The Harry M. Zweig Memorial Fund for Equine Research

Management of Horses with Partial Paralysis of the
Left Arytenoid Cartilage

Dr. Normand G. Ducharme

The clinical problem that we seek to address in this project is partial paralysis of the voice box, a common variant of “roaring”. We hope to find a solution that does not risk worsening the condition and one that would reduce to lay-up time to 2 weeks.

The voice box (larynx) is made of cartilages, two of which serve as flappers (arytenoid cartilages) that open the airways and tense the vocal cords when the horse needs air and closes the airway when swallowing to protect the windpipe (trachea) from feed contamination.  Laryngeal hemiplegia (roaring) is a disease in which one-half of the larynx becomes completely paralyzed thus the flapper and the attached vocal cord obstructs airflow when the horse inhales.  Laryngeal hemiplegia has been diagnosed for over two centuries and affects 5-8% of Thoroughbred racehorses. The current preferred treatment is a surgical procedure called a “tie - back” (laryngoplasty).  This procedure involves suturing the paralyzed cartilage in an open position such that the airflow can be restored—a surgery that is successful in only 50 to 60% of racehorses. A larger study published this year documented that in successful cases, the airway only stays about 75% open after the surgery.  Most complications are associated with aspiration of feed material in the airway due to the permanently open position of the larynx.

Trainers and veterinarians treating performance horses are far more commonly called upon to address the problem of horses with partial paralysis. The clinical dilemma they often face is a horse that has a partial paralysis and is able to be trained but is not competing at an acceptable level or is inconsistent. Although this complaint can be made of many horses, the problem here is that there is clearly some compromise to the diameter of the airway. To illustrate the significance of this problem, in one recent study 96% of horses (out of 375 horses) with laryngeal disease had partial paralysis and only 4% were completely paralyzed. The practical dilemma is that it is difficult for a veterinarian to assess based on an exam at rest if the size of the airway during exercise is better or worse that what might be obtained after surgery (75% of opening of the airway in successful cases). Although high speed treadmill examination can answer this question, it does so in a more limited number of horses because of the small number of equine sports medicine clinics. What must be considered in the decision is that the partial paralysis will be either worsened or not be improved by surgery in 30 to 50% of racehorses. Furthermore, to add to the decision making dilemma as to whether or not to perform surgery is the observation that the procedure seems less successful in horses that retain significant movement of the flapper, presumably because the remaining movement pulls against the suture causing it to cut through the cartilage. Finally, the lay-up period following flapper surgery is 6 to 8 weeks. This leads most individuals to defer surgery until the performance is not at all acceptable or the paralysis is completed (many months) causing a significant financial loss.

A better approach would be to develop a procedure that would improve the airway of horses with partial paralysis that has little risk of causing further deterioration of the degree of movement of the flapper and hopefully would require a shorter recuperation period (~ 2 weeks). We propose to divide this study into two years. In the first year, multiple potential surgical treatments will be tested in a cadaveric study. In parallel with this study, we will develop a model of partial paralysis in horses exercising on a high speed treadmill. Investigation of the problem of partial paralysis has been hampered by the lack of a model for this disease. Finally we will determine the effect of various degrees of the voice box opening using flow modeling techniques. The latter is done by using a computer that recreates a 3-D version of a real horse larynx using a CT scan and then through engineering software manipulates the degree of the opening of the flapper to predict the airflow limitation created. This will allow us to map the mechanical implications on airflow of different degrees of closure of the larynx (voice box). In the second year of the study, two of the successful treatments from the cadaveric study in year 1 will be developed into a clinical procedure and performed in live horses; and we will evaluate their efficacy during high speed treadmill and also ensure patient safety.

The five potential treatments we will evaluate are: 1) Thermal capsulorrhaphy (a procedure in which the joint capsule is tightened by increasing the temperature of the tissue causing an immediate contraction of the tissues). It is commonly used in people for treatment of shoulder instability. Rationale: This would tighten the joint capsule on the open side of the voice box resisting the closing pressure during inhalation, thus helping to keep the top of the voice box open. 2) Laser removal of the vocal cord. We first reported this surgical technique last year (March 2002) and have clinical evidence that healing would be completed by 2 weeks. Rationale: the vocal cord obstructs the bottom of the voice box because it is not tensed when the flapper is partially paralyzed. Removing the vocal cord would help to keep the bottom of the voice box open. 3) Creating tension in the vocal cord by changing the position of two cartilages at the bottom of the voice box. This procedure is done in people to tense the vocal cord and increase the pitch of the voice. Rationale: if we could tense the vocal cord this would help to keep open the bottom of the voice box. This procedure is similar to a “tie-forward” procedure used for treatment of palate displacement in horses and requires only 2 weeks of healing port-operatively. 4) Place a blocking device across the flapper joint to prevent the flapper from moving below a certain level. Rationale: Most research into surgical treatment of laryngeal hemiplegia involves manipulation designed to pull the flapper open. The important goal is to prevent the flapper from closing during inhalation. Therefore if we could limit the range of movement of the flapper joint, we could prevent closure of the airway in horses with partial paralysis that develop progressive flapper closure as they fatigue at the end of exercise. 5) Tying the vocal cord to the side of the larynx using a noninvasive procedure. This procedure was developed in humans within the last 5 years and seems to give good results for up to 2 years. Rationale: This would allow to keep open the bottom of the voice box.

The second phase of the project (Year 2) would be to test two of the most promising treatments using the model that most reliably approximates the clinical situation of partial paralysis of the flapper. We will finalize the technical aspect of the surgical procedure(s), determine that procedure is effective during the maximal exercise and verify stability of the procedure over a 3 month period. This is critical to ensure patient safety before field implementation in a clinical population.

In summary, we believe that this project would add significant information to a very important and common clinical problem. Year 1 investigations are logical steps in finding a solution and Year 2 would not be implemented unless both a model and a solution can be established.