Functional Electrical Stimulation of Intrinsic Laryngeal Muscles under Varying Loads in Exercising Horses

Bilateral vocal fold paralysis (BVCP) is a life threatening condition and appears to be a good candidate for therapy using functional electrical stimulation (FES). Developing a working FES system has been technically difficult due to the inaccessible location and small size of the sole arytenoid abductor, the posterior cricoarytenoid (PCA) muscle. A naturally-occurring disease in horses shares many functional and etiological features with BVCP. In this study, the feasibility of FES for equine vocal fold paralysis was explored by testing arytenoid abduction evoked by electrical stimulation of the PCA muscle. Rheobase and chronaxie were determined for innervated PCA muscle. We then tested the hypothesis that direct muscle stimulation can maintain airway patency during strenuous exercise in horses with induced transient conduction block of the laryngeal motor nerve. Six adult horses were instrumented with a single bipolar intra-muscular electrode in the left PCA muscle. Rheobase and chronaxie were within the normal range for innervated muscle at 0.55±0.38 v and 0.38±0.19 ms respectively. Intramuscular stimulation of the PCA muscle significantly improved arytenoid abduction at all levels of exercise intensity and there was no significant difference between the level of abduction achieved with stimulation and control values under moderate loads. The equine larynx may provide a useful model for the study of bilateral fold paralysis

Recurrent laryngeal neuropathy : current treatment and future possibilities

This case report will feature a 2 year old, Thoroughbred racehorse who presented to Cornell University Equine Hospital on 10/24/2010. Chief complaint consisted of poor performance and a loud noise made during exercise. The referring veterinarian passed an endoscope and diagnosed recurrent laryngeal neuropathy. On presentation no significant physical exam abnormalities were found and a standing endoscopic examination confirmed the diagnosis of grade III laryngeal hemiplegia most likely due to recurrent laryngeal neuropathy. Laryngoplasty with left ventriculectomy and right aryepiglottic fold resection was recommended and performed. Prognosis for return to successful racing is good, although there are many complications possible post-operatively

Titration of pulse duration and stimulation frequency with left abduction of the left arytenoid for horses with induced motor conduction block at 90% HRmax demonstrating significantly more abduction with longer pulse duration (p = 0.0037) and higher frequency (p<0.002).

<p>Linear contrast (*) indicates increased abduction at 49 Hz with pulse duration of 4.27 ms versus 0.427 ms (p = 0.03).</p

Strength-duration curve for innervated equine posterior cricoarytenoid muscle at rest.

<p>Data shown are mean and standard deviation for six horses.</p

Method measuring arytenoid abduction.

<p>Briefly a line is drawn connecting the dorsal- and ventral-most points of the <i>rima glottidis.</i> This line is then extended dorsally for a distance of one third of the dorsoventral height of the <i>rima glottidis</i>. A tangential line to the arytenoid cartilages is drawn, and the angle between the dorsoventral line and the tangential line is measured.</p

Illustration to demonstrate position of vidoendoscope in nasopharynx during resting and exercising endoscopy.

<p>Endoscope secured to horse's halter (not shown).</p

Relationship between stimulation and arytenoid abduction during inspiration at increasing loads (exercise intensity).

<p>(A) Increasing load produced increased abduction under control conditions but decreased abduction with block and block+stimulation. Stimulation at 4.27 ms pulse duration, 49 Hz, 6mA. Data shown are Least Squares (Adjusted) means with standard error. (B) Same data plotted to demonstrate restoration of function by stimulation at moderate exercise intensities (50–90%HRmax). Letters denotes significant differences with Tukey's <i>post hoc</i> test at p<0.05.</p

Illustration of KY5 electrode.

<p>Screw length 1.4 mm with 1.5 turns; screw diameter 1.6 mm; screw surface area 7.2 mm². Screw is Eligiloy with Iridium oxide coat. Total electrode length 580 mm.</p