As previously found by several authors [18–20], all horses showed very good acceptance of the endoscopic examination during exercise. Dynamic stenotic disorders of the upper airways can only be reliably diagnosed during exercise (on the treadmill or using exercise endoscopy). The influence of head-neck position on upper airway function has already been examined by many authors [21–24]. A higher incidence of dynamic stenosis could be proven particularly in a severely overbent head-neck position. A possible cause for this effect might be that, if the head is bent, the airflow no longer follows a straight line but has to make its way at an increasingly smaller angle with increasing bend [25]. Additionally there is a change in pharyngeal diameter [11, 12]. To show these changes on the endoscopic image during exercise poses new challenges for evaluation. During exercise, particularly during a change of head-neck position, a rostral or caudal movement of the endoscope cannot be avoided. Due to the change in head-neck position, the soft tissue boundaries of the nasopharynx rotate around the larynx, which acts as an extension of the trachea [10]. As a consequence, there is a change in the distance of the endoscope from the larynx as well as in the angle at which the assessed structures are viewed. In addition, none of the structures viewed has a known size, and the distance of the endoscope from these structures is unknown. Thus, an absolute measurement to determine pharyngeal width is impossible, which is why the above method was developed for the present study. Under the assumption that the width of the epiglottis remains constant, this method permits to compensate for changing distances and viewing-angle between the scope and the larynx.
In the present study, only horses without any identifiable disorders of the upper respiratory tract were included, in order to test the new measuring method. In ten horses, the PE-ratio in the different head-neck positions in canter was determined during both expiration and inspiration. These ten horses were selected, because of the high quality of their endoscopic videos, in which the fixed points were easily visible during the entire breathing cycle. No significant difference with regard to the respiratory phase could be detected. Opposed to this, in horses with dynamic pharyngeal collapse the pharyngeal walls will collapse during inhalation [26] and a detectable difference in the PE-ratio measured during inspiration and expiration could be expected. Due to the higher quality of the images, for further evaluation freeze frames from the endoscopic videos were exclusively taken during expiration in all horses.
Through the simultaneous start of recording exercise endoscopy as well as lateral video footage, it was ascertained that the freeze frames, which were to be assessed were obtained within the respective contact position to be investigated. A completely synchronous extraction of freeze frames from exercise endoscopy and lateral video footage was not possible since not all analysable freeze frames from exercise endoscopy corresponded with orthograde lateral images of horse and rider. Measurement of the GA and WA enabled objective assessment and verification of the different head-neck positions, even if different riders presented the horses and the required contact positions were implemented differently.
The selected contact positions were derived from the FEI guidelines [13]. For the purposes of the present study, the head-neck positions were selected to be practicable for most of the horses, even if they are previously not trained accordingly. Thus, the used contact positions had to be slightly modified and we laid down a new nomenclature. Figure 3 shows the reviewed contact positions and their accompanying combination of GA and WA. Note that in the case of hyperflexion only the GA is decisive for definition. The FEI states that hyperflexion is a working technique to provide a degree of longitudinal flexion of the neck [14]. Therefore, the condition of hyperflexion could be considered fulfilled if the nose-line of the horse is behind vertical (GA > 90°). In contrast, the extreme yielding of the poll in the sense of the so-called “Rollkur”, as described by Meyer et al. [9], is only achieved if the nose-line is 20 degrees and more behind the vertical. In the present study, only two horses could show this posture.
A significant difference in the relative change in PE-ratio between the unrestrained head position and the other head-neck positions investigated could be confirmed. Similar results were already reported [21–24, 27]. In all these studies, the horses were examined in an unrestrained head position and with side-reins or in a contact position determined by the rider respectively. The aim of our study was to differentiate further these head-neck positions previously only described as contact, and to investigate their effect on pharyngeal width. Contrary to the hypothesis that PE-ratio would be smallest in hyperflexion, a change in the percentage PE-ratio corresponding to the increasing degree of contact could not be confirmed. A possible explanation might be the choice of horses and their largely varied level of schooling. Thus, some of the horses were ridden in elevation and/or hyperflexion for the first time in the course of this study.
In addition, the fact, that only approximately half of the data could be further evaluated after assessment of the head-neck position is due to a relatively heterogeneous subject group in terms of age and level of schooling. Half of the freeze frames from the lateral video footage of horse and rider each were obtained in suspension phase or support phase respectively. In the elevation head-neck position, particularly young horses tended to lean on the bit during the support phase due to their insufficient self-carriage, thus getting their nose-line markedly further behind the vertical than during suspension phase. Hence, the variation in GA and WA was above average and for at least one of those angles the predetermined limit of the standard deviation was exceeded. Within each group of working discipline, the groups of varying age and level of schooling were too small for a well-founded statistical statement. However, the group of dressage horses showed a significant higher rate of correctly implemented contact positions compared to the other two groups. This may be an indication of the impact of the level of schooling. No significant dependence between the choice of rider (A or B) and the proper performance of contact positions could be detected.
In the present study, the choice of the ranges of angles used to define the different head-neck positions (Figure 3) still needs to be critically assessed. Particularly, in the reference head position the chosen ranges of angles overlap with those of hyperflexion and elevation. For performing the reference head position the riders were simply asked to ride the horses similar to a common warm-up-phase. Ideally, an extension posture with a low neck and the nose-line slightly in front of the vertical should have been achieved. However, the majority of the horses were presented with the nose-line slightly behind the vertical, which leads to an overlap with the definition of hyperflexion. This probably stemmed from the way they were used to be ridden. The change in PE-ratio was investigated in relation to the reference position in order to enable a comparison between the horses. Hence, the outcome of the present study may be influenced by the choice of investigated contact positions and we cannot completely exclude, that a choice of head-neck positions defined by clearly distinguished ranges of angle may have led to a deviating result. Furthermore, the results of our study are based on the assumption, that the width of the epiglottis remains unaltered regardless of changing head-neck positions. A further verification of this hypothesis was not possible in the course of our study.
Between the calculated percentage change in PE-ratio and the measured angles (GA and WA) only a weak correlation could be confirmed. A possible explanation might be, that in this study, head-neck positions were merely defined by two angles in the sagittal plane and the level at which the horse’s head was held at that time was not taken into account. Thus, the degree of flexion was converted to a measurable unit, but the level of head and neck in space described as “Einstellung” by Meyer et al. [9] was not recorded. However, Cehak et al. [11] showed in their study that in horses at rest, there is a higher correlation between pharyngeal width and the degree of flexion than between the pharyngeal width and the level of head and neck in space. Likewise, a previous study showed that in horses at rest, there is a significant correlation between the radiographic pharyngeal diameters and the GA or WA applied respectively [16].
Furthermore, it is apparent that in the study on horses at rest, the determined pharyngeal diameter in the flexed head-neck position was significantly smaller than in the other two head-neck positions examined (neutral and extended). In the present study on horses during exercise, on the other hand, a significant difference between the unrestrained head-neck position and the other head-neck positions examined was proven, but not between the different degrees of contact. An explanation for this is the changing muscular tone of the upper airways during exercise. As there is no osseous or cartilaginous basic support structure particularly in the caudal part of the nasopharynx of the horse, its stability is solely dependent on the tone of the associated muscles. The stylopharyngeal muscle, upon contraction, elevates the dorsal pharynx and prevents a pharyngeal collapse [28]. Its activity is modified by the glossopharyngeal nerve [28]. Upon increased respiration during exercise, mechanoreceptors in the upper airway mucosa are activated and lead to a higher activity of the muscles of the upper airways [29]. Due to the general assumption that sedation leads to a change in muscular function and because the mechanoreceptors are not activated at rest, the results of this study, where horses were examined during exercise, differed from those of previous studies, where sedated horses were examined at rest.