Endoscopy of the Laryngeal Vault of the Dog and Cat

Laryngoscopy is the visual examination of laryngeal anatomy and movement for the purpose of evaluating laryngeal function. The most common symptom of laryngeal disease in dogs and cats is upper airway noisy breathing (stridor), although many symptoms may result from disorders of laryngeal structure or function including exercise intolerance, respiratory distress, increased inspiratory effort, prolonged inspiratory time, cyanosis, voice change, loss of bark/purr, and coughing after eating or drinking. Laryngoscopy should also be performed before any bronchoscopic examination.

Instrumentation

Laryngoscopy is a technically easy procedure. Any combination of instruments can be used as long as adequate illumination of the larynx is generated, and the epiglottis and dorsal pharyngeal wall are kept from the visual field. The veterinarian can generally get a good view of the larynx using an overhead examination light, a penlight, and a laryngoscope. The value of an endoscope is that the light is more focused, the surrounding structures (tongue, epiglottis, and soft palate) are minimally displaced, and the veterinarian can obtain excellent pictures or movies of the larynx for later examination (e.g., with clients and students). A standard laryngoscope blade or a common wooden tongue depressor is most often used to depress the epiglottis and improve the view of the ventral portion of the larynx. The real key to successful and safe laryngoscopy is to know (1) what normal anatomy and normal anatomic variations are expected and (2) common and predictable complications of the procedure.

Patient Preparation

Laryngoscopy is most easily performed when the patient is lightly anesthetized. Food should be withdrawn for 6 to 12 hours to minimize the risk of aspiration during the examination. Preanesthesia laboratory tests are performed as indicated by the patient’s age, health status, and the good sense of the laryngoscopist. Laryngoscopy can be performed as an emergency diagnostic tool before surgical correction; however, the best results are, of course, obtained when the procedure is performed in a stable patient.

Patient Restraint and Positioning

Laryngoscopy can be performed in a matter of minutes. That being said, the patient should nevertheless be examined under a light plane of general anesthesia so that the procedure can be performed in a thorough manner and in a stress-free environment (for the laryngoscopist!). The patient’s plane of anesthesia should be deep enough to allow the oral cavity to be held open without fear that the endoscopist may be bitten, but light enough to retain some degree of normal laryngeal function. An animal that has been properly anesthetized for laryngoscopy still displays a strong gag reflex subsequent to pharyngeal or laryngeal stimulation and will also demonstrate a rapid withdrawal reflex to deep pain. If the patient’s level of anesthesia allows simple gag-free intubation, normal laryngeal motion may be altered because of the anesthesia; this is not ideal, but laryngoscopy should still be performed (see later discussion on the use of doxapram).

Injectable anesthetics are most typically used for laryngoscopy. My preference is propofol used to effect. The dose of propofol needed to generate a proper plane of anesthesia commonly ranges from 1 to 4 mg/kg IV. The drug should be given slowly, over 30 seconds to 2 minutes, to effect. Hypoxemia is absolutely predictable, and every patient should receive supplemental oxygen by mask before and immediately after the procedure. It is also appropriate to intermittently stop laryngoscopy to supplement the patient with oxygen, as needed. Atropine or glycopyrrolate should be used as a premedication to prevent bradyarrhythmias that are sometimes induced by laryngeal manipulation and subsequent vagal stimulation. Topical 1% lidocaine may occasionally be needed to decrease reflex responses (e.g., laryngospasm, swallowing, or gagging) that may be encountered during the procedure. This is much more commonly required in the feline species. Laryngoscopy should be performed with the patient in a sternal recumbent position. A mouth gag should be used to facilitate safe oral examination and to ensure good visualization of the larynx.

Procedure

A thorough laryngoscopic evaluation includes a complete anatomic examination and an assessment of normal laryngeal function and motion. To begin, care should be taken to gently depress the epiglottis from the visual field and gently elevate the soft palate, as needed. Forceful maneuvers can inappropriately fix the laryngeal cartilages in place and artifactually distort otherwise normal laryngeal motion.

Evaluation of Laryngeal Structure

Normal structures that should be evaluated during laryngoscopy include the cricoid, thyroid, and arytenoid cartilages (also the corniculate and cuneiform processes), vestibular folds, vocal folds (cords), laryngeal saccules (lateral ventricles), epiglottis, and aryepiglottic folds (Figure 10-1). Normal mucosa should be pink in color, and superficial vessels may be visible. Secretions are minimal in the area of the normal laryngeal vault. Mucosal hyperemia and edema, excessive secretions, and redundant pharyngeal mucosa are commonly encountered abnormalities. If the lumen of the glottis is smaller than normal, the resulting airflow is turbulent rather than laminar. Turbulent airflow is irritating to mucosa, and this commonly causes at least some of the mucosal erythema and edema that is encountered during laryngoscopy.

Figure 10-1 Normal laryngeal anatomy.

Common structural abnormalities include everted saccules and laryngeal collapse. These abnormalities usually develop as secondary phenomena as a result of more chronic disease and increased negative inspiratory pressures resulting from unilateral or bilateral laryngeal paresis or paralysis. Laryngeal webbing and granuloma formation both typically are secondary to previous surgery or trauma. Both conditions result in stenosis of the glottic lumen, and these fixed lesions often cause severe signs of airway obstruction. Less commonly a pharyngeal or laryngeal rannula, tumor, or foreign body may be found. (Laryngeal abnormalities are depicted in Figures 10-3 through 10-17 in the “Atlas” section). The physical characteristics of secretions or blood should be noted, and the examiner should attempt to determine the anatomic source of the abnormal fluid accumulation. The examiner should recall that secretions (including blood) found in the region of the larynx might have originated in the trachea, lower airways, or lung parenchyma.

Ancillary Tools and Procedures

Radiographic evaluation of the larynx does not add significant diagnostic or prognostic information and is not routinely advocated as part of the complete evaluation of laryngeal disorders. A number of investigators have demonstrated that ultrasound of the larynx may be used to diagnose abnormal laryngeal structure or function. However, the practitioner will still need to directly visualize the larynx to make a proper diagnosis. Tidal breathing flow-volume loops (TBFVLs) have been used to document the presence of upper airway obstruction (Figure 10-2). However, an abnormal TBFVL still requires laryngoscopy to confirm the presence and extent of structural and functional laryngeal disorders. A laryngeal electromyogram or biopsy procedure may be used to delineate a specific cause, but these tests are properly performed after the initial laryngoscopic evaluation has been made.

Figure 10-2 A tidal breathing flow-volume loop (TBFVL) from a working-breed dog presented for decreased exercise ability during hunting. A, Note the loop shape in a normal dog. B, The loop shape on inspiration (arrow) is flattened.

(From Amis TC, et al: Upper airway obstruction in canine laryngeal paralysis, Am J Vet Res 47:1008, 1986).

Chest radiographs are usually indicated before laryngoscopy for a few reasons. First, laryngeal disease is commonly a disease of older patients. If surgery to correct abnormalities of the larynx is being considered, radiographic evaluation of the thorax will be valuable in the decision to operate or not. Second, patients with laryngeal disease may be forcefully inspiring against a somewhat closed glottic opening. Interpretation of the lung pattern seen in these patients will be complicated by underinflation and some degree of interstitial bleeding caused by negative inspiratory pressure–induced interstitial capillary rupture. These findings will be valuable in determining the indications for and value and timing of the laryngoscopic procedure.

Complications

A properly performed laryngoscopic examination is a low-risk procedure. However, the clinician should be aware that emergencies might occur during and immediately after laryngoscopy. In a small number of patients, stimulation of the larynx may provoke a very strong vagal response that may result in profound bradyarrhythmias. Administering anticholinergic medication before anesthesia is induced largely prevents this. Laryngeal mucosal hyperemia and edema are commonly encountered in a variety of laryngeal disorders. The manipulation that occurs during laryngoscopy may cause additional mucosal irritation, edema, and, potentially, further narrowing of the airway (a complication that is more common in cats). This is the most important reason why the laryngeal structures should be treated with great respect and should be physically manipulated only as much as is absolutely needed to complete the examination. In addition, some patients with laryngeal disease and upper airway obstruction may have great difficulty maintaining a patent airway during the immediate postanesthetic recovery phase. Patients with upper airway obstruction have adapted a breathing strategy that minimizes discomfort. However, in the immediate postanesthetic recovery phase, a semiaware patient may begin to breathe with increased effort, and this lowers the intraglottic pressures. Lowered intraglottic pressure increases medial movement of the laryngeal cartilages (adduction) and worsens the airway obstruction. A vicious cycle develops in which increased airway obstruction causes panic, increased effort to breathe, and further worsening of airway obstruction. Therefore endotracheal and tracheostomy tubes and instruments should be available (in suspected cases). If a surgically correctable disorder of the larynx is suspected (e.g., laryngeal paralysis), the laryngoscopic examination should be scheduled so that surgical correction can immediately follow the examination, thereby avoiding a second anesthetic procedure.

Differences Between Feline and Canine Laryngoscopy

The laryngeal vault of dogs and cats are anatomically, grossly different. Specifically, the corniculate and cuneiform processes of the arytenoid cartilages are well developed and are much more prominent in dogs than in cats. Furthermore, mucosa lining the feline laryngeal vault becomes edematous after manipulations that cause no edema in canine patients. This edematous response may be dramatic and can result in serious airway obstruction (see Figure 10-5 in the “Atlas” section). The feline larynx may respond to light touch with spasm, further occluding the upper airway. For these reasons it is critical to place a few drops of 1% lidocaine on the surface of the feline laryngeal structures before laryngoscopy is performed. It is also important for the operator to be very gentle when manipulating this area.

Atlas for Laryngoscopy Pages 334-338

Laryngeal Edema

Laryngeal Foreign Bodies

Laryngeal Sacculitis

Laryngeal Collapse

Nonneoplastic Growths

Laryngeal Neoplasia

Figure 10-3 A 12-year-old female spayed domestic short hair was evaluated for change in the quality of the “meow.” A, Profound laryngeal edema was found; however, no cause was determined. The patient’s signs improved but were not completely resolved after treatment with oral and inhaled corticosteroids. B, Normal feline larynx for comparison.

Figure 10-4 A 2-year-old spayed female Siamese cat was seen 3 weeks after smoke inhalation. The majority of signs were resolved; however, the cat continued to have noisy breathing. Edema is present at the ventral margin of the laryngeal vault (arrows).

Figure 10-5 A, Severe laryngeal edema and obstruction that developed within 5 minutes of a traumatic attempt at intubation in a 4-year-old domestic short hair cat. B, Normal feline larynx for comparison.

Figure 10-6 A 4-year-old spayed female Springer Spaniel was seen for acute signs of gagging. A string (yellow structure) was found circumferentially around the dorsal and lateral margins of the larynx. The owner later determined that the string was from a recent pot roast served for dinner the previous evening.

Figure 10-7 A 6-year-old neutered male Bijon Friese was presented for noisy breathing. The saccules are everted and edematous and obstruct more than 75% of the glottic lumen. Surgical removal of the saccules was successful; however, the laryngeal cartilages and associated vocal folds were then found to adduct on inspiration (laryngeal collapse). This was likely the cause of the saccular eversion.

Figure 10-8 Saccular edema and eversion in a cat.

Figure 10-9 A 5-year-old intact male German shorthaired pointer was presented for exercise intolerance. The arytenoid cartilages and vocal folds adducted bilaterally on inspiration.

Figure 10-10 Partial laryngeal collapse in a 12-year-old Labrador retriever with a 6-month history of noisy breathing. Only partial bilateral abduction of the arytenoid cartilages occurs during maximal inspiration. The left vocal fold is visible in the lumen of the normally clear laryngeal opening. Note also hyperemia, excessive secretions, and edema distributed throughout the laryngeal vault.

Figure 10-11 A 10-year-old neutered male Labrador retriever was presented for exercise intolerance and loss of bark. A, The vocal folds adducted on inspiration and the arytenoid cartilages were fixed in place during inspiration and expiration during tidal breathing. B, All structures adducted during doxapram stimulation causing complete laryngeal collapse. The patient was sedated and placed in oxygen and recovered uneventfully from the procedure.

Figure 10-12 Vocal fold granuloma in a 6-year-old Boxer. The dog’s owners had noticed a gradual change in the quality of the animal’s bark.

Figure 10-13 Laryngeal granuloma in a dog presented for noisy breathing 4 months after a “de-barking” surgical procedure.

Figure 10-14 Vocal fold tear in an 8-year-old Yorkshire terrier presented for chronic intermittent coughing, gagging, and change in quality of bark. Resection of the tear resulted in resolution of symptoms. The cause of the tear was not determined.

Figure 10-15 A 6-year-old spayed female domestic short hair with a 2-month history of noisy breathing and raspy quality to the “meow.” The mass (arrow) was growing lateral to the left arytenoid laryngeal cartilage, forcing the cartilage medially. The diagnosis was undifferentiated carcinoma.

Figure 10-16 A 2-year-old neutered male domestic short hair was evaluated for gagging, choking, and weight loss. The patient was referred from a shelter. The mass completely distorts the normal laryngeal architecture and profoundly narrows the glottic opening. The patient’s results were positive for feline leukemia virus, and the histologic diagnosis was lymphoma.

Figure 10-17 A 14-year-old spayed female mixed breed hunting dog was seen for severe exercise intolerance and absence of bark. A mass was found that obstructed more than 75% of the glottic lumen. The histologic diagnosis was squamous cell carcinoma.

Figure 10-18 Thyroid carcinoma causing significant airway obstruction and exercise limitation in an 8-year-old terrier.

Atlas for Laryngeal Edema

Atlas for Foreign Bodies

Atlas for Laryngeal Sacculitis

Atlas for Laryngeal Collapse

Atlas for Nonneoplastic Growths

Atlas for Laryngeal Neoplasia

Tracheobronchoscopy of the Dog and Cat

In 1904 Dr. Chevalier Jackson developed the first rigid bronchoscope; however, it took 64 more years (1968) before fiberoptic bronchoscopy became part of clinical pulmonary medical practice in human medicine. Over the past 20 years, respiratory endoscopy has also become an important diagnostic technique in the practice of high-quality veterinary medicine. Before the development of the bronchoscope, veterinary patients with clinical signs referable to the respiratory system were evaluated with the techniques of chest auscultation, thoracic radiography, and transtracheal washing. Bronchoscopy advanced our ability to examine these patients; with a bronchoscope we can better understand the nature, vascularity, extent, and distribution of many pathologic changes in airway wall structure and function. A microbiology brush can be easily passed into areas of focal infection without contamination of “healthy” sites, which avoids a common complication that can occur during unguided transtracheal or bronchopulmonary washing through an endotracheal tube. Exfoliated cells from primary or metastatic neoplastic lesions can be identified with the technique of bronchoalveolar lavage, which has made it possible to stage and treat cancer more effectively. Left atrial compression of a mainstem bronchus can be identified, which can help confirm the presence of cardiomegaly as a cause of coughing and can help distinguish cardiac- and respiratory-related disorders. The general experience of most clinicians who use bronchoscopy in their practices of veterinary medicine and surgery can be summarized in one line: “How did we ever get along without it?”

Indications

The two most frequent indications for performing bronchoscopy in veterinary patients are (1) acute or chronic cough that is unanticipated or unresponsive to standard medical therapy and (2) unexplained radiographic infiltrates (Box 10-1). Bronchoscopy can also be invaluable in distinguishing cardiac from respiratory causes of a cough and for staging metastatic lung cancer. Tracheobronchoscopy should also be considered in veterinary patients with noisy breathing or stridor for which laryngoscopy fails to confirm the cause.

Bronchoscopy is particularly valuable in collecting samples for microbiological, cytologic, and histologic analysis. Tracheobronchial biopsy is primarily performed to obtain a histologic diagnosis for abnormal endobronchial growths or parenchymal infiltrates. Microbiological sampling of infected areas within the respiratory tree is greatly assisted by bronchoscopic guidance because the sampling brush can be directed into specific areas of interest. This technique offers the best chance of retrieving diagnostic samples and minimizes the risk of cross-contamination of less involved sites, as can occur when nonspecific washing techniques are used. Bronchoalveolar lavage is a technique that can be used to assess the morphology and distribution of cells that line individual lung segments, and the retrieved samples can be processed to include total and differential counts of the cells obtained.

Bronchoscopy itself has no specific contraindications. However, the procedure should not be performed in patients that are not candidates for general anesthesia. Importantly, all standard bronchoscopes are larger in diameter than standard endotracheal tubes used for cats; therefore bronchoscopy in the feline patient must be done with great speed and skill to avoid predictable hypoxemia and respiratory compromise.