Causes

Laryngeal paralysis is a common unilateral or bilateral respiratory disorder that primarily affects older (>9 years) large- and giant-breed dogs. However, a congenital form does occur in certain breeds such as Bouvier des Flandres, Siberian huskies, and bull terriers. A laryngeal paralysis-polyneuropathy complex has been described in dalmatians, rottweilers, and Pyrean mountain dogs. For the more frequently encountered acquired laryngeal paralysis, the Labrador retriever is the most common breed reported, but golden retrievers, Saint Bernards, Newfoundlands, and Irish setters are also overrepresented. Acquired laryngeal paralysis is caused by damage to the recurrent laryngeal nerve or intrinsic laryngeal muscles from polyneuropathy, polymyopathy, accidental or iatrogenic trauma, or intrathoracic or extrathoracic masses. In most dogs the cause remains undetermined, and these cases are traditionally classified as idiopathic. Recently, it was shown that many dogs develop systemic neurologic signs within 1 year following diagnosis of laryngeal paralysis, which is consistent with progressive generalized neuropathy (Stanley et al, 2010). Abnormalities in the results of electrodiagnostic tests and histopathologic analysis of nerve and muscle biopsy specimens reflecting generalized polyneuropathy have been documented in a small number of dogs with acquired laryngeal paralysis (Thieman et al, 2010).

Clinical Signs

In laryngeal paralysis, the arytenoid cartilages, and consequently the vocal folds, remain in a paramedian position during inspiration, creating upper airway obstruction. Dogs typically present with noisy inspiratory respiration and exercise intolerance. Early clinical signs include voice change and mild coughing and gagging. Severe airway obstruction results in respiratory distress, cyanosis, and collapse. Dogs may also have signs of dysphagia.

Progression of clinical signs is highly variable, and dogs may have clinical signs for several months to years before significant respiratory distress ensues. However, clinical signs are worsened by heavy exercise or increasing environmental temperature or humidity, which results in an acute exacerbation of a chronic condition. As respiratory rate increases, the mucosa covering the arytenoids obstructing airflow may become inflamed and edematous, which leads to further airway obstruction. A vicious cycle ensues that if unaddressed may become life threatening.

Diagnosis

Routine diagnostic evaluation for dogs thought to have laryngeal paralysis includes physical examination, neurologic examination, complete blood count, biochemical profile, urinalysis, thyroid function screening, thoracic radiographs, and laryngeal examination. Dogs with bilateral laryngeal paralysis are at risk of aspiration pneumonia both before and after surgery. Therefore thoracic radiographs are a necessary part of the diagnostic workup in dogs suspected to have laryngeal dysfunction. For dogs that present with dysphagia or vomiting, an esophagram should be obtained to rule out esophageal dysfunction or megaesophagus, which may not be apparent on plain thoracic radiographs. Severe progressive esophageal dysfunction has been reported in a set of dogs with idiopathic laryngeal paralysis and is likely reflects the proposed generalized progressive polyneuropathy (Stanley et al, 2010). Hypothyroidism may be found concurrently with laryngeal paralysis, although a direct causal link has yet to be established. Regardless, thyroid function screening is performed routinely in the workup for laryngeal paralysis. Thyroid supplementation should be instituted if indicated, although this does not seem to improve clinical signs associated with laryngeal paralysis.

Definitive diagnosis of laryngeal paralysis requires visual examination of the larynx. However, laryngoscopy can be confounding, and false-positive results are common because of the influence of anesthetic agents on laryngeal function. Laryngeal paralysis should not be diagnosed based solely on the lack of arytenoid movement; inflammation and swelling of the laryngeal cartilages also should be apparent. Diagnosis may additionally be confused by the presence of paradoxical movement of the arytenoids, resulting in a false-negative result. In this situation, the arytenoid cartilages move inward during inspiration because of negative intraglottic pressure that is created by breathing against an obstruction. The cartilages then passively return to their original position during the expiratory phase, which gives the impression of abduction. An assistant can state the phase of ventilation during laryngoscopy to help in distinguishing normal from abnormal motion.

Intravenous thiopental administered to effect is thought to be the best choice to allow assessment of laryngeal function. When a butorphanol-glycopyrrolate premedication is used, either thiopental or propofol allows excellent visualization of the larynx. The recent lack of availability of thiopental leaves propofol as the most appropriate induction agent for laryngeal examination in dogs. Doxapram HCl (1 mg/kg IV) has been advocated for routine use during laryngoscopy to increase respiratory rate and effort and improve intrinsic laryngeal motion, and should be administered if the diagnosis is in doubt. Some clinicians include doxapram administration as part of every laryngeal examination. Transnasal laryngoscopy, ultrasonography, and computed tomography (CT) all have been described as methods to diagnose laryngeal paralysis; however, none appears superior to traditional oral laryngeal examination under heavy sedation or light anesthesia.

Emergency Treatment

For dogs in acute respiratory distress, initial treatment is directed at improving ventilation, reducing laryngeal edema, and minimizing the animal’s stress. A typical treatment regimen involves oxygen supplementation and administration of short-acting steroids (e.g., dexamethasone 0.2 to 1 mg/kg IV) and sedatives (e.g., acepromazine 0.02 mg/kg IV). Additional administration of buprenorphine (0.005 mg/kg IV) or butorphanol (0.25 mg/kg IV) also may be considered. These dogs are often also hyperthermic, and appropriate cooling procedures should also be instituted. If respiratory distress cannot be abated, intubation or a temporary tracheostomy should be considered. However, the use of a temporary tracheostomy tube in dogs with laryngeal paralysis has been shown to be a negative prognostic indicator following surgery, because dogs that received a temporary tracheostomy preoperatively were more likely to experience major complications.