UPDATE ON RESPIRATORY DISEASES

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DISEASES OF THE LARYNX Anjop J. Venker-van Haagen, DVM, PhD

The larynx is a musculocartilaginous organ that forms a passageway between the pharyngeal cavity and the tracheobronchial tree. Its principal function is the opening and closing of the passage. The larynx protects the tracheobronchial tree from leakage of food and water during the swallowing proc~ss. This is largely a function of the extrinsic laryngeal muscles, and the corresponding dysfunction, leading to dysphagia, is more appropriately discussed as a pharyngeal-esophageal dysfunction than as a laryngeal disease. The function of the larynx as part of the upper airway consists of the activity of the intrinsic laryngeal muscles in opening and closing the laryngeal inlet during respiration. Associated functions are vocalization and the closing and sudden opening of the glottis during coughing. All of these functions of the larynx are reflected by the corresponding dysfunctions in laryngeal disease. In addition, there are receptors in the laryngeal mucosa that initiate coughing when irritated. Laryngeal disease is suggested by the following signs: Stridor is a wheeze or noise that occurs during inspiration or during both inspiration and expiration. The sound is soft and rasping if there is mild obstruction and becomes high toned if there is severe, life-threatening near closure of the laryngeal opening. Dyspnea (respiratory distress) occurs together with the stridor. As the loudness of the stridor increases, the dyspnea typically becomes more prominent. A high-pitched stridor usually is accompanied by severe dyspnea, cyanosis, and often vomiting of mucus. Loss or change of voice indicates a dysfunction of the vocal folds. It is less often a sign of inflammatory disease in dogs and cats than in humans, and more often a sign of neoplasia of the vocal folds . Vocal From the Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, University of Utrecht, Utrecht, The Netherlands

VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 5 • SEPTEMBER 1992

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changes may also be noted in laryngeal paralysis cases. Coughing occurs upon irritation of the laryngeal mucosa and usually indicates inflammatory disease. The cough is dry. When the irritation is severe, it may be followed by gagging. CLINICAL EXAMINATION

The procedure for examining the patient with laryngeal disease depends largely upon the degree to which its respiration is impaired. If the patient is in a state of severe dyspnea or is in rapidly increasing distress, emergency laryngotracheal intubation precedes clinical examination, after intravenous anesthesia using low dose, short-acting barbiturate. In these cases, the differential diagnosis of dyspnea must be made on the basis of the combined signs of dyspnea and stridor. Laryngoscopy can be completed when the patient is stabilized. When dyspnea is not the predominant sign, clinical examination begins with listening to the patient's spontaneous stridor or cough . In listening to the stridor, not only the sound and its continuity are important: it should also be noted whether it occurs during inspiration alone or during both inspiration and expiration. (The latter is indicative of a more severe obstruction.) When, according to the owner, the sounds are only produced by certain circumstances, the circumstances should, if possible, be reproduced for purposes of the examination (e.g., mild exertion or exercise). Palpation of the larynx can reveal information about the degree of irritation of the laryngeal mucosa. If it is severely irritated, a harsh dry cough is produced immediately following laryngeal palpation. The tone of the stridor will change when soft pressure is applied to the larynx. Palpation also can reveal a change in location or a deformation of the larynx itself. Deformation can be caused by a tumor developing in or around the larynx or by metastatic involvement of the superficial laryngeal lymph nodes. In chronic laryngitis, the cartilages of the larynx can become so indurated by ossification that palpation reveals the larynx to be "as hard as stone." A lateral radiograph can be helpful in detecting cartilage ossification and neoplasia9 but will not usually be helpful in diagnosing functional disorders. Even moderate respiratory distress can influence the configuration of air pockets in and around the laryngeal structures, and care must be taken in the interpretation of these radiographs. Direct inspection of the larynx via the oropharyngeal cavity (laryngoscopy) is the most informative diagnostic test. When care is taken in the choice of anesthesia, the respiratory movements of the arytenoid cartilages and the vocal folds may be assessed. The movements of the glottis are suppressed by cortical and vagal depression. A satisfactory method of anesthesia in dogs is direct intravenous administration of a short-acting barbiturate without premedication. Laryngeal inspection should take place as soon as the dog is just losing resistance to opening the mouth. In cats, laryngeal movements also are observed when there

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is just loss of resistance to opening the mouth during the induction of anesthesia with ketamine hydrochloride and xylazine hydrochloride. The use of lidocaine spray is indispensable in cats. The larynx of the cat should be examined with a minimum of touching of the laryngeal mucosa because it is prone to the development of edema. When the clinical signs and the insufficient glottic movements revealed by laryngoscopy both indicate laryngeal paralysis, electromyography of the intrinsic laryngeal muscles may be used to confirm the diagnosis. Anesthesia is required for this procedure, but when the detection of normal action potentials is of importance, as in checking the placement of the electrode or to diagnose partial paralysis, a superficial level of anesthesia is necessary. Anesthesia does not interfere with the detection of denervation potentials, pseudomyotonia, or myotonia. After the initial dose of barbiturate has been administered, the dog is placed in dorsal recumbency. The tip of the laryngoscope is firmly placed over the dorsal side of the epiglottis and the larynx is exposed and slightly lifted. The bipolar needle electrode is introduced orally, and its tip is inserted through the laryngeal mucosa into the intrinsic laryngeal muscles in a sequential fashion (Fig. 1). The preferred level of anesthesia is maintained by administering additional doses of barbiturate intravenously. An intratracheal tube is not inserted because it would be a major hindrance during this procedure. In adult or adolescent dogs, the trauma caused by the needle electrode is of no consequence to the function of the larynx. In very young puppies and

Figure 1. Oral view of the larynx in the dog. The mucosa (left) is omitted to show the underlying muscles. The black dots indicate placements of the electrodes. D = dorsal cricoarytenoid muscle; V = ventricular muscle; T = thyroarytenoid muscle. (From Venker-van Haagen AJ, Hartman W, Goedegebuure SA: Spontaneous laryngeal paralysis in young Bouviers. J Am Anim Hosp Assoc 14:714, 1978; with permission.)

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in cats, however, there is a serious risk of causing obstructive laryngeal edema, hence, this procedure should only be performed when the information is of such importance that it justifies a possible tracheostomy.*

CONGENITAL LARYNGEAL MALFORMATION

Laryngeal hypoplasia is most common in brachycephalic dogs. The inadequate development of the cartilaginous structures of the larynx results in a small and unusually flexible laryngeal skeleton, leading to a narrow laryngeal opening. In addition, there is inadequate abduction of the vocal folds during inspiration, and eversion of the lateral ventricles occurs as a result of airflow obstruction combined with other malformations. Surgical correction of the malformed larynx in brachycephalic dogs has been mentioned, but the risk of further collapse and contraction by scar tissue resulting in a more severe obstruction is very high. 5 In young dogs and cats with increasing respiratory distress and stridor, an anomaly of the larynx may be suspected. The malformation can affect all three parts of the laryngeal cavity: the vestibule, the glottis, and the infraglottic cavity. 16 Laryngoscopy provides the necessary detailed information for diagnosis and for deciding whether surgical intervention may result in functional improvement. In cases of serious underdevelopment resulting in major functional obstruction, euthanasia may be justified. When corrective surgery is needed in dogs or cats under 5 months of age, it should be remembered that trauma to the cartilage may adversely influence the development of the cartilaginous skeleton of the larynx. At around 8 months of age, the larynx usually is fully developed, so minor corrections that are thought necessary can be done with less risk at this age or older.

INFLAMMATION

Acute laryngitis is characterized by continuous coughing. When the irritation is severe, paroxysmal coughing often leads to gagging. The dog's attempt to bark, or the eat's to purr, may also elicit the characteristic dry cough. The cause of the disease determines the progression of the clinical signs; in other words, whether the cause is a viral or bacterial infection or a local irritation. The most common cause of acute laryngitis in dogs is infectious tracheobronchitis (kennel cough). This viral/bacterial disease causes inflammation of the laryngeal, tracheal, and, sometimes, the bronchial *Editor's note: The recent development of tidal-breathing flow-volume loop analysis and techniques (see the article in this issue entitled "Pulmonary Function Testing," ) has proved of assistance in the noninvasive assessment of upper airway obstructive disorders.

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mucosa.* The clinical signs are those of acute laryngitis and tracheitis. There is usually no fever or other signs of systemic illness with this mucosal disease. The signs can persist for 3 weeks or longer. Therapy consists of rest and avoidance of excitement. Pediatric cough syrups usually are very effective. A moist environment and additional intake of water diminish the irritation of the mucosa and hence the coughing. There is no indication for corticosteroids. If there is no fever, there is no indication for antibiotics. In cats, viral rhinotracheitis and calicivirus may affect the la~yngeal mucosa. This seldom results in a dry cough but rather in stridorous breathing caused by edema of the laryngeal mucosa. The dominant symptoms in cats are fever, salivation, conjunctivitis, and general distress. The treatment consists of antibiotic therapy, parenteral fluids, and symptomatic therapy. The laryngeal disease seldom leads to lifethreatening obstruction that would necessitate tracheostomy. Local irritation of the larynx in the dog can be caused by a day of continuous barking and panting and in both dogs and cats by intratracheal intubation during anesthesia. The voice is hoarse, and sometimes there is spontaneous coughing. Therapy consists of rest and avoiding exercise. Laryngeal irritation by inhalation of caustic gases can occur in both dogs and cats. Therapy for the laryngeal disease is symptomatic and usually secondary in importance to management of the more severe damage to the tracheobronchial tree and the lungs. Acute inflammation and severe edema can be caused by insect bites. There is rapidly increasing inspiratory and expiratory dyspnea and stridor. The progress is unpredictable and may be life-threatening. Administration of corticosteroids, preferably intravenously, is the first step. Preparations should be made for intubation and tracheostomy if the dyspnea worsens. Tracheostomy provides relief to the patient and should be considered at an early stage. Prolonged strenuous breathing can lead to the development of lung edema, after which an accumulation of complications can lead to death. With a tracheostomy tube in place and maintenance of homeostasis, the prognosis generally is good. The tube can be removed in 3 to 5 days after inspection of the larynx ensures that the airway is patent. A laryngeal abscess is a rare finding. It can be caused by a penetrating foreign body such as a needle, fish bone, or stick. The clinical signs are determined by the location and the size of the abscess and are usually characterized by a slowly progressing laryngeal stridor and dyspnea. The abscess is diagnosed by laryngoscopy, and it is treated by incision and drainage followed by a short period of antibiotic therapy. The prognosis generally is good. Chronic laryngeal inflammation is rather common in dogs. According to the clinical manifestations, a mild and a severe form can be distinguished. The mild form may exist for years, characterized by coughing on exertion, coughing when straining at the leash, and *Editor's note: The reader is referred to the article in this issue entitled " Bordetella and Mycoplasma Infections in Dogs and Cats" for additional information on infectious tracheobronchitis.

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gagging in association with coughing. There is no dysfunction other than a mild hoarseness. Laryngoscopy reveals reddened and thickened laryngeal mucosa. The vocal folds are also voluminous and red. The cause of these changes is not always apparent, but in some cases they may be attributed to ~ habit of frequent panting, frequent barking, constant straining against the collar, and comparable excesses. The therapy therefore should start with the use of a harness instead of a collar. When possible, changes should be made in the other contributing habits. Nothing is to be expected from medication. The prognosis is dubious in cases in which no radical change of habits is possible. Only in rare cases does the disease lead to a more severe form of laryngitis.* Severe laryngitis is characterized by impairment of vocalization, stridor on exertion, and frequent swallowing attempts. Palpation elicits coughing and dyspnea, and the larynx is hard and appears to be enlarged. Radiography reveals ossification of the cartilage to a greater degree than is common in older dogs. During laryngoscopy, the cartilage feels hard, and the epiglottis is often less flexible. The mucosa is red, more often thin than thickened, and, often edematous. The vocal folds often are hyperplastic. There is no therapy other than alternate-day treatment with corticosteroids, but this may suppress the signs, bring comfort to the dog, and prolong its life. When the inflammation is severe and affects the pharyngeal area as well, swallowing may be difficult, and regurgitation and emaciation can occur. · A different form of laryngitis that also has radical consequences is the form seen in dogs undergoing attack training. These dogs can develop laryngeal spasm during stress. The dyspnea is acute and severe and is accompanied by salivation, vomiting, and, sometimes, loss of consciousness. Recovery usually occurs within minutes. Laryngoscopy reveals mild laryngitis, often with voluminous vocal folds. In the case history, the stress is characteristically that of aggressive training for attacking humans. In other circumstances when no stress of this kind is involved, the dog has excellent staying power, but once these signs develop, they will always recur under the same stressful circumstances. No treatment has been successful; hence, complete and permanent, and cessation of the training is usually indicated.

TUMORS

Primary laryngeal tumors occur occasionally in dogs and cats. In dogs, leiomyoma, rhabdomyosarcoma (Fig. 2), and squamous cell carcinoma, among other types of tumors, have been reported.1- 4• 7• 8• 12 In cats, lymphosarcoma and squamous cell carcinoma are the usual tumors.12 Squamous cell carcinomas often invade the laryngeal tissues rapidly and are usually inoperable without removal of a large part of *Editor's note: Mild laryngitis may be encountered in clinical cases of chronic bronchitis as a secondary inflammatory process; that is, as a result of the chronic coughing itself.

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the larynx or total laryngeal resection. Lymphosarcoma also usually infiltrates the laryngeal structures and is felt to be as inoperable, as is squamous cell carcinoma. Rhabdomyosarcoma may be localized and develop over a longer period. This tumor seems to be less aggressive, and reports about regrowth after attempted removal are rare. Surgical removal is still possible when the tumor originates from one of the vocal folds. The removal of the tumor and the vocal fold can have a satisfactory long-term prognosis (Fig. 3). The ventral midline approach to the laryngeal lumen through the thyroid cartilage is the pr~ferred method.

Surgical Procedure in Ventral Midline Approach to the Vocal Folds

Figures 4 through 7 show the tumor, the surgical approach, and postoperative apperance of a laryngeal polyp. The dog is premedicated. After induction of anesthesia with an intravenous barbiturate, an endotracheal tube is inserted for the administration of anesthetic gases and oxygen. The dog is placed in dorsal recumbency and the skin of the ventral surface of the neck is prepared for tracheostomy and

Figure 2. Rhabdomyosarcoma originating from the right vocal fold in a dog.

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Figure 3. Postoperative view after removal of the tumor shown in Figure 2 via the ventral midline approach.

Figure 4. Fibroangioblastic tissue

(laryngeal polyp) attached to the right half of the glottis.

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Figure 5. Prior to resection, the fibroangioblastic mass shown in Figure 4 is exposed via a ventral midline incision in the thyroid cartilage. (From Venker-van Haagen AJ : A laryngeal polyp in a young dog. Companion Animal Practice, Japan 8:22, 1987; with permission.)

Figure 6. After the removal of the mass shown in Figures 4 and 5, the thyroid cartilage is closed with interrupted absorbable sutures. (From Venker-van Haagen AJ: A laryngeal polyp in a young dog. Companion Animal Practice, Japan 8:22, 1987; with permission.)

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Figure 7. Two months after the surgery shown in Figures 5 and 6, there was no regrowth of the laryngeal polyp. (From Venkervan Haagen AJ: A laryngeal polyp in a young dog. Companion Animal Practice, Japan 8:22, 1987; with permission.)

laryngeal surgery. The tracheostomy is made midway between the larynx and the thoracic inlet. An endotracheal tube is introduced through the incision to replace the previous intralaryngeal tube, which is removed. A skin incision on the ventral midline over the thyroid and cricoid cartilages is followed by freeing of the ventral part of the thyroid and cricoid cartilages and the cricothyroid muscles. With a pointed scalpel, the thyroid cartilage and the cricothyroid ligament are opened exactly in the midline, leaving the cranial 2 mm of the thyroid cartilage intact. Using a small retractor to separate the halves of the thyroid cartilage, an opening is provided for inspection and eventual surgical removal of the vocal fold and the tumor. After this procedure, the thyroid cartilage is closed with interrupted absorbable sutures placed superficially in the cartilage. The skin wound is closed routinely. The endotracheal tube in the tracheostoma is removed and replaced by a tracheal cannula, which is left in place for 3 to 5 days. Total laryngectomy for laryngeal tumors has been described in dogs3 but is not within the author's experience.

LARYNGEAL PARALYSIS

Laryngeal paralysis is a complete or partial loss of function of the larynx caused by neurologic, muscular, neuromuscular, or ankylotic

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(cricoarytenoid articulation) disease. The severity of the clinical signs (dyspnea and stridor) is predominantly correlated with the degree of abductor dysfunction. Laryngeal paralysis of neurologic origin can be complete or partial. The intrinsic laryngeal abductor and adductor muscles are innervated by the recurrent laryngeal nerves. Trauma in the region dorsolateral to the cervical trachea on one or both sides can cause unilateral or bilateral laryngeal paralysis. Unilateral laryngeal paralysis caused by interruption of one recurrent laryngeal nerve usually does not lead to clinicat signs because of the compensatory activity of the contralateral side (Figs. 810). Bilateral loss of innervation by the recurrent laryngeal nerves leads to insufficient abduction and adduction and thus dyspnea during exertion. Neurogenic diseases can result in severe distress; namely, adduction during inspiration and laryngeal spasm. Most commonly, laryngeal paralysis presents as a slowly progressive disease in middle-aged to older dogs. In these cases, there is progressive loss of innervation of the intrinsic laryngeal muscles, resulting in partial denervation. Electromyographic recordings are most useful to diagnose this type of disease because there are normal motor unit potentials together with denervation potentials (fibrillation and pseudomyotonia), which are usually in all muscles (Figs. 11-14). The disease may progress for a year or longer. It is characterized by

Figure 8. Oral view of a dog's larynx with left-sided laryngeal paralysis. No asymmetry is apparent.

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Figure 9. The larynx shown in Figure 8 in the first state of adduction. Note the action of the right-sided adduction.

Figure 10. The same larynx shown in Figures 8 and 9. The adduction is complete and the glottis is closed by the compensatory activity of the right side.

DISEASES OF THE LARYNX

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~ Figure 13. Electromyogram showing pseudomyotonia in the right ventricular muscle. Paper speed 5 em/sec, sweep speed 10 msec/div, amplitude 200 f.LV/div. (From Venker-van Haagen AJ, Hartman W, Goedegebuure SA: Spontaneous laryngeal paralysis in young Bouviers. J Am Anim Hosp Assoc 14:714, 1978; with permission.)

Figure 14. Electromyogram showing normal activity together with denervation in the left thyroarytenoid muscle. Paper speed 5 em/sec, sweep speed 10 msec/div, ampliiude 200 f.LV/div. (From Venker-van Haagen AJ, Hartman W, Goedegebuure SA: Spontaneous laryngeal paralysis in young Bouviers. J Am Anim Hosp Assoc 14:714, 1978; with permission.)

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increasing stridor and decreasing exercise tolerance until dyspnea becomes severe and life-threatening. Laryngeal spasm may be one of the signs. This form can occur in all types of dogs, but in the Netherlands, the Leonberger, the Labrador Retriever, and the Irish Setter are overrepresented. Bouvier des Flandres are also afflicted with a progressive laryngeal paralysis, but the clinical signs have an onset at the age of 5 to 8 months. 13 In these dogs, the laryngeal paralysis is a hereditary (autosomal dominant) disease, and the pathogenesis is based on loss of motoneuron cells in the nucleus ambiguus. 14• 15 : Laryngeal paralysis caused by a muscular disease is rare but has been demonstrated in Bull Terriers and dogs of various other breeds. Laryngeal paralysis is never the sole disorder in these cases but rather one aspect of a polymyositis or general muscular disease. Similarly, a neuromuscular disease, myasthenia gravis, may involve the intrinsic laryngeal muscles in some cases. Laryngeal paralysis caused by ankylosis of the cricoarytenoid articulations is rare. The diagnosis can be made when there is no abduction during inspiration even though electromyography reveals normal motor potentials during inspiration. Older dogs with a history of recurrent or continuous laryngitis are prone to develop this disorder. With the exception of muscular and neuromuscular laryngeal paralysis, which should be treated according to the diagnosis of the disease, the only way to restore the obstructed laryngeal passage is by surgical intervention. Lateral fixation of one vocal fold is the least traumatic method and usually results in a satisfactory result, especially when the larynx has a normal cartilaginous structure.

Surgical Technique

The surgical technique of lateral fixation of one vocal fold is preceded by a tracheostomy and the administration of anesthetic gases and oxygen via the tracheostomy tube, as described earlier in the ventral approach to the larynx. The dog is placed in dorsal recumbency, and a paramedian skin incision is made 1 em from the ventral midline. The larynx is approached in a dorsal direction by identifying the sternohyoid and sternothyroid muscles. The dorsal edge of the thyroid cartilage is then identified by palpation through the thyropharyngeal muscle. By lifting the dorsal edge of the thyroid cartilage with the index finger, the caudal part is identified and the thyropharyngeal muscle is opened over a distance of 1 to 2 em to expose the caudodorsal part of the dorsal edge of the thyroid cartilage. The caudodorsal part of the thyroid cartilage is attached to the cricoid cartilage by the cricothyroid articulation. This articulation is separated, giving a clear view of the intrinsic laryngeal muscles when the thyroid cartilage is lifted. Atrophy of the dorsal cricoarytenoid muscle, as usually occurs in laryngeal paralysis, facilitates the identification of the muscular process of the arytenoid cartilage. This muscular process is attached to

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the cricoid cartilage by the cricoarytenoid articulation. When this articulation is severed, the arytenoid cartilage, with the vocal fold, is further mobilized by separation from the cricoid cartilage and separation from the contralateral arytenoid cartilage. This in turn interrupts the ligaments and a cartilaginous fragment between the two arytenoid cartilages. The arytenoid cartilage and the attached vocal fold should be free to move laterally without pulling the opposite arytenoid cartilage and vocal fold. This should be checked by oral inspection. The muscular process is then attached to the most caudal point on the dorsal edge of the thyroid cartilage with a nonabsorbable (e.g., polyfilament stainless steel) suture. The second suture attaches the muscular process at a more cranial position on the same dorsal edge to give an extra lateral pull to the arytenoid cartilage and to prevent the arytenoid from pivoting around the first suture. The effect of the fixation should be confirmed by oral inspection. When a satisfactory laryngeal opening has been accomplished, the thyropharyngeal muscle and the subcutis and skin incision are closed routinely. The endotracheal tube is removed and replaced by a tracheal cannula, which is left in place for 5 days following surgery. Meticulous care of the cannula is required while it is in place. There have been reports of slight variations of this procedure, which have similarly satisfactory results. 6 • 10• 11 • 17 The permanently opened glottis results in a loss of voice, but in the author's experience, insufficient closure of the glottis rarely causes leakage of food and

Figure 15. Laryngeal webbing after bilateral vocal fold resection in a dog.

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water into the trachea. Coughing is enhanced by the open glottis, and the dog will produce more force in the effort of coughing. Laryngeal function is not restored, and exercise must be restricted on hot days. The surgery is performed mostly in dogs but has been reported to be applicable in the rare case of a cat with laryngeal paralysis. LARYNGEAL TRAUMA

Accidental trauma to the larynx may cause a life-threatening situation when hemorrhage and edema prevent normal airflow. Immediate intratracheal intubation under anesthesia, followed by tracheostomy, is the best approach in the management of these cases. The damage to the larynx is difficult to evaluate during the first few days after trauma and a tracheostomy carefully managed for several days following injury is usually the key to spontaneous recovery from the damage. Perforation of the mucosa, on the other hand (which usually results in subcutaneous emphysema), should be repaired surgically. Iatrogenic trauma includes complications after laryngeal surgery and damage to the laryngeal nerves. One of the most feared complications is laryngeal webbing: the formation of scar tissue that narrows the laryngeal air passage (Fig. 15). Webbing occurs when the mucosa of the glottic part of the larynx is interrupted. Stenosis that results from webbing is difficult to treat, because new scar tissue (a web) often forms when the initial web is removed. The prognosis in these situations is guarded at best.* In all types of laryngeal surgery, this complication should be prevented, for example, by avoiding bilateral interruption of the glottic mucosa at one time. References 1. Bright RM, Gorman NT, Goring RL, et a!: Laryngeal neoplasia in two dogs. J Am Vet Med Assoc 184:738, 1984 2. Calderwood Mays MB: Laryngeal oncocytoma in two dogs. J Am Vet Med Assoc 185:677, 1984 3. Crowe DT, Goodwin MA, Greene CE: Total laryngectomy for laryngeal mast cell tumor in a dog. J Am Anim Hosp Assoc 22:809, 1986 4. Flanders JA, Castelman W, Carberry CA, eta!: Laryngeal chondrosarcoma in a dog. JAm Vet Med Assoc 190:68, 1987 5. Harvey CE, Venker-van Haagen AJ: Surgical management of pharyngeal and laryngeal airway obstruction in the dog. Vet Clin North Am 5:515, 1975 6. LaHue TR: Treatment of laryngeal paralysis in dogs by unilateral cricoarytenoid laryngoplasty. J Am Anim Hosp Assoc 25:317, 1989 7. Lightfoot RM, Bedford PGC, Hayward AHS: Laryngeal leiomyoma in a dog. J Small Anim Pract 24:753, 1983 8. Ndikuwera J, Smith DA, Obwolo MJ: Malignant melanoma of the larynx in a dog. J Small Anim Pract 30:107, 1989 9. O'Brien JA, Harvey CE, Tucker JA: The larynx of the dog: Its normal radiographic anatomy. JAm Vet Radiol10:38, 1969 10. Payne JT, Martin RA, Rigg DL: Abductor muscle prosthesis for correction of laryngeal paralysis in 10 dogs and one cat. JAm Anim Hosp Assoc 26:599, 1990 *Editor's note: The use of a silicone intralaryngeal stent (Laryngeal Keel, Hood Laboratories, Pembroke, MA) has been used in the treatment of laryngeal webbing.

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11. Rosin E, Greenwood K: Bilateral arytenoid cartilage lateralization for laryngeal paralysis in the dog. JAm Vet Med Assoc 180:515, 1982 12. Saik JE, Toll SL, Diters RW, et a!: Canine and feline laryngeal neoplasia: A 10-year survey. JAm Anim Hosp Assoc 22:359, 1986 13. Venker-van Haagen AJ, Hartman W, Goedegebuure SA: Spontaneous laryngeal paralysis in young Bouviers. JAm Anim Hosp Assoc 14:714, 1978 14. Venker-van Haagen AJ: Investigations on the pathogenesis of laryngeal paralysis in the Bouvier [Thesis]. Utrecht, The Netherlands, University of Utrecht, 1980 15. Venker-van Haagen AJ, Bouw J, Hartman W: Hereditary transmission of laryngeal paralysis in Bouviers. JAm Anim Hosp Assoc 17:75, 1981 16. Venker-van Haagen AJ, Engelse EJJ, van den Ingh TSGAM: Congenital subglottic stenosis in a dog. JAm Anim Hosp Assoc 17:223, 1981 17. White RAS: Unilateral arytenoid lateralization: An assessment of technique and longterm results in 62 dogs with laryngeal paralysis. J Small Anim Pract 30:543, 1989 ~ddress reprint requests to Anjop J. Venker-van Haagen, DVM, PhD Department of Clinical Sciences of Companion Animals Faculty of Veterinary Medicine University of Utrecht P.O. Box 80.154 3508 TD Utrecht The Netherlands