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LARYNGEAL HEMIPLEGIA
Overview
Laryngeal hemiplegia, also known as roaring, is a condition of the upper respiratory tract that results in respiratory noise and decreased performance in the horse. This condition is most commonly reported in the racing Thoroughbred, but also occurs in other large breed performance horses including Warmbloods, Draft horses, and Standardbreds. It is estimated that about 8% of large sport horses are affected with laryngeal hemiplegia.
A basic understanding of the laryngeal anatomy is necessary to understand what structures are affected in horses with laryngeal hemiplegia (Fig 1). The larynx (similar to the voice box or Adam’s apple in people) is the structure that connects the nasal passage to the trachea (also known as the windpipe). It consists of a group of cartilages that allow air to pass into the lung and protect the airway during swallowing. Laryngeal hemiplegia can be defined as paralysis of one side of the larynx, particularly one side of the arytenoid cartilage. The left arytenoid is the most common side affected (up to 95%). In a normal horse, the arytenoids (commonly called flappers) allow maximal airflow into the trachea during abduction (the outward movement of the arytenoid cartilages to open the entrance into the trachea). Abduction of the arytenoid cartilage is controlled by the recurrent laryngeal nerve, which innervates the cricoarytenoideus dorsalis (CAD) muscle. It is believed that the recurrent laryngeal nerve undergoes degeneration and results in decreased function of the laryngeal musculature, particularly the CAD muscle. This can result in failure of the arytenoid cartilages to abduct, thereby decreasing airflow into the lungs resulting in respiratory noise and exercise intolerance.
Figure 1. This is an upper respiratory tract endoscopy of a normal horse. The important anatomical structures are pointed out in this image. Images such as this are taken by passing an endoscope (long tube like structure with a camera and a light source) up the nostril of the horse to the level of the eye.
Signs/Clinical Presentation
Laryngeal hemiplegia is often seen in tall (usually greater than 15 hands), male horses, generally between the ages of 3 to 7 years. Thoroughbreds usually are affected at younger ages (3-5 years old) compared to draft horses which are slightly older at presentation (4-7 years old). Owners or trainers will often complain of exercise intolerance that has progressed over weeks to months. Often a classic whistling or roaring noise is produced during exercise (usually during cantering or higher activity) that may be heard by the rider and those observing the horse. As the condition progresses the sound of the horse’s whinny may change due to impaired vocal ability. The horse may even appear to gasp for breath after light exercise. Veterinarians may also note atrophy (shrinking of the musculature) of the CAD muscle when palpating the throat latch over the larynx.
Risk Factors
Often horses diagnosed with laryngeal hemiplegia have no known cause for developing this condition. Although the exact cause of recurrent laryngeal nerve degeneration is unknown, several factors have been implicated in some cases. These include trauma to the cervical (neck) region, prior neck surgery, perivascular (around, but not into the vein) injection of an irritating drug, esophageal obstruction/rupture, guttural pouch infections, organophosphate intoxication, lead poisoning, Streptococcus equi infection (strangles), various diseases of the central nervous system, and thiamine deficiency. Involvement of the right arytenoid is usually associated with direct trauma to the nerve, including perivascular injection on the right side of the neck.
Diagnostic Tests
Classically, laryngeal hemiplegia is diagnosed and graded (I – IV) by upper airway endoscopy. Standing endoscopy (Fig 2) will generally provide enough information for diagnosis; however, high-speed treadmill endoscopy (Fig 3) may be necessary to further evaluate horses and to ensure that no other upper respiratory problems are contributing to the exercise intolerance and/or respiratory noise. During standing endoscopy, arytenoid abduction is evaluated by nasal occlusion, which will cause the horse to take a deep breath (inspiration). Nasal occlusion should result in full and even/equal abduction of the right and left corniculate process (the portion that is visualized during endoscopy) of the arytenoid cartilages. If abduction does not occur or is not symmetrical then the diagnosis of laryngeal hemiplegia can be made. Also during the standing endoscopic examination the veterinarian may perform a “slap test”. The slap test evaluates the adductor (the inward motion toward the opening of the trachea) function of the arytenoids. The test is conducted by gently slapping the upper right chest near the withers with and open hand. This should result in a flickering inward movement of the left arytenoid. The test is repeated on the opposite side. If this motion does not occur or is reduced compared to the other side a diagnosis of laryngeal hemiplegia is suggested. Severe cases (grade IV) generally show no movement of the affected arytenoid during inspiration or expiration. Horses with grade III laryngeal hemiplegia may show some slight movement, but not full abduction. Grade II horses may display only a slight hesitation or flutter prior to abduction during inspiration. Horses with grade II or grade III may require further evaluation with endoscopy while being exercised on a high speed treadmill.
Photo courtesy of Dr. Jeremy Hubert
Figure 2. This image demonstrates how standing upper airway endoscopy is performed
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Photo courtesy of Dr. Jeremy Hubert
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Figure 3. Horses may need to be evaluated on a high speed treadmill as seen above. Once the horses have adapted to the treadmill the endoscope is placed up the nostril and secured. Images are stored on a video recorder for later evaluation.
Treatment Options
Treatment options vary depending on the severity/grade, the breed, the age and the use of the horse affected with laryngeal hemiplegia. There are four treatment options for this condition: prosthetic laryngoplasty (also known as a tie-back or flapper surgery), ventriculectomy and/or cordectomy, arytenoidectomy, and neuromuscular pedicle graft.
The first treatment, prosthetic laryngoplasty, is the most common of the four treatments performed. This surgical procedure is performed under general anesthesia and is the treatment of choice for all grade IV and select grade III cases of laryngeal hemiplegia. The procedure involves making an incision in the throat latch area. The airway is not entered during surgery but the outside of the laryngeal cartilages are identified (particularly the muscular process of the arytenoids and the cricoid cartilage). Suture (the prostheses) is place into the cartilage from the cricoid to the muscular process. This suture is placed to mimic the pull of the CAD muscle that has atrophied, resulting in abduction of the arytenoid at all times (not just during inspiration). Over time the suture adheres down with scar tissue providing stability and maintenance of abduction which will decrease respiratory noise and improve airflow to the lungs.
The second treatment, ventriculectomy/cordectomy, may be performed alone or along with prosthetic laryngoplasty. Paralysis of one side of the arytenoid cartilage often results in collapse of the vocal cords and interference of the airway by the ventricles (a pocket of excess tissue next to the vocal cord). The ventriculectomy/cordectomy is a surgical procedure that involves the removal of the ventricle and/or the vocal cord on the affected side. The removal of these structures can be done by two different approaches. One approach is done surgically through an incision under the jaw (known as a laryngotomy) into the airway or it can be done using an endoscopic guided laser. Laryngotomy incisions require general anesthesia and are left open to heal on their own. Laser techniques can be performed either under general anesthesia or under standing sedation. No incision is necessary as the laser and the endoscopic camera is passed up the nose to the larynx. The laser technique performed standing is ideal for large Draft breeds that may have difficulty recovering from general anesthesia. This procedure alone may improve the exercise intolerance and respiratory noise in Draft breeds because they do not need to perform at high rates of speed.
The third treatment for laryngeal hemiplegia is known as an arytenoidectomy. The purpose of this surgical procedure is to remove part of the affected arytenoid cartilage which will increase the cross-sectional area of the tracheal opening. Arytenoidectomies are also performed through a laryngotomy incision under general anesthesia. This surgery is not the first treatment of choice and is usually reserved for those that have had a failed laryngoplasty. Overall this procedure has increase risk of complications and may not allow the horse to return to their previous level of competition; therefore, it is not commonly recommended.
The last treatment for laryngeal hemiplegia is a neuromuscular pedicle graft. Young horses with grade III hemiplegia are considered good candidates for this procedure. This surgical treatment involves reinnervation of the CAD muscles. A nerve (the 1st cervical nerve) is isolated from one of the neck muscles (omohyoideus muscle) and a branch of that nerve is placed in the atrophied CAD muscle. Reinnervation requires approximately 6 to 12 months. Grade III horses will respond faster to reinnervation than grade IV horses. Horses that have had a previous laryngoplasty sustain damage to the 1st cranial nerve; therefore they are not candidates for this procedure.
When to Seek Referral
The diagnosis of laryngeal hemiplegia is not difficult and can be determined by any veterinarian that has the ability to perform standing upper airway endoscopy. Treatment, however, requires surgery; therefore, horses are usually sent to a veterinary surgeon for further evaluation and surgery. Further evaluation may involve high speed treadmill endoscopy and/or lateral skull radiographs.
Potential Complications of Surgery
Complications and their rate of occurrence depend on the surgical procedure. Complications after prosthetic laryngoplasty occur in 9 to 47% of cases. They include coughing, aspiration of food or dirt particles resulting in tracheitis or aspiration pneumonia, incisional infection or dehiscence, seroma formation (excessive swelling and fluid accumulation at the incisional site) arytenoid chondritis (inflammation of the cartilage resulting in deformation of the arytenoids), infection of the suture/prostheses, breakage of the prosthesis and failure to maintain abduction. Of these complications, coughing is the most common and is often seen immediately following surgery; however, only 5 to 10% will remain chronic coughers. If the suture/prosthesis fails (either from infection, breakage or from cartilage pull out) a second laryngoplasty can be performed; however, success rates decrease and chance of subsequent failure increases. Complications from ventriculectomy/cordectomy are few as the laryngotomy site usually heals with minimal problems. Rarely granulomas (excessive scaring tissue) may form at the site of ventricle/vocal cord removal, but this can be managed with anti-inflammatories and/or removal. Complications are common with arytenoidectomy and can include coughing, difficultly swallowing, and aspiration pneumonia. Complications for the neuromuscular pedicle graft are minimal and include seroma formation at the surgical site, infection of the surgical site and failure for the CAD muscle to reinnervate.
Aftercare
Post operative care depends on the surgical procedure performed. Antibiotics are typically administered prior to surgery and may continue after surgery for duration of 3 to 7 days depending on the surgeon. Anti-inflammatory medications (such as phenylbutazone or banamine) are typically administered post operatively for 3 to 7 days. Monitoring of the surgical incision for swelling, heat, and drainage will be necessary and skin sutures or staples may need to be removed after laryngoplasty or neuromuscular pedicle graft. Horses that have laryngotomy incisions will require twice daily cleaning of the surgical site and application of petroleum jelly around the site and down the neck. Horses that undergo a ventriculectomy/cordectomy may also be administered a throat spray to help decrease upper airway inflammation.
Stall rest and time to return back to exercise will also vary depending on the surgeon’s preference, the surgical procedure performed, and the grade of laryngeal hemiplegia. Post operative rest following laryngoplasty typically includes 30 days of stall rest with hand-walking/grazing followed by small paddock turnout at 30 days. Gradual return to exercise is usually allowed 45 to 60 days post operatively. It is very important to prevent a horse that has undergone laryngoplasty from exercising until advised by the surgeon, as this will cause increased movement of the suture/prosthesis and could increase the risk of suture breakage or pull out. Horses that have had a laryngotomy performed require stall rest with hand walking or small paddock turnout until their surgical site has completely healed, typically around 30 days. Post operative stall rest is also required for horses undergoing the neuromuscular pedicle graft surgery. Recommendations usually include stall confinement for 2 to 3 weeks followed by small paddock turnout only for approximately 12 weeks. Gradual return to training can resume after this time.
Recheck evaluations may or may not be required by the surgeon. The time of the recheck will depend on the surgical procedure performed. If any complications are noted, the surgeon will typically request to re-evaluate the horse so he or she can initiate appropriate treatment.
Prevention and Prognosis
Since most cases of laryngeal hemiplegia have no known cause for development, it is a difficult disorder to prevent. Care should be taken when administering irritating drugs (such as phenylbutazone) to ensure that perivascular leakage does not occur, as this could result in damage to the recurrent laryngeal nerve. Other traumatic injuries and infections of the upper respiratory tract and surrounding tissues of the neck should also be addressed in a timely manner to limit damage to the recurrent laryngeal nerve. Overall prevention for development of laryngeal hemiplegia is difficult if not impossible.
Prognosis for horses that have laryngeal hemiplegia often depend on the horse’s desired level of performance and surgical treatment performed. Overall success of treatment ranges from 50 to 90% but the definition of successful treatment varies depending on the horse’s job. For example, racehorses will require full abduction and stabilization of the arytenoid to remove airflow obstruction and perform successfully, whereas a hunter may only require stabilization of the arytenoid to eliminate noise that is produced during exercise. Therefore, successful treatment after laryngoplasty in racehorses is reported to be between 50-70%. Neurovascular pedicle graft surgery is a fairly new procedure; however, reported success is around 50%.
—Britta Leise, MS, DVM
Large Animal Resident
Editor: Charles McCauley, DVM
Diplomate ACVS
Posted 5/2/2007
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