By Nicola Lynch, Staff Clinician in Equine Surgery, and Justin Perkins, Senior Lecturer in Equine Surgery

Intermittent dorsal displacement of the soft palate (iDDSP) occurs when the caudal border of the soft palate moves dorsal to the epiglottis during exercise. (Fig. 1) Affected horses typically present due to upper respiratory noise, poor performance or exercise intolerance. Riders often report that the horses pull up suddenly and make a characteristic expiratory or inspiratory “gurgle” or “snore”. Displacement of the soft palate usually occurs at maximum speed and usually at the end of intense exercise.

Definitive diagnosis of the condition can be challenging. Resting endoscopy is known to be a poor predictor of iDDSP, however should be performed in all cases where iDDSP is suspected to rule out other causative factors. Overground exercising endoscopy and high-speed treadmill endoscopy (HSTE) are required to confirm the diagnosis where there is a suspicion based on clinical signs.

Currently several treatments, both conservative and surgical, (Table 1) are used in horses following a diagnosis of iDDSP, with similar success rates reported for each. Part of the reason for the average and comparable success rates is our lack of understanding of the pathogenesis of naturally occurring disease.

Recently, an exciting collaboration between researchers at the RVC and Cornell University has provided new insight into naturally occurring iDDSP by evaluating the role of the thyrohyoideus muscle in displacement of the soft palate.

The thyrohyoideus muscle is a paired flat muscle that extends from the thyrohyoid bone to the thyroid cartilage. It draws the larynx forward and the basihyoid caudally. Dysfunction of this muscle results in a more caudal and ventral position of the larynx relative to the basihyoid bone and a more ventral position relative to the soft palate. Transection of this muscle can result in DDSP.

In this study entitled ‘Investigation into pathophysiology of naturally occurring palatal instability and intermittent dorsal displacement of the soft palate (DDSP) in racehorses: Thyrohyoid muscles fatigue during exercise’, electromyographic (EMG) traces from the thyrohyoideus muscle of nine racehorses were measured during exercise. EMG allows investigation of muscle recruitment, degree of activation and fatigue, through measurement of the signal amplitude and power spectrum.

Intramuscular electrodes were implanted into the thyrohyoideus muscles of nine racehorses including four with confirmed iDDSP. In the four iDDSP affected racehorses it was shown that at the highest exercise intensity there was clear evidence of fatigue of the thyrohyoideus muscles. More specifically the DDSP group had minimum activation of the fast twitch muscle fibres consistent with these fibres developing fatigue faster than in control horses.

This increased fatigability may be further explained by the muscle fibre distribution in iDDSP affected horses. Histological analysis of the thyrohyoideus muscle has shown that in normal horses it is composed of predominantly fast twitch (type II) muscle fibres with a lesser proportion of slow twitch – fatigue resistant (type I) muscle fibres. During inspiration and at intense exercise the type I fibres are activated before the type II fibres.

In iDDSP horses we have found a significantly lower proportion of type I fibres. This may result in increased fatigability due to earlier recruitment of type II fibres at slower speeds leading to them being exhausted before maximum exercise intensity. Once these fibres are exhausted there are insufficient type I fibres to maintain contractility and thus failure of the thyrohyoideus muscle occurs.

These findings potentially guide the way for new treatments for horses with iDDSP. aimed at increasing the fatigue resistance of the thyrohyoideus muscle by increasing the number of type I muscle fibres and delaying recruitment of type II fibres at less intense exercise. It is possible that this may be achieved through the use of conservative treatments, such as the use of a grackle noseband or Cornell collar to allow continued training in horses whilst these physiologic muscle adaptations take place.

Reference: 

Cercone, M., Olsen, E., Perkins, J.D., Cheetham, J., Mitchell, L.M., Ducharme, N.G. (2019) ‘Investigation into pathophysiology of naturally occurring palatal instability and intermittent dorsal displacement of the soft palate (DDSP) in racehorses: Thyro-hyoid muscles fatigue during exercise’. PLoSONE 14:10