34th Annual Scientific Meeting proceedings

Laryngeal paralysis (LP) is a well-recognized condition encountered mainly in older middle size breeds and was first described in detail by Harvey and Van Haagen (1975) who proposed management of the condition using unilateral arytenoid lateralization (UAL). Although the technique of ‘tie-back’ became the standard procedure for LP, many authors have subsequently reported complications rates as high as 20 – 30% and consequently many, much less-effective techniques have been proposed as alternatives. Analysis of these complications indicates that the most commonly-reported complications were i) aspiration and ii) regurgitation and likely stemmed from a failure of surgeons to understand the impact of their intervention on the functional anatomy of the larynx. A 3-D animated model of the canine larynx was therefore created to permit a more in-depth understanding of laryngeal functional anatomy.

Laryngeal functions can be summarized as:

1. Control of upper airway diameter during the respiratory cycle
2. Protection of the lower airway during the swallowing phase
3. Vocalization - in the dog, this function is considered of lesser significance than 1 & 2.

Upper airway control during the respiratory cycle
Upper airway diameter (rima glottidis) is determined by the position of the paired arytenoid cartilages and the vocal folds attached to the vocal processes of the arytenoids. Rotational movement of the arytenoids in a dorso-lateral plane dilates the dorsal rima glottidis whilst the ventral aspect is dilated by the passive movement of the attached vocal folds. Movement of the arytenoids is the result of activity of the intrinsic laryngeal muscles; the major muscle responsible for abducting the arytenoids being the dorsal crico-arytenoid muscle whilst adduction is caused by activity in several muscles, the most important being the lateral crico-arytenoid and vocalis muscles. Innervation is via the caudal laryngeal nerve (branch C.N. X) which supplies ALL intrinsic muscles.

Lower airway protection during swallowing
Airway protection during the swallowing phases results in closure of the posterior nares by upward displacement of the soft palate and a series of mechanisms that prevent food from entering the larynx, these include:

1. Epiglottic covering: the epiglottis is a triangular-shaped cartilage attached at its base to the cranio-ventral aspect of the thyroid cartilage and capable of a hinge-like movement in a cranio-caudal plane which results in covering the laryngeal aditus during swallowing and directs food over and around the larynx. The epiglottis therefore provides an important hood-like protection for the larynx during the swallowing phases.
2. Ventral laryngeal displacement: the hypoid apparatus (HA) is attached ventrally to the cranial cornu of the thyroid cartilages and dorsally to the base of the skull in the region of the mastoid bone. The HA is capable of considerable compression and extension facilitating the ventro-caudal displacement of the larynx during swallowing. The movement of the larynx during swallowing is facilitated by the ventral extrinsic laryngeal muscles which comprise the thyrohyoid, sternohyoid and sternothyroid muscles. This mechanism therefore provides protection for the larynx by its ventral displacement during swallowing to allow the food bolus to pass dorsally.
3. Glottic closure: during swallowing, contraction of the intrinsic adductor muscles causes the rima glottidis to narrow or even close. Glottic closure has therefore been widely considered as playing a part in the airway protection mechanism system.

Aspiration
Rational evaluation of the airway protection mechanisms suggests that 3. (glottic closure) plays only a very minor, if indeed any, role. Patients with LP most commonly suffer from diseases of the caudal laryngeal nerve which is responsible for innervation of all intrinsic laryngeal muscles. We can infer therefore that BOTH the laryngeal abduction (dilation of the rima) and abduction (constriction of the rima) functions are lost. Failure of the abducting function results in stridor, reduced exercise tolerance and in severe cases, cyanosis typically associated with LP. Failure of the adducting function should, if glottic closure is an important airway protection mechanism results in aspiration and pneumonia, but these are only rarely seen. The implication of this being that Mechanisms 1 & 2 (epiglottic covering and ventral laryngeal displacement) are the major airway protection mechanisms and that, contrary to the very widely perceived view, the absence of glottic closure in dogs that have undergone UAL or other ‘arytenoid tie-back’ procedures for LP does NOT explain complications such as aspiration.

UAL requires the disarticulation of the crico-arytenoid joint and separation of the sesamoidean band. The arytenoid can then be repositioned laterally to allow attachment to the thyroid cartilage. Although successful in experienced hands, this technique i) does not mimic the physiologic plane of arytenoid movement and also ii) allows for potential overdilation of the rima. Aspiration is therefore most likely to occur when Mechanism 1 (epiglottic covering) fails due to over-dilation of the rima.

For this reason, we recommend that UAL is modified to unilateral arytenoid rotation (UAR) which i) mimics the natural plane of arytenoid movement and ii) prevents the risk of over dilation.

Regurgitation
Mechanism 2 (ventral laryngeal displacement) is brought about through the function of the ventral extrinsic muscles (thyrohyoid, sternohyoid and sternothyroid muscles) whilst the dorsal group (hyopharyngeus, thryopharyngeus and cricopharyngeus muscles), better termed the pharyngeal muscles, initiate esophageal activity through their sequenced contraction which moves the food bolus into the cranial esophagus. Both UAL and UAR necessitates lateral access to the arytenoid cartilage thereby unilaterally disrupting the thyropharyngeus muscle. Sectioning the muscles fibers horizontally is known to result in significant pharyngeal dysphagia and affected patients exhibit regurgitation of varying duration. Separating the muscle fibers in the vertical plane however, results in significantly less postoperative muscle dysfunction and minimal impact of swallowing function.

For this reason, we therefore recommend that the fibers of the thyropharyngeus muscle should be separated instead of sectioned.

Summary
Most complications in ‘tie-back’ surgery in the dog stem from the surgeon’s failure to understand the critical role that laryngeal functional anatomy plays in the outcome of the procedure.

Surgeons should therefore endeavour to

1. have a thorough understanding of laryngeal function,
2. use arytenoid rotation rather than lateralization,
3. minimize disruption to swallowing function by preserving the thyropharyngeus muscle.