Determination of the optimal conditions for laryngeal pacing with the Itrel II implantable stimulator.

OBJECTIVE: To determine the optimal stimulus paradigm, electrode orientation, and configuration of an implantable stimulator used to reanimate the posterior-cricoarytenoid (PCA) muscle in case of bilateral vocal fold paralysis (BVFP). STUDY DESIGN: Acute studies were conducted on 13 canines implanted with Itrel II systems with or without PCA innervation. PCA stimulus-response characteristics were obtained by measuring stimulated vocal fold displacement endoscopically. RESULTS: The denervated PCA was only 10% to 25% as responsive to stimulation as the innervated PCA. However, the response could be increased to 38% and 61% if the Itrel was modified to deliver 1 and 2 msec pulses, respectively. Stimuli delivered centrally to the muscle 5 mm from the median raphe improved performance. CONCLUSION AND SIGNIFICANCE: The optimal stimulus paradigm identified in this study (1 msec pulses delivered at 30 to 40 Hz and 2 to 8.5 mA) has been applied to implanted BVFP patients and improved outcome. Information regarding optimal electrode orientation could also be important to future clinical trials.

The effects of chronic electrical stimulation on laryngeal muscle reinnervation.

The present study examined the effects of functional neuromuscular stimulation (FNS) on reinnervation of the posterior cricoarytenoid (PCA) muscle. In 4 canines, the recurrent laryngeal nerve (RLN) was sectioned and anastomosed and a patch electrode array implanted for stimulation and recording at multiple PCA sites. Following implantation, FNS was applied to 2 canines for a period of 6 weeks. Two additional animals served as nonstimulated controls. In each animal, histomorphometric analysis of the RLN was used to assess the quality of nerve regeneration and the potential for muscle reconnection. The magnitude of reinnervation was monitored by electromyographic (EMG) potentials evoked by RLN stimulation. The appropriateness of reconnection was determined by the pattern of spontaneous EMG activity and recovery of vocal fold abduction. Results of this preliminary study indicated that FNS caused an overall repression of reinnervation. However, the repression preferentially inhibited reconnection by foreign nerve fibers, promoting selective reinnervation and preventing synkinesis.

Electrical pacing of the paralyzed human larynx.

This study represents the first attempt to electrically pace the paralyzed human larynx. The goal was to determine if electrical stimulation of the posterior cricoarytenoid muscle could produce functional abduction of the vocal fold in pace with inspiration. An external apparatus was used to sense inspiration and reanimate the unilaterally paralyzed larynx of a thyroplasty patient. Stimuli were delivered through a needle electrode to locate and pace the abductor muscle. The magnitude of electrically induced abduction was comparable to spontaneous movement on the normal side. The abduction was appropriately timed with inspiration: this finding demonstrated that this simple pacing system could effectively modulate stimulation with patient respiration.

Technical approach for reanimation of the chronically denervated larynx by means of functional electrical stimulation.

Functional electrical stimulation (FES) of the posterior cricoarytenoid (PCA) muscle to produce vocal fold abduction offers an alternative approach to current surgical therapies for bilateral vocal fold paralysis. The purpose of this study was to characterize the application of FES to chronically denervated PCA muscles. Specific goals were to develop a stimulus delivery system for the PCA muscle, determine a practical means of implantation, and identify stimulus parameters effective in activating chronically denervated muscle. Seventeen dogs were implanted with planar electrode arrays 3 months after unilateral recurrent laryngeal nerve resection. A nail-bed electrode array allowed discrete activation of the PCA muscle and gave the greatest abductions, with minimal charge dissipation. Muscle mapping revealed hot-spot regions on the PCA muscle surface, in which stimulation produced maximum abduction. A conservative stimulus paradigm effective in activating chronically denervated muscle was a 1-second pulse train of 2-millisecond-duration pulses, delivered at a tetanizing frequency of 30 Hz and an amplitude of 4 to 14 mA.

Awake evoked electromyography recording from the chronically implanted rat.

Investigation of the effects of various factors on nerve regeneration has been compromised by the lack of an accurate and objective technique which can monitor dynamic changes in the status of nerve-muscle innervation over the entire course of regeneration. The approach of evoked electromyography (EEMG) was adopted to obtain temporal and quantitative data during nerve regeneration. Initially, transcutaneous nerve stimulation and percutaneous muscle recording was performed, but the approach was abandoned because of the high interest variability (20% average deviation) and requirement for anesthesia during testing. A new approach using chronically implanted stimulation and recording electrodes was adopted in an attempt to circumvent these problems. Initial acute studies performed in the hindlimb of the anesthetized rat identified stable EEMG recording sites with sciatic nerve stimulation. In a second study conducted in chronically implanted unanesthetized unrestrained animals, EEMG recording from these sites demonstrated remarkable stability with an average interest variability of only 5%. Preliminary results have been obtained with this technique in monitoring the progression of hindlimb reinnervation following crush and transection nerve injuries.

Facial nerve recording from the eardrum: a possible method for evaluating idiopathic facial nerve paralysis.

A noninvasive technique has been developed for recording from the the facial nerve within the fallopian canal. Following transcutaneous stimulation of the nerve on the face, an antidromic nerve potential can be detected with electrodes placed on the eardrum and enhanced by the technique of averaging. In studies conducted on cats and normal human subjects it has been determined that the primary recording site of the nerve potential is along the tympanic segment of the nerve just distal to the geniculate ganglion. Disturbances in nerve conduction caused by an experimental lesion produced changes in the recorded nerve potential. Thus it seems likely that this technique could assess a developing lesion in a patient with idiopathic facial nerve paralysis. A more accurate and earlier prognosis could be obtained than with conventional peripheral nerve testing techniques, since the severity of the developing lesion itself could be assessed.