Dynamic Laryngeal Collapse Research Articles

Traumatic nasogastric intubation as a suspected cause of laryngeal obstruction in a horse recovering from emergency laparotomy

AbstractAn 18‐year‐old mare, admitted for colic, underwent an exploratory laparotomy for right dorsal colon displacement. During recovery from anaesthesia, a severe acute onset laryngeal obstruction occurred at standing, requiring orotracheal re‐intubation, which relieved the obstruction. This was followed by secondary pulmonary oedema, treated with furosemide (2 mg/kg intravenously) and oxygen. At re‐extubation, attempted 30 minutes later, the horse re‐obstructed immediately and collapsed, prompting an emergency tracheostomy. Furosemide (2 mg/kg intravenously) and dexamethasone (0.1 mg/kg intravenously) were also administered. The horse stood up thereafter with a patent airway. In the history, to note, nasogastric intubation was particularly difficult at admission, and was identified as a potential cause of laryngeal trauma that may have precipitated laryngeal obstruction due to severe local inflammation. Dynamic laryngeal collapse probably also played a role due to high inspiratory negative pressure generated at the precise timing of standing, similar to exercise.

Unilateral and bilateral compression of the epiglottis during poll flexion in harness racehorses

SummaryBackgroundHorses exercised ‘onto the bit’ with periods of induced poll flexion can demonstrate a unique array of upper respiratory tract (URT) disorders.ObjectivesTo describe a previously unreported exercise‐related disorder of the epiglottis associated with poll flexion in harness racehorses.Study designRetrospective observational study.MethodsMedical records of all harness racehorses that presented for exercising URT endoscopy between 2005 and 2022 were reviewed. These horses were exercised on a high‐speed treadmill using a previously standardised protocol with alternating 1‐min phases of free head carriage (no rein tension on the bit) and poll flexion (driven onto the bit with long reins) until they could no longer maintain the trotting gait.ResultsSeven Standardbreds and 11 Norwegian‐Swedish Coldblooded trotters were diagnosed with unilateral or bilateral compression of the epiglottis during exercise in poll flexion. These horses demonstrated progression of this disorder during the phases of induced poll flexion and showed no signs of epiglottic compression during phases exercised with free head carriage.Main limitationsRetrospective nature of study and limited sample size due to low prevalence.ConclusionsExercise‐related compression of the base of the epiglottis is a URT disorder only evident videoendoscopically when horses are driven onto the bit leading to poll flexion. It is most often seen in association with dynamic laryngeal collapse but can also be witnessed as a solitary disorder. Videoendoscopically, it appears that the compression is initiated by the rostral advancement of the larynx and hyoid apparatus within a progressively narrowing intermandibular space during poll flexion. Visually, this leads to local inward compression of the lateral nasopharyngeal walls and base of the epiglottis. Further studies are ongoing to confirm this hypothesis and to objectively quantify the degree of URT obstruction caused by this conformational narrowing of the epiglottis during poll flexion.

Association between dynamic upper airway obstructions and fitness parameters in Standardbred racehorses during high-speed treadmill exercise.

To evaluate the association between the severity of dynamic upper airway obstruction (DUAO) and selected fitness parameters in Standardbred racehorses measured by treadmill test. 191 Standardbred racehorses that underwent a standardized diagnostic protocol for poor performance evaluation between 2002 and 2021 at the Veterinary Teaching Hospital, University of Milan. Horses underwent a diagnostic protocol including clinical examination, lameness evaluation, laboratory analyses, ECG, resting upper airway endoscopy, metabolic incremental treadmill test, high-speed treadmill endoscopy, postexercise tracheobronchoscopy, and bronchoalveolar lavage fluid cytology. On the basis of findings of the high-speed treadmill endoscopy, horses were divided into the following groups: no DUAO, mild DUAO (medial deviation of the aryepiglottic folds or epiglottic entrapment), severe DUAO (dorsal displacement of the soft palate, nasopharyngeal collapse, dynamic laryngeal collapse, or epiglottic retroversion), and multiple DUAOs (different forms of DUAO occurring concomitantly). Fitness parameters, measured by incremental treadmill test, included speed at a heart rate of 200 bpm (V200), speed and heart rate at a lactate concentration of 4 mmol/L (VLa4, HRLa4), peak lactate, maximum speed, maximum hematocrit, and minimum pH. Data were compared between groups by use of descriptive and statistical analyses. Horses affected by severe or multiple DUAOs showed lower V200 and VLa4, compared to the no-DUAO group; horses with multiple DUAOs also had higher peak lactate concentrations compared to horses without DUAOs. While mild forms of DUAO do not affect athletic capacity, severe forms of DUAO and multiple DUAOs may lead to reduced fitness in Standardbred racehorses by impairing aerobic capacity.

Evaluation of Overground Endoscopy Findings in Colombian Criollo Paso Horses

The dynamic upper airway functional (URT) problems of Colombian paso horses (CPH) have not been investigated in literature up to date. These horses perform special walking gaits with high poll flexion of the neck. Our goal is to evaluate the upper airway mechanics in CPH, showing abnormal respiratory sounds and poor performance during exercise. Resting and overground endoscopy was performed in 40 CPHs. Statistical analyses were performed using the sciPy package. One-tailed Fisher exact tests were used to check for positive contingency between each pair of upper respiratory tract disorder (P < .05). Arytenoid cartilage collapse was observed in 35 of 40 cases during exercise. Among these, dynamic laryngeal collapse (DLC) was the most significant finding, but ventromedial luxation of the apex of the corniculate process of the arytenoid and recurrent laryngeal neuropathy was also observed. Dorsal displacement of the soft palate was only detected in 4 of 40 cases. DLC was significantly associated with vocal cord collapse, nasopharyngeal collapse, and medial collapse of the margins of the epiglottis, and medial collapse of the margins of the epiglottis was associated with nasopharyngeal collapse. DLC is only a typical feature in some special breeds worldwide. We suggest that the extensive poll flexion and the relatively small laryngeal lumen and high intensitivity workload are the most important predisposing factors of DLC. Ventromedial luxation of the apex of the corniculate process of the arytenoid was over-represented in our caseload, compared with other studies. Whereas, despite the intensive workload, dorsal displacement of the soft palate was relatively uncommon. We presume that this could also be tailored to the high poll flexion performed during the special gaits.

A bitless bridle does not limit or prevent dynamic laryngeal collapse.

Bits have often been incriminated as a cause of upper respiratory tract obstruction in horses; however, no scientific studies are available to confirm or refute these allegations. Clinical signs of dynamic laryngeal collapse associated with poll flexion (DLC) are induced when susceptible horses are ridden or driven into the bit. To determine whether use of Dr Cook's™ Bitless Bridle, instead of a conventional snaffle bit bridle, would reduce the severity of DLC in affected horses measured objectively using inspiratory tracheal pressures. Intervention study using each horse as its own control in a block randomised order. Nine Norwegian Swedish Coldblooded trotters previously diagnosed with DLC were exercised on two consecutive days using a standardised high-speed treadmill protocol with either a conventional bridle with a snaffle bit, or Dr Cook's™ Bitless Bridle. Head and neck position, rein tension, inspiratory tracheal pressure measurements, and laryngeal videoendoscopy recordings were obtained. A heart rate greater than 200bpm, and similar degrees of poll flexion/head height, had to be achieved in both bridles for the individual horse's data to be included for comparison. Seven horses' data met the inclusion criteria. The change in mean inspiratory tracheal pressure between free and flexion phases in the bitless bridle (-15.2±12.3cmH2 O) was significantly greater (P<.001) than in the snaffle bit bridle (-9.8±7.9cmH2 O). Mean inspiratory pressure during the free phase was significantly (P<.001) more negative with the snaffle bit bridle (-32.3±6.3cmH2 O), vs the bitless bridle (-28.5±6.9cmH2 O). Mean pressures in flexion phase, snaffle bridle (-42.1±10.8cmH2 O), vs bitless bridle (-43.7±15.6cmH2 O) where not significantly different between bridles (P=.2). Small sample size due to difficulty recruiting suitable clinical cases. This study could not provide any clear evidence that the effect of a snaffle bit in a horse's mouth influences the development or severity of DLC. Instead, head and neck angles induced by rein tension seem to be the key event in provoking DLC in susceptible horses.

Exercise Induced Laryngeal Obstruction in Humans and Equines. A Comparative Review.

Dynamic obstructions of the larynx are a set of disorders that occur during exercise in equines and humans. There are a number of similarities in presentation, diagnosis, pathophysiology and treatment. Both equines and humans present with exercise intolerance secondary to dyspnea. During laryngoscopy at rest, the larynx appears to function normally. Abnormalities are only revealed during laryngoscopy at exercise, seemingly triggered by increased ventilatory demands, and quickly resolve after cessation of exercise. Lower airway disease (asthma being the most prevalent condition), cardiac disease and lack of fitness are the major differentials in both species. Laryngoscopic examination during exercise should be performed from rest to peak exertion to allow for a comprehensive diagnosis, including where the airway collapse begins, and thereafter how it progresses. Dynamic disorders with most visual similarity between humans and equines are: aryepiglottic fold collapse (both species); equine dynamic laryngeal collapse (DLC) relative to some forms of human combined supraglottic/glottic collapse; and epiglottic retroversion (both species). Quantitative grading techniques, such as airway pressure measurement, that have proven effective in veterinary research are currently being piloted in human studies. Conditions that appear visually similar are treated in comparable ways. The similarities of anatomy and certain types of dynamic collapse would suggest that the equine larynx provides a good model for human upper respiratory tract obstruction during exercise. Thus, close collaboration between veterinarians and medical personal may lead to further advancements in understanding pathophysiologic processes, and enhance the development of improved diagnostic tests and treatments that will benefit both species.

Exploring the genetics underpinning dynamic laryngeal collapse associated with poll flexion in Norwegian-Swedish Coldblooded Trotter racehorses.

Dynamic laryngeal collapse (DLC) associated with poll flexion is the most common disorder of the upper respiratory tract (URT) in the Norwegian-Swedish Coldblooded Trotter (NSCT). The disorder, which has also been diagnosed in other breeds of trotters and gaited horses, appears to be related to anatomic phenotypes and only occurs during poll flexion when the horse is exercised 'on the bit'. Identify genomic regions associated with DLC in the NSCT by combining a rigorous phenotyping protocol with genomic data from a high-density equine genotyping array. Prospective case/control study. High-speed treadmill endoscopy was used to phenotype horses (n=61) for DLC, distinguishing between cases and controls. Genome-wide association (GWA) analysis of DLC status was then performed using a principal component approach (PCA) with haplotype analyses subsequently performed for regions containing single-nucleotide polymorphisms (SNPs) above the suggestive genome-wide significance (GWS) threshold (P<1.0×10-5 ). One region containing 10 SNPs (Equus caballus chromosome [ECA] 7: 89,601,935-94,647,192) was above the suggestive GWS threshold. Two inferred haplotypes in this region demonstrated significant differences (P<0.001) between cases and controls, with the most frequent haplotype resulting in a significantly increased risk of DLC. Small sample size as a result of stringent phenotyping protocols. The current study highlights a candidate genomic region on ECA7 as potentially important with regard to the manifestation of DLC. Further exploration of this region and the genes included within it will bring veterinarians and researchers closer to fully understanding the biological mechanisms underlying DLC in horses.

Exploring the genetics of dynamic laryngeal collapse associated with poll flexion in Norwegian-Swedish Coldblooded Trotter racehorses

Physical exercise induces several adaptations involving modulation of the energetic pathways to meet the energy demand during physical exercise. The branched-chain amino acids, leucine in particular, are considered as the most relevant amino acids, especially for exercise physiology. In this study, the change of leucine, valine, and tryptophan concentration was investigated in athletic horses before and after different exercise type. A total of 40 sport horses were equally divided into 4 groups according to the exercise type: group A (jumping), group B (reining), group C (32 km ride), and group D (72 km ride). Blood samples were collected before (TPRE), immediately after (TPOST), and 30 minutes after (TPOST30) the race to assess leucine, valine, and tryptophan concentration. Two-way analysis of variance showed an effect of time and/or exercise on the serum leucine, valine, and tryptophan values in all groups (P < .01). Lower leucine and valine values were found in groups A, B, and D at TPOST and TPOST30 respect to TPRE, whereas group C showed higher values at TPOST and TPOST30 compared with the values found at TPRE. All groups showed increased tryptophan values at TPOST and TPOST30 compared with TPRE. Lower leucine, valine, and tryptophan values were found in group D at TPOST30 compared with the values found at TPOST. These dynamic changes suggest the involvement of amino acid metabolism during different exercise types. These findings are probably related to the complex physiological adaptations to exercise stressor that allow reestablishment of the homeostatic equilibrium of the organism.

Exercising upper respiratory videoendoscopic findings of 50 competition draught horses with abnormal respiratory noise and/or poor performance.

Upper respiratory tract (URT) endoscopy at rest is commonly used to evaluate competition draught horses with URT conditions. Overground endoscopy might be preferred for draught horse URT evaluation as it allows the horses to be driven with harness, overcheck and cart-load under similar conditions to those experienced in the show ring where airway conditions are most prominent. To describe the exercising URT findings of competition draught horses with abnormal respiratory noise and/or poor performance. Case series. Medical records of competition draught horses undergoing overground endoscopic evaluation between January 2013 and January 2018 with a presenting complaint of abnormal respiratory noise and/or poor performance were reviewed. Video recordings of resting and overground endoscopy were evaluated in all horses. Spearman's rank correlation coefficient was calculated between laryngeal function at rest and at exercise. Fifty competition draught horses were examined. Thirteen had previously undergone URT surgery. There was significant correlation between resting and exercising laryngeal function (ρ=0.77, P<0.01). Abnormalities were detected in 46 horses and included arytenoid cartilage collapse (n=31), vocal fold collapse (n=27), palatal dysfunction (n=14), epiglottic disorders (n=11), dynamic laryngeal collapse (n=1), rostral deviation of the palatopharyngeal arch (n=3) and medial deviation of the aryepiglottic folds (n=16). The majority of horses had a complex of abnormalities (n=31) or required exercising examination for identification (n=41). Incidental upper oesophageal incompetence was observed in nine horses. Retrospective collection of data. Overground endoscopic evaluation was a useful technique for identifying URT disorders in competition draught horses. The spectrum of upper airway conditions identified in exercising draught horses supports the use of overground endoscopy as a diagnostic technique and could influence treatment considerations. The Summary is available in Portuguese - see Supporting Information.

Histopathological assessment of intrinsic laryngeal musculature in horses with dynamic laryngeal collapse

The pathogenesis of bilateral dynamic laryngeal collapse associated with poll flexion (DLC) of horses is unknown but might be associated with intrinsic laryngeal muscle weakness. To investigate histopathological characteristics of the cricoarytenoideus dorsalis, the cricothyroid (CT) and the cricoarytenoideus lateralis muscles in DLC-affected horses and compare these with unaffected controls. Our hypotheses were that evidence of neurogenic atrophy of the CT or cricoarytenoideus dorsalis muscles would be found in DLC-affected horses and that observed changes would be symmetrically (left/right) distributed, or that muscle fibre diameter would be significantly reduced in DLC-affected horses compared to unaffected controls, reflecting an underlying paresis. Case-control study. Five DLC horses and 8 controls were included. Muscle samples were harvested immediately following euthanasia. Fibre type proportions and size were evaluated by multiple immunofluorescence labelling of cryosections, and compared between sides (left/right) and groups (DLC-affected cases/ unaffected controls). Subjective and objective assessments of fibre type grouping were compared between sides and groups. Fibre type proportions, fibre size and the subjective assessment of fibre type grouping did not reveal any statistically significant differences between the groups. Objective assessment of fibre type grouping revealed significantly more large clusters of T1 fibres within the left cricoarytenoideus lateralis muscle of DLC-affected cases versus controls, and within the right CT muscle of control horses compared to the DLC-cases. The absence of bilateral symmetric fibre type grouping, fibre type loss and fibre atrophy in the DLC-affected cases do not support a neuromuscular component within the pathogenesis of DLC. The objective assessment of fibre type grouping revealed some statistical differences between the DLC-affected cases and the unaffected controls; however, these findings were inconsistent with regard to DLC. An alternative aetiology of DLC seems likely.

Results of upper airway radiography and ultrasonography predict dynamic laryngeal collapse in affected horses

The pathogenesis of dynamic bilateral laryngeal collapse (DLC) associated with poll flexion is unknown. Diagnosis is dependent upon exercise endoscopy while replicating the flexed head position harness racehorses experience during racing. To describe the effects of poll flexion on rostrocaudal laryngeal positioning and laryngeal lumen width in resting horses diagnosed with DLC compared to controls, and to establish diagnostic criteria for DLC by use of diagnostic imaging. Case-control study. Fifty harness racehorses were prospectively included in the study: 25 cases diagnosed with DLC by treadmill endoscopy and 25 controls in which treadmill endoscopy revealed no abnormal findings. Laryngeal radiography and ultrasonography were obtained in neutral and flexed head positions. Laryngeal positioning and laryngohyoid conformation were compared between the groups and head positions. Poll flexion induced a greater rostral advancement of the larynx in relation to the hyoid apparatus in resting harness racehorses affected with DLC compared to controls (P = 0.007). At the level of the vocal folds, poll flexion resulted in a smaller laryngeal lumen width in horses affected with DLC compared to controls (P = 0.04). Horses were significantly more likely to be affected with DLC when the thyrohyoid bone to thyroid cartilage distance was ≥12 mm in poll flexion (odds ratio 21.3, 95% confidence interval 3.65-124.8, P = 0.004) and when laryngeal lumen width at the level of the vocal folds was less in poll flexion than in the neutral head position (odds ratio 8.4; 95% confidence interval 1.6-44.1, P = 0.012). In DLC horses, poll flexion advanced the larynx more rostrally and resulted in a decreased airway lumen width compared to control horses. Laryngeal ultrasound and radiography may facilitate the diagnosis of DLC at rest.

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

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.

Relative prevalence of upper respiratory tract obstructive disorders in two breeds of harness racehorses (185 cases: 1998–2006)

Two genetically and phenotypically distinct horse breeds are used for harness racing in Scandinavia: the Standardbred (SB) and Coldblooded Trotter. These racehorses have identical environmental, management and racing conditions. Therefore, this study was undertaken to identify and compare the relative prevalence of upper respiratory tract (URT) obstructive disorders in these 2 breeds. To determine whether these 2 phenotypically different breeds of harness racehorses have different predispositions for URT disorders. Retrospective study of 88 Norwegian Coldblooded Trotters (NCT) and 97 SBs referred to this hospital for URT evaluation between 1998 and 2006. Case records of all horses diagnosed with an URT disorder during resting endoscopy, and all horses undergoing high-speed treadmill videoendoscopy (HSTV) with one or more periods of induced poll flexion were evaluated. The relative prevalence of URT disorders between the 2 breeds was analysed using a Fisher's exact test. There was a significant (P<0.05) breed predisposition regarding 6 URT disorders. Bilateral dynamic laryngeal collapse associated with poll flexion and flaccid epiglottis was significantly more frequent in the NCT. Alar fold collapse and nasopharyngeal collapse were significantly more frequent in SBs. Epiglottic entrapment and nasal flutter were only diagnosed in the SBs. Dynamic disorders were more common than resting disorders in both breeds. URT obstructive disorders (dynamic laryngeal collapse associated with poll flexion, flaccid epiglottis, pharyngeal collapse, alar fold collapse, nasal flutter and epiglottic entrapment) are breed related, indicating an anatomic or functional cause. Periods of induced poll flexion during HSTV was essential to declare harness racehorses free of URT disorders. Further anatomic or physiological studies comparing these breeds could potentially provide insight into the pathogenesis of certain URT obstructive disorders. Induced poll flexion should be included in routine HSTV examinations of all harness racehorses.

A novel treatment for dynamic laryngeal collapse associated with poll flexion: The modified checkrein

Dynamic laryngeal collapse (DLC) associated with poll flexion is a performance limiting dynamic upper airway disorder commonly diagnosed in Coldblooded trotters. An inspiratory obstruction occurs when affected horses are driven with tension on the reins, inducing poll flexion. To date, surgical treatment and conservative management have failed to improve racing performance in affected horses. To test the efficacy of an external device, the modified checkrein, in limiting poll flexion, diminishing laryngeal collapse and improving airway mechanics when horses are driven on a treadmill with tension on the reins and test the hypothesis that the modified checkrein will limit poll flexion thereby improving endoscopic appearance of the larynx and reduce airway obstruction in harness racehorses affected with DLC. Fourteen Norwegian Coldblooded trotter (NCT) racehorses previously diagnosed with DLC associated with poll flexion exercised on a high-speed treadmill at speeds that resulted in heart rate >200 beats/min. Upper airway videoendoscopy recordings, tracheal pressures and video recordings of the head and neck position were collected while horses exercised with the conventional checkrein on Day 1 and the modified checkrein on Day 2. The modified checkrein successfully limited poll flexion as measured by no change in head position parameters between phases of no rein tension and phases of rein tension. Endoscopic scores were significantly improved (arytenoids cartilage collapse P<0.0001; vocal fold collapse P<0.0001) and tracheal peak inspiratory pressures were significantly less negative (P = 0.0162) when horses were fitted with the modified checkrein compared to the conventional checkrein. The modified checkrein successfully limited poll flexion and also prevented upper airway obstruction in NCT horses affected with DLC associated with poll flexion. The modified checkrein seems promising in offering a method of conservative management in harness racehorses affected with this disorder.

Correlation of resting and exercising endoscopic findings for horses with dynamic laryngeal collapse and palatal dysfunction

To correlate resting and exercising endoscopic grades of laryngeal function in horses undergoing high-speed treadmill endoscopy (HSTE) using the Havemeyer grading system. To correlate dorsal displacement of the soft palate (DDSP) seen at rest with palatal function during exercise. Records of horses that underwent HSTE examination (1999-2009) were reviewed. Resting laryngeal function score and other abnormalities noted on resting endoscopy were recorded as were results of HSTE. Results of resting and exercising endoscopic findings were correlated. 281 horses underwent HSTE. There was significant correlation between grade of laryngeal function at rest (grades 1-4) and exercise (ρ=0.53, P<0.001) and between resting subgrades 3.1, 3.2 and 3.3 and exercising grades of laryngeal function (ρ=0.43, P=0.0017). DDSP was observed at rest significantly more often in horses that developed DDSP during HSTE than those without DDSP during HSTE (RR=4.1, P<0.001). The sensitivity and specificity of DDSP seen during resting endoscopy as a test for DDSP occurring during exercise were 25.5 and 95.1% respectively (positive predictive value 0.57, negative predictive value 0.83). The results of the current study support the use of the Havemeyer system for grading laryngeal function in the resting horse, and corroborate findings of previous studies correlating resting and exercising palatal abnormalities. Studies that use the presence of spontaneous DDSP during resting endoscopic examination as an inclusion criterion for investigating efficacy of treatments for DDSP are likely to have a low proportion of horses with false positive diagnoses.

Comparisons of overground endoscopy and treadmill endoscopy in UK Thoroughbred racehorses

To date there is no information on the comparison of the more recently documented technique of performing endoscopy during ridden exercise in the field, with the more traditional method of endoscopy during high-speed treadmill exercise. To compare the results of upper respiratory tract endoscopy in UK Thoroughbred racehorses performed during ridden exercise in the field with those obtained during exercise on the treadmill. A direct comparison was undertaken in 4 horses whereby both procedures were performed in the same horse within 10 days of each other. An indirect comparison was also undertaken whereby the results of overground endoscopy performed in 50 racehorses was compared to the results obtained during treadmill endoscopy in a further 50 racehorses. Horses were matched for age, gender, use (National Hunt vs. Flat) and presenting complaint (abnormal respiratory noise vs. poor performance). Dorsal displacement of the soft palate was diagnosed less frequently during overground endoscopy than during treadmill endoscopy. There was no significant difference in the diagnosis of dynamic laryngeal collapse between the 2 techniques. The treadmill exercise test was performed over longer distances at higher inclines, albeit at lower speeds than the overground test. In contrast to the treadmill test, the overground test was frequently performed in intervals. The results of both the direct and indirect comparisons suggest that dorsal displacement of the soft palate is diagnosed less often during overground endoscopy than during treadmill endoscopy. Strenuous exercise tests may be more easily performed on a treadmill than by performing multiple exercise intervals in the field. Care should be taken in interpreting negative findings during both procedures, but particularly during overground endoscopy if racing conditions have not been appropriately replicated.

Dynamic obstructions of the equine upper respiratory tract. Part 1: Observations during high-speed treadmill endoscopy of 600 Thoroughbred racehorses

To review the prevalence of single and complex forms of dynamic airway obstructions within a large group of Thoroughbred horses in training referred for investigation of poor performance. Video-endoscopic recordings of the upper respiratory tract made during a standardised treadmill exercise test of 600 Thoroughbred racehorses were reviewed and analysed in real time and slow motion to identify dynamic collapse by the tissues bordering onto the pharyngeal and laryngeal airways. Dynamic collapse within the nasopharynx or larynx was confirmed in 471 of the 600 horses. Dorsal displacement of the soft palate (DDSP; 50%) and palatal instability (33%) were the disorders most frequently identified. It was concluded that deglutition is not a significant event in the triggering of DDSP. Complex forms of dynamic collapse were present in 30% of the horses with upper respiratory tract obstructions. A significant influence of age on the prevalence of DDSP and dynamic laryngeal collapse was identified. There was an increased risk of DDSP in younger horses, and of laryngeal collapse in older horses. No association with gender or format of racing was identified. Palatal instability and DDSP comprised the most frequently encountered forms of dynamic collapse within the upper respiratory tract of the Thoroughbred racehorses in this study and are probably expressions of the same nasopharyngeal malfunction. Complex obstructions, i.e. where more than one structure collapses into the airway, occur frequently and therefore treatments that address solitary disorders may often be unsuccessful. Younger horses were found to be at greater risk of sustaining DDSP while older horses seemed more at risk to vocal cord collapse but not to collapse of the arytenoid cartilage itself.

Dynamic obstructions of the equine upper respiratory tract. Part 2: Comparison of endoscopic findings at rest and during high-speed treadmill exercise of 600 Thoroughbred racehorses

The reliability of diagnoses of obstructive conditions of the upper respiratory tract (URT) based on examinations performed at rest vs. at exercise is controversial. To compare diagnosis of URT by endoscopy at rest with that achieved during high-speed treadmill exercise (HSTE). Endoscopy of URT at rest, when performed in isolation from other simpler techniques is unreliable in the prediction of dynamic respiratory obstructions. Endoscopic findings of 600 Thoroughbred racehorses during quiet breathing were compared with findings during high-speed treadmill exercise. Other parameters were also assessed for their specificity in diagnosis. Endoscopy of the resting horse showed low sensitivity (0.15) in the diagnosis of dorsal displacement of the soft palate (DDSP) and palatal instability (PI). When endoscopy and reported noises were taken together there was still a 35% misdiagnosis rate. Although there was significant association between resting laryngeal function score (LFS) and dynamic vocal cord and/or arytenoid cartilage collapse at exercise, 19% of horses with a grade 4/5 LFS were able to attain and maintain full abduction during exercise and 7% of those with 'normal' grades 1 or 2 LFS at rest showed dynamic laryngeal collapse when exerted. Sensitivity of the diagnostic model was greatly increased (80%) when a history of inspiratory noise and palpable intrinsic muscle atrophy were included. Endoscopy of the upper respiratory tract of static horses is unreliable in the diagnosis of dynamic obstructions of the URT and should not be used in isolation in surgical decision-making or in the assessment of horses at the time of sale.

Effect of poll flexion and dynamic laryngeal collapse on tracheal pressure in Norwegian Coldblooded Trotter racehorses

Dynamic laryngeal collapse (DLC) associated with poll flexion is a newly diagnosed upper respiratory tract obstructive disorder that causes poor racing performance. To determine if Norwegian Coldblooded Trotters (NCTs) affected with DLC associated with poll flexion differ from normal, elite NCTs based on simple airway mechanics measurements. Five normal elite NCTs and 6 NCTs diagnosed previously with DLC underwent treadmill videoendoscopy while tracheal pressures were measured continuously. Alternating head positions were used such that horses were exercised with free head carriage and induced poll flexion at heart rates >200 beats/min. Peak inspiratory tracheal pressures were significantly more negative for horses with DLC compared to the elite horses. This difference was only significant during the exercise phases when the poll region was flexed, P = 0.0015. Head position significantly affected peak inspiratory pressure for both elite and affected horses, P < 0.0001. Induced poll flexion significantly affected peak inspiratory pressure (PIP) in all horses; however, PIPs were significantly more negative in those affected with DLC. Based upon the tracheal pressure measurements recorded in this study, DLC in NCTs is a severe obstructive upper respiratory tract disorder that is induced by poll flexion.

Surgical and Conservative Management of Bilateral Dynamic Laryngeal Collapse Associated with Poll Flexion in Harness Race Horses

To test the hypothesis that in bilateral dynamic laryngeal collapse associated with poll flexion, vocal fold collapse (VFC) is the initial abnormal event that induces further laryngeal collapse, and that racing performance would therefore be substantially improved after bilateral ventriculocordectomy in affected individuals. Retrospective study. Twenty-six horses. Medical records (1998-2006) of harness racehorses admitted for high-speed treadmill videoendoscopy (HSTV) that had bilateral dynamic laryngeal collapse associated with poll flexion were reviewed. Race records, owner interviews, and follow-up HSTV were used to evaluate outcome after either surgical treatment including bilateral ventriculocordectomy or conservative management. Bilateral dynamic laryngeal collapse, defined as bilateral VFC with concurrent arytenoid cartilage collapse (ACC), was identified in 26 horses. Norwegian Coldblooded Trotters (NCT) were overrepresented. Sixteen horses had surgical treatment and 10 were treated conservatively. Return to racing and racing performance was not improved within or between groups after the treatment. On follow-up HSTV of 6 surgically treated horses, there was no residual soft tissue collapse in the ventral portion of the rima glottidis; however, ACC and other abnormalities were still evident. Bilateral ventriculocordectomy resolved VFC, but failed to stabilize the arytenoid cartilages or to significantly improve racing performance. Bilateral dynamic laryngeal collapse associated with poll flexion is a serious performance-limiting upper respiratory tract disorder that is overrepresented in NCT racehorses. Our results suggest that VFC is not the initiating event in this complex obstructive airway disorder for which there is currently no consistently effective treatment.