Asymmetric recurrent laryngeal nerve conduction velocities and dorsal cricoarytenoid muscle electromyographic characteristics in clinically normal horses

Abstract

The dorsal cricoarytenoid (DCA) muscles, are a fundamental component of the athletic horse's respiratory system: as the sole abductors of the airways, they maintain the size of the rima glottis which is essential for enabling maximal air intake during intense exercise. Dysfunction of the DCA muscle leads to arytenoid collapse during exercise, resulting in poor performance. An electrodiagnostic study including electromyography of the dorsal cricoarytenoid muscles and conduction velocity testing of the innervating recurrent laryngeal nerves (RLn) was conducted in horses with normal laryngeal function. We detected reduced nerve conduction velocity of the left RLn, compared to the right, and pathologic spontaneous activity (PSA) of myoelectrical activity within the left DCA muscle in half of this horse population and the horses with the slowest nerve conduction velocities. The findings in this group of horses are consistent with left sided demyelination and axonal loss, consistent with Recurrent Laryngeal Neuropathy (RLN), a highly prevalent degenerative disorder of the RLn in horses that predominantly affects the left side. The detection of electromyographic changes compatible with RLN in clinically unaffected horses is consistent with previous studies that identified "subclinical" subjects, presenting normal laryngeal function despite neuropathologic changes within nerve and muscle confirmed histologically.

Figure 1

Schematic of the Nerve Conduction study. (a) Approach to determine nerve conduction velocity (NCV) in the proximal and distal portions of the equine recurrent laryngeal nerve (RLn). (b,c) Representative compound motor action potentials (CMAP) detected through the recording electrode (RE) in the dorsal cricoarytenoid (DCA) muscle following supramaximal stimulation of the RLn distally (red arrows) and proximally (green arrows). Differences in onset time (O) used to calculate NCV.

Figure 2

Nerve Conduction Velocity and Motor Unit Potential analysis results. (a) Nerve conduction velocity (NCV) is decreased in the longer left recurrent laryngeal nerve (RLn) and in the distal compared with proximal segments of ipsilateral RLn. Different letters indicate statistically significant differences between groups (p < 0.001, n = 20). (b) Motor Unit Potential (MUP) Size Index is significantly higher in the left dorsal cricoarytenoid (DCA) muscle (*p = 0.031, n = 20).