Treatment and prognosis for laryngeal hemiplegia

Abstract

Parente discusses treatment and prognosis for laryngeal hemiplegia, one of the most common problems of the upper respiratory tract in horses. There are multiple methods of treatment, and the tieback (laryngoplasty) procedure is still the most common method of treatment. Performing a tieback to abduct and lock the left arytenoid in an open position is a simple concept but much more complex than it appears. Part of the complexity is taking what should be a dynamic functional larynx, and making it into a somewhat static structure without complications.

Laryngeal hemiplegia (recurrent laryngeal neuropathy) has been recognized for many years and is one of the most common problems of the upper respiratory tract but is still challenging to effectively treat. There are multiple methods of treatment, and the tieback (laryngoplasty) procedure is still the most common method of treatment. Performing a tieback to abduct and lock the left arytenoid in an open position is a simple concept but much more complex than it appears. Part of the complexity is taking what should be a dynamic functional larynx, and making it into a somewhat static structure without complications. Furthermore we are working with tissues (cartilage) that have significant variability to their shape and stiffness and thus the ability to construct and maintain a rigid structure is difficult. This leads to significant variability of success and success that is greatly dependent upon the primary complaint and the expected level of performance for the individual horse. The success rate for treatment of nonracehorses is generally very high (>80%), while the success for a racehorse is reported to be from 25-70% depending on the criteria and methodology used to determine success.

The primary reason for the limited success is the inability to maintain an adequate airway during exercise. Other reasons involve complications as a result of the surgery. Aspiration or coughing is a significant concern but anecdotally appears to be a function of the surgeon's experience and not directly associated with the degree of abduction. The occurrence of seroma formation is <10%, and infection at the surgical site even lower. While many surgical infections with implants require suture removal, if addressed early the laryngoplasty suture does not always need to be removed. Finally a small percentage of horses (particularly those well abducted) can develop granulation tissue on their corniculate without a history of coughing postoperatively. This stresses the importance of a follow up endoscopy at 4 weeks before resuming training.

The first step to success is obtaining an accurate diagnosis. The diagnosis is most often made with resting endoscopy. Several different laryngeal grading systems have been developed for horses examined while standing loosely restrained in a box stall or in stocks, with a twitch applied and without tranquilization or sedation. The majority of grading systems are similar to what is described here with minor variations. Grade I (synchronous and symmetrical abduction) and grade II (symmetrical but asynchronous abduction) are considered normal, in that the horse is capable of obtaining and maintaining complete abduction of the arytenoid cartilages during sustained exercise. Laryngeal grades III and IV are abnormal. Any horse with grade IV laryngeal movements clearly demonstrates progressive and severe axial collapse of the paralyzed arytenoid cartilage during exercise. Its clinical significance will depend on the level of exercise and how the horse is judged (performance or noise). Most racehorses with grade III (asynchronous and asymmetrical movement) dysfunction will also experience dynamic arytenoid collapse but many show horses will not.

Since laryngeal dysfunction is almost always left sided, any right sided observations should cause the clinician to pause and reevaluate the horse. Right sided dysfunction is most commonly secondary to 4th branchial arch disease or general dysplasia. This should be confirmed prior to considering surgery since structural abnormalities may prohibit the chance of a successful laryngoplasty procedure. It can be confidently diagnosed with ultrasound or MRI. Causes of trauma to the recurrent laryngeal nerve will result in dysfunction but should also be confirmed prior to considerations for surgery.

The first challenge of laryngoplasty surgery is attaining the exact position of the arytenoid desired at the time of surgery. Intraoperative videoendoscopy is essential to more precisely position the arytenoid. Despite videoendoscopy, less than two thirds of the horses had the ideal position of abduction on the morning following surgery in one retrospective study. Futhermore, the most common complication postoperatively is loss of abduction. One study reported a loss of abduction of one grade or more in almost half of the horses over a 6 week period. While initially it is intolerable to think that this could happen it is consistent with all the experimental studies. Critical evaluation of the experimental studies published indicates a significant postoperative loss of abduction. There are likely multiple causes for loss of abduction, and many different causes have been proposed but none have been clearly defined. There is no evidence that the prosthesis material, the horse's age, or the preoperative degree of dysfunction impacts laryngoplasty failure. Thus there is no benefit of waiting until a horse is completely paralyzed before pursuing surgery. There is actually more recent data to show that horses undergoing laryngoplasty that were not completely paralyzed returned to a higher level of performance than horses that were completely paralyzed. It is possible that the inability to consistently maintain abduction is inherent in the technique and the tissues we are working with. This prompted us to further evaluate those possibilities.

Initially we demonstrated the great variability in the cricoid cartilage, which is an important structure in anchoring the laryngoplasty suture. We have also shown significant variability in distraction of the suture anchored in the cricoid. These factors could in part explain the variable loss of abduction over time. To compact these factors we developed a different approach to minimize abduction loss and improve stability with a modified laryngoplasty approach. There are two significant modifications from the standard laryngoplasty. The first is approaching the muscular process caudal to the cricopharyngeus muscle rather than between the crico and thyropharyngeus. The second is transecting the insertion of the cricoarytenoideus dorsalis muscle from the muscular process and entering the cricoarytenoid (CA) joint to debride part of the articular cartilage and facilitate ankylosis of the CA joint. An experimental study demonstrated the effectiveness of this technique. We completed a retrospective study to assess our technique clinically. We evaluated horses over several years and looked at quarterly earnings. Our data indicate that the horses treated by modified laryngoplasty improved significantly relative to performance while suffering from hemiplegia and earned as much over time, and competed as long or longer than their cohorts that previously raced better than them.

While laryngoplasty may never be the perfect procedure, if the goal is to create a stable partially abducted arytenoid, methods as described above to improve the probability of achieving this goal could be employed. The only potential disadvantage of this technique is if the appropriate degree of abduction is not achieved, there will be a limited time of opportunity to change the position of the arytenoid before a joint ankylosis ensues. If several months after surgery the horse exhibits significant aspiration, or on the other extreme a lack of adequate abduction, repeat laryngoplasty is unlikely to be an effective option for surgical correction. An arytenoidectomy would be recommended.

Other potential reasons for limited improvement in performance after laryngoplasty do not just revolve around structural stability but involve dynamic function. There is evidence that the horse with limited resting abduction after taryngoptasty can often undergo further dynamic collapse of the affected arytenoid, or have other soft tissue structures obstructing the airway during strenuous exercise. If there is not adequate abduction of the Left arytenoid it is common that the left aryepigtottic fold and/or the right vocat cord wilt deviate axiaLLy during inspiration and result in partiat obstruction and noise. If the arytenoid is poorly abducted it is tikety that the left arytenoid witt undergo further dynamic cottapse during exercise. Resting evatuation of the arytenoid cannot predict stabitity under exercising conditions accuratety. There is atso experimentat evidence that horses after taryngoptasty may not have gross evidence of aspiration but can have microscopic contamination of their tower airway. The impact of this tower airway contamination on performance is unknown but it is easy to specutate that it coutd initiate a comptex of tower airway inflammation that does impact gas exchange.

Given the chattenge of obtaining a positive outcome with taryngoptasty, other atternative treatments of cordectomy, ventricutectomy, arytenoidectomy, or reinnervation have been considered as viabte treatment options. These different surgicat options are taitored more to the use of the horse and the degree of taryngeat dysfunction. Arytenoidectomy is generatty reserved for chondropathies, and reinnervation has timited success but vocat cordectomies stitt can be successfut in setect cases of taryngeat dysfunction.

The vocat cordectomy (sometimes in conjunction with the ventricutectomy) is reserved for horses that maintain some abduction capabitity and that are not racehorses. The goat is to minimize noise production for show horses. It is rarety recommended as a sote treatment for a racehorse. Vocat cordectomy can be performed in the standing horse with transendoscopic taser resection or via taryngotomy (standing or anesthetized). White bitaterat ventricutocordectomy is advocated by some ctinicians to decrease abnormat noise, and can be performed successfutty with traditionat surgery, it shoutd not be attempted with transendoscopic taser surgery at one time because it witt tikety tead to ventrat scar/webbing formation in the tarynx. The vatue of a ventricutectomy is arguabte. If the vocat cord is comptetety resected, the ventricutectomy tikety has tittte benefit.