Laryngeal paralysis is one of the most common causes of respiratory dysfunction and distress in older large breed dogs. It may present either as a chronic progressive increase in upper airway noise and exercise intolerance or as a respiratory emergency with severe inspiratory distress. Fortunately, surgical treatment is highly successful with infrequent complications and a high degree of client satisfaction.
Causes and pathophysiology
The specific causes of acquired laryngeal paralysis remain uncertain but are associated with dysfunction of the laryngeal muscles and recurrent laryngeal or vagus nerves. The primary muscle responsible for abduction of the arytenoid is the cricoarytenoideus dorsalis which is innervated by the recurrent laryngeal nerve. This neuromuscular mechanism may undergo idiopathic failure in older large breed dogs leading to inspiratory difficulty. The clinical signs are exacerbated by heat and exercise due to the effects of increased respiratory effort on the rima glottidis.
The rima glottidis is the opening between the arytenoids and represents a narrowing in the airway when laryngeal paralysis occurs. Heat and exercise increase the rate of flow through the upper airway and the narrowing at the rima glottidis results in a region of low pressure at the rima due to the Venturi effect. The result is that increased respiratory effort results in further narrowing of the rima glottidis and further dyspnea. The patient experiences increased stress and temperature resulting in an additional increase in respiratory effort and further narrowing of the rima glottidis. Ultimately the end results of laryngeal paralysis particularly in periods of increased heat and exercise are hypoxia, hyperthermia and moderate to severe stress. A secondary result can be aspiration due to ineffective laryngeal adduction and eventually aspiration pneumonia.
The association between acquired laryngeal paralysis and generalized neurologic disease remains controversial. Several studies have suggested that a significant percentage of dogs with laryngeal paralysis may have or develop generalized neurologic signs and that those with generalized neurologic disease at the time of diagnosis have a greater risk of complication and poorer prognosis than those without.
Infrequently, dogs may present with congenital laryngeal paralysis. The most common breeds identified with laryngeal paralysis are Bouviers des Flandres and Siberian huskies. Many dogs with congenital laryngeal paralysis are younger and frequently show signs of other neurologic disease. Many of these patients will have progressive neurologic deterioration and owners should be cautioned of this likelihood prior to electing surgery.
There is a broad variety of presentations of laryngeal paralysis although most dogs are older and males are two to four time more commonly affected than females. The Labrador retriever is the most common breed presented although other breeds such as golden retrievers and spaniels are also over-represented. Traumatic laryngeal paralysis secondary to bite wounds or other cervical trauma may present in any breed of dog or cat.
Owners often report progressive exercise intolerance, increased respiratory effort in the heat, increased respiratory noise and intermittent gagging. Mild cases may only result in clinical signs or increased noise during significant exertion. We commonly see two groups of presentations. In the first group the patients present with a normal respiratory effort and attitude but a distinct raspy upper airway noise is evident. There may be a history of gagging and increased respiratory noise when sleeping. The second group presents as a respiratory emergency with severe upper airway distress and possible hyperthermia.
Treatment of patients in severe upper airway distress is directed at supplementing oxygen, sedation, and cooling if indicated by hyperthermia. Treatment of the stress by sedation is probably the single most important action to stop the cycle of hypoventilation and subsequent increased respiratory effort triggered by stress. In the most severe cases, we will anesthetize and intubate the patient and either recover them from the anesthesia slowly and continue sedation or take them immediately to surgery for a laryngeal tie back.
Thoracic radiographs should be obtained in all cases of suspected laryngeal paralysis to evaluate for aspiration pneumonia or co-morbidities.
Definitive diagnosis of laryngeal paralysis is made by laryngoscopy with the patient under light anesthesia. We avoid all premedications during this procedure to decrease the risk of false positive results. We generally use i.v. propofol alone and want the patient at a plane of anesthesia where they are actively taking deep breaths on their own. The oral cavity and naso- and oropharynx are examined for any masses or other disease. The arytenoid function is then carefully evaluated in conjunction with monitoring the patient’s inspiration and expiration. It is critical to coordinate the respiration with the movement of the arytenoids to avoid a false negative test. Other methods of diagnosis including ultrasound or flexible endoscopy have not proven as effective as laryngoscopy.
Medical treatment of mild laryngeal paralysis is often directed at modification of lifestyle. Exercise moderation should be prescribed particularly in warmer weather and weight management may aid in reducing the risk of hyperthermia and overexertion. Elevated feeding and decreasing the speed of water consumption with water bowl obstacles may decrease the risk of aspiration.
Surgical treatment is directed at enlarging the rima glottidis or bypassing the larynx by permanent tracheostomy. Most surgeons elect unilateral arytenoid lateralization (tie back procedure) due to the high success rate and lower complication rate than other procedures.
A permanent tracheostomy should not be necessary for treatment of laryngeal paralysis except in rare cases of laryngeal collapse. Tracheostomies require regular care by the owners and may contribute to additional tracheal disease.
Partial laryngectomy may be performed by an oral approach or ventral approach and may utilize laser or routine hand instruments. Complications including webbing and subsequent airway obstruction can be severe and the degree of airway opening is less predictable than with the tie back procedure.
Unilateral arytenoid lateralization (tie back) is the most common surgical procedure for laryngeal paralysis. A lateral skin incision is made over one side of the larynx and the cricoarytenoidius dorsalis muscle is replaced with a non-absorbable suture that holds one of the arytenoids in permanent abduction. Bilateral lateralization is rarely indicated. The surgical procedure is approximately 45 minutes to an hour and the rima glottidis is checked during the procedure to ensure adequate opening. We use a non-absorbable braided suture due to its strength and the size and curvature of the needle. Unlike many surgical procedures, the tie back procedure is based more on palpation than vision and the nature of working in a very tight anatomical region necessitates a perfect needle size and curvature.
Postoperative care and complications
Patients undergoing arytenoid lateralization have instant relief of the airway obstruction and demonstrate a dramatic difference in attitude and respiration within minutes of anesthetic recovery. We take numerous precautions to limit aspiration pneumonia during the recovery period including the use of antacids, broad spectrum antibiotics and metoclopramide in addition to recovering the patients with an elevated head and thorax. These patients demonstrate very little surgical pain and, like most Labradors, are eager to eat as soon as permitted. We feed soft meatball shaped food for the first two weeks and then gradually return them to their normal diets.
The most common complication of the tie back procedure is seroma formation at the incision site due to microvasculature and dead space formation. Aspiration pneumonia is reported any time postoperatively in 10 to 20% of cases although our records suggest our hospital rate is closer to 6%. Most aspiration pneumonia can be medically treated on an out patient bases. We are aware of only two cases of elective euthanasia due to aspiration pneumonia from patients in our hospital. Over a 10-year period we have re-operated 2 patients after failure of the cartilage or suture with successful revision in both cases. Initial failure may be due to inappropriate suture placement or brittle cartilage. Owners should also be informed that following the tie back procedure, dogs will not be able to generate a normal bark sound although many have had a voice change preoperatively as well.
The overall survival rate following tie back surgery is 94%, 90% and 85% at 1,2 and 3 years postoperatively respectively. The overall client satisfaction rate is greater than 95% in our hospital. Successful surgical treatment of laryngeal paralysis can eliminate or drastically reduce the risk of upper airway distress and hyperthermia and significantly improve exercise tolerance. The decrease in stress to the owners and the patient is immeasurable. While the risk of complications with tie back surgery can never be completely eliminated, the high client satisfaction rate and significant survival times support the use of surgical treatment in the majority of patients.