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..
Descriptions of Pharyngeal Dysfunction have often been restricted to the single act of `dorsal displacement of the soft palate'(DDSP).It is time we examined and took into account that which in my opinion is the more common expression of P.D. which is the passage of air orally during inspiration without DDSP.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. As air is directed upwards from the floor of the oropharynx past the intrapharyngeal ostium vibration of the aryepiglottic folds 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.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 usually the loudest.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;
Ahern T J: Acquired pharyngeal dysfunction (APD). J of Equine Vet Sci 1993; 13:125-128.
Ahern T J : Oral palatopharyngoplasty. J of Equine Vet Sci 1993;13:185-188.
Ahern T J : Oral palatopharyngoplasty, asurvey of one hundred post-operative raced horses. J of Equine Vet Sci 1993;13:670-672.
Ahern T J : The elastic pharynx. J of Equine Vet Sci 1994;14462-463.
Ahern T J : Epiglottic hypoplasia; a surgical approach to reduce the incidence of associated dorsal displacement of the soft palate (DDSP). Centaur 1996;x111:1-4.
Ahern T J : Abnormalities of the rostral region of the soft palate: A three case study.
(Jpn) J of Equine Sci;7:55-58.
