Evaluation of the correlation between laryngohyoid conformation and electromyographic activity of thyrohyoideus muscle in horses with intermittent dorsal displacement of the soft palate (DDSPi)

INTRODUCTION DDSPi seems to be related with dysfunction of Thyrohyoideus (TH) muscle possibly caused by incorrect laryngohyoid conformation (LC). Evaluation of TH muscle activity at rest and LC by means of surface electromyography (sEMG) and ultrasonography (US) respectively, could give more information concerning DDSPi pathogenesis. OBJECTIVES To evaluate the electromyographic activity of TH muscle and its possible correlation with LC in both normal and DDSPi affected horses METHODS 12 racehorses that underwent treadmill endoscopy were selected. In 5 horses DDSPi was detected (Group A) while normal pharyngo-laryngeal function was observed in the other 7 horses (group B). For each patient sEMG of TH muscles at rest during deglutition and US of the laryngohyoid region were performed. A Shapiro-Wilk test was used to assess the normal distribution of the data followed by a t-test to compare data between group A and B. Pearson correlation coefficients (r) were used to establish a possible correlation between US measurements and EMG activity of the TH muscles in the two groups. Multiple linear regression analysis was used to investigate the effect of US measurements, age, weight, sex and attitude on the EMG activity of TH muscles. The effect of the DDSPi status (group A vs B) on US variables was investigated with a different linear regression analysis. Significant values were set at P < 0.05. RESULTS sEMG did not show any significant differences of TH muscle activity in DDSPi horses vs controls. US showed a significant correlation among several laryngohyoid measurements in DDSPi horses only. Concerning the correlation between EMG activity and LC, a significant effect (p = 0.037) of the US depth of the thyroid cartilage on the mean of EMG activity of TH muscle was found in both group of horses. DISCUSSION Functional abnormalities of TH muscles activity in DDSPi affected horses at rest was substantially ruled out. The correlation in the laryngohyoid measurements found in the DDSPi horses suggested in this group an incorrect LC. The correlation between US measure and EMG found in both group, suggested that the different LC does not result in alteration of EMG at rest. It seems more plausible that the abnormal conformation leads to TH fatigue during exercise with subsequent onset of DDSPi. CONCLUSION LC is not secondary to TH muscle activity at rest. Further research concerning the EMG activity of TH muscles during strenuous exercise may provide more information on DDSPi pathogenesis.