Complex dynamic upper airway collapse: Associations between abnormalities in 99 harness racehorses with one or more dynamic disorders

Abstract

Many horses demonstrate dynamic collapse of more than one upper respiratory tract (URT) structure during high-speed treadmill videoendoscopy (HSTV). To report the frequency of complex dynamic URT collapse in harness racehorses and determine if an association exists between occurrence of certain disorders. Retrospective study of 99 Standardbreds and Coldblooded trotters that had one or more dynamic URT disorders identified during HSTV between 1998 and 2006. The horses underwent HSTV using a protocol that included periods of free head carriage and poll flexion until fully fatigued. Dynamic abnormalities were classified as: dynamic laryngeal collapse (DLC) associated with poll flexion; axial deviation of the aryepiglottic folds (ADAF); dynamically flaccid epiglottis (FE); caudal palatal instability (PI); intermittent dorsal displacement of the soft palate (iDDSP); pharyngeal collapse (PC); alar fold collapse (AFC) and nasal flutter (NF). Cluster analysis and Fisher's exact test was performed between groups. Significance was set at P<0.05. The overall incidence of complex dynamic URT collapse was 69.7%. Axial deviation of the aryepiglottic folds was significantly associated with PI (P<0.0001) and FE (P<0.0001); iDDSP was significantly associated with PI (P = 0.004) only. Dynamic laryngeal collapse associated with poll flexion was significantly associated with FE (P = 0.002). Cluster analysis confirmed strong associations between the above diagnosis groups and weaker associations between others. Caudal PI seems to result in 2 diagnosis groupings: one that leads to ADAF and the other to iDDSP. Dynamic collapse of the margins of the epiglottis was a common finding in this study strongly associated with DLC, PI and ADAF. The associations between certain dynamic diagnoses identified in this study seems best explained as due to local anatomic and functional weaknesses that influence other structures through the Bernouilli principle, rather than being due to a generalised neurological disorder.