Laryngeal reinnervation using the spinal accessory nerve: Electromyographic study of the sternomandibularis muscle

Background Selective laryngeal reinnervation using the first and second cervical nerve (C1C2) is a treatment option for recurrent laryngeal neuropathy that aims to restore the function of the cricoarytenoideus dorsalis (CAD) muscle. Despite the technique's satisfying success rate, it has several limitations. These triggered the search for another potential donor nerve that could reduce CAD muscle fatigue and shorten rehabilitation. The ventral branch of the spinal accessory nerve, providing motor innervation to the sternomandibularis (SM) muscle, was identified as a potentially well-adapted nerve. Objectives To gather normative data from SM muscle activity induced by physiological spinal accessory nerve stimulation at rest and during exercise, and to determine SM muscle activity and fibre type recruitment relative to posture, gait and respiratory cycle. Study design Clinical observational study. Methods Surface electromyography (EMG) of the SM muscle was performed in nine horses: trained warmbloods, Thoroughbreds and Standardbreds. Signals were recorded in different feeding postures and at exercise, which included standardised treadmill exercise tests, lungeing and ridden work. Results Timing of the SM muscle contraction coincided with inspiration at gallop. Intra-individual mean SM muscle activity increased with exercising speed (four times higher in Thoroughbreds at gallop than walk and seven times higher in Standardbreds at high-speed trot than walk). Moreover, the SM muscle was strongly activated at rest when the horse was grazing (seven times more than when it was eating out of a hay net or stable feeder). Frequency domain analysis revealed a predominant type I muscle fibre recruitment during feeding and at exercise (type I muscle fibre activity was at least 2.5 times higher than type II muscle fibre activity). Main limitations Lack of reference data on equine EMG. Conclusions This study confirmed the potentially advantageous properties of the spinal accessory nerve for laryngeal reinnervation.