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

Abstract

Exercise induced intermittent dorsal displacement of the soft palate (DDSP) is a common cause of airway obstruction and poor performance in racehorses. The definite etiology is still unclear, but through an experimental model, a role in the development of this condition was identified in the dysfunction of the thyro-hyoid muscles. The present study aimed to elucidate the nature of this dysfunction by investigating the spontaneous response to exercise of the thyro-hyoid muscles in racehorses with naturally occurring DDSP. Intramuscular electrodes were implanted in the thyro-hyoid muscles of nine racehorses, and connected to a telemetric unit for electromyographic monitoring implanted subcutaneously. The horses were recruited based on upper airway function evaluated through wireless endoscopy during exercise. Five horses, with normal function, were used as control; four horses were diagnosed as DDSP-affected horses based on repeated episodes of intermittent dorsal displacement of the soft palate. The electromyographic activity of the thyro-hyoid muscles recorded during incremental exercise tests on a high-speed treadmill was analyzed to measure the mean electrical activity and the median frequency of the power spectrum, thereafter subjected to wavelet decomposition. The affected horses had palatal instability with displacement on repeated exams prior to surgical implantation. Although palatal instability persisted after surgery, only two of these horses displaced the palate after instrumentation. The electromyographic traces from this group of four horses showed, at highest exercise intensity, a decrease in mean electrical activity and median power frequency, with progressive decrease in the contribution of the high frequency wavelets, consistent with development of thyro-hyoid muscle fatigue. The results of this study identified fatigue as the main factor leading to exercise induced palatal instability and DDSP in a group of racehorses. Further studies are required to evaluate the fiber type composition and metabolic characteristics of the thyro-hyoid muscles that could predispose to fatigue.

Fig 1. Schematic illustration to show the position of the intramuscular electrodes and electromyography transmitter.

The diagram illustrates the left side of the horse, and the electrodes within the two thyro-hyoid muscles. The electrodes leads are tunneled subcutaneously and connected to the electromyography transmitter implanted subcutaneously over the left proximal neck.

Fig 2. Electromyographic activity of the thyro-hyoid (TH) muscle in an exercising horse.

TH electrical activity (TH emg) is synchronous with inspiration (2a), increases with swallowing and is absent immediately prior to dorsal displacement of the soft palate (DDSP) (2b). 2b) Representative raw electromyographic activity of the thyro-hyoid muscle and pharyngeal pressure in a horse affected with intermittent DDSP. Black asterisks indicate swallows; the red asterisk indicates the DDSP episode. Note the marked reduction in the breathing related EMG activity prior to development of DDSP. The Y-axis is limited to +/-1000mV to facilitate viewing of EMG trace.

Fig 3. Mean electrical activity (MEA) of the thyro-hyoid muscle decreases at the highest exercise intensity in horses affected by palatal instability and intermittent dorsal displacement of the soft palate (DDSP).

3a) MEA showed significant difference between control and DDSP horses at lower exercise intensity. 3b) MEA normalized to the value recorded at HR_max80 (nMEA), was significantly lower in horses diagnosed with DDSP compared to control at HR_max100. Data are expressed as mean with SD. *, p<0.05.

Fig 4. Median Power Frequency (MF) of the thyro-hyoid muscle electrical activity decreases at highest exercise intensity in horses affected by palatal instability and intermittent dorsal displacement of the soft palate (DDSP).

4a) The DDSP group horses show a significantly lower MF at HR_max100 compared to normal horses. 4b) A significant drop in MF normalized to the frequency recorded at HR_max80 (nMF) was recorded before developing palatal displacement in 2 horses. Data are mean and SD. * p<0.05. Different letters indicate statistically significant difference between nMF recorded at different speed intervals in the DDSP horses.

Fig 5. Wavelet decomposition of the thyro-hyoid muscles electromyographic activity in exercising horses.

Relative contribution (% of the total intensity) of the center frequency of the 16 wavelets to the thyro-hyoid muscles electromyographic activity shows a relative shift towards lower frequencies in horses affected by palatal instability and intermittent displacement of the soft palate (DDSP) compared to normal horses (control). Data expressed as mean ±SD.

Fig 6. Exercise induced variation in intensity of the two frequency domains identified after wavelet decomposition of the thyro-hyoid muscles electrical activity during exercise.

The control group (black bars) showed a significant increase in the intensity of the two frequency domains during exercise, reaching the maximum value at HR_max100. The DDSP group (red bars) showed a non-significant initial increase followed by a non-significant decrease in the intensity of both domains during exercise. Intensity of each frequency domain is normalized to the value recorded at exercise intensity corresponding to HR_max80 (nmV²). Data are expressed as mean ±SD. Different letters indicate statistically significant difference between intensity values recorded at different speed intervals.

Fig 7. Relative contribution (% of the total intensity) of the high frequency (330–465 Hz) domain to the thyro-hyoid muscles electromyographic activity during exercise.

Horses affected by palatal instability and dorsal displacement of the soft palate (DDSP) show a significant lower contribution of the high frequency wavelet domain at the highest exercise intensity interval (HR_max100). Data expressed as mean ±SD. *, p<0.05.

Fig 8. Relative contribution (% of the total intensity) of the 16 wavelets to the thyro-hyoid muscles electromyographic activity during each exercise intensity interval.

8a) Normal horses (control) showed a uniform contribution of each wavelet throughout the exercise. 8b) Horses affected by palatal instability and intermittent dorsal displacement of the soft palate (DDSP), showed a progressive decrease in the contribution of the high frequency wavelets with shift toward a higher low frequency wavelets contribution at highest exercise intensity (100% HRM), and minimum of high frequencies contribution in two horses in the interval preceding the displacement of the soft palate (DDSP). Data expressed as mean ±SD.