Effects of bilateral hypoglossal and glossopharyngeal nerve blocks on epiglottic and soft palate position in exercising horses.

This research paper investigates how blocking certain nerves in horses – the hypoglossal and the glossopharyngeal – affects the positioning of their epiglottis and soft palate, two structures in their throat that are integral to swallowing and breathing. The study, conducted on five Standardbreds, found that blocking these nerves made the tracheal pressure more negative while inspiratory pressure in the pharynx (back of the horse’s throat) was less negative. Despite resulting in an unstable epiglottis, this nerve block did not displace the soft palate from its position.

Research Goal

• The objective of this study was to determine the effect of bilateral hypoglossal and glossopharyngeal nerve blocks on the position of the epiglottis and soft palate, as well as tracheal and pharyngeal pressures in exercising horses.

Methods and Design

• The study involved five Standardbreds, a breed of horse known for its harness racing capabilities.
• The researchers measured the tracheal and pharyngeal pressures in these horses at various intensities of exercise – 50%, 75%, and 100% of maximal heart rate – with and without nerve blocks.
• The nerve blocks were performed using video-endoscopic guidance for precise injection of mepivicaine hydrochloride, a local anaesthetic, into the space between the glossopharyngeal and hypoglossal nerves located in the area known as the guttural pouch (a large air-filled diverticulum from the Eustachian tube).

Findings and Conclusion

• The results showed that the nerve block made the tracheal pressure more negative and the pharyngeal pressure less negative, indicating changes in airflow and pressure during inspiration.
• Respiratory frequency was also found to be lower with the nerve block.
• The experiment-induced nerve block destabilised the epiglottis, causing it to move backwards (retroflex) into the rima glottis during inhalation. However, this retroflexion did not lead to dorsal displacement of the soft palate, contrary to what might be expected.
• These results suggest that a functional soft palate may not rely on the epiglottis for support or positioning. Thus, conditions resulting in dorsal displacement of the soft palate in horses, a commonly observed abnormality, may not be solely due to epiglottic dysfunction as previously thought.
• This could implicate other neural or muscular elements, like the hyoepiglotticus or geniohyoideus muscles (muscles assisting in swallowing and tongue movement), or the hypoglossal nerve, in causing epiglottic retroflexion in horses. The finding opens up new areas to explore in researching this displacement condition.