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Equine airway narrows with head and neck flexed, study shows

Lateral radiographs of the head were obtained in three head and neck positions. Left: extended head and neck position. Middle: neutral head and neck position. Right: flexed head and neck position. The pharyngeal diameter was defined as the shortest distance between the epiglottis and the roof of the pharynx (double-headed arrow). The metallic markers (M) were used to calculate the amplification factor as described by Cehak and others.

Lateral radiographs of the head were obtained in three head-and-neck positions. Left: extended head-and-neck position. Middle: neutral head-and-neck position. Right: flexed head-and-neck position. The pharyngeal diameter was defined as the shortest distance between the epiglottis and the roof of the pharynx (double-headed arrow). The metallic markers (M) were used to calculate the amplification factor as described by Cehak and others.

German researchers have found further evidence of a narrowing of the airway in horses in a flexed neck position.

Researchers Li-mei Go, Ann Kristin Barton and Bernhard Ohnesorge used radiograph imaging in their study, in which they measured the opening of the airway in the pharyngeal region – the area of the throat behind the nasal cavity and above both the esophagus and larynx, which sits atop the windpipe.

Their findings, published in the journal, BMC Veterinary Research, found significant narrowing at the pharyngeal area in the flexed position, compared with extended and neutral neck positions.

The trio, from the Clinic for Horses at the University of Veterinary Medicine Hannover, said some head-and-neck positions in sport horses were significant as they can interfere with upper airway flow mechanics during exercise.

Until now, research had focused on subjectively described head-and-neck positions. The researchers set out in their study to develop an objective, reproducible method for quantifying head-and-neck positions accurately.

They precisely determined three pre-selected head-and-neck positions, using the relationships between the ridge of the nose and the horizontal plane, together with the angle between the ridge of the nose and the line connecting the neck and the withers. They comprised a flexed position, an extended head-and-neck position, and a neutral head-and-neck position.

A total of 35 German warmblood horses were used for the study, nine of whom were used for dressage, 13 for show jumping and 13 for pleasure riding.

All of the horses underwent an exercise test, and x-rays of the head were obtained at rest.

For the exercise test, the horses were ridden at trot and canter in each of the specific head-and-neck positions, all of which were achieved without force.

Withers angle. The withers angle (at right) was defined as the angle between the ridge of the nose (at left, line A) and the line (at left, line B) connecting the neck (point 1) and the withers (point 2).

Withers angle: The withers angle (at right) was defined as the angle between the ridge of the nose (at left, line A) and the line (at left, line B) connecting the neck (point 1) and the withers (point 2).

Occurrence of abnormal respiratory noises, times of appearance, as well as the characteristics and alterations in dependence upon the head-and-neck positions, were noted.

For the remaining radiographic examination, all the horses were sedated and a series of x-ray images were taken in the three equivalent head-and-neck positions.

The images were analysed and the pharyngeal diameter calculated in each position.

The smallest diameter noted – 7 millimetres – was found in the flexed head-and-neck position. The largest diameter – 79.1 mm – was found in the extended head-and-neck position.

In the flexed head-and-neck position, the diameter ranged from 7mm to 44.8mm, the mean being 28.5mm, plus or minus 9.6mm).

In the neutral head-and-neck position, it ranged from 34.6mm to 76.8mm (the mean being 51.3mm, plus or minus 8.87mm).

For the extended head-and-neck position, it ranged from 38.4mm to 79.1mm (the mean being 55.6mm, plus or minus 8.9mm).

“There was a significant difference in the pharyngeal diameters between the flexed head-and-neck position and the neutral and extended head-and-neck positions,” they reported, noting that there was no significant difference in diameter in the neutral and extended head-and-neck positions.

There were no significant differences in pharyngeal diameter at each head-and-neck position between horses with and without respiratory noise during exercise.

The researchers, in discussing their findings, stressed 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 was missing.

Ground angle. The ground angle (at right) was defined as the angle between the ridge of the nose (at left, line A) and the horizontal ground plane (at left, line B).

Ground angle. The ground angle (at right) was defined as the angle between the ridge of the nose (at left, line A) and the horizontal ground plane (at left, line B).

They said the aim of the first part of the study was to develop an objective, reproducible method for quantifying various head-and-neck positions accurately, which they said they had achieved.

This approach involved measurement of what they called the ground angle (GA), defined as the angle between the ridge of the nose and the horizontal ground plane, and the withers angle (WA), was defined as the angle between the ridge of the nose and the line connecting the neck and the withers.

These angles may provide an opportunity to differentiate the various forms of hyperflexion, defined by the FEI, free from subjective bias.

Applying their new approach meant using head-and-neck positions commonly used during training and competition in sport horses could be clearly measured and differentiated.

They said a previous study using x-ray imaging had already proven an association between pharyngeal diameter of horses and the position of the head and neck. They had now succeeded in showing that changes in diameter related to the positions objectively measured in this study.

Further studies were needed to investigate the impact of objectively assessed head-and-neck positions on the upper airways during exercise, they said.

Li-mei Go, Ann Kristin Barton and Bernhard Ohnesorge.
Objective classification of different head and neck positions and their influence on the radiographic pharyngeal diameter in sport horses.
BMC Veterinary Research 2014, 10:118 doi:10.1186/1746-6148-10-118

The full study can be read here

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