Laryngeal Problems - Hocus pocus or cutting edge science?

Diagnosis of Laryngeal Problems: hocus pocus or cutting edge science? Celia M. Marr, Equine Veterinary Journal.   << EVJ logo near here>>   Recurrent laryngeal neuropathy (RLN) is the correct term for the condition better known as roaring or laryngeal hemiplegia. It is extremely common in Thoroughbreds and represents one of the major causes of poor performance or jockey-reported noise. Because it is so important, many young horses are scoped at sales looking for laryngeal asymmetry to try to identify those that may be at risk of having this condition. But scoping at rest is fraught with difficulty – the larynx may be normal at rest only to show signs of weakness during exercise, so positive cases can be missed. Conversely, tired young horses can have apparently poor laryngeal function at rest that in fact is of little significance. Many question whether subjecting foals and young horses to endoscopy at sales is reasonable, although in part this latter concern is reduced now that videoendoscopy is available. Furthermore, the dynamic endoscopy, overground or on a treadmill, is widely accepted as the best way to evaluate horses suspected of having upper airway disorders leading to dynamic obstruction of the airway during exercise, a population that may or may not have obviously abnormal throats when examined at rest. Nevertheless, there is a need for better tools to evaluate the larynx for clinical application and also to allow researchers to study the condition in more detail. Two recent studies published in Equine Veterinary Journal have addressed this issue. << Overground endoscopy image near here>> << RLN endoscopy image near here>> What is recurrent laryngeal neuropathy? RLN is actually due to damage to the nerves supplying the larynx rather than being an abnormality of the structure of the larynx itself. The larynx is a cartilage structure which is opened and closed by a collection of muscles that encircle it. Damage to the laryngeal nerves leads to atrophy of the main laryngeal adductor, the cricoarytenoid lateralis (CAL) muscle; and the main arytenoid abductor, the cricoarytenoid dorsalis (CAD). Atrophy (or wasting) of the left CAD muscle means that it can no longer open the arytenoid cartilage during inspiration.  This in turn causes dynamic airway collapse at exercise, reducing the amount of air entering the lungs and, if severe, reducing the arterial oxygen concentration. Traditionally, the diagnosis of RLN has been made using endoscopy to determine the degree of arytenoid abduction or asymmetry.   How reliable is endoscopy in RLN? A team from University College Dublin led by Dr. Charlotte McGivney set out to answer this question, and because it had not been looked at before, they were particularly interested in determining whether different clinicians would have different opinions when examining overground endoscopy images. Dr. McGivney collected video material from 43 Thoroughbreds in flat race training that had undergone overground endoscopy examination, on the same gallop. Videos from individuals representing a range of common upper airway abnormalities and a few horses judged to be normal at the initial examination were included. Four experienced clinicians, all of whom held advanced professional qualifications, then assessed videos of each horse, at rest and at exercise. One of the key findings the assessors were looking for was arytenoid asymmetry because this is relevant to the diagnosis of recurrent laryngeal neuropathy. To add to the challenge, Dr. McGivney duplicated all the videos; the observers knew that there were some duplicates in the study sample, but they did not know how many, and the identity of each horse was removed. In other words, the assessors were unaware if the horses were thought to be normal or not, or whether they had looked at the same animal already. The data was then analyzed to find out how consistent the assessors were; i.e. whether they arrived at the same assessment when they looked at the same horse twice and how much agreement there was between assessors.   Are individual clinicians consistent when they assess the larynx? The assessment made when the same person looked at the same horse twice was virtually identical in horses with arytenoid asymmetry at exercise. It was not quite perfect but still substantial for arytenoid asymmetry at rest. Overall the results for other conditions were good, although agreement about some of the less common abnormalities such as vocal fold collapse, ventromedial luxation of the apex of the corniculate process of the arytenoid, nasopharyngeal collapse, and grading the epiglottis at rest produced only moderate agreement.   Do experienced clinicians agree with each other when they look at the larynx? Fortunately, the results here were also fairly encouraging although not quite as reliable as assessments made by one individual. When assessors were deciding whether or not a specific disorder was present or absent, there was substantial agreement for arytenoid asymmetry at rest and exercise. However, when Dr. McGivney looked at specific grades of arytenoid asymmetry assigned by the four independent assessors, the grades assigned for arytenoid asymmetry during exercise were moderately repeatable between observers but there was much less agreement at rest, suggesting there is a degree of individual subjectivity being introduced into the grading of disorders, supporting difference in clinical experience and opinions. There was more grading disparity with some of the other upper airway problems, suggesting that there is a need for better defined grading systems which can be universally applied.   Ultrasonography of the larynx Transcutaneous ultrasound has been used to evaluate laryngeal structure and vocal fold movement of the equine larynx for a few years now, and in particular ultrasonography can be used to image the CAD muscle.  A study published in Equine Veterinary Journal in 2011 by Dr. Katherine Garrett from Rood and Riddle Equine Hospital in Kentucky showed that using ultrasonography to look at the muscles of the larynx through the skin was effective for diagnosis of RLN. The key ultrasonographic features that identify RLN are an increase in the echogenicity (brightness) of the muscle and decrease in its size.  Comparing ultrasonography findings with the diagnosis made during endoscopy during exercise on a high-speed treadmill in a group of 79 Thoroughbred racehorses showed that laryngeal ultrasound was very accurate.   << RLN ultrasound near here>>   Novel approaches to laryngeal imaging An important limitation of transcutaneous ultrasonography is that direct assessment of the CAD muscle is very limited because the muscles above the larynx make it difficult to access. There is a need to have better ways to assess the CAD muscle, not only to improve diagnosis but also because novel treatments are being developed that aim to restore nerve function and glottic opening, and these rely on preventing further muscle atrophy, restoring muscle mass, or improving contractile force. Recognizing this need, a research team lead by Dr. Jonathan Cheetham based at Cornell University has been working on improved imaging techniques to determine the geometry of the CAD muscle and to characterize the relationship between CAD geometry and laryngeal function.   The Cornell group used computed tomography (CT) to create 3D reconstructions of the equine larynx and determine volume and midbody dorsal-ventral thickness and cross-sectional area of the left and right CAD muscles. By comparing CT findings with autopsy, Dr. Cheetham’s team have confirmed that there is a very close correlation between CT estimates and actual muscle mass. Determining this relationship is important, as there is a close relationship between a muscle’s volume and its ability to generate force. This is clinically relevant as the degree of force of the CAD muscle determines the muscle’s ability to abduct the arytenoid cartilage and open the rima glottidis. For the research study the procedure was performed under general anesthesia, but it may also be possible perform the same technique in standing sedated horses.   << CT larynx near here>> << Standing CT near here – optional image>>   The other innovative technique used in the Cornell study is transesophageal laryngeal ultrasound.  For this procedure, horses are sedated and a video endoscope is placed into the right nostril to confirm correct placement of a human pediatric transesophageal probe via the left nostril across the nasopharynx into the esophagus. The probe is advanced to image the left CAD muscle ventrally through the esophageal wall. The procedure takes 10-15 minutes to perform and horses typically tolerate the procedure well. To look at transesophageal ultrasonography findings, the procedure was performed in 112 horses with a spectrum of laryngeal function and compared with findings in the same horses with conventional resting endoscopy. In 90 of these 112 horses, endoscopy was also performed during high-speed treadmill exercise. The ratio of left:right thickness in the mid body and caudal body of the CAD muscle was significantly reduced in horses with resting grade III laryngeal function compared to grades I and II. Likewise, the ratio of left:right thickness in the mid and caudal body of the CAD muscle was significantly reduced in horses with grade B  or C laryngeal function compared to grade A (normal). Resting laryngeal function does not perfectly predict laryngeal function on exercise and horses evaluated at rest as grade II and III are the most challenging. The addition of transcutaneous evaluation of CAL muscle echogenicity improves the ability to predict abnormal arytenoid movement during exercise but it is not 100% accurate. Therefore the transesophageal technique has great potential to further enhance our ability to predict which horses will develop arytenoid collapse during exercise. The researchers also concluded that together these methods show great promise for monitoring atrophy and its resolution in response to reinnervation therapies.   Summary It has long been suspected that resting endoscopy is a highly subjective business; individual vets are fairly consistent but there is some variability when different vets assign grades. However, this research has shown that agreement is perhaps a bit better than one might fear. Advanced diagnostic imaging techniques, CT, and transesophageal ultrasonography are showing great promise as better tools to address how clinicians predict and monitor laryngeal function.   Captions   Overground endoscopy is currently considered the definitive method to diagnose recurrent laryngeal neuropathy. Image provided by Rossdales LLP, Newmarket, Suffolk. RLN endoscopy. The left side of the larynx is typically affected by RLN: when viewed with a scope the left arytenoid, a cartilage forming the roof of the larynx, collapses into the airway. Because the larynx is viewed from the front, the left side of the horse is on the right side of the image. RLN ultrasonography. The top image shows the left, abnormal laryngeal muscle between the white arrowhead that is smaller and brighter than the normal muscle on the right side. TC, CC, and AC indicate the different components of the laryngeal cartilage that are operated by these muscles. Image provided by EVJ Ltd. CT larynx. CT reconstruction of CAD muscle: structures are delineated on the 2D CT slices, and individual slices are then combined to produce 3D reconstructions of these muscles. Here the normal right muscle is green, the smaller abnormal left one is yellow. Standing CT. For the Cornell study on laryngeal CT, horses were examined under general anesthesia, but it may also be possible to perform this technique under sedation. Image provided by Rossdales LLP, Newmarket, Suffolk.

Recurrent laryngeal neuropathy (RLN) is the correct term for the condition better known as roaring or laryngeal hemiplegia. It is extremely common in Thoroughbreds and represents one of the major causes of poor performance or jockey-reported noise.

Because it is so important, many young horses are scoped at sales looking for laryngeal asymmetry to try to identify those that may be at risk of having this condition. But scoping at rest is fraught with difficulty – the larynx may be normal at rest only to show signs of weakness during exercise, so positive cases can be missed.

Conversely, tired young horses can have apparently poor laryngeal function at rest that in fact is of little significance. Many question whether subjecting foals and young horses to endoscopy at sales is reasonable, although in part this latter concern is reduced now that videoendoscopy is available.

Furthermore, the dynamic endoscopy, overground or on a treadmill, is widely accepted as the best way to evaluate horses suspected of having upper airway disorders leading to dynamic obstruction of the airway during exercise, a population that may or may not have obviously abnormal throats when examined at rest.

Nevertheless, there is a need for better tools to evaluate the larynx for clinical application and also to allow researchers to study the condition in more detail. Two recent studies published in Equine Veterinary Journal have addressed this issue..

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