Dorsal displacement of the soft palate has been diagnosed traditionally on the basis of the clinical history and of resting endoscopic observations to eliminate other URT abnormalities. Horses with DDSP are frequently described as making a characteristic gurgling sound during expiration (Cook 1965; Heffron and Baker 1979; Haynes 1983; Robertson 1991; Beard 1996). However, recently it has been suggested that up to 30% of horses with DDSP make no audible abnormal respiratory sounds at exercise (Lumsden et al. 1995; Ahern 1999; Martin et al. 2000) rendering it likely that afflicted horses remain undiagnosed under field conditions. In this study, 85% of horses diagnosed with DDSP and 75% horses diagnosed with PI had a history of abnormal respiratory noise. Of these, characteristic ‘gurgling’ sounds were reported in 62% of horses (DDSP = 67%, PI = 50%). However, other horses that were not diagnosed with any form of palatal malfunction during treadmill exercise were reported to make similar sounds. It has been suggested that some of these might have been horses that were susceptible to DDSPoverground under racing conditions but that defied diagnosis on the treadmill (Parente et al. 2002; Lane et al. 2006). During treadmill exercise, 89% of horses with DDSP were found to make characteristic rough expiratory sounds, including 26 horses with no history of untoward noises reported by their trainers. In contrast, horses with PI make no expiratory ‘gurgling’sound but may make low grade harsh inspiratory noises. Clearly there are limitations in the evaluation of respiratory sounds by ear and the perception or interpretation of noise by jockeys is not always reliable. However, objective assessment of sounds using spectral analysis may be a more useful method to diagnose DDSP (Derksen et al. 2001; Franklin et al. 2003).
Previous authors have suggested that the presence of prolonged dorsal displacement of the palatal arch at rest (Cook 1965), provoked by nasal occlusion (Heffron and Baker 1979; Robertson 1991; Beard 1996), or induced by swallowing (Hobo et al. 1995) point to a diagnosis of DDSP during exercise. However, Haynes (1983) reported that intermittent DDSP may occur at rest in normal horses. Similarly, Parente and Martin (1995) found that horses with DDSP confirmed during exercise were no more likely to exhibit displacement at rest than ‘normal’ horses. Robertson (1991), Kannegieter and Dore (1995) and Tulleners (1997) mention that some horses with DDSP have ulceration of the caudal border of the soft palate, which may arise due to the repeated trauma of a vibrating soft palate. Epiglottic entrapment and abnormalities of the epiglottic cartilage have also been associated with intermittent or persistent DDSP (Haynes 1983). A number of authors have implicated epiglottic hypoplasia or flaccidity in the development of DDSP (Haynes 1981; Linford et al. 1983; Robertson 1991; Hobo et al. 1995). However, this has been refuted by Parente (1996) who found no association with epiglottic length and by experimental studies that reproduced epiglottic retroversion without precipitating DDSP, thereby demonstrating that loss of contact between the epiglottis and the
soft palate does not influence the position of the free border of the palate (Holcombe et al. 1997). In the current study, palatal and epiglottic abnormalities were observed in only 70 horses at rest. Although the specificity of resting endoscopy as a diagnostic test was high (0.95), the sensitivity was very low (0.15) indicating that, if used alone, resting endoscopy would result in a false negative diagnosis in 85% of cases. Other studies have also reported a weak link between a resting endocopic diagnosis of DDSP and confirmation by HSTE (Kannegieter and Dore 1995; Parente et al. 2002).
Much contention exists regarding the usefulness of resting endoscopic observations in the diagnosis of RLN. Duncan et al. (1977) suggested that laryngeal asymmetry or asynchrony at rest represents a subclinical form of laryngeal ‘hemiplegia’(sic) which may result in a performance-limiting malfunction under exercise conditions, or that horses with such laryngeal motility will deteriorate later. Evidence to support the progressive nature of RLN was provided recently by Dixon et al. (2002) who showed that isolated individuals from all LFS grading groups may show deterioration of laryngeal function. Previously Baker (1983) had suggested that for the overwhelming majority of horses laryngeal function, as perceived endoscopically at rest, remains unchanged throughout life. Also, Anderson et al. (1997) concluded that asynchronous laryngeal movements are common in young racehorses and that, although laryngeal movements may interchange between what is considered normal and abnormal, the proportion of horses that show more obvious asynchrony and eventually develop laryngeal ‘hemiplegia’ (sic) is low.
There is conflicting evidence whether horses with consistent asymmetry at rest, but which can achieve full abduction of the left arytenoid on occasions, typically after swallowing or in response to nostril closure - grade 3 LFS - are likely to sustain dynamic laryngeal collapse during exercise. Morris and Seeherman (1990) found that all 32 horses in their study with laryngeal asynchrony or ‘hemiparesis’ could fully abduct the rima glottidis during exercise. The results of the present study are in agreement with that conclusion because the majority of horses (61%) with grade 3 LFS at rest were able to maintain full abduction throughout exercise and a lesser proportion sustained a form of DLC. This figure cannot be taken to represent the equine population at large because the horses in this study were already a select group referred for HSTE in many cases because untoward respiratory noises had been heard during exercise. Therefore, it is likely that rather less than 39% of Thoroughbreds with grade 3 LFS are afflicted with a performance-limiting laryngeal malfunction. A small number of horses with grade 4 LFS - 6/32 (19%) in the present study - are able fully to dilate the rima glottidis during strenuous exercise. Hammer et al. (1998) and Martin et al. (2000) showed similar proportions of horses - 1/26 (4%) and 7/36 (19%), respectively - exhibiting the equivalent of grade 4 LFS at rest and yet capable of full symmetrical abduction when exerted. In contrast Rakestraw et al. (1991), reported that 5/6 horses that were unable to achieve full symmetrical arytenoid abduction at rest were able to do so during treadmill exercise, albeit at relatively slow speed. How do horses that are unable to achieve full symmetrical abduction of the arytenoids at rest manage to do so when exerted? Apossible explanation for the laryngeal function of these exceptional horses may lie in the response of the crico-arytenoideus dorsalis muscles to the chemical and mechanical stimuli of respiration which occur during intense exercise (Brancatisano et al. 1991).
406 Dynamic obstructions of the equine upper respiratory tract. Part 2
While all horses with true hemiplegia, grade 5 LFS, showed dynamic restriction of the rima glottidis during treadmill exercise, at the other end of the scale, 7% of ‘normal’horses, grades 1 and 2LFS at rest, were shown to become abnormal under exercise conditions. Such findings have been reported previously in a small proportion of horses (Morris and Seeherman 1991; Kannegieter and Dore 1995). Here, it was notable that all of the horses that sustained dynamic laryngeal collapse during exercise made an abnormal inspiratory noise during exercise and the majority (79%) had palpable atrophy of the CAD muscle. It is logical to conclude that the laryngeal function of horses that make no audible noise at exercise, is likely to be normal, i.e., that full symmetrical abduction is maintained at exercise, regardless of the LFS grade at rest. To this should be added the provisos that the hearing acuity of the clinician is satisfactory and that the environmental conditions are suitable. Therefore, there is an urgent need to develop reliable means of sound recording and analysis to match the advances made in the dynamic imaging of the airways. At the very least, the results of this study confirm that assessments of laryngeal function of horses that rely on endoscopy of the static horse, and which do not include an exercise test and palpation, are inadequate and should not be used as a basis for decisions regarding treatment or suitability for purchase.
In the present study, 32% of horses with URT obstructions were diagnosed with multiple forms of dynamic airway collapse during exercise. It was not possible to predict from resting observations which horses would sustain complex as opposed to simple dynamic collapse of a single structure. Until technological advances allow endoscopy during exercise over ground, examinations during treadmill exercise must be considered to be the definitive method to discriminate between simple and complex forms of dynamic obstructions of the URT of horses.
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