The Harry M. Zweig Memorial Fund for Equine Research

New Field Diagnosis Modality for Upper Airway Obstructions in Horses

Dr. Normand G. Ducharme

Upper airway problems have been recognized in horses since at least 1866. These disorders affect both racehorses and horses used for other purposes. In racehorses, the complaint is ordinarily that the horse's athletic performance is diminished. Abnormal respiratory noise is the more common problem in show horses. The two most common causes of upper airway obstruction and noise are laryngeal hemiplegia (roaring) and dorsal displacement of the soft palate (DDSP). Laryngeal hemiplegia is a common cause of diminished athletic performance and abnormal respiratory noise. This disorder is readily diagnosed by resting endoscopy and affects 5-8% of racehorses. The percentage of horses affected by palate displacement is difficult to pinpoint precisely since most horses with this disorder are affected only during strenuous exercise and, hence, appear normal on resting endoscopy. Palate displacement typically leads to markedly impaired performance and abnormal respiratory noise (choking down) in racehorses. With the advent of video endoscopy during high speed treadmill exercise, the diagnosis of throat problems can now be made with greater confidence. In addition, treadmill endoscopy has led to diagnosis of dynamic upper respiratory disorders, such as displaced epiglottis and axial displacement of aryepiglottic folds, that are evident only at high speed work. In one such study, 53% of horses presented to a sport medicine clinic with impaired performance were found to have an upper airway obstruction during treadmill exercise. Furthermore, If one uses the frequency of tongue-ties or the performance of the Llewellyn procedure as a measure, then the prevalence of palate problems is considerably higher.

As early as 1866, diagnostic methods for upper airway obstructive disorders consisted of subjective criteria such as type and characteristics of respiratory noise as well as the "grunt test". In the 1980's, objective measurements of upper airway pressure and airflow were introduced for assessment of the upper airway in horses. In 1991, we introduced a laryngeal grading system which was based on correlation of a video endoscopy of the upper airway at rest and at exercise on a high speed treadmill. Treadmill endoscopy is now considered the gold standard for diagnoses of upper airway obstruction. However, high speed equine treadmills are available only at university hospitals or large private clinics. Development of a field test to screen horses for airway obstruction at the track or on the farm would be a major breakthrough in diagnosing these problems.

To this day, there are no objective criteria for evaluating upper airway noise in horses. This is important since noise usually accompanies obstructive problems leading to decreased performance in racehorses and may be the only clinical sign of concern to a show horse owner. Additionally, elimination of noise has been used as an indicator of treatment success for various airway disorders. Assessment of noise has been entirely subjective. We are no further ahead than the classical 1936 presentation of Sir Hobday where in his experience with 4,000 roarers, he felt he could render 85% of hunters useful, and "20% sufficiently sound to pass the average hunting man without comments". This is still the current standard for noise!

Joint time-frequency analysis (JTFA) is a significant mathematical advance in sound analysis techniques. This technology is now well established and is used in human speech analysis. We propose to adapt this technique to equine upper airway sounds recorded during exercise. We hope to develop a method to precisely characterize equine upper airway sounds. Additionally, we intend to develop a technique which enables equine practitioners to record sounds during exercise using a microphone placed on a horse's nostril and connected to a FM tape recorder. Recordings could then be played back and submitted to JTFA analysis to screen for airway disease or to make a diagnosis of a specific upper airway obstruction. Eventually, we plan to use objective analysis of respiratory noise to assess the outcome of various upper airway surgeries.