Axial deviation of the aryepiglottic folds () has been recognized as a cause of dynamic upper respiratory obstruction in horses since the first use of high-speed treadmill exercise testing to evaluate poor performance. The membranous portions of the aryepiglottic folds, which extend from the abaxial margin of the epiglottis to the corniculate processes at the lateral aspect of the arytenoid cartilages, collapse axially to occlude the glottis during inspiration (). Horses with axial deviation of the aryepiglottic folds have poor performance and are often reported to “finish poorly” or “stop” near the end of a race. During inspiration at exercise, some affected horses make an abnormal noise that may sound similar to the “roar” associated with laryngeal hemiplegia. The cause is unknown, although immaturity may be a factor in younger horses and should be suspected if concurrent dynamic upper respiratory abnormalities are present.
Clinical Signs And Diagnosis
Affected horses are typically presented with a chief complaint from the owner of poor performance. Horses with axial deviation of the aryepiglottic folds may or may not make an abnormal upper respiratory noise during exercise. No breed or gender predisposition exists, and the condition has been diagnosed in Thoroughbreds, Standardbreds, and racing Arabians. The condition has been reported in racehorses from 2 to 8 years of age, but the percentage of 2- and 3-year-old horses that were diagnosed with axial deviation of the aryepiglottic folds in one hospital population was significantly greater than in the overall hospital population evaluated for poor performance.
Physical examination and endoscopic examination of the resting horse typically do not yield any abnormalities related to the condition. At endoscopic examination at rest, the membranous portion of the aryepiglottic folds of affected horses has no visible structural or functional abnormalities. Nasal occlusion during endoscopic examination, which mimics airway pressures generated during exercise, does not induce axial deviation of the aryepiglottic folds in horses that subsequently demonstrate the condition during treadmill exercise. Endoscopic examination during high-speed treadmill exercise is required to diagnose axial deviation of the aryepiglottic folds. axial deviation of the aryepiglottic folds most often occurs as a distinct clinical problem but also can occur with other upper airway abnormalities. Horses may be unilaterally or bilaterally affected. No association has been identified between the development of axial deviation of the aryepiglottic folds and subsequent dorsal displacement of the soft palate or other causes of dynamic upper respiratory abnormalities.
Severity of axial deviation of the aryepiglottic folds is evaluated based on the extent to which the membranous portion of the aryepiglottic folds collapse across adjacent structures of the larynx. With mild collapse, the fold remains abaxial to the vocal fold. Moderate cases have collapse of the fold beyond the vocal fold but less than halfway between the vocal fold and the midline. In severe collapse, the fold reaches or crosses the midline of the glottis. Mild collapse results in less than or equal to 20% obstruction of the glottis and may not be of clinical significance in some cases. Horses with moderate collapse have 21% to 40% obstruction of the glottis and those with severe collapse have been reported to have 41% to 63% obstruction.
Axial Deviation of the Aryepiglottic Folds: Treatment
Horses with moderate and severe cases of axial deviation of the aryepiglottic folds and those with clinically significant mild axial deviation of the aryepiglottic folds are candidates for surgical treatment. Transendoscopic laser excision of the aryepiglottic folds (TLEAF) to remove a 2-cm isosceles right triangle of tissue from each collapsing aryepiglottic folds with use of a neodymium/yttrium-aluminum-garnet or diode laser in contact fashion is recommended. This approach is easier to perform in a sedated, standing horse with topical anesthesia, but it may be performed successfully with the horse anesthetized in lateral recumbency. The procedure may also be performed with the horse under general anesthesia through a laryngotomy with conventional instruments. The disadvantage for the clinician of performing the procedure this way is the inability to see the exact tissue being resected relative to its normal position to the larynx. If surgical resection is performed through the laser with general anesthesia, the horse is nasotracheally intubated and heliox (70% helium, 30% oxygen) should be mixed with 100% oxygen to achieve a fraction of oxygen in inspired air equal to 0.4 to prevent ignition while the laser is activated.
For surgery with TLEAF in the standing animal, horses are sedated with xylazine hydrochloride (0.4 mg/kg IV). Additional doses of xylazine hydrochloride (0.2 mg/kg IV) may be required. A videoendoscope is inserted into the nasal passage ipsilateral to the target aryepiglottic fold and held in place by an assistant. Topical anesthesia is achieved with an aerosolized solution that contains benzocaine hydrochloride (14%), butyl aminobenzoate (2%), and tetracaine hydrochloride (2%; Cetacaine) administered through polyethylene tubing (PE-240; Becton Dickinson, Sparks, Md.) passed through the biopsy channel of the videoendoscope.
Bronchoesophagoscopic forceps (Richard Wolf Medical Instrument, Vernon Hills, 111.), 60 cm in length and bent manually to conform to the curve of the equine nasal passage and pharynx, are used to provide traction on the aryepiglottic folds during excision. These forceps are passed into the nasal passage contralateral to the target aryepiglottic fold and are manipulated by a second assistant. The free margin of the membranous portion of the aryepiglottic fold is grasped halfway between the arytenoid and epiglottic attachments and elevated caudodorsally (). The laser is set to 18 W of power and excision of the tissue is performed in contact fashion. Beginning rostrally and immediately adjacent to the epiglottic attachment, the clinician makes a horizontal incision in the mucosa by sweeping the fiber side to side and gradually cutting tissue in a rostral to caudal direction. The grasping forceps are then rotated to apply traction to the aryepiglottic fold in a rostromedial direction. A vertical incision is then made from dorsal to ventral to cut the tissue adjacent to its attachments on the corniculate process of the arytenoid cartilage. The vertical incision is extended ventrally to intersect the horizontal incision and the tissue is removed with the grasping forceps. For bilateral excision, the videoendoscope and forceps are positioned in reverse for excision of the contralateral aryepiglottic fold.
To excite the aryepiglottic fold with the horse under general anesthesia, the horse’s mouth is held open with a mouth speculum and the soft palate is manually displaced dorsally. Active suction is used to evacuate smoke from the pharynx. The videoendoscope, grasping forceps, and suction tubing are all positioned in the oral cavity to perform the same surgical procedure. Surgical excision has been performed through a laryngotomy; however, this approach does not afford the same visual perspective of the surgical field as does the videoendoscopic approach.
Broad-spectrum antimicrobial therapy is given preoperatively and continued for 7 days postoperatively because of the open mucosal wound created in the larynx by excision of the aryepiglottic fold. Antiinflammatory therapy is recommended and should consist of tapering courses of phenylbutazone (2 mg/kg orally twice daily for 3-4 days, then once daily for 3-4 days), prednisolone (0.8 mg/kg orally once daily for 7 days, then 0.8 mg orally every other day for 3 treatments then 0.4 mg/kg orally every other day for 3 treatments), in addition to a topical pharyngeal spray (37 ml nitrofurazone solution [0.2%], 12 ml dimethyl sulfoxide [DMSO; 90%], 50 ml glycerine, and 0.2 ml prednisolone acetate [5%]; 10 ml twice daily for 7 days). The pharyngeal spray is administered though a 10-Fr male dog urinary catheter (Monoject, division of Sherwood Medical, St Louis, Mo.) that is placed up the ventral meatus of the nasal passage to a point level with the medial canthus of the eye. The pharyngeal spray is given slowly. If the horse swallows during administration, the catheter is correctly placed in the pharynx.
Postoperative management instructions for horses that have TLEAF should include at least two weeks of daily hand-walking or turnout in a small paddock. Additional rest may be indicated if other surgical procedures are performed for concurrent airway problems. Follow-up endoscopy is recommended before returning the horse to training. Postoperatively, the edge of the tissue will look slightly more fibrous and concave but not dramatically different than the preoperative appearance.
Some horses, especially younger animals and those with multiple upper respiratory abnormalities, may benefit from conservative management with prolonged rest. Additionally, these horses may benefit from longer periods of time between races when returned to training.
Prognosis of Axial Deviation of the Aryepiglottic Folds
In a retrospective study of racehorses with an exclusive diagnosis of axial deviation of the aryepiglottic folds as the cause of their poor performance, 75% of horses that had surgical excision of the aryepiglottic folds and 50% of the horses managed with rest had improved performance. Improvement of the upper respiratory noise is more likely to occur with surgical treatment. No complications have been recognized after surgical excision, and no adverse effects on deglutition or laryngeal or pharyngeal function have been reported.