Coughing is a non-specific response to inflammation or stretch of the airways. Normally, the mucociliary escalator, alveolar macrophages, and bronchus-associated lymphoid tissue are the most important protective mechanisms of the lower airways. The cough reflex comes into play when these responses have been overwhelmed by an increased volume of exudate or mucus, or by the presence of foreign material. The cough reflex may also be triggered by repeated local trauma or stretch, such as might occur in dogs with structural abnormalities such as collapsing trachea or compression of the left mainstem bronchus.
The cough reflex is triggered by local inflammation or compression of the airways, and controlled by cough centers in the brainstem. A cough begins as a maximal inspiration, followed by initial forced exhalation against a closed glottis. Sudden opening of the glottis results in rapid expulsion of air under considerable pressure, which assists in removal of debris, foreign material, and mucus from the respiratory tract. This is further assisted by simultaneous contraction of the bronchial smooth muscle, which narrows the airways, further increasing the force with which material is expelled.
Coughing may be defined as productive or non-productive. A productive cough occurs when material is expectorated from the trachea into the pharynx. In dogs and cats this material is usually swallowed, but it can occasionally be expectorated to the exterior. Clinically, a productive cough sounds moist and low-pitched, and the animal often swallows immediately afterwards. In contrast, non-productive coughing is usually harsh, high-pitched or even honking. Expectoration of mucus may occur occasionally, but is usually not a feature. Dogs with collapsing trachea usually have non-productive coughing, which can be paroxysmal, often triggered by excitement or exercise.
Collapsing Trachea
Tracheal collapse is a progressive degenerative disorder of the tracheal cartilage seen in predominantly small breed dogs. Although numerous breeds can be affected, it is very common in miniature poodles, Yorkshire terriers and Pomeranians. The disorder is associated with abnormalities of the cartilage of the tracheal rings, the cartilage is softer than normal because it is composed of deficient glycosaminoglycans and contains decreased numbers of chondrocytes. The normal ring-shaped cartilages become C-shaped and flattened. The dorsal tracheal membrane becomes stretched and floppy, and to varying degrees falls down and obstructs the tracheal lumen. The physical changes in the airway can be accompanied by variable inflammation of the tracheal mucosa, resulting in edema and increased mucous accumulation, which further exacerbates the clinical signs.
Tracheal collapse is categorized by its location and severity. Specifically, collapse can be extrathoracic, at the thoracic inlet, intrathoracic, or mainstem bronchus. The extent of collapse is also graded by the severity of airway obstruction: grade 1 collapse obstructs 25% of the airway, grade 2 obstructs 50% of the lumen, grade 3 obstructs 75% of the lumen, and grade 4 completely obstructs the tracheal lumen.
The history of patients with tracheal collapse usually includes coughing, which is often characterized by a loud, harsh, goose-honk quality. The coughing is often paroxysmal and precipitated by excitement or exercise. Dogs may have stable disease or may experience gradual progression of the airway disease over time. Acute exacerbations of clinical signs may be precipitated by excitement or by concurrent problems such as pneumonia.
Physical Examination of the Collapsing Trachea Patient
Observation
Most patients with mild to moderate collapsing trachea are normal at rest between paroxysms of coughing. In contrast, patients with severe airway obstruction often have increased respiratory effort at rest. They may have a considerable abdominal component to their respiration, with nasal flare and postural adaptation. The most severely affected patients have paradoxical respiration and signs of respiratory muscle fatigue.
Palpation
In normal dogs and cats, the trachea is cylindrical and the dorsal membrane can only be palpated with difficulty. Dogs with collapsing trachea may have obvious softening of the tracheal cartilage and airway deformity (rings become C-shaped). A brief, dry cough can be induced in most normal dogs and cats when the trachea is compressed. In contrast, paroxysms of honking coughing and wheezing may be precipitated in patients with tracheal collapse.
Auscultation
Auscultation is a vital part of the evaluation of any patient with a cough. Coughing can be an early sign of left-sided congestive heart failure, but the mere presence of a murmur is not enough to prompt a diagnosis of congestive heart failure. Many patients that are actually suffering from chronic bronchitis or collapsing trachea also have some degree of mild mitral endocardiosis, but are not actually in heart failure. Therapy for heart disease in such patients will not result in resolution of the cough, which instead should be treated with anti-tussives and bronchodilators. Some patients with mitral regurgitation may have significant enlargement of the left atrium. In this instance, compression of the left mainstem bronchus may result in coughing that is unrelated to heart failure. The most common finding in patients with collapsing trachea is increased upper airway sounds, which are loudest when the bell of the stethoscope is placed over the cervical trachea.
Diagnostic Testing
Every patient that has been coughing for more than 2 months deserves at least a basic workup to determine the best course of management. In particular, management of disorders such as collapsing trachea, chronic bronchitis, and congestive heart failure can be extremely frustrating. Before committing to life-long therapy for these chronic illnesses it is vital that a correct diagnosis is made, and that reversible or curable disorders are ruled out.
Blood Tests
Most of these patients deserve a basic clinical workup including a complete blood count, chemistry panel, urinalysis, and heartworm testing. The intent is to determine the presence of organic or systemic disease that may be contributing to chronic cough. If therapy with drugs such as corticosteroids, angiotensin-converting enzyme inhibitors, or digoxin is to be considered, then knowledge of liver and kidney function is vital.
Radiographs
Thoracic and cervical radiographs are vital. Dogs with chronic bronchitis or collapsing trachea usually have normal radiographs or a peribronchial pattern. Sometimes a collapsing trachea can be demonstrated by radiographs obtained during inspiration and during exhalation to document phasic changes in tracheal diameter, or by using flexed and extended neck views to document collapse during neck movement. Caution should be exerted in interpretation of these views, however. Some collapse of the trachea can be normal during neck hyperextension. Patients with chronic tracheal collapse or bronchitis usually do not have evidence of pulmonary alveolar disease. If there are any signs of alveolar disease, other disorders such as bronchopneumonia, neoplasia, or congestive heart failure should be considered. Bronchiectasis can be evident as a cylindrical dilation of bronchi as they extend to the periphery of the lung lobes, rather than their usual tapering. Masses may be evident in lung lobes or compressing the airways. Intraluminal masses, abscesses, parasitic nodules or foreign bodies may be outlined by the negative contrast of air in the major airways.
Fluoroscopy
Fluoroscopy is a very useful additional tool to confirm a diagnosis of collapsing trachea or mainstem bronchus because it provides a non-invasive, dynamic, real-time representation of the motion of the airways. Fluoroscopy is more useful than plain radiography, because it allows more specific documentation of the location and dynamic quality of tracheal collapse, particularly if the images are obtained while the patient is actively coughing. None of the plain radiographic and fluoroscopy studies distinguish whether the obstruction of the airway is occurring because of mucosal thickening or excess mucous accumulation, rather than because of anatomic changes in the cartilage or membrane. Similarly, they do not allow assessment of the presence of concurrent diseases such as laryngeal paralysis or chronic bronchitis.
Fecal Flotation for Parasites
Pulmonary migration of ascarids can cause chronic cough, especially in heavily infested puppies or kittens. In areas where lungworms are endemic, a Baermann preparation should be performed to detect the presence of lungworm larvae in the stool. Usually fecal samples are evaluated three days in a row, and if there is a strong suspicion of the presence of lungworms, anthelminthic therapy may be initiated even if the results are negative.
Tracheobronchoscopy
The final and most definitive method of diagnosis is by performing tracheobronchoscopy. Although anesthesia is somewhat risky in these patients, definitive diagnosis of the problem is a priority prior to performing invasive therapeutic procedures such as tracheal stent or extraluminal ring placement. Tracheobronchoscopy initially allows confirmation that laryngeal function is normal, ruling out concurrent laryngeal paralysis. Then tracheobronchoscopy allows imaging of the lumen of the trachea and grading of the severity and location of tracheal collapse. Finally, imaging the bronchi allows the clinician to determine the extent to which mainstem bronchus collapse or chronic bronchitis might be contributing to the clinical signs. Finally, tracheobronchoscopy samples to be obtained that can be submitted for cytology and culture.
Other
Left-sided congestive heart failure is a common cause of chronic coughing in dogs, and may require evaluation with echocardiography and electrocardiography. Some dogs or cats with chronic coughing may have mass lesions in the lung, which may require surgical exploration and resection. Some of these patients may also benefit from additional imaging modalities such as computed tomography or magnetic resonance of the thorax.
Medical Management
Anti-Tussives
Anti-tussive agents are one of the cornerstones of therapy; they are especially important when the cough is non-productive, and are often of considerable benefit when long term coughing is interfering with the patient's ability to exercise and even to sleep. In such cases, the continued airway irritation caused by coughing can lead to more coughing, and thus can perpetuate a vicious cycle, which can be temporarily broken by anti-tussive agents. The primary drugs effective as anti-tussives are centrally acting opiate derivatives, which act on the cough center of the brain to depress its response to cough stimuli. Hydrocodone bitartrate (1.25-5 mg PO up to QID) is effective and widely used. Its main side effect is mild sedation, which can, however, prove to be helpful in some patients. Other prescription drugs, such as butorphanol tartrate (0.05-0.1 mg/kg PO, BID-QID) are also effective, with less central nervous system depression. Non-prescription drugs such as dextromethorphan (1-2 mg/kg PO, TID-QID) are available in various human proprietary cough mixtures, and play a useful role in symptomatic treatment of chronic cough.
Bronchodilators
Two classes of bronchodilators are widely used: methylxanthine derivatives and beta 2 agonists. Methylxanthine derivatives such as aminophylline (4-5.5 mg/kg PO TID) are well absorbed from the gastrointestinal tract. Recent studies have shown that they also act at the level of the diaphragm to increase its contractility and to render it less susceptible to fatigue. Thus, these agents may also prove useful in cases of chronic respiratory tract disease for reasons other than bronchodilation. The beta 2 agonists such as terbutaline sulfate (1.25-5 mg BID-TID) and albuterol (50 mg/kg PO BID-TID) are also effective bronchodilators. In some cases, administration of aerosolized bronchodilators can be helpful.
Corticosteroids
Corticosteroids play an important role in therapy, but considering their negative side-effects, their use should be undertaken with caution. Anti-inflammatory doses of prednisone (0.5 mg/kg SID) can be beneficial. This dose can be effective in decreasing the inflammatory response, leading to reduction of secretions, and decreases in mucosal edema, airway thickening and bronchospasm. This results in clinical improvement in many patients, with decreased coughing and better exercise tolerance. Corticosteroids can also be administered by inhalation, thereby limiting their adverse systemic effects.
Surgical Management
Surgical management of collapsing trachea should be reserved for those animals in which medical management has failed. Usually, these will be patients with grade III or IV tracheal collapse. Patients should be carefully selected by performing a careful evaluation to ensure that tracheal collapse, rather than laryngeal paralysis or other lung disease, is the main cause of the dyspnea. Surgical options include placement of extraluminal tracheal rings or intraluminal stents.