Introduction

Upper and lower airway disease is common in dogs and cats, which can present with similar signs regardless of the location. Following stabilization of the patient with oxygen, radiography plays a very valuable role in determining the cause of airway distress. In addition to radiography, computed tomography (CT) is a cross-sectional modality for examining pathology when radiography is equivocal. This lecture will provide the participant an overview of how to differentiate upper from lower airway disease in the coughing dog and discuss how to arrive at a practical diagnosis with many case examples.

Techniques in Radiography

If the animal is stable enough with its respiratory rate and effort, I advocate a 4-view thoracic radiograph (R and L lateral, VD and DV). A dorsoventral radiograph instead of a ventrodorsal radiograph is often beneficial in dyspneic patients, as they are often more comfortable resting on their sternum. If diseases of the upper airways are suspected, a lateral projection of the larynx/pharynx area and cervical trachea is mandatory. A foreign body or mass in this region can be immediately recognized. Thoracic studies should always be obtained during inspiration. An additional exception to the rule is the comparison of diameters of the trachea and mainstem bronchi in patients with tracheal collapse, which requires both respiratory phases and even fluoroscopy.

Upper Airway Diseases

Larynx and Trachea

Lateral views of the laryngeal and cervical region are usually sufficient. Typical changes in dogs are increasing mineralization of the laryngeal and tracheal cartilages with age. Mineralization can already be present in dogs as young as 2 years of age, especially in chondrodystrophic breeds. The most commonly diagnosed diseases of the larynx are larynx paralysis, eventration of the ventricle, and larynx collapse. These can usually be diagnosed by oral inspection. Radiographic findings are often unremarkable; however, decreased diameter of the air-filled oro- and nasopharynx may be detected. In brachycephalic syndrome, elongation of the soft palate, tracheal hypoplasia, and increased soft tissues of the larynx can be identified radiographically. The normal ratio of tracheal diameter to thoracic inlet diameter is 0.07–0.21 in bulldogs and averages 0.16 in non-bulldog brachycephalic breeds. Non-brachycephalic breeds have a ratio of 0.2. Note: The trachea will appear narrower prior to one year of age in many breeds, especially brachycephalic ones.

In cats, the pharyngeal area should be scrutinized for soft-tissue masses associated with nasopharyngeal polyps.

The rigidity of the trachea decreases with age in small-breed dogs and has an effect on the diameter during inspiration and expiration, which can be shown radiographically. This makes the diagnosis of collapsing trachea in small-breed dogs sometimes difficult. The difference between normal and pathologic is not always clear. Therefore, dynamic views of the trachea can be very helpful for a more definitive diagnosis. This is best performed with fluoroscopy so that the change in diameter of the cervical and thoracic trachea can be observed during deep inspiration and expiration, as well as when the dog coughs. Unfortunately, this is only available at referral centers. Static radiographs, however, are a minimum data base and should be performed if the survey radiographs are negative. Both in inspiration and expiration have to be compared to detect many types of tracheal collapse. Typically, the extrathoracic trachea will collapse during inspiration, the intrathoracic trachea and stem bronchi during expiration. Collapse may also occur focally at the thoracic inlet. The significance of a soft-tissue opaque shadowing of the dorsal tracheal border is subject to discussions. One possible explanation for the appearance is a redundant tracheal membrane, meaning the protrusion into the tracheal lumen.

Small-breed dogs with large livers and a cranially displaced diaphragm can have direct compression of the stem bronchi, which can have a similar effect as an enlarged left atrium in dogs with left heart failure. Reduction in the diameter of the trachea may also occur due to compression by mediastinal masses, enlarged tracheobronchial lymph nodes, or an enlarged left atrium. Collapse of the stem bronchi is commonly the cause of coughing in small-breed dogs with mitral insufficiency due to endocardiosis and enlargement of the left atrium. Tracheal collapse and compression should be differentiated from stenosis secondary to space-occupying lesions. Granulomas (foreign body, parasites) are rare. Neoplasia is also very uncommon. Tumors such as osteochondroma, osteosarcoma, and chondrosarcoma appear as soft-tissue opaque lesions that contrast with the air-filled tracheal lumen. Inhaled foreign bodies such as small pebbles are not uncommon, especially in cats. Foreign bodies are usually readily visible due to contrast with the air-filled lumen. Stenosis can occur due to prior foreign body or aspiration of gastric juices. They present radiographically as a focal narrowing of the tracheal lumen and can be present anywhere along the length of the trachea.

Tip: For squirming small-breed dogs, where you are trying to get inspiratory and expiratory views in lateral recumbency and respect radiation safety at the same time, make a doggie burrito: swaddle the tiny dog in a towel with its legs pulled forward, and it won't struggle on its side.

Lower Airway Diseases

Peribronchial infiltrates and edema, narrowing of the bronchial lumen due to either thickening of the bronchial wall or buildup of secretions, as well as enlargement of the bronchi are common consequences of dogs and cats with lower airway disease.

Radiographic findings of lower airway disease are rings with a relatively small air-filled lumen representing transverse sections of the affected bronchi along with an increased number of linear structures throughout the lung. The lung will appear to have a diffusely increased opacity due to the presence of thickened bronchi, bronchial secretions, or peribronchial infiltrates. The difficulty lies in differentiating disease from age-related changes of the bronchial tree, which can appear similar. Mineralized bronchial walls due to age appear thin and finely mineral opaque and sharply delineated. Thickening of the bronchial walls leading to "doughnuts" and "tramlines" is a sign of chronic bronchial inflammation. Primary differential diagnoses are chronic bronchitis, eosinophilic infiltrates, or parasitic infections.

Thickening of the peribronchial tissues (bronchial cuffing) due to edema or inflammation can mimic bronchial wall thickening but belong to another list of differentials (bronchopneumonia, cardiogenic edema in large-breed dogs, or allergic reactions).

Chronic bronchitis is an exclusion diagnosis. Thickened bronchial walls and their increased visibility is a reliable sign of chronic bronchitis in dogs. In severe cases, the bronchi can be completely opacified by mucus and can be confounded with vessels or even small nodules. In cases of acute bronchitis, the thoracic study may be inconspicuous or resemble chronic cases.

Bronchiectasis is much less common and appears as widened, irregularly shaped bronchial branches with a thickened wall. This represents end-stage bronchial disease, usually following chronic bronchitis. Soft-tissue opacities may also be present in the periphery of the lung due to secretion buildup and appear as a peripheral alveolar pattern, either in one or multiple lobes. Bronchiectasis may only be evident in one or two lung lobes or can be generalized.

Acute bronchitis may appear unremarkable radiographically. Bronchitis is an interstitial disease, so the radiographic findings are often mixed. It can result in air trapping and emphysema, which will result in an increased air volume in the lung, which appears hyperlucent. Atelectasis of the right middle lung lobe may occur in cats and appears as a consolidation and decreased volume of that lung lobe (best seen in VD view).

A relatively common cause of cough is inhaled foreign bodies such as grass awns, wooden pieces, and the like. The radiographic appearance depends on the degree of bronchial obstruction and if the foreign body has irritating properties.

The most common form of pneumonia is unspecific bacterial infection and often follows primary lung disease such as hemorrhage, viral infection, or chronic bronchitis. Patients suffering immunodeficiency or having reduced ciliary apparatus motility are prone to pneumonia. In cases of pneumonia, three different projections are recommended to visualize all parts of the lung field. The most common radiographic sign is an alveolar pattern affecting an entire lobe or just its tips ventrally. The most often affected areas are the cranioventral parts of the lung and the right middle lobe. An asymmetric distribution is also possible. Aspiration pneumonia most often affects the cranioventral regions, whereas inhaled, high-speed foreign bodies tend to lodge in the caudodorsal region.

Neoplasia of the bronchial walls is relatively rare. The most common tumor type is bronchial wall carcinoma. Also in neoplasia the radiographic appearance may be manifold (solitary nodules, miliary to alveolar consolidation). The accumulation with cells and fluids in the interstitium may lead to an increased opacity without complete obliteration of the air-containing spaces. Therefore, the vessels, caudal vena cava, and the cardiac silhouette may still be visible but ill defined.

Canine Tracheobronchomalacia

Tracheal chondromalacia and collapse are common in middle-aged and older small- and toy-breed dogs. Collapse can be static or dynamic and can occur anywhere from mid-trachea to the stem bronchi and can even involve the entire trachea. Extrathoracic tracheal collapse is most pronounced during inspiration, while intrathoracic collapse is most pronounced during expiration. Therefore, both inspiratory and expiratory radiographic images should be performed that include the entire length of the trachea. Ideally, video fluoroscopy should be performed if screening inspiratory and expiratory static radiographs fail to detect collapse.

Static Tracheal Collapse

This is identified radiographically as a static dorsoventral narrowing of the tracheal lumen. An undulating appearance of the dorsal wall or non-uniform diameter of the tracheal lumen is also suggestive of chondromalacia and indicates the need for dynamic examination.

Inspiratory and Expiratory Radiography

Dynamic radiography to diagnose tracheal collapse takes patience and effort. Small dogs with breathing difficulty are stressed, and quiet surroundings and attention to their comfort are necessary for a successful outcome. Radiographs can be exposed during induced coughing after applying pressure to the trachea. With the dog in lateral recumbency, the trachea is palpated until coughing is induced. An attempt should be made to make exposures as the dog breathes deeply in and then again on deep expiration. However, this can be difficult to do, and oftentimes small dogs are not cooperative or are stressed during the procedure. A good alternative is to calmly cover the nose and mouth while talking to it, and this will gradually lead to deeper and deeper inspiration and consequently expiration so that exposures can be made at these moments.

Confirming a Diagnosis of Collapse

Less than 25% decreased luminal diameter is generally considered insignificant in chondrodystrophic-breed dogs and usually doesn't correlate to collapse fluoroscopically. Furthermore, intrathoracic collapse can be detected fluoroscopically when dynamically performed radiographs are negative. Decreased luminal diameters should be further classified as 50%, 75%, or 90–100% during inspiration and expiration and whether or not the collapse is cervical, intrathoracic, static, or dynamic. However, in another study, bronchomalacia and sublobar airway collapse in the absence of cervical tracheal collapse were common in medium- and large-breed dogs examined, providing further evidence for the utility of bronchoscopy in the diagnostic evaluation of dogs with cough. This underscores the necessity for including bronchoscopy in coughing dogs, even if the tracheal collapse is not evident radiographically.

Bronchopneumonia

Pneumonia may occur due to inhalation of either airborne or fluid-borne pathogens or material. Aspiration pneumonia is inhalation of liquids and ingesta into the trachea and bronchi.

Aspiration pneumonia classically results in a ventral distribution of air-space disease. This is recognized as an alveolar pattern with air bronchograms and potentially a lobar sign as the disease progresses from the periphery to the central region of the affected lobe or lobes. The most commonly affected lungs are the right middle and cranial, the caudal subsegment of the left cranial lobe, and the left cranial and accessory lobes, in that order.

Heartworm disease

The thoracic radiographic findings are extremely variable, from unremarkable to severe pulmonary hypertension with mixed pulmonary patterns and enlarged and tortuous pulmonary arteries. A bronchial or peribronchial increased soft-tissue opacity is most often present in cats and dogs.

Inflammatory Airway Disease

Radiographs are very helpful for determining whether a bronchoscopy is indicated as the next course of action in a coughing dog where tracheal collapse has been ruled out with static and dynamic radiographs. The presence of a bronchial pattern indicates airway disease, past or current. It is important to concentrate on the peripheral regions of the lungs when searching for bronchial disease. Centrally the airways are large and the bronchial walls are easily visible. The peripheral third of the lung should be devoid of linear and ring-like markings due to bronchi. Start by tracing the airways out from the carina to each lung lobe. Scrutinize the adjacent vessels, which should be sharply marginated in normal dogs. If small linear and ring-like bronchial markings are evident in the periphery of the lung, then bronchial disease is present. Such markings can be seen in older animals that are not coughing, and the decision to perform bronchoscopy should be made from a combination of clinical signs and presence of an airway pattern on the radiograph.

Airway Disease in Cats

Lower airway disease can appear radiographically normal or have varying severities of airway pattern. Tracing the trachea to the carina and then tracing each main bronchus of each lobe should be performed. Two-thirds of the way out from the carina, the visualization of the bronchial walls and vessels should slowly disappear. If larger numbers of branching structures are visible, then an airway pattern is present. However, clinical signs are not always present. The clinical signs of airway disease may wax and wane, but chronic airway patterns are persistent on thoracic radiographs, regardless of clinical activity. This is where reader bias can sway the importance placed upon the presence of an airway pattern, or even lead the reader away from other abnormalities due to tunnel vision. Airway disease is usually due to allergic airway disease, asthma, and heartworm infection. A recent study confirmed that the most common radiographic abnormality is a bronchial pattern, but an unstructured interstitial pattern can be present in many cats. More than half of the cats in that study had lung hyperinflation also. Bronchiectasis can be identified in a smaller number of cats. Right middle lobar atelectasis can be seen, as can small nodules throughout the lung and represent mucous plugs with granuloma formation.

Severe inflammatory lower airway disease can lead to hyperinflation with a flattened diaphragm. The bronchial pattern can be mixed with small nodules due to mucous plugging and exudates.