Intermittent dorsal displacement of the soft palate (DDSP or “palate displacement”) results in a upper–airway obstruction associated with abnormal (i.e. gurgling) sound4–6 and poor performance in racehorses.7,8 Anatomically, two muscles (i.e. the thyrohyoid or TH muscles) located on each side of the voice box (i.e. larynx) have been shown to have decrease activity during exercise a few seconds prior to the occurrence of DDSP. Prior study in our group has shown that the TH muscles when they contract they move the voice box upward and forward. Furthermore if these muscles are removed it results in DDSP at exercise only. This has been the basis of the introduction of the laryngeal tie–forward by our team.

However we do not know what causes the dysfunction of TH muscles in naturally occurring cases: is it muscle fatigue or lack of power/strength? In a preliminary study we instrumented 2 normal horses and 2 horses with naturally occurring DDSP with electromyographic electrodes (EMG) to investigate the TH muscle activity during exercise. We found that in normal horses the TH muscles contraction increases with exercise intensity: that is the TH muscle works harder the faster it exercised. This was contrary to what was recorded in the 2 horses with DDSP: after an initial increase in muscle activity, a marked decrease in TH muscle recruitment preceded by a few seconds the DDSP episodes. Therefore the EMG data obtained suggest that DDSP is the result of muscle fatigue. The reasons for fatigue to occur in the TH muscle are unknown but could rely in a different fiber–type proportion in the TH muscles, between normal horses and horses with DDSP: that is fewer fatigue resistant muscle fibers type (type I) in the DDSP horses.

This hypothesis requires investigation through histologic evaluation of the muscle characteristics in both normal and DDSP horses. Currently there is no available data regarding the fiber type composition of the TH muscles. This study would give us the chance to obtain information missing to date on the normal functionality of the TH muscle during exercise and on the role of its dysfunction on the development of DDSP. We treated one DDSP horse with different protocols of TH muscle electrical stimulation (note: This is because electrical stimulation of the muscle is a known way to effectively change the muscle composition. Eventually once we have proven the hypothesis we would focus on training methods to achieve the same results). The protocol that supposedly increases muscle strength but not its fatigue resistance caused a gradual increase in palate displacement rate (i.e. DDSP got worse). To the contrary, after a few weeks of switching to a protocol to induce an increase in fatigue resistant fibers, the horse finally exercised without developing DDSP (Appendix, Fig. 5). These preliminary findings suggest a potential rationale for a new management approach of horses with DDSP, by increasing the muscle strength and fatigue resistance through specific training protocols and/or FES of the TH muscles.