EVALUATION OF ELECTROMYOGRAPHIC ACTIVITY OF THYROHYOIDEUS (TH) MUSCLE AND ULTRASONOGRAPHIC ASSESSMENT OF THE POSITION OF THE HYOID APPARATUS AT REST AND THEIR POSSIBLE ROLE IN THE PATHOGENESIS OF INTERMITTENT DORSAL DISPLACEMENT OF THE SOFT PALATE (DDSP) IN RACEHORSES.

Introduction Dorsal displacement of the soft palate (DDSP) is one of the most common intermittent dysfunctions of the nasopharynx in racehorses. The DDSP etiopathogenesis is, still today, not clearly understood. Recently was induced DDSP in horses during exercise after bilateral resection of thyrohyoideus (TH) muscles. DDSP was also correlated to a more ventral position of the basihyoid bone evaluated using ultrasonographic scan of the laryngohyoid region. Objectives The purpose of the present work is to investigate TH activity at rest in horses affected with DDSP using surface electromyography (EMG) and correlate the results with the position of hyoid apparatus using ultrasonographic scan of the laringohyoid region. Materials and method 12 racehorses (10 Standardbreds and 2 Thoroughbreds), referred with history of poor performance ranged in age between 2 and 4 years (mean age 3.1 ± 0,7 years) with a mean body weight of 468,9 ± 52,3 kg. All horses underwent a thorough diagnostic protocol, including examination at rest and during exercise of different body systems, ultrasonographic scan of the laryngeal-hyoid portion and EMG evaluation of TH muscles. Exercise tests were realized using an high-speed treadmill (SATO-I, Sweden) and all the exercise tests was performed using a standardized protocol. On day 4 the treadmill endoscopy using a videoendoscope (Stortz®-Germany) held in position with Velcro straps was performed. After videoendoscopy of the upper respiratory tract on treadmill horses were classified into one of two groups based: DDSP when the caudal border of the epiglottis was situated dorsal to the epiglottis for a minimum of 8 s and horses without DDSP if DDSP wasn’ t discovered.. Ultrasonographic scan of the laryngohyoid region comprised two acoustic windows the rostroventral and the midventral, scanned in longitudinal and transverse planes. Using the electronic caliper of the machine were measured the depth of the basihyoid bone at the base of lingual process, the depth of the caudal aspect of the basihyoid bone, the depth of the rostral aspect of the thyroid cartilage, and the distance from the caudal aspect of the basihyoid bone to the rostral aspect of the thyroid cartilage. The EMG activity of the TH muscles was evaluated at rest with surface electrodes and a ground obtained with a subcutaneous needle electrode. The was applied on both sides at the level of the topographic area corresponding to the TH muscle, which was clipped and cleaned with alcohol. TH activity was measured during deglutition obtained following administration of 50 ml of water in the mouth using a 50 ml syringe. This operation was repeated at least five time for each side and every activity connected with deglutition recorded. Electromyographic signals were processed and analyzed and the mean electrical activity (MEA) of the muscles was evaluated. The statistical analysis of the results was made comparing the two groups, horses with DDSP (wDDSP) and without DDSP (nDDSP), considering all the measures of the laryngohyoid position, the MEA of TH muscle on the left side (MEAl), the MEA of TH muscle on the right side (MEAr) and mean MEA obtained from the two sides (MEArl) for every horse. The analysis was realized with a statistical software SIGMAPlot®12. Using SIGMAPlot®12 was realized a Shapiro-Wilk test to assess the normal distribution of the data followed by a t-test to compare every data between the group wDDSP and nDDSP. Next a Pearson Product Moment Correlation was used to evaluate the correlation between the different data in nDDSP and wDDSP groups. Finally the relationship between age, sex and weight on MEAr, MEAl, MEArl and laringohyoid measures was analyzed using a linear regression test. For all statistical analysis, values were considered significant at P ≤ 0.05. Results The videoendoscopy of the upper airways during exercise on a high speed treadmill permitted to diagnose DDSP in 5 horses (4 standardbred and 1 Throughbred), pharyngeal collapse in 1 horse, pharyngeal collapse associated with axial deviation of the aryepiglottic folds (ADAF) in 2 horses and no dynamic abnormality of the upper respiratory tract (URT) in 4 horses. The statistical analysys, using Pearson Product Moment Correlation test, showed only in the wDDSP group some correlation between the data analyzed. A strong correlation (p. 0,0019) was found between the depth of the basihyoid bone at the base of lingual process and depth of the rostral aspect of the thyroid cartilage on the midventral window. Another strong correlation (p. 0,0092) was found between depth of the rostral aspect of the thyroid cartilage and depth of the caudal aspect of the basihyoid bone in the midventral window. Finally, strong correlations were found between MEAr and MEAl (p. 0,0039), MEAr and MEArl (p. 0,0005) and MEAl and MEArl (p. 0,0004). No other difference or correlation were found. Discussion and conclusion: Our results didn’t identify any statistical difference in laringohyoid measures between nDDSP and wDDSP groups. Anyway, in wDDSP group we found a high correlation between some ultrasonographic measures obtained confirming the importance of the connection between hyoid apparatus and larynx in DDSP patients. We didn’t find any difference between MEA in nDDSP and wDDSP patients at rest. Probably, our results are directly conditioned by the type of test realized. Probably more information regarding the activity of TH muscles could be obtained with the evaluation during exercise. The results obtained provide informations regarding the pathogenesis of intermittent DDSP. Future results obtained with the evaluation of the same parameters under exercise conditions may aid the clinician to identify the actual causes for DDSP.