PHARYNGEAL DYSFUNCTION AND ABNORMAL NOISE

Laryngeal Dysfunction is the inability of the horse to maintain adequate laryngeal airway during exercise.The most common cause is the inability to maintain adequate abduction of the corniculate processes of the arytenoid cartilages (usually unilateral).Accompanying this condition is an inspiratory stridor created

with the passage of air into one or both laryngeal ventricles. NB the word strider = a high pitched whistling sound.. or harsh high pitched inspiratory noise.


Descriptions of Pharyngeal Dysfunction (PD) have often been restricted to the single act of `dorsal displacement of the soft palate'(DDSP) ‘gurgling’.It is time we examined and took into account that which in my opinion is the more common expression of PD. which is the passage of air orally during inspiration without DDSP. This abnormailty of function has been described by British workers as palatal instability (PI) [Lane G, et al].

The horse as a species rates `flight' as its main defence against predators.The horses throat thence evolved in such a manner as to allow it to `eat and run'.That is the oropharynx which could contain food at those most often unexpected moments was isolated from the airway and `uninterrupted flight 'was possible.The maintenance of a palato-laryngeal seal free of any form of leakage, was of crucial importance to the survival of the species.Any form of breakdown of this seal had the potential to reduce respiratory performance and could thus be life threatening.Therefore any condition which predisposes to the oral passage of air(which necessitates a leakage) should be looked upon as abnormal and potentially deleterious in particular with reference to performance.

Abnormal pharyngeal noises are not restricted to that which occur with DDSP.

In fact the most common noise heard at track work is that created as the horse works with his mouth slightly or largely open and air is taken orally during inspiration. This can occur after the vacuum, created caudal to the Isthmus Faucium with the apposition of the the root of the tongue and ventral mucosa of the soft palate, is broken. The horse has only to push its tongue forward which then flattens and allows air to enter the oropharynx. At this time the cheeks are often drawn between the upper and lower molars which can result in quite deep abrasions.The noise is created by the vibration of the soft palate as air passes both under and over this structure. This noise is described as a harsh low pitched inspiratory noise. Then as air is directed upwards from the floor of the oropharynx through the intrapharyngeal ostium and then courses around the aryepiglottic folds and into the larynx, vibration of these folds and or epiglotti with flaccid boarders may also result in abnormal respiratory noises.DDSP is not a necessary consequence of this dysfunction but may occur at the point of fatigue. Thus a harsh inspiratory noise may turn to a gurgle at the end of work. During expiration gases now encounter in particular the hill like rostral portion of the soft palate which ventrally is no longer in apposition with the dorsal surface of the tongue and therefore is free to vibrate..A significant noise can then be produced as gases encounter this structure.The latter expiratory noise is often just as loud as the inspiratory component.This situation may exist continually during exercise if required, or only when or if the horse senses that its nasal or nasopharyngeal air supply is not adequate and thence supplementation with orally derived air is necessary.

Where a horse is presented for poor performance a complete history of all abnormal respiratory noises should be obtained including those which are no longer apparent.The latter may be explained by a formally diagnosed disease or condition.However,if this is not the case, the horse may now be avoiding maximal performance and associated respiratory distress.Hence with the elimination of any other significant findings associated with the onset of poor performance one should not rule out a diagnosis of P.D.

Another consideration is that if treadmill testing is to include an assessment of nasopharyngeal airway maintenance and thence function under exercise conditions, shouldn't we then be excluding the abnormal passage of air orally.

To achieve this one would need to apply a nose band and tighten this to a point where upper and lower teeth were in firm contact.The danger would be if DDSP occurred and the horse was unable to utilise its emergency oral air supply.

A release pin mechanism would need to be built into the nose band and held by an assistant who would keep a close eye on the monitor.



References;

Lane J.G. et al: Dynamic obstructions of the equine upper respiratory tract. Part 1: Observations during high-speed treadmill endoscopy

of 600 Thoroughbred racehorses. Equine vet. J. (2006) 38(5) 393-399.


Ahern T J: Acquired pharyngeal dysfunction (APD). J of Equine Vet Sci 1993; 13:125-128.