Use of overground endoscopy for detection of upper airway abnormalities in Thoroughbred racehorses in South Africa

Abstract

Upper airway endoscopy at rest has been the diagnostic method of choice for diagnosing equine upper respiratory tract (URT) disease since its development in the 1970´s. The development of high-speed treadmill endoscopy (HSTE) improved the sensitivity of URT endoscopy by allowing the examiner to observe the horse’s nasopharynx and larynx during exercise. However, the level of exertion achieved during HSTE may not always represent that achieved during normal exercise as surface, rider, tack, and environmental variables are altered. Recently, the development of dynamic overground endoscopy (DOE) has addressed some of those shortcomings. A retrospective study was undertaken to describe the upper airway abnormalities detected during DOE in horses presenting with poor performance and/or abnormal respiratory noise in South Africa. Patient records of Thoroughbred racehorses undergoing DOE from November 2011 to August 2012 by the Onderstepoort Veterinary Academic Hospital were reviewed. Data collected included signalment, primary complaint, distance exercised, maximum speed attained, and dynamic airway abnormalities detected. XIII A second study was carried out to evaluate agreement within and among examiners of three grading systems for laryngeal function at exercise. The grading systems assessed were an existing system for grading axial deviation of aryepiglottic folds (ADAF), a modified system for grading recurrent laryngeal neuropathy (RLN) at exercise, and a proposed system for grading vocal cord collapse (VCC). For investigation of intra-observer variability, recordings were watched by two of the investigators at the same time, on two different occasions, in real time, slow motion, and at frame-by-frame speed. To evaluate inter-observer variability, recordings were watched by four investigators on one occasion, as described for investigation of intra-observer variability, and scoring sheets completed. Kappa agreement was calculated for both intra- and inter-observer sessions. Fifty-two horses that underwent DOE for investigation of poor performance and/or abnormal respiratory noise were identified. The main abnormalities detected included dorsal displacement of the soft palate (DDSP) (13/52 horses, 25%); recurrent laryngeal neuropathy (RLN) (17/52 horses, 33%); axial deviation of the aryepiglottic folds (ADAF) (21/52 horses, 40%) and vocal cord collapse (VCC) (18/52 horses, 35%). A total of 40 horses presented one or more abnormalities of the URT (77%). Fifteen horses (29%) had a single abnormality, and 25 horses (48%) had multiple abnormalities. Results at frame-by-frame speed from the intra-observer evaluations for all the conditions showed substantial agreement for RLN by both observers (K = 74 - 80). Intra-observer evaluations for VCC were moderate to substantial (K 53 – 63). ADAF was the most difficult URT abnormality to assess for both observers, and agreement within observers was only fair to moderate (K = 36 - 52). Inter-observer evaluations for RLN showed substantial to moderate agreement (K = 62). Inter-observer evaluations for VCC showed moderate agreement (K = 47 – 54), and inter-observer evaluations for ADAF showed only slight to fair agreement and were the lowest for all the conditions (K = 14 – 22). This study showed that DOE is a useful technique for providing valuable information about disease of the URT. Finding multiple abnormalities in 48% of horses examined using DOE suggests that DOE may be indicated even for those horses with an obvious abnormality found during resting endoscopy. The intra-observer evaluations showed that RLN had higher agreement values than those for ADAF and VCC at all speeds, and that ADAF had lower agreement values than those for VCC and RLN at all speeds. Inter-observer agreement was less than intra-observer agreement, presumably because more observers were involved in the inter-observer assessment.