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TECHNICIAN UPDATE
Diagnosing and Managing Respiratory Abnormalities in the Equine Athlete
By Molly Cripe Birt, BS, RVT, VTS-EVN
During exertion, the horse’s airflow will increase nearly 20 times than when at rest, which changes the shape and rigidity of the airway. For the equine athlete, particularly horses competing in high-speed events, the upper airway is vital for its performance and success. Three common abnormalities in the pharynx and larynx can affect that performance: 1. dorsal displacement of the soft palate, 2. laryngeal hemiplegia and 3. entrapped epiglottis. These conditions occur more often in young Thoroughbreds or Standardbreds. However, Warmbloods and Draft horses are also affected more than other horses. Signs of abnormalities affecting the structures of the pharynx and larynx are expiratory or inspiratory noise, exercise intolerance and poor performance.
Diagnosing Upper Airway Abnormalities
The primary presenting complaints of an upper airway abnormality include poor performance or exercise intolerance, combined with an expiratory or inspiratory noise during respiration. Many presumptive diagnoses are made based on history alone and confirmed with a careful physical examination and the use of endoscope, ultrasonography and diagnostic imaging. In all cases of poor performance or exercise intolerance, the veterinarian must rule out pulmonary, cardiac or neuromuscular disease, or lameness.
The physical examination should include assessing air flow through both nostrils, percussion on the sinuses, auscultation of lung sounds, a rebreathing examination and digital palpation of the larynx, which is done by facing the shoulder of the horse with its nose resting on the shoulder of the technician. The index fingers should then palpate the larynx until they rest above the dorsal larynx. The left and right muscular processes are palpable in this position.
A “slap test” can also be performed to evaluate arytenoid function. Another technician can slap, with a flat hand, just ventral to the right or left withers; if the arytenoid is functioning normally the technician palpating should feel the corresponding cricoarytenoideus dorsalis muscle contract. When the left side of the withers is tapped, the right arytenoid should move.
Flexible endoscopic examination will definitively diagnose an upper airway abnormality and has become a common tool in many practices. The horse must be restrained for its and the staff’s safety. Sedation alters the muscular function of the soft palate and larynx, so is contraindicated during the endoscopic examination—unless the patient is fractious. A nose twitch is preferred if the patient resists the passing of the scope. Stocks, if available, do offer more control because they limit movement.
With the veterinarian holding the hand piece and directing the exam, the technician passes the flexible endoscope by placing a hand on the bridge of the nose of the horse and lifting the alar fold of the false nostril. The other hand passes the endoscope into ventral meatus, directing slightly medial. The endoscope should be passed deliberately to avoid unnecessarily agitating the horse, but without haste to avoid striking the ethmoid turbinate and causing epistaxis.
The veterinarian should be able to fully examine the nasal passages, pharynx and larynx to rule out any abnormalities. The nasal septum, meati, nasomaxillary sinus opening, ethmoid turbinate, pharyngeal lymphoid tissue, pharyngeal openings to the guttural pouch, and the guttural pouches can be seen. Using a flexible stylet passed through the biopsy channel, the soft palate, the epiglottis and aryepiglottic folds, arytenoids and the trachea can be seen.
The normal endoscopic examination of the pharynx will reveal a soft palate posi- tioned ventrally to the epiglottis. The soft palate may intermittently displace dorsally, shortly returning to its original position when the patient swallows. Pharyngeal hyperplasia is common in younger patients, but excessive inflammation is considered abnormal. The arytenoids should abduct with inspiration and adduct with swallowing; both can be elicited by holding of the airway and introducing water into the airway.
Dynamic endoscopy may be indicated when no abnormality is identified, or when the extent of the problem must be examined more closely. Dynamic endoscopy is done using a high-speed treadmill or a telemetric endoscope. This allows the veterinarian to evaluate the soft palate, epiglottis and arytenoids during maximal exercise, which may reveal abnormalities that are not visible while at rest. If indicated, blood sampling before, during and after exercise can be performed to evaluated blood lactate, carbon dioxide and creatinine kinase levels.
Ultrasonography can be used to evaluate laryngeal and soft palate function and identify airway inflammation. Radiographs of the skull that focus on the throatlatch are useful in identifying the cartilage structures of the larynx, the hyoid apparatus, guttural pouches and paranasal sinuses.
3 Common Upper Airway Abnormalities
Dorsal Displacement of the Soft Palate Dorsal displacement of the soft palate (DDSP) occurs when the caudal free margin of the soft palate billows dorsally over the epiglottis, resulting in an airway obstruction. Mechanical causes include the presence of subepiglottic or palatal granulomas or cysts. DDSP is likely caused by excessive sternohyoideus and sternothyroideus muscles contraction due to excitement, nervousness or head positioning, which results in the caudal movement of the larynx. This pulls the epiglottis away from the soft palate. Another potential cause of DDSP is a neuromuscular deficit of the pharyngeal branch of the vagus nerve or intrinsic muscles.
DDSP decreases expiratory air flow, resulting in gurgling or “choking down” during exhalation as air becomes caught in the billowing soft palate, resulting in exercise intolerance and poor performance. Owners often complain the horse is “choking” or “swallowing its tongue” during exercise.
DDSP occurs more often in the exercising horse, and it has been suggested that 10% to 20% of the 2- to 3-year-old racehorses were affected. One research study found that only 1.3% of 479 horses had DDSP during resting endoscopic exams.
A definitive diagnosis is made by history, physical and endoscopic examination. Endoscopic examination reveals a dorsally displaced soft palate that prevents visualization of the epiglottis. It should be noted that DDSP is classified as either persistent or intermittent. Intermittent DDSP at rest might or might not worsen during exercise, and some cases will present with a normally positioned soft palate but with a history of exercise intolerance Therefore, the practitioner may recommend dynamic endoscopy to confirm DDSP during exercise. This confirmation is vital to proper treatment recommendations, as there are management changes that can be implemented prior to surgery.
The trainer may elect to change the head tack used during training and performances to alter head position or prevent mouth opening. There are several options, including nosebands...its mouth, and W-bits and spoon bits to prevent caudal retraction of the tongue. Some trainers, particularly in the Standardbred circuit, will tie the tongue forward; however, research suggests that this practice is not as successful as trainers think. Finally, several companies sell a throat support device to position the larynx rostrally.
Pharyngitis and inflammatory diseases within the guttural pouch can cause neuromuscular deficits and can be treated with systemic anti-inflammatory medication, such as phenylbutazone. A pharyngeal spray comprised of glycerin, dimethyl sulfoxide, prednisolone and nitrofurazone can be administered topically through a long polyethylene catheter inserted into the nasal passaged to the pharynx.
Treatment over 30 days has been found to significantly decrease inflammation. Overall, there is a 60% success rate with medical and non-surgical options.
However, with persistent DDSP diagnosed at rest or through dynamic endoscopy, there are several surgical treatment options that the veterinarian can recommend, including thermal palatoplasty, which causes fibrosis and stiffening of the soft palate, which is done with a laser on the end of an endoscope.
Thermal palatoplasty can be performed on a standing patient as well as under general anesthesia. For a standing procedure, the patient must be restrained properly and sedated adequately. The soft palate must be displaced dorsally over the epiglottis to perform thermal palatoplasty, which can be achieved by inducing swallowing. If DDSP cannot be maintained, bronchoesophageal forceps can grasp the epiglottis and retract it caudally. Topical local anesthetic spray administered through an endoscopic sprayer, will desensitize the soft palate. Some surgeons prefer thermal palatoplasty with the patient under general anesthesia.
The endotracheal tube will automatically displace the soft palate dorsally, which removes one of the greatest difficulties of the procedure in the standing patient. However, the patient’s anesthesia must be maintained using total IV anesthesia, as oxygen ventilating through the endotracheal tube will cause a catastrophic airway fire if exposed to the laser beam.
Overall, thermal palatoplasty is a simple surgical procedure with a good prognosis for return to activity. Postoperatively, a systemic anti-inflammatory is administered for two days. Topical pharyngeal spray containing glycerin, prednisolone, nitrofurazone and dimethyl sulfoxide is administered transnasally via a polyethylene catheter for up to 10 days. Postoperative dysphagia and coughing are common in the first 7 to 10 days and generally resolves.
Thermal palatoplasty is often combined with another surgical procedure to achieve the best outcome possible. A standard myectomy of the sternohyoideus and sternothyroideus muscles will prevent the caudal retraction of the larynx. A sternothyroideus myotenectomy and staphylectomy likely will not be performed with thermal palatoplasty; this procedure transects the sternothyroideus tendons and trims a small crescent of the caudal free margin of the soft palate.
The laryngeal tie forward, however, is considered a superior procedure for treating DDSP, particularly when combined with thermal palatoplasty. The larynx moves rostral and dorsal, and the basihyoid bone will move dorsal and caudal; hence the name “tie-foreward” surgery. Prior to surgery, systemic antimicrobials and anti-inflammatory drugs are administered.
Once induced under general anesthesia, the patient is intubated. Should thermal palatoplasty be performed, total IV anesthesia will be administered until the laser is no longer used. Once complete, inhalant anesthesia can begin. The patient is positioned in dorsal recumbency with the head and neck extended fully. The ventral cervical and intermandibular regions are clipped and aseptically prepared. The incision will be located 2 cm caudal to the cricoid cartilage and 2 cm rostral to the basihyoid bone. The incision is made, and tissue dissected to expose the larynx. Heavy, nonabsorbable suture is placed in the caudal aspect of the wing of the thyroid cartilage.
Dorsal displacement of the soft palate
Photos courtesy of Molly Cripe Birt
The surgeons at Purdue University Large Animal Hospital have implemented the use of Arthrex Suture Buttons to distribute the pressure of the suture, preventing it from tearing through the cartilage. Another method is to pass the suture 2 to 4 times through the cartilage. Using a wire passer, the suture is passed around the basihyoid bone.
These steps are repeated on the contralateral side. Upon completion of the suture placement, the horse’s nose is tilted 90⁰ while the sutures are snugly tied. The surgeon confirms the cricoid cartilage in relation to the basihyoid bone, and once satisfied, the head is lowered into a semi-flexed position with a pad supporting the nose. Once the surgical site is closed, a bandage using elastic tape is placed to support the soft tissue and prevent seroma formation.
Systemic antimicrobial and anti-inflammatory drugs are continued for several days. An important aspect of postoperative care is reducing the tension on the prosthetic sutures. Elevate feed and water to shoulder height to prevent undue stress on the neck. The same considerations must be made for pasturing postoperative patients. After 2 weeks, regular feeding, water, turn out and training can resume.
Laryngeal Hemiplegia
Laryngeal hemiplegia or recurrent laryngeal neuropathy is considered idiopathic, although the loss of large, myelinated nerve fibers in the left recurrent laryngeal nerve can cause neurogenic atrophy of the circoarytenoideus dorsalis muscle. This results in progressive loss of abduction and adduction function of the arytenoid.
Many owners report that clinical signs developed acutely, but it is a slow, insidious progress until the muscles just fatigue. However, trauma to the recurrent laryngeal nerve or guttural pouch mycosis, strangles and neoplastic tumors have been reported to cause laryngeal hemiplegia.
This causes the exercise intolerance and poor performance because of hypoxemia, hypercarbia and metabolic acidosis, and the classic “roaring” inspiratory noise resulting from air passing over the affected vocal cord and ventricle. Large breed horses and racehorses are most affected, although the highest incidence is seen in young Thoroughbreds. Often, laryngeal hemiplegia is discovered in the workup examinations for yearling sales, or as 2- and 3-year-old horses enter training.
A definitive diagnosis is made through history, physical examination and endoscopic examination.
Palpation of the larynx will reveal atrophy of the cricoarytenoideus dorsalis muscle. Endoscopic examination will reveal asymmetric and asynchronous abduction or complete paralysis of the arytenoids, which can be graded on a subjective scale. There are several treatments for laryngeal hemiplegia, with laser ventriculocordectomy and laryngoplasty being common surgical corrections currently performed.
The veterinarian will make treatment recommendations based upon the severity of the presenting complaint, the horse’s work and the grade of laryngeal movement. Invasive surgery may not be the optimum choice for some equine athletes.
FIG 2. Laryngeal hemiplegia
FIG 3. Entrapped epiglottis
However, the noise associated with laryngeal hemiplegia is undesirable in the show arena, and disciplines that require an elevated head or an intense flexion of the neck, such as dressage or carriage driving, may exacerbate the inspiratory noise.
In this instance, a ventriculocordectomy, which removes the mucosal lining of the ventricle and part of the vocal fold, might be offered. It was once performed by a laryngotomy under general anesthesia. However, advances in laser ventriculocordectomy via the nasal passage or oral cavity have largely eliminated the need for ventral laryngotomy. The surgeon uses the laser to ablate the ventricle and vocal cord. Laser ventriculocordecomy can be performed on a standing patient or under general anesthesia and is frequently combined with prosthetic laryngoplasty.
Standing ventriculocordectomy is achieved using proper restraint by an assistant with the head elevated. IV sedation, possibly with a combination of detomidine hydrochloride and butorphanol tartrate, is necessary to prevent horse movement during the procedure. Topical anesthesia is applied to the nasal passages, pharynx and larynx using an endoscopic sprayer.
Should the procedure be performed under general anesthesia, special precautions must be taken. Endotracheal intubation must be delayed so that the vocal cords are visible, and laser safety considerations are respected. Once the horse has been induced and positioned in the proper recumbency, anesthesia is maintained with total intravenous anesthesia.
A flexible 600-micron laser fiber is passed through the biopsy channel of the endoscope. The surgeon may use a combination of non-contact and contact techniques when performing the laser ventriculocordectomy. Starting ventral to the vocal process, the vocal cord is ablated as the fiber is moved carefully over its surface. The ventricle is then ablated with the laser fiber in direct contact with the mucosa. Many surgeons prefer specific settings for their practice.
Specialized instruments can be used to aid in the ventriculocordectomy. Bronchoesophageal forceps are passed through the nostril opposite the endoscope. Bronchoesophageal forceps grasp and evert the ventricle, placing it in a position for excision using a laser fiber. Lower wattage is used for excision. Alternatively, some surgeons prefer the use of a custom-made roaring burr, which is passed through the nostril opposite the endoscope. The burr is pressed into the ventricle and rotated with gentle traction placed to evert the ventricle. Then the laser fiber is used for surgical excision.
Complications include intraoperative hemorrhage and iatrogenic damage to the vocal process and arytenoid. Postoperatively, the patient should receive phenylbutazone intravenously or orally and dexamethasone intravenously. Laser ventriculocordectomy patients typically heal in 30 to 45 days.
A standalone ventriculocordectomy would not be sufficient to correct the problems associated with a grade III-C and IV laryngeal hemiplegia. A laryngoplasty is the recommended treatment for horses who experience exercise intolerance and inspiratory noise. Laryngoplasty is the placement of one or two prosthetic sutures between the cricoid and arytenoid cartilages with the intention of permanently retracting the affected arytenoid out of the airway—hence, the other name for the surgery; tie-back surgery. The surgical “art” of the laryngoplasty is achieving laryngeal opening that is adequate for performance, but not so wide as to cause chronic aspiration of feed and saliva.
FIG 4. Normal equine laryngeal structures
FIG 5. Surgically "tying back" the left arytenoid in a horse under general anesthesia
Preoperative nonsteroidal anti-inflammatory drugs and antimicrobials are administered prior to induction. Presuming the horse has a left-sided laryngeal hemiplegia, the horse is induced under general anesthesia and positioned in right lateral recumbency. The head and neck are extended fully. If a laser ventriculocordectomy is to be performed during general anesthesia, the procedure would begin at this point with safety considerations in place that were described previously. The incision of the palatoplasty is 10 to 12 cm long, ventral and parallel to the linguofacial vein and begins 4 cm cranial to the ramus of the mandible.
The area of interest is clipped and aseptically prepared. Using basic instrumentation, the skin and muscle layers are incised and dissected until the lateral and dorsal aspects of the larynx are visualized. Strong retractors, such as Saurbach, Richardson or Kelley retractors, expose the larynx.
Two large-caliber, non-absorbable sutures are placed through a trocar point needle and used for the laryngoplasty. Trocar point needles will not create large, traumatizing hole in the cartilage and will likely not break off within the cartilage. The needle and suture are passed through a notch in the cricoid cartilage and then advanced cranially to avoid piercing the trachea. At this point, an endoscope should be advanced caudally to the arytenoids and tilted dorsally to visualize the roof of the cricoid cartilage and trachea. This verifies that there is no suture piercing the airway, thus contaminating the prosthetic suture. Next, the suture is drawn under the cranial aspect of the cricopharyngeus muscle. A similar, second suture is placed 1 cm caudal in the same fashion. The ends of the suture are tensioned, abducting the arytenoid. The endoscope is pulled rostrally to visualize the degree of abduction. Once the desired degree is reached, the surgeon will tie the sutures. Special instrumentation by Securos includes a tensioning device, crimps and crimper to achieve a very specific level of tensioning and securement; some surgeons will use this special instrumentation.
Also within the surgical procedure is the surgical debridement of the cricoarytenoid joint using curettes or ablation by the CO2 laser. These limit or completely prevent the movement of the joint associated with arytenoid movement, enhancing the permanent abduction of the arytenoid and reducing post-operative prosthetic failure. The CO2 laser is prepared by setting the laser for a defocused beam, inserting the sterile laser hand piece and draping the articulating laser arm with sterile adhesive wraps. Surgeons at Purdue University Large Animal Hospital prefer setting the CO2 laser at a continuous wave for 15 watts with a 5-mm circular spot.
Once the cricoarytenoid joint is exposed and opened, the surgeon will position the laser hand piece three centimeters from the joint surface, which is then ablated. The entirety of the cricoarytenoid joint is not fully accessible with the CO2 laser; completion of fusing the joint is achieved using a 2-0 curette. If a CO2 laser is not available, curettage of the joint is acceptable.
The incision is closed, and a bandage is placed over the throatlatch for recovery. Postoperative nonsteroidal anti-inflammatory drugs and antimicrobial therapy are continued. Hay and water should be lowered to prevent aspiration. Thirty days of stall rest with light handwalking is recommended. By weeks 5 and 6, light, controlled exercise can begin. After 6 weeks, a return to training can resume. Complications of laryngoplasty include hemorrhage from a needle piercing through laryngeal veins, needle breakage in the cricoid or arytenoid cartilage, development of a large seroma over the surgical site, dysphagia and chronic cough, aspiration pneumonia, incisional infection, infected prosthetic and prosthetic failure.
Many horses who cough immediately postoperatively will resolve within seven to ten days. Coughing and dysphagia beyond ten days is considered abnormal and warrants a recheck examination. If the prosthetic laryngoplasty fails, the surgeon may elect for another laryngoplasty or would consider an arytenoidectomy. An arytenoidectomy performed via laryngotomy will fully remove the affected arytenoid. Despite the list of potential complications associated with laryngoplasty, research reports that nearly 50 to 70% of racehorses treated will have improved racing performances.
Entrapped Epiglottis
The epiglottis becomes entrapped when the ventrally located aryepiglottic folds become dorsally malpositioned over the epiglottis. Entrapped epiglottis will cause respiratory noise, exercise intolerance and coughing. Nasal exudate has been described but is not as common. Some epiglottic entrapments are intermittent and relieved by swallowing and, patients will show minimal clinical signs. Other entrapments are persistent; the longer the epiglottis remains entrapped, the more inflamed the tissue becomes, causing thickening and ulceration of the tissue. Epiglottic hypoplasia is a risk factor that contribute to persistent epiglottic entrapment. The most affected horses are Thoroughbreds and Standardbreds, often because of the effect on their performance. Some horses are unaffected by entrapped epiglottis and live normally.
Entrapped epiglottis is diagnosed by endoscopic examination, sometimes coincidentally as no clinical signs may be present. On endoscopic examination, the general shape of the epiglottis is visible, but the distinct margins and classic vascular pattern are obscured. The veterinarian might describe the membranes as being thin or thick, narrow or wide, ulcerated or nonulcerated, with mild to extensive ulceration. Approximately 95% of cases have aryepiglottic folds that are described as persistent, thick and wide.
A laryngotomy with resection of redundant sub epiglottic tissue is a viable surgical option, but DDSP might develop because of altered epiglottic function. A concurrent staphylectomy might reduce the chance of DDSP from occurring after this surgery. A more modern, less invasive approach to managing entrapped epiglottis is the axial division of the aryepiglottic fold with a laser or epiglottic hook bistoury knife. 75% to 80% of patients can be treated in this method if the aryepiglottic fold is thin without inflammation. Patients with chronically thickened ulcerated entrapments are good candidates for resection of sub epiglottic tissue via a laryngotomy.
Axial division of the aryepiglottic folds is completed using a shielded, curved bistoury knife through a transoral or transnasal approach, or by using a laser through an endoscope. The transoral approach is performed with the patient under general anesthesia. Transnasal axial division is completed in a standing patient that is properly restrained and sedated. The most dangerous complication from the transnasal approach is the iatrogenic laceration and damage to the soft palate, which can destroy the athletic animal’s career. The transoral approach requires a mouth speculum and is considered the safest, as it is nearly impossible to damage the soft palate. Laser axial division of the aryepiglottic fold is performed
standing. The patient must have proper restraint and sedation, as previously described. A flexible 600-micron laser fiber is passed through the biopsy channel of an endoscope. A topical anesthetic, such as Cetacaine, is sprayed over the epiglottis and aryepiglottic fold using an endoscopic sprayer. Surgeons at Purdue University Large Animal Hospital prefer the laser settings at 15-25 watts at three seconds on and one second off. The laser fiber is used in contact fashion, incising layer by layer from caudal to rostral across the aryepiglottic fold. Swallowing by the patient will progressively retract the aryepiglottic folds. Once the division is complete, the patient is made to swallow to confirm the epiglottis will not become re-entrapped.
Complications from laser axial division of the aryepiglottic fold include iatrogenic damage to the tip of the epiglottis, the development of DDSP and prolonged sub epiglottic swelling. Postoperatively, the patient is administered a systemic anti-inflammatory drug and steroid for several days. Stall rest is ordered for 21 days with a slow return to work. Returning to training too soon will lead to prolonged edema and inflammation, thus reducing the healing time.
The technician working in an equine practice is in a unique position to assist with the workup and treatment of equine athletes presenting with complaints of upper respiratory noise and poor performance. With a basic understanding of the bones, cartilage, tendons and muscles that are vital to the function of the upper respiratory tract, the technician will have a better insight into the diagnosis and treatment methods of dorsal displacement of the soft palate, laryngeal hemiplegia, entrapped epiglottis and a variety of other abnormalities.
Becoming experts in handling and using endoscopic equipment and laser safety and surgical treatment methods can help make the technician an invaluable member of the veterinary team.
Teaching Points
Care and cleaning the endoscope is important to maintain the equipment and to prevent infections.When the endoscopic examination is complete, the endoscope is dismantled.As each endoscope is different, it is paramount that the technician refers to the manufacturer’s cleaning instructions.
Basic cleaning procedures, however, include flushing a diluted enzymatic detergent through the chambers, brushing the chambers with a special bristle brush and rinsing with deionized water. Some endoscopes can be submerged in water so long as a waterproof cap is in place over the camera connection.
If the endoscope was used with surgical laser or if a suspected trauma occurred, the technician should leak test the endoscope.
Fill the inside of the endoscope with air, and air bubbles will stream from the defect if damage is present. This not only alerts the technician of damage, but also prevents water from leaking inside of the endoscope. Water contamination in an endoscope is catastrophic.
Once the endoscope is disinfected and rinsed, it should be connected to the air source so that any water droplets can be blown out of the chambers to prevent water from collecting and sitting in the chambers,
About the Author
Molly Cripe Birt, BS, RVT, VTS-EVN, is a large animal surgery Technician, at the Purdue University Veterinary Teaching Hospital PURDUE, University College of Veterinary Medicine. She presented this information at the 2017 Fall Conference.
For more information:
Auer J. A., Stick J. A. Equine Surgery, Third Edition. St. Louis: Saunders Elsevier, 2006.
Couetil, L., Hawkins J. Respiratory Diseases of the Horse: A problem-oriented approach to diagnosis & management. London: Manson, 2013.
Hawkins J. Advances in Equine Respiratory Surgery. Wiley-Blackwell, 2015;3-8.
