For objective interpretation of the head and neck position, two newly defined angles were established (GA and WA). These two angles were chosen to characterize atlanto-occipital flexion (WA) and cervicothoracic flexion (GA). As shown in Table 1, the combination of the WAe and the GAe provides an unambiguous method for judging the various head and neck positions in exercising horses.
The head and neck positions reviewed in the present study were named differently for the investigations at rest and those during exercise. The reason for this distinction is that a sedated horse at rest cannot have the same muscle tension as a horse in motion, and the contact between the bit in the horse’s mouth and the rider’s hands is missing. The head and neck position termed “elevated neck” indicates that the neck is raised, forming a harmonious curve from the withers to the poll (the highest point) with the head slightly in front of vertical. This position can be achieved only by a horse being ridden. When the head and neck position was “unrestrained”, most horses showed forward and slightly downward stretching—which could not be put on a level with the head and neck being maximally distended forward in a resting horse. Because of these unequal circumstances, the head and neck positions were named differently, and the ranges of angles in similar but not identical head and neck positions show slight differences.
During the exercise test all requested head and neck positions were achieved without using force. It should be noted, that the non-use of force was not determined by objective measurable parameters, but all requested head and neck positions were achieved without noticeable resistance against the riders aids and without the use of auxiliary reins. That and the various working disciplines and levels of training of the horses are the reasons why not all horses were able to perform all of the required head and neck positions. Hence, some horses had to be excluded from further analysis of one or more head and neck positions during exercise. Especially the head and neck position “hyperflexion” may have a negative connotation, not only because it has been subject of many studies but also the press discussed it extensively. In the current study the head and neck position “hyperflexion” was achieved, if the ridge of the nose was behind vertical. This should not be equated with the extreme overbending of the poll and neck –also called “Rollkur” - which has been banned by the FEI [17, 18] and could only be implemented by horses trained accordingly. The aim of the present study was to develop a method for categorizing head and neck positions objectively. In order to test this method it was not of importance which head and neck positions were investigated but rather that the selected head and neck positions could be differentiated unambiguously. As confirmed by the results of our study the two newly introduced angles are suitable tools describing head and neck position in an objective way. Therefore these angles may provide an opportunity to differentiate the various forms of hyperflexion, defined by the FEI , free from subjective bias.
No significant dependence of the pharyngeal diameter on the existence of a respiratory noise during exercise could be detected. The fact that the majority of horses showing an abnormal respiratory noise during exercise were diagnosed with laryngeal and not pharyngeal instability, could be an explanation.
In the second part of the current study, the GAr and the WAr were used to determine head and neck positions in resting horses. Three head and neck positions (neutral, extended, flexed) and the influence of each on the pharyngeal diameter in horses at rest were probed. There was a significant correlation between the pharyngeal diameter and the analyzed head and neck position and between the pharyngeal diameter and both the GAr and the WAr.
Figure 6 shows that increasing the WAr results in an increasing pharyngeal diameter. The maximum pharyngeal diameter is reached at approximately 115°. Beyond that point, despite continuing to increase the WAr, the pharyngeal diameter slightly decreased. This phenomenon occurs because hyperextension of the head and neck leads to greater hyperextension of the atlanto-occipital joint than of the atlantoaxial joint owing to the disparity in the range of motion of these two joints .
Radiographs were obtained from all of the sedated horses, regardless of the respiration phase. This method was chosen because an earlier study found no detectable relation between the pharyngeal diameter and the phase of respiration or the sedation status in horses at rest . In the same study, a value of 29.6 ± 11.3 mm (mean ± SD) was published for the smallest pharyngeal diameter detected from the radiographs of healthy horses at rest. In the current study, the smallest measured pharyngeal diameter (7.0 mm) occurred in the flexed head and neck position in a healthy horse that did not develop abnormal respiratory noise during the exercise test. This minimum pharyngeal diameter appeared in combination with the maximum measured GAr of 103.7°, which implies that the ridge of the nose must be more than 10° behind vertical. In the studies mentioned before, flexion of the head and neck was performed only to the point where the ridge of the nose reaches vertical. Perhaps the higher flexion of the poll led to severe reduction of the pharyngeal diameter in this horse. Another possible explanation for this phenomenon is that the radiograph was obtained immediately after the head and neck of the horse was flexed—before the horse had swallowed to compensate for the positive pressure in the guttural pouches. The floor of the guttural pouches form the roof of the nasopharynx, and positive pressure within the guttural pouches could result in collapse of the roof of the nasopharynx . It is also possible that the sedation status and the phase of respiration are more important than has been assumed. Some authors have postulated that the nasopharynx is more likely to collapse in a sedated horse than in a conscious horse because of the effect of sedation on muscular function. The major muscle preventing dorsal pharyngeal collapse is the stylopharyngeus muscle. Glossopharyngeal nerve anesthesia leads to stylopharyngeal dysfunction and consequently induces collapse of the dorsal pharyngeal walls . Thus, sedation and the resulting relaxation of the stylopharyngeus muscle may lead to a similar effect. However, a previous study showed no statistically significant difference between the radiographic pharyngeal diameter determined with and without sedation in 10 horses .
In accordance with previous reports, the smallest pharyngeal diameter in resting horses was found in the flexed head and neck position [16, 22]. Thus, the head and neck position in horses affects upper airway flow mechanics [2, 9]. Decreasing the pharyngeal diameter leads to increased resistance in respiratory airflow. Particularly during exercise, when negative peak inspiratory airway pressures are reached, this may result in dynamic collapse of elements of the larynx or pharynx. Therefore, it is hypothesized that head and neck flexion is an important contributing factor for the development or exacerbation of dynamic collapse of the upper respiratory tract in horses during exercise [4, 23–25]. Dynamic pharyngeal collapse and several cases of dynamic laryngeal collapse have been reported in association with head and neck flexion during exercise [10, 26, 27].
Further studies are needed to investigate the impact of objectively assessed head and neck positions on the upper airway tract during exercise.