Laryngeal Paralysis

The function of the larynx is to regulate airflow, protect the lower airway from aspiration during swallowing, and control phonation. Laryngeal paralysis is the failure of the arytenoid cartilages to properly abduct secondary to muscle or nerve damage. This results in upper airway obstruction because of interference with airflow, particularly during inspiration. Although laryngeal paralysis can occur in cats, this condition is far more common in dogs and should be considered as a possible differential diagnosis for animals presenting with respiratory distress and stridor.

Functional Anatomy

The larynx is a term used to describe the cartilages that surround the rima glottidis and that are responsible for control of airflow during respiration. The four cartilages that make up the larynx are the paired arytenoids, and the unpaired epiglottis, cricoid, and thyroid cartilages. The base of the epiglottis attaches to the cranial trough of the thyroid cartilage. Laterally the epiglottis is connected to the cuneiform processes of the arytenoid by mucosa that makes up the aryepiglottic fold. The apex of the epiglottis extends into the oropharynx just beyond the caudal edge of the soft palate. The thyroid cartilage is a U-shaped structure with dorsal horn-shaped projections on each end known as the rostral and caudal cornu. The rostral cornu articulate with the thyrohyoid bones of the hyoid apparatus, whereas the caudal cornu connect with the cricoid cartilage. The cricoid cartilage is a ring-shaped structure that is wider dorsally than it is ventrally. The caudal aspect of the cricoid cartilage connects with the trachea. The paired arytenoid cartilages are irregular structures with four sets of processes: the corniculate process dorsally, the cuneiform process rostrally, the muscular process caudally, and the vocal process ventrally. The rima glottidis is the elongated opening between the arytenoid cartilages, and it is the narrowest part of the larynx. The glottis consists of the rima glottidis dorsally and the vocal folds ventrally, which are created by vocal ligaments extending from the vocal processes of the arytenoid cartilage.

The cricoarytenoideus dorsalis muscle is solely responsible for enlarging the glottis. The muscle originates on the dorsolateral surface of the cricoid and inserts on the muscular process of the arytenoids. Contraction of the muscle results in external rotation and abduction of the arytenoids that then pulls the vocal processes laterally. The recurrent laryngeal nerve innervates all of the intrinsic muscles of the larynx except the cricothyroid muscle, which is supplied by the cranial laryngeal nerve. The right side branches from the vagus nerve at the level of the middle cervical ganglion in the thoracic inlet, curves dorsocranially around the right subclavian artery, and then courses cranially along the dorsolateral surface of the trachea. The left side leaves the vagus nerve at about the level of the middle cervical ganglion, but travels caudally and then medially around the aortic arch before coursing cranially along the ventrolateral aspect of the trachea.

Etiology

Laryngeal paralysis is a common unilateral or bilateral respiratory disorder that primarily affects older large breed dogs. However, a congenital form does occur in certain breeds. Laryngeal paralysis is caused by damage to the recurrent laryngeal nerve or cricoarytenoideus dorsalis muscle resulting in failure of arytenoid abduction during inspiration.

Congenital Laryngeal Paralysis

Congenital laryngeal paralysis has been reported in a variety of canine breeds. An autosomal dominant trait has been documented in the Bouvier des Flandres, resulting in Wallerian degeneration of the recurrent laryngeal nerves and abnormalities of the nucleus ambiguous [1,2]. Although a precise mode of inheritance has not been established, a hereditary predisposition has also been identified in Siberian husky dogs, Alaskan malamutes, and crosses of those two breeds [3-5]. Laryngeal paralysis-polyneuropathy complexes have been described in Dalmatians, Rottweilers, and Leonberger dogs [6-8]. Other reported breeds with congenital laryngeal paralysis include bull terriers and white-coated German shepherd dogs [9,10]. Respiratory signs are often apparent within the first few months of life, but most dogs are diagnosed at less than a year of age. The prognosis for dogs with congenital laryngeal paralysis is poor, especially in those with concurrent polyneuropathy.

Acquired

Acquired laryngeal paralysis is a common condition of older large and giant breed dogs. The Labrador retriever is by far the most common breed reported, but Golden retrievers, Saint Bernards, Newfoundlands, and Irish setters are also over-represented [11-16]. Average age of onset ranges from 9 to 11 years. In most studies, male dogs appear to be more commonly affected [12,14,15]. Laryngeal paralysis has also been described in cats [17]. Proposed causes of laryngeal paralysis include accidental trauma, intrathoracic masses, cervical masses, and neuromuscular disease (Table 52.1). There is an association between acquired laryngeal paralysis and hypothyroidism, but a direct link has not been determined [18]. For dogs that are diagnosed with concurrent hypothyroidism, thyroid supplementation should be instituted, but this typically does not improve the clinical signs of laryngeal paralysis. An underlying cause of laryngeal paralysis often cannot be determined, and these cases are then deemed idiopathic.

Diagnosis

Suspicion of laryngeal paralysis arises from the history of the dog as well as from consistent clinical signs. A routine work-up for a dog suspected to have bilateral laryngeal paralysis includes physical examination, neurologic examination, complete blood count, biochemical profile, urinalysis, thoracic radiographs, thyroid function screening, and laryngeal examination. Definitive diagnosis of laryngeal paralysis requires visual examination of the larynx. However, laryngoscopy is a poorly specific diagnostic test, as false positives are common owing to the influence of anesthetic agents on laryngeal function. By definition, laryngeal paralysis is the absence of arytenoid abduction on inspiration. In particular, the corniculate process of the arytenoid cartilages that frame the rima glottidis dorsally are observed for purposeful motion (Fig. 52-1). However, an absolute diagnosis should not be made based solely on the lack of arytenoid movement. Laryngeal paralysis is a fixed upper airway obstruction that results in inflammation and swelling of the laryngeal cartilages. Diagnosis may also be confused by the presence of paradoxical movement of the arytenoids, which can lead to a false negative result. In this situation, the arytenoid cartilages move inward during inspiration owing to negative intraglottic pressure that is created by breathing against an obstruction. The cartilages then return to their original position during the expiratory phase, giving the impression of abduction. An assistant can state the stage of respiration during laryngoscopy to help distinguish normal from abnormal motion.

Figure 52.1. Laryngeal anatomy as visualized during laryngeal examination: A = Right corniculate process of the arytenoid cartilage; B = Right cuneiform process of the arytenoid cartilage; C = Rima glottidis; D = Epiglottis.

Treatment

Dogs often do not have severe clinical signs until they have bilateral laryngeal paresis or paralysis. For dogs with bilateral laryngeal paralysis, the decision to institute surgical treatment is based on the quality of life of the dog, the severity of clinical signs, and the time of the year. Surgical treatment of laryngeal paralysis is primarily directed at increasing the functional area of the rima glottidis. This is most commonly performed by unilateral arytenoid lateralization, but other methods include variations of partial laryngectomy and castellated laryngofissure. Permanent tracheostomy is also described as a surgical treatment as a means of bypassing the upper airway obstruction entirely. Bilateral arytenoid lateralization is no longer considered a reasonable option for this disease because of the number of postoperative complications; also, it is not necessary to enlarge the area of the rima glottidis to that degree to alleviate clinical signs [15,19]. According to Poiseuille's law, air flow through the glottis is directly proportional to the radius of glottis to the fourth power (Fig. 52-2). For example, if the radius of the rima glottidis is doubled, resistance to air flow through the glottis is reduced by a factor of 16. However, this equation is used most appropriately to describe laminar low velocity flow through a straight tube, generally the small peripheral airways. Movement of air through the trachea, rima glottidis, nose, and mouth is high velocity turbulent flow with heavy resistance. Yet, different methods of unilateral arytenoid lateralization have resulted in varying degrees of increase in rima glottidis area without any difference in short-term clinical outcome or complication rate [20]. Cricoarytenoid laryngoplasty, in which the suture is passed from the muscular process of the arytenoid cartilage to the dorsocaudal aspect of the cricoid cartilage, results in a 207% increase in the area of the rima glottidis. This is compared with a 140% increase in rima glottidis area for dogs with thyroarytenoid lateralization, in which the suture travels from the muscular process of the arytenoid cartilage to the caudodorsal aspect of the thyroid cartilage.

Figure 52.2. Schematic representation of Poiseuille’s (Q = πr4P/8ζL) law where Q = flow rate, r = radius of the tube, L = Length of the tube, P = pressure within the tube, and ζ = viscosity.

The effect of the degree of suture tension on the area of the rima glottidis has also been evaluated in canine cadavers [21]. It was found that a low-tension suture (the suture was tied until resistance was felt from the cranial part of the cricoarytenoid articulation joint capsule) increased the area of the rima glottidis when the epiglottis was open without an increase in the area of the rima glottidis not covered when the epiglottis is closed. A high-tension suture (the suture was tied as tight as possible) resulted in a significant increase in the area not covered by the epiglottis in a closed position (467% larger than the low-tension suture). It was suggested that the use of a low-tension suture in clinical cases may reduce the potential risk of postoperative aspiration pneumonia.

Surgical Complications

Following surgical treatment of laryngeal paralysis, postoperative aspiration pneumonia has been reported to occur in 8 to 19% of dogs following unilateral arytenoid lateralization [15,22]. Although aspiration pneumonia is most likely to occur in the first few weeks following surgery, it has been recognized that these dogs are at risk for aspiration pneumonia for the rest of their lives [15]. Factors that have been significantly associated with a higher risk of developing complications include preoperative aspiration pneumonia, postoperative megaesophagus, temporary tracheostomy placement, and concurrent neoplastic disease [15].

The use of a temporary tracheostomy tube before surgery has been found to be a significant negative risk factor in dogs with laryngeal paralysis. Of dogs that had a temporary tracheostomy preoperatively, 50% developed complications postoperatively. The increased risk was not related to whether the dog was presented as an emergency. The presence of a tracheostomy tube results in loss of cilia, epithelial ulceration, submucosal inflammation, and retained secretions. The trachea becomes colonized with bacteria from oropharyngeal flora within 24 hours after tracheostomy tube placement. It is theorized that tracheostomy tubes significantly interfere with the normal function of the mucociliary apparatus such that these dogs are then unable to effectively clear aspirated substances [23].

Prognosis

Without surgical complications, unilateral arytenoid lateralization for treatment of canine laryngeal paralysis results in less respiratory distress and stridor and improved exercise tolerance. Owner satisfaction with this procedure has been reported as excellent, with the majority of owners believing the quality of their dog's life was dramatically improved [14,22,24].

Laryngeal Paralysis in Cats

Laryngeal paralysis is an uncommon condition in the cat. Clinical presentation is similar to that of the dog and both unilateral and bilateral laryngeal paralysis have been reported. Cats with unilateral laryngeal paralysis can present with significant clinical signs, unlike dogs, which are rarely symptomatic. There also appears to be a prevalence of left-sided unilateral laryngeal paralysis in cats, which is similar to what is reported in humans and horses. The specific etiology of laryngeal paralysis in cats is unknown, but several cases have been associated with trauma, neoplastic invasion, and iatrogenic damage. Successful surgical treatment primarily utilizing unilateral arytenoid lateralization has been reported [17].