Neuroprosthesis in Upper Airway Surgery

Neuroprosthesis in upper airway surgery

A neuroprosthesis is an electrical stimulator that generates a signal whose amplitude (mA), frequency (Hz), pulse duration (micro sec), and pulse characteristic (phase) can be controlled. Early investigation in laryngeal neuroprosthesis proves that reanimation of the arytenoid cartilage and vocal fold was possible in dogs (Sanders et al., 1991, Zealar et al., 1994). More recently, positive results were reported in humans in a small clinical study (Zealar et al., 2003). The aim of our project is twofold: 1) test the hypothesis that restoration of abduction can be obtained through electrical stimulation of the recurrent laryngeal nerve or the cricoaryteneoid dorsalis muscle (CAD) in horses, 2) determine stimulation-response curves for electrical stimulation under each condition in horses.

The focus of our investigation is the motor limb of neuroprosthesis. For a fully physiological laryngeal reanimation we would need to explore the sensory limb or trigger of the contraction. Ex-vivo experiments have indicated that maximal opening of the rima glottitis is obtained with stimulation of the medial and lateral compartments of the CAD; Stimulation of the lateral compartment produced more lateral displacement of the arytenoid cartilage, while stimulation of the medial compartment produced more dorsal movement of the arytenoid cartilage (Cheetham et al., 2008).

In experimental horses, when the pacing electrode was placed on the recurrent laryngeal nerve, stimulation resulted in full arytenoid cartilage abduction but also resulted in medial bowing of the vocal cord due to stimulation of the vocalis muscle. Therefore proper laryngeal pacing for respiratory function required pacing of the recurrent laryngeal nerve after transection of its adductor branch. Stimulation of the entire recurrent laryngeal nerve after transection of the abductor branch of the recurrent laryngeal nerve resulted in various degrees of arytenoid cartilage abduction: full abduction was obtained in horses with laryngeal grade I or II (n=7) and laryngeal grade III (n=2, Figure 2), while poor abduction was obtained in long-standing (<1 year) grade IV horses (n=2). The threshold for stimulation of recurrent laryngeal nerve stimulation was ~0.587 mA. Constant stimulation of the recurrent laryngeal nerve for 60 minutes led to full abduction without evidence of muscle fatigue (n=2). A complication associated with the procedure was electrode cuff displacement, and this occurred as late as 14 months after implantation.

This led to the need to explore intra-muscular stimulation. The intra-muscular course of the medial (i.e., abductor) branch of the recurrent laryngeal nerve was then established using Sihler’s stain in 2 horses. Likewise the location of the CAD motor endplates was identified using acetylcholinesterase staining (Cheethan et al., 2008). [...]