Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
A cough is considered a defense mechanism to clear the (predominantly upper) airways. A cough clears the larynx, trachea, and large bronchi of secretions, infectious organisms, and foreign particles. The process of coughing is quite complex as are the pathways involved in the initiation of a cough. A cough occurs in 3 phases: an inspiratory phase, a forced expiration occurring against a closed glottis, and the opening of the glottis with rapid expiration which creates the audible sound of a cough.1 The afferent pathway of coughing is initiated by activation of “cough receptors” which are a somewhat controversial entity that exist in primarily the larynx, trachea, and large bronchi.1-4 It is generally considered that more “cough receptors” are present in higher airways like the trachea and larynx.
Along the efferent path of cough receptors, an afferent receptor becomes stimulated travelling through various fibers to the vagus nerve to cough centers in the medulla and cerebral cortex which then stimulate various efferent motor nerves to affect the cough. A mechanosensory receptor that responds to mechanical presence of aspiration (particulates or the physical presence of gastric content), post-nasal drip, or a pharyngeal bolus is reliably experimentally induced and is likely associated with rapidly adapting receptors (RARs) and travel through myelinated Aδ fibers. A chemoreceptor that responds to infectious or non-infectious inflammatory or irritant-induced lung injury can be induced by inhalation of capsaicin, an irritant. This receptor is likely associated with unmyelinated C-fibers.2,3,5
A chronic cough (existing for >8 weeks) is considered to have a lower threshold for afferent activation of a cough. Therefore, the stimulus for a cough is less than that in a non-chronically affected animal.1,2 This hyperresponsiveness and sensitization of the upper airway and bronchi that is a hallmark of a chronic cough is often due to asthma, rhinitis, and/or gastroesophageal reflux in people.5-7 Very few studies have investigated the causes of dogs presenting with a clinical history of coughing.8
The upper airway has an overabundance of afferent cough receptors. Coughing, though important for protecting and clearing airways, can become excessive with deleterious effects that perpetuate inflammation.4 Also, given that the cough reflex can initiate from multiple anatomical sites, a variety of underlying causes could instigate a chronic cough. Because of this, chronic coughing is often considered a syndrome rather than a diagnosis9 with a multifactorial approach needed to determine underlying cause.
Imaging Steps for Approaching the Coughing Dog
The foundation for the approach to a coughing dog should be based on the anatomical sites for receptors involved in the afferent pathway of cough initiation. Clinical history such as frequency of cough, environmental exposures, and activity during coughing episodes may assist with differentiation of potential causes of a cough. In people, using an “anatomic diagnostic protocol” results in diagnosis of cough in >90% of cases; some steps in the protocol involve imaging, while history, clinical signs, and other testing are important to incorporate into the work-up.10
Differential Diagnoses for the Coughing Dog
Differential diagnoses for cough are numerous, many of which are thoracic diseases; however, upper respiratory disease is also a possibility. The most common causes for 115 dogs presenting for cough were large airway disease (chronic bronchitis, 77%; and tracheobronchomalacia, 51%; many dogs fell into both categories).8 Other pleural, pulmonary, mediastinal, and cardiovascular diseases can cause a chronic cough, as well, so evaluation must be aimed at ruling out causes as much as definitively diagnosing disease. Given that coughing is most commonly associated with airway related disease, this will be the focus of imaging descriptions discussed below.
Primary Bronchial Disease
The primary radiographic finding identified with chronic bronchitis is thickening and irregularity of bronchial walls and increased visibility of bronchial markings, especially in the lung periphery where airways are typically not identifiable. Irregular and thickened bronchial walls create the so-called tram tracks and rings within the lungs. The finding of a bronchial pattern implies airway related disease that is either noninfectious inflammatory (allergy or irritant environmental exposure) or infectious (secondary to viral infection or bacterial). Rarely neoplasia, such as bronchoalveolar carcinoma, and cardiogenic edema can cause a bronchial pattern.
When the margins of the bronchial walls are indistinct and hazy, a more active disease or disease that extends or originates in peribronchial interstitium can be assumed. With chronicity, the bronchial walls tend to be better defined, while being irregularly marginated. Most often, when a true bronchial pattern exists, it is present in all lung lobes.
With chronic disease, bronchi can fail to taper, due to bronchiectasis. Bronchiectasis is important to recognize as an irreversible sequela of chronic airway related disease and as a predisposing factor for future airway related symptoms and bronchopneumonia.11
Tracheal collapse is a common cause of cough in small breed, often obese dogs and is caused by weakened cartilaginous rings that have undergone chondromalacia and collapse in the dorsoventral dimension.12,13 It may result in diffuse or segmental narrowing of the tracheal lumen, depending on severity. Severity is graded using percentage of tracheal lumen reduction where 0–25% collapse is likely normal, 50% is considered moderate, greater than 75% reduction in tracheal luminal diameter is severe.12,14 Views obtained on expiration and inspiration may assist with diagnosis of this dynamic collapse. On inspiration, the cervical trachea is expected to collapse, and on expiration, the intrathoracic trachea is expected to collapse.
Bronchomalacia is the weakness and collapse of the principle bronchi or other small airways, which are supported by cartilage; this condition may be accompanied by tracheal collapse and results in excessive narrowing of the airways.13,15,16 In 83% of dogs with tracheal collapse, concurrent bronchial collapse was observed via bronchoscopy.15 Bronchomalacia as a single entity is less commonly reported in the veterinary literature when compared to combined tracheobronchial collapse or tracheal collapse as a single entity with a relatively unknown prevalence.17
The Normal Thoracic Radiograph
“Thoracic radiography is the principal imaging method for investigating chronic cough in dogs because it is a versatile method for detecting various pulmonary, pleural, mediastinal and cardiac lesions, not because it usually enables a specific diagnosis.”18 This should highlight the point that diagnostic tests and treatments should be geared towards the patient, not the radiographs, so the choice of performing an airway wash on a coughing dog with normal thoracic radiographs is certainly the correct one. Even more so when no other cause of coughing was detected on the radiographs.
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