchronized responses to electrical stimulation and refractory effects (Wilson et al., 1994). One way to improve signal representation is to reduce the level of across-fiber synchrony and hence, the level of the amplitude alternation. To accomplish this goal, we have examined EAP responses in the presence of Gaussian noise added to the pulse train stimulus. Addition of Gaussian noise at a level approximately -30 dB relative to EAP threshold to the pulse trains decreased the amount of alternation, indicating that stochastic resonance may be induced in the auditory nerve. The use of some type of conditioning stimulus such as Gaussian noise may provide a more 'normal' neural response pattern.

Auditory nerve responses to monophasic and biphasic electric stimuli.

Charge-balanced, biphasic stimulus pulses are commonly used in implantable cochlear prostheses as they can be safely delivered to living tissue. However, monophasic stimuli are more efficient (i.e. producing lower thresholds) and likely provide more spatially selective excitation of nerve fibers. We examined the neural responses to monophasic, 'pseudomonophasic', and biphasic stimuli to better understand the inherent tradeoffs of these stimuli. Using guinea pig and cat animal models, we compared the auditory nerve responses to both 40 micros monophasic and 40 micros/phase biphasic stimuli using both electrically evoked compound action potential and single-fiber recordings. We also made comparisons using a computational model of the feline auditory nerve fiber. In all cases, our stimuli were cathodic monophasic and cathodic-first biphasic pulses. As expected, monophasic stimuli provided lower thresholds relative to biphasic stimuli. They also evoked responses with relatively longer latencies. We also examined responses to charge-balanced biphasic pulses composed of two phases of differing duration (i.e. pseudomonophasic stimuli). The first phase was fixed at 40 micros, while the second phase was systematically varied from 40 to 4000 micros. With a relatively long second phase, we hypothesized that these stimuli would provide some of the beneficial features of monophasic stimuli. Both the gross-potential and single-fiber data confirmed this and indicate that the largest incremental effects of changing the second-phase duration occur for durations less than 500 micros. Consideration of single-fiber data and computer simulations suggest that these results are consistent with the neural membrane acting as a leaky integrator. The computer simulations also suggest that the integrative properties at least partially account for the difference between our monophasic-biphasic results and previously published data. Our results apply to cathodic-leading stimuli; due to differing patterns of membrane depolarization, they may not be applicable to situations using anodic-leading stimuli. Finally, we observed differences between the guinea pig and cat response patterns. Compared to cats, guinea pigs produced smaller monophasic vs. biphasic threshold differences. This interspecies disparity may be due to differences in cochlear anatomy.

Electrically evoked single-fiber action potentials from cat: responses to monopolar, monophasic stimulation.

We recorded action potentials from single auditory-nerve fibers of cats using monophasic current pulses delivered by a monopolar intracochlear electrode. These simple stimuli provided a means of investigating basic properties and hypotheses of electrical excitation. Standard micropipette recording techniques were used. Responses to anodic (positive) and cathodic (negative) stimulus pulses were recorded separately to evaluate stimulus polarity effects. Mean spike (action potential) latency was polarity dependent, with greater latencies for cathodic stimulation. Threshold stimulus level was also polarity dependent, with relatively lower cathodic thresholds. Both effects are consistent with trends reported in the compound action potential. Variability in single-fiber latency (i.e., jitter) was dependent upon stimulus polarity. In contrast, the slope of single-fiber input-output functions failed to show a clear polarity dependence, although such trends have been seen in the compound action potential data. We also observed a relatively greater degree of adaptation over time with anodic stimulation. Bimodal post-stimulus-time histograms were recorded in a small number (2%) of fibers, supporting the hypothesis that both the peripheral (dendritic) and central axonal processes are excitable with the same stimulus polarity, in a limited number of cases. This observation, together with analyses of interactions among measures of latency, threshold, and jitter, is consistent with the hypothesis that, with monopolar intracochlear stimulation, most fibers are stimulated at axonal (modiolar) sites and a minority of fibers nearest the electrode are stimulable at their peripheral processes.

Pseudospontaneous activity: stochastic independence of auditory nerve fibers with electrical stimulation.

We describe a novel signal processing strategy for cochlear implants designed to emphasize stochastic independence across the excited neural population. The strategy is based on the observation that high rate pulse trains may produce random spike patterns in auditory nerve fibers that are statistically similar to those produced by spontaneous activity in the normal cochlea. We call this activity 'pseudospontaneous'. A supercomputer-based computational model of a population of auditory nerve fibers suggests that different average rates of pseudospontaneous activity can be created by varying the stimulus current of a fixed-amplitude, high-rate pulse train, e.g. 5000 pps. Electrically-evoked compound action potentials recorded in a human cochlear implant subject are consistent with the hypothesis that such a stimulus can desynchronize the fiber population. This desynchronization may enhance neural representation of temporal detail and dynamic range with a cochlear implant and eliminate a major difference between acoustic and electric hearing.

Electrically evoked compound action potentials of guinea pig and cat: responses to monopolar, monophasic stimulation.

We recorded electrically evoked compound action potentials (EAPs) from guinea pigs and cats using monophasic current pulses delivered by a monopolar intracochlear electrode. By using simple stimuli, we sought results that could shed light on basic excitation properties of the auditory nerve. In these acute experiments, the recording electrode was placed directly on the auditory nerve. Responses to anodic and cathodic stimulus pulses were recorded separately to evaluate stimulus polarity effects. Several polarity-dependent properties were observed. Both EAP morphology and latency were polarity-dependent, with greater latencies for cathodic stimulation. Threshold stimulus level was also polarity-dependent, but in different directions in the two species: cats had lower cathodic thresholds while guinea pigs had lower anodic thresholds. We also observed that the slopes of the EAP amplitude-level functions depended upon stimulus polarity. In most cases where EAP saturation amplitude could be measured, that amplitude was similar for anodic and cathodic stimuli, suggesting that either stimulus polarity can recruit all fibers, or at least a comparable numbers of fibers. The common findings (e.g., EAP morphology and polarity-dependent latency) observed in these two species suggest results that can be extrapolated to responses obtained in humans, while the species-specific findings (e.g., dependence of threshold on polarity) may point to underlying anatomical differences that caution against overgeneralization across species. Some of our observations also bear upon hypotheses of how electrical stimuli may excite different sites on auditory nerve fibers.

Surgical management of Bell's palsy.

OBJECTIVES: Incomplete return of facial motor function and synkinesis continue to be long-term sequelae in some patients with Bell's palsy. The aim of this report is to describe a prospective study in which a well-defined surgical decompression of the facial nerve was performed in a population of patients with Bell's palsy who exhibit the electrophysiologic features associated with poor outcomes. In addition, management issues related to Bell's palsy including herpes simplex virus typel etiology, the natural history, electrodiagnostic testing, and efficacy of surgical strategies are reviewed. STUDY DESIGN AND METHODS: A multicenter prospective clinical trial was designed utilizing electroneurography (ENOG) and voluntary electromyography (EMG) to identify patients with Bell's palsy who would most likely develop poor return of facial function, as suggested by Fisch and Esslen. Patients who displayed electrodiagnostic features of poor outcome, >90% degeneration on ENOG testing and no voluntary motor unit EMG potentials within 14 days of onset of total paralysis, were offered a surgical decompression of the facial nerve through a middle cranial fossa surgical exposure, including the tympanic segment, geniculate ganglion, labyrinthine segment, and meatal foramen. Control subjects were those who displayed similar electrodiagnostic features and time course. RESULTS: Subjects who did not reach 90% degeneration on ENOG within 14 days of paralysis all returned to House-Brackmann grade I (n = 48) or II (n = 6) at 7 months after onset of the paralysis. Control subjects self-selecting not to undergo surgical decompression when >90% degeneration on ENOG and no motor unit potentials on EMG were identified had a 58% chance of developing a poor outcome at 7 months after onset of paralysis (House-Brackmann grade III or IV [n = 19]). A group with similar ENOG and EMG findings undergoing middle fossa facial nerve decompression exhibited House-Brackmann grade I (n = 14) or II (n = 17) in 91% of the cases. An exact permutation test confirmed that the surgical group had a significantly higher proportion of patients with a good outcome (House-Brackmann grade I or II) (P = .0002). CONCLUSION: Electroneurography in combination with voluntary EMG successfully identified patients who will most likely return to normal from those who had a greater chance of long-term sequelae from Bell's palsy. Surgical decompression medial to the geniculate ganglion significantly improves the chances of normal or near-normal return of facial function in the group that has a high probability of a poor result. Surgical decompression must be performed within 2 weeks of onset of total paralysis for it to be effective.

Axon termination conditions for electrical stimulation.

The cable model for electrical stimulation near the terminal of a passive fiber is derived for excitation by an arbitrary, time-varying, applied extracellular field. Unless the termination impedance is comparable to that of mammalian node of Ranvier, the end-conditions require the longitudinal intracellular current at the fiber terminal to be negligibly small. This requirement substantially alters the membrane potential profile from that obtained with a fiber of infinite length. Stimulation near the end of a fiber may result in lower thresholds and may reverse the anodal/cathodal threshold ratio obtained with stimulation in the mid-portion of the fiber. Chronaxie for stimulation near the terminal may be much smaller than at a distance from the terminal and the strength-duration curve may be nonmonotonic. These differences may have significant implications for any application of electrical stimulation where fiber terminations may play a role in the excitatory process.

In vitro measurement and characterization of current density profiles produced by non-recessed, simple recessed, and radially varying recessed stimulating electrodes.

Potential fields induced by nonrecessed, simple recessed, and radially varying recessed electrode designs were measured in vitro. Comparison of experimental results with theoretical analyses substantiated the experimental measurement technique and emphasized the importance of considering both nonuniform charge injection and surface electrochemistry when designing implantable stimulating electrodes. Radially varying recesses produced uniform charge injection at the electrode surface and at the aperture-tissue interface. In general, the radially varying recessed electrodes provided a combination of uniform charge injection and flexibility in design and fabrication that warrants their incorporation into all appropriate planar stimulating electrode designs.

Three-dimensional geometric modeling of the cochlea using helico-spiral approximation.

In this paper, the three-dimensional geometry of the human cochlea is modeled by the helico-spiral seashell model. The 3-D helico-spiral model, the generalized representation of the Archimedian spiral model, provides a framework for measuring cochlear features based on consistent estimation of model parameters. Nonlinear least square minimization based algorithms are developed for the identification of rotation, center and intrinsic parameters of the helico-spiral representation. Two algorithms are designed for the rotation axis aligned to the modiolar axis: one is more susceptible in the presence of noise, while the other allows applicability to two-dimensional data sets. The estimated center and intrinsic parameters allow the calculation of length, height and angular positions needed for frequency mapping of multichannel cochlear implant electrodes. Model performance is evaluated with numerically synthesized curves with different levels of added random noise, histologic data and real human cochlear spiral computed tomography data.

Analysis of monophasic and biphasic electrical stimulation of nerve.

In an earlier study, biphasic and monphasic electrical stimulation of the auditory nerve was performed in cats with a cochlear implant. Single-unit recordings demonstrated that spikes resulting from monophasic and biphasic stimuli have different thresholds and latencies. Monophasic thresholds are lower and latencies are shorter under cathodic stimulation. Results from stochastic simulations of a biophysical model of electrical stimulation are similar. A simple analysis of a linear, "integrate to threshold" membrane model accounts for the threshold and latency differences observed experimentally and computationally. Since biphasic stimuli are used extensively in functional electrical stimulation, this analysis greatly simplifies the biophysical interpretation of responses to clinically relevant stimuli by relating them to the responses obtained with monophasic stimuli.

Digital X-ray stereophotogrammetry for cochlear implantation.

Multielectrode, intracochlear implant systems are effective treatment for profound sensorineural hearing loss. In some cases, these systems do not perform well, which may be partially due to variations in implant location within the cochlea. Determination of each electrode's position in a patient's inner ear provides an in vivo basis for both the cochlear modeling of electrical fields and the future design of electrode arrays that deliver electrical stimulation to surviving auditory neurons, and may improve speech processor programming for better speech recognition. We developed an X-ray stereophotogrammetric approach to localize implanted electrodes in three dimensions. Stereophotogrammetry of implanted electrodes is formulated in weak perspective geometry, with knowledge of a three-dimensional (3-D) reference structure and electrode positions in each of two digital stereo-images. The localization error is theoretically, numerically, and experimentally quantified. Both numerical and experimental results demonstrate the feasibility of the technique.

The effects of interpulse interval on stochastic properties of electrical stimulation: models and measurements.

It is known that some cochlear implant users have improved speech perception using higher rates of interleaved pulsatile stimulation. There are, however, significant limitations on their performance presumably due in part to temporal and spatial interactions. To address these limitations, we have examined refractory characteristics of the auditory nerve using experimental animal models and computational simulations. A stochastic model of the node of Ranvier modified for mammalian sodium channel kinetics has been developed to calculate the masked input-output (I/O) functions for different interpulse intervals (IPI) [26]. The model is based upon 1000 voltage-gated sodium channels and incorporates parameters such as nodal resistance and capacitance. The relative spread (RS) [35] calculated from the I/O functions was typically 0.03 for 17 different IPIs between 450 micros and 6 ms for cathodal stimuli. For IPI = 830 and 870 micros, the RS was ten times greater than those for other IPIs. Although it is not fully understood how the electrically evoked compound action potential (EAP) data are related to single fiber data, the RS of single fibers is a partial contributor [19]. We have measured the EAP using a monopolar intracochlear stimulating electrode and a recording electrode placed directly on the nerve and have observed changes in slope of EAP growth functions consistent with the theoretical RS values. These results have significant implications for speech coding in a cochlear implant since they suggest an increased membrane noise for pulse trains of specific rates.

Acoustic neuromas in the elderly.

OBJECTIVE: To determine if an "observation" protocol with serial scanning is a safe and effective management paradigm for acoustic neuromas in the elderly. STUDY DESIGN: A retrospective case review was performed. SETTING: This study was performed in an academic, tertiary care center. PATIENTS: Forty-one patients over the age of 65 years were identified with the primary diagnosis of unilateral acoustic neuroma, without prior treatment or observation. INTERVENTION: The patients were followed with serial, gadolinium-enhanced magnetic resonance imaging (MRI) scans performed at 6 months and then yearly, if no significant growth occurred. MAIN OUTCOME MEASURES: The patients were monitored for tumor growth, cranial nerve deficits, and hydrocephalus. RESULTS: The patients were followed for an average of 3.5 years (range, 6 months to 9 years). The average tumor size at presentation was 1.14 cm, with a range of growth rates from 0 to 1.2 cm per year. Twenty-one patients demonstrated tumor growth at an average rate of 0.322 cm per year. Only five patients (12%) required further intervention. Three patients underwent translabyrinthine excision, and two patients were treated with radiation. No patients developed significant complications during the observation period. CONCLUSIONS: Acoustic neuromas in the older population can be managed safely using serial MRI scanning. No correlation could be made between initial tumor size and subsequent growth rate.

Rheumatoid arthritis of the temporomandibular joint with herniation into the external auditory canal.

Previous authors have shown that soft tissue can present in the external auditory canal via a patent foramen of Huschke. One case represented a patient with psoriatic arthritis and a polyp in the external auditory canal. Typically, neoplastic, inflammatory, or degenerative lesions of the temporomandibular joint do not present in the external auditory canal. We present a patient with rheumatoid arthritis of the temporomandibular joint and soft tissue herniation into the external auditory canal. The case, and a discussion of possible causes, are presented.