The upper airway encompasses all of the respiratory anatomy outside the pleural space. In practical terms, the upper airway refers to the nose, pharynx, larynx and extra thoracic trachea. Nasal and palatal issues have been covered in a prior lecture. The current lecture will consider disorders of the posterior pharynx including the adenoids, the larynx and the trachea.
The most important part of the physical examination of the respiratory system is inspection, to determine whether the respiratory difficulty can be identified primarily during inspiration or during expiration. If this isn't obvious during the exam, more subtle clues in evaluating abnormal breathing patterns include inspection of the nares for "flaring", or abduction of the legs to increase the size of the thoracic cage.
Cats and dogs with disorders within the pharynx, larynx or trachea will exhibit reverse sneeze, noisy breathing or difficulty breathing primarily during the inspiratory phase of breathing. Experienced clinicians know that the patient with severe stridor is not a candidate for a leisurely, prolonged physical examination. However, for most patients, a comprehensive examination should be performed during which all organ systems are thoroughly evaluated.
Palpation of the larynx may reveal significant distortion, suggesting a malignant process. Similarly, palpation of the trachea may be valuable to elicit cough. The quality of the cough can help determine the cause of the cause. For example, the cough due to tracheal narrowing is usually high pitched and honking, while the cough due to pneumonia is soft and wet. It is important to recognize that any cause of cough will sensitize the trachea. Therefore, the fact that you can elicit cough by palpation of the trachea does not imply tracheal disease!
Auscultation is a classic method of examining the patient with signs of respiratory disease. In general, I don't rely upon information gained by chest auscultation to make diagnoses of respiratory disease in small animals. Physicians always ask their human patients to breathe deeply with an open mouth, and this increases the quality and quantity of respiratory sounds. We can't do that successfully with our patients, and the amount of air moving in and out of a resting animal is often not enough to produce audible sounds of diagnostic significance. In the specific case of laryngeal disease, you may auscult harsh sounds over the larynx especially during inspiration. We refer to these sounds as stridorous. Additionally, in some cases of tracheal narrowing you may hear increased whistling sounds in the area of the thoracic inlet.
Signs of pharyngeal, laryngeal and tracheal disease include noisy breathing, reverse sneeze, change in bark, exercise intolerance and coughing. Diagnostic techniques which are frequently used alone, or in conjunction with other tests to determine the cause(s) of these signs include, radiographic imaging and pharyngoscopy, laryngoscopy or tracheoscopy. I emphasize that in many cases a strong presumptive diagnosis can be made by the history, signalment, clinical signs and plain film radiography.
Plain film imaging is generally not useful to diagnose problems within the laryngeal vault. Tracheal disorders are of course more easily defined by radiography. In general, tracheal collapse can be identified with a single lateral view of the neck. It is important to consider that during imaging of the trachea using the standard right lateral approach, the esophagus often lies directly on top of the trachea. This may cause a shadow artifact thru the lumen of the trachea that mimics the appearance of tracheal collapse. If you can see the dorsal tracheal membrane dorsal to what appears to be a narrowed tracheal lumen, the esophageal shadow artifact may be occurring. If you are in doubt, reexamine the tracheal lumen with the patient in left lateral recumbency. In this position the esophagus often lies lateral to the trachea and the artifact is removed.
Pharyngoscopy Laryngoscopy and Tracheoscopy
Patients with chronic reverse sneeze are excellent candidates for pharyngoscopy to determine the cause of this clinical sign. Adenoiditis is a common cause of reverse sneeze in canine patients. Interestingly, this anatomic region is rarely discussed in veterinary medicine. Adenoid tissue lies within the wall of the caudal nasopharynx. To visualize the adenoids you need a flexible endoscope. Adenoiditis is seen as a thickening and reddening of the adenoid tissue, often with small (1-3mm) polypoid tissue formation.
Small animals with stridor or voice change should have laryngoscopy performed to determine if laryngeal dysfunction, including laryngeal neoplasia is present. To perform laryngoscopy, the animal should be lightly anesthetized. A laryngoscope blade or tongue depressor can be used to depress the epiglottis, and a pen light can be used to visualize the area. A diagnosis of laryngeal paresis or paralysis is reasonable if either or both laryngeal folds and/or associated arytenoids cartilages fail to abduct during normal breathing. It is important not to depress the epiglottis too forcefully, or tilt the head up at an acute angle, because normal laryngeal architecture can be artificially altered, resulting in an inappropriate diagnosis of laryngeal disease. When in doubt, it is appropriate to use Dopram, 1mg/kg i.v. to induce increased rate and depth of respiration. In this way you may avoid the confounding issue of anesthesia-induced respiratory depression that might reduce the normal movement of the vocal folds and associated cartilage.
General Comments: bacteria may be found in material obtained from the tracheobronchial tree of most healthy dogs and cats. This apparent paradox is easily explained, the tracheobronchial tree is not routinely sterile.
As in almost all situations in clinical medicine, the culture results obtained from any of the methods described below should be interpreted in light of the patient's clinical history, physical signs, and other diagnostic test results.
Trans tracheal wash (TTW) is a time honored method of obtaining uncontaminated material from the airway for culture (to bypass the oropharyngeal flora). This technique was first described for use in conscious human patients, and is a suitable technique for medium to large sized dogs. Better methods than TTW exist for retrieval of airway material for culture (see below) and the author does not advocate the TTW technique in cats. If alternative methods are not feasible, TTW can be safely performed in small animals by placing a long dwelling 21 g catheter through the cricothyroid membrane to the level of the thoracic inlet, followed by injection of 0.5 cc/kg body weight of non-bacteriostatic saline (previously warmed to 37°C). Placement of the animals head and thorax in a dependent position makes it easier to retrieve the instilled fluid. Alternatively, a sterile endotracheal tube may be passed and used as the conduit for the flush solution.
Use of the guarded microbiology brush (Microvasive, Milford Mass) is an alternative to the TTW that is a reliable method of retrieving airway material for culture. The brush is designed to be passed through the biopsy port (2 mm diameter) of a previously positioned adult (5.0 mm outer diameter) bronchoscope. If a bronchoscope is not available the brush may be passed through a sterile endotracheal tube that has been previously placed in the anesthetized patient. The distance from the mouth to the thoracic inlet should be measured. The brush can then be passed this distance, or until any resistance is felt. The inside brush can then be extruded, gently massaged within the airways, and re-sheathed. Finally, the brush can be withdrawn from the bronchoscope and processed by cutting off the end of the brush into a sterile red top tube, with 0.25-0.5 ml non-bacteriostatic sterile water to prevent the brush from drying out.
Secretions obtained using this brush may be cultured routinely or in a quantitative fashion. Quantitative bacterial cultures have been used to distinguish colonization from infection in human beings with pneumonia. In these cases, bacterial growth at a concentration of less than 104 CFU/ml is believed to represent non pathologic colonization and antibiotic therapy is not recommended. Healthy small animals may harbor an aerobic bacterial population within their main stem bronchi at a concentration of as high as 1x103 CFU/ml.
General comments: Cytological evaluation of respiratory secretions obtained from small animals with signs of tracheal disease is most helpful to confirm suspected infectious organisms and exfoliated neoplastic cells. In practice, I rarely perform TTW to collect cytology samples from patients with suspected pure tracheal disease.
Tracheoscopy is indicated for animals with suspected tracheal disease when the symptoms of chronic cough are not responsive to standard treatment. This technique is valuable to visualize masses within the trachea and to assess the structural integrity of the tracheobronchial tree. Additionally, abnormal mucus secretion, collapsing airways and mucosal appearance can be seen. Tracheoscopy can be performed with either a rigid or a flexible fiberoptic bronchoscope (FB). The rigid bronchoscope is usually less costly to purchase than a FB and requires little formal training to use. Additionally, the rigid scope is easily passed through an endotracheal tube adapter (Bodai Swivel "Y", Sontek Medical, Dallas TX.) so that gas anesthesia and oxygen may be administered during the procedure. In general, rigid scopes deliver a better visual image than FB's, although the image seen through a FB is certainly adequate for any diagnostic study. In practice, FB's are much more clinically useful. The adult sized FB (5 mm outer diameter) has a 2.0 mm channel that is wide enough to allow passage of biopsy, retrieval and culture instruments. It is very important to recognize that these FB's occlude > 50% of the airway of small animals, these studies should be performed only by persons very familiar with their use. Pediatric sized FB (3.5 mm outer diameter) can be passed through a 4.5 FR endotracheal tube and cause less airway obstruction. Because these FB's are smaller the operator can visualize smaller airway branches. The disadvantage of the pediatric FB is the smaller biopsy channel (1.3 mm) which permits only lavage.
The limitation of this procedure is the safety of the patient. These patients have often adopted a specific breathing strategy to minimize the dynamic airway collapse that occurs in the upper airway during forceful breathing during inhalation. During recovery from anesthesia they may breathe with greater force and generate significant negative inspiratory pressures. This can cause a serious breathing disorder that can be life threatening. Therefore, tracheoscopy should be performed in these patients only by veterinarians that are well trained in this procedure.
Bacterial infection of adenoid tissue is a common finding in young children with obstructed upper airway breathing and chronic tonsillitis. In veterinary medicine, adenoiditis is a common cause of reverse sneeze, and is most commonly allergic in origin.
Rhinoscopy is diagnostic for the condition. Biopsy of allergic adenoid tissue reveals hyperplastic lymphoid tissue with invasion of plasma cells. This condition responds well to antihistamine therapy. When antihistamines are not effective, short courses of systemic corticosteroids will usually alleviate the clinical signs. In chronic cases, inhaled steroids are very effective to control clinical signs.
There are no good medical treatments for this disease. When laryngeal paresis or paralysis is discovered, the patient should be directed to surgery. This recommendation is of course tempered by the age of the patient and co-existing disorders that may alone or in concert suggest a short life span post diagnosis. In my experience, even mild unilateral laryngeal paresis can develop into bilateral paralysis and laryngeal collapse within a 3-6 month period of time. The problem with waiting to do surgery is that the longer the wait, the greater the progression of disease and the decreased chance of a good long term surgical outcome.
Multiple surgical approaches have been used to correct laryngeal dysfunction. The data is pretty clear that unilateral tie-back is the most effective surgical approach, with the least post operative complications. Having said that, aspiration pneumonia will occur at some point after surgery (perhaps one day, one month or one year) in 15-30% of patients undergoing this procedure. Peri-operative and post operative treatment with metoclopramide or Cerenia may lessen the immediate threat of this complication. Additionally, because increased airflow through the glottic opening is proportional to the increase in the radius of the glottic opening, small changes in the size of the glottic opening produce large increases in the amount of air that flows through that opening. For this reason, some surgeons are now considering incomplete rather than complete tie-back procedures. The clinical outcomes are equivalent, and the post operative chances of aspiration are reduced. It is worth talking to your surgeon about this at the time of referral for your patient.
This is the most common disorder of the trachea seen in clinical practice. This is a structural abnormality of the cartilaginous matrix and is not due to bronchoconstriction. There is a theoretical argument that can be made regarding use of bronchodilators for small airway constriction. However, the airways of canine patients are enormous relative to body size when compared with humans, and bronchoconstriction even in dogs with primary chronic bronchitis is not a significant clinical problem.
In contrast, dogs with tracheal collapse and chronic cough often have mucosal erosions within the trachea itself. These erosions stimulate the cough receptors and eventually there develops a cough-causes-erosions-cause cough cycle. Most patients with tracheal collapse and cough can be managed successfully with variable doses of Hydrocodone. The only significant side effect of long term use of hydrocodone is constipation. This side effect can be effectively treated with fiber supplements or stool softeners as needed. The doses of hydrocodone used in this disease are often 5 times the doses listed in pharmacology texts, and will be discussed in more detail during the lecture.
Patients for whom medical management is not successful may be candidates for tracheal stenting. This aspect of the lecture will be covered by another author.