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.

Evoked cortical activity and speech recognition as a function of the number of simulated cochlear implant channels.

OBJECTIVES: (1) To determine if consonant-vowel-consonant (CVC) syllables [Hillenbrand J, Getty L, Clark M, Wheeler K. Acoustic characteristics of American English vowels. J Acoust Soc Am 1995;97:3099-3111] could be used to evoke cortical far field response patterns in humans, (2) to characterize the effects of cochlear implant-simulated channel number on the perception and physiological detection of these same CVC stimuli, and (3) to define the relationship between perception and the morphology of the physiological responses evoked by these speech stimuli. METHODS: Ten normal hearing monolingual English speaking adults were tested. Unprocessed CVC naturally spoken syllables, containing medial vowels, as well as processed versions (2, 4, 8, 12, and 16 spectral channels) were used for behavioral and physiological testing. RESULTS: (1) CVC stimuli evoked a series of overlapping P1-N1-P2 cortical responses. (2) Amplitude of P1-N1-P2 responses increased as neural conduction time (latency) decreased with increases in the number of spectral channels. Perception of the CVC stimuli improved with increasing number of spectral channels. (3) Coinciding changes in P1-N1-P2 morphology did not significantly correlate with changes in perception. CONCLUSIONS: P1-N1-P2 responses can be recorded using CVC syllables and there is an effect of channel number on the latency and amplitude of these responses, as well as on vowel identification. However, the physiological detection of the acoustic changes does not fully account for the perceptual performance of these same syllables. SIGNIFICANCE: These results provide evidence that it is possible to use vocoded CVC stimuli to learn more about the physiological detection of acoustic changes contained within speech syllables, as well as to explore brain-behavior relationships.

Development of a biophysical model for vestibular prosthesis research.

Physiologic properties of primary vestibular neurons are compared and contrasted with properties of primary auditory neurons. The differences and similarities suggest possible coding strategies for a vestibular implant. The degree of spike rate variability, or coefficient of variation (CV), is a prominent physiological property of vestibular neurons with undetermined functional significance. At the very least, CV is highly correlated with threshold to electrical stimulation in the intact vestibular labyrinth. If CV is also important for vestibular coding, then electrical stimulation strategies should be designed to restore relatively physiologic patterns of CV. Simulations using a stochastic model of primary afferent vestibular neurons reveal that this should be possible using combinations of low and high-rate pulsatile stimulation. They also demonstrate that differences in the number and independence of synaptic inputs can significantly affect CV.

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.

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.

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.

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.

Semiautomatic segmentation of the cochlea using real-time volume rendering and regional adaptive snake modeling.

The human cochlea in the inner ear is the organ of hearing. Segmentation is a prerequisite step for 3-dimensional modeling and analysis of the cochlea. It may have uses in the clinical practice of otolaryngology and neuroradiology, as well as for cochlear implant research. In this report, an interactive, semiautomatic, coarse-to-fine segmentation approach is developed on a personal computer with a real-time volume rendering board. In the coarse segmentation, parameters, including the intensity range and the volume of interest, are defined to roughly segment the cochlea through user interaction. In the fine segmentation, a regional adaptive snake model designed as a refining operator separates the cochlea from other anatomic structures. The combination of the image information and expert knowledge enables the deformation of the regional adaptive snake effectively to the cochlear boundary, whereas the real-time volume rendering provides users with direct 3-dimensional visual feedback to modify intermediate parameters and finalize the segmentation. The performance is tested using spiral computed tomography (CT) images of the temporal bone and compared with the seed point region growing with manual modification of the commercial Analyze software. Our method represents an optimal balance between the efficiency of automatic algorithm and the accuracy of manual work.

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.

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.

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.

Three-dimensional modeling and visualization of the cochlea on the Internet.

Three-dimensional (3-D) modeling and visualization of the cochlea using the World Wide Web (WWW) is an effective way of sharing anatomic information for cochlear implantation over the Internet, particularly for morphometry-based research and resident training in otolaryngology and neuroradiology. In this paper, 3-D modeling, visualization, and animation techniques are integrated in an interactive and platform-independent manner and implemented over the WWW. Cohen's template shape with mean cross-sectional areas of the human cochlea is extended into a 3-D geometrical model. Also, spiral computer tomography data of a patient's cochlea is digitally segmented and geometrically represented. The cochlear electrode array is synthesized according to its specification. Then, cochlear implantation is animated with both idealized and real cochlear models. Insertion length, angular position, and characteristic frequency of individual electrodes are estimated online during the virtual insertion. The optimization of the processing parameters is done to demonstrate the feasibility of this technology for clinical applications.

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.

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.

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.

Pre-lingually deaf children can perform as well as post-lingually deaf adults using cochlear implants.

Both pre-lingually deaf children and post-lingually deaf adults benefit from cochlear implants. These adults have a memory for speech sounds, but the children do not. In this preliminary investigation, we asked whether it was possible for these children to obtain high scores similar to the adults. We compared 21 prelingually deaf children to 81 post-lingually deaf adults using multichannel cochlear implants on the same test of sentence recognition. The children were selected to have the vocabulary and language sufficient to complete the test. Scores from the children, ranged from 0 to 97% correct. About one half of the children scored above 70% correct. Scores from the adults ranged from 0 to 100% correct with about one half of the adults scoring over 60%. Many of the children are scoring as well or better than adults are scoring. We conclude that the auditory system of pre-lingually deaf children is sufficient to obtain levels of speech recognition similar to post-lingually deaf adults.