Introduction

Coughing is one of the most common presenting complaints for dogs in small animal practice . A plethora of disorders of the respiratory tract but also of other thoracic structures can result in cough. Although computed tomography is gaining popularity in veterinary medicine, cervical and thoracic radiographs remain a mainstay in the diagnostic workup of these patients. A lateral view of the neck is usually most helpful for evaluation of pharynx, larynx and trachea. However, even though these structures are superimposed over the cervical spine and difficult to evaluate on a VD view, a VD view should also be obtained to allow full evaluation of the cervical soft tissues. The radiographic series of a thorax should consist of a right lateral, a left lateral and a VD or DV view. Acquisition of both the right and left lateral view is paramount as lateral recumbency quickly results in atelectasis of the dependent lung lobes which may obscure pulmonary infiltrates, nodules or mass lesions. In some instances acquisition of both a VD and DV view may be indicated as the VD view allows improved assessment of the thoracic structures in case of pleural effusion and easier evaluation of the cardiac silhouette, while the DV view results in improved visualization of the caudal dorsal lung fields. A radiographic diagnosis in a dog presented with cough can be challenging as some diseases especially in the early stage may not result in radiographic abnormalities, physiologic variations or aging changes may mimic pathologic lesions and result in an erroneous diagnosis, concurrent problems may be present all of which could be responsible for the clinical signs, and as imaging findings in some thoracic (especially pulmonary) diseases overlap. A systematic approach to the thoracic radiograph should include assessment of the thoracic wall, diaphragm, mediastinal structures, pleural space, cardiovascular system and lung.

Causes of Coughing and Imaging Findings (1–3): Inflammatory, Parasitic and Allergic Conditions

Pharyngitis and tonsillitis do usually not result in significant radiographic abnormalities although severe inflammation may result in noticeable soft tissue swelling of laryngeal and pharyngeal structures on radiographs. Similarly, radiographs in dogs with tracheitis are often unremarkable or may show diffuse tracheal narrowing in severe cases. Lower airway disease (infectious bronchitis, asthma and pulmonary parasites) is often characterized by a diffuse mixed bronchial and unstructured interstitial pulmonary pattern with diffuse increased opacity to the lung with thickened and sometimes irregularly shaped bronchial walls. In extreme and chronic cases, irreversible and irregular widening of bronchi may occur (bronchiectasis). Bacterial pneumonia (bronchopneumonia or aspiration pneumonia) typically results in a cranial ventrally distributed alveolar pattern which may be unilateral or bilateral and is characterized by increased opacity of the affected lung lobe(s) with complete loss of visualization of the pulmonary vascular margins. If bronc hi within these lung lobes remain air-filled, air bronchograms will be visible. An abnormal lung lobe bordering normal air-filled lung will result in a lobar margination sign. Pulmonary abscessation is rare but may occur for instance following aspiration of a foreign body. A pulmonary abscess usually appears as a round or oval variable size soft tissue opacity pulmonary mass which may contain gas opacity foci. Viral pneumonia may not result in radiographic abnormalities or may manifest as a diffuse unstructured interstitial pattern. Fungal disease has a variable appearance on radiographs and may be associated with an unstructured interstitial pattern, multiple pulmonary nodules, masses, or irregular infiltrates. Intrathoracic lymphadenopathy, especially tracheobronchial lymphadenopathy, is common. Heartworm disease primarily results in cardiovascular changes (enlargement of the main pulmonary artery segment, right heart enlargement, dilation, blunting and tortuosity of pulmonary arteries) but is also often associated with concurrent mixed pulmonary patterns indicative of eosinophilic bronchopneumopathy.

Neoplasia

Even advanced pulmonary neoplasia may not be associated with clinical signs. However, both primary pulmonary neoplasms (e.g., adenocarcinoma) and metastatic disease can result in cough. A primary pulmonary neoplasm is characterized by a variable size usually sharply marginated mass. The lesion is most commonly homogeneously soft tissue opaque but may be cavitary. Primary lung tumours do not usually result in visibly enlarged tracheobronchial lymph nodes on thoracic radiographs. Pulmonary metastases result in numerous variable size and usually round pulmonary nodules. Round cell neoplasia (lymphoma, histiocytic sarcoma) have a variable radiographic appearance ranging from unstructured interstitial patterns over pulmonary nodules and masses to diffuse infiltrates. Concurrent thoracic lymphadenopathy is common.

Airway associated neoplasia (e.g., originating from pharynx, larynx or trachea) is relatively uncommon but usually readily identified as focal alteration of the normal air-filled lumen of the airway by a wall associated mass. Other mass lesions associated with the neck or thorax (e.g., thyroid carcinoma, heart base mass or oesophageal mass) can result in airway irritation and cough.

Traumatic and Physical Causes

Tracheal and/or mainstem bronchial collapse is common in small breed dogs. If severe it may be visible on survey radiographs. In other cases dynamic imaging (radiographs or fluoroscopy during inspiration, expiration and during cough) may be needed for diagnosis. Hypoplastic trachea is a congenital condition and is most commonly seen in bulldogs. Affected animals are presented at a young age and have generalized narrowing of the trachea on survey radiographs. Concurrent aspiration pneumonia is common. Inhaled foreign bodies are usually easily detected as they contrast with the normal air-filled lumen of the trachea or bronchi. Inhalation of irritating gases, liquids or solids causes variable radiographic changes ranging from normal to severe caudodorsal unstructured interstitial to alveolar patterns indicative of noncardiogenic pulmonary oedema. The intrathoracic trachea is well protected from trauma; however, trauma to the neck is common (e.g., bite would or excessive pull on leash) and may result in tracheal irritation and cough. If a perforating wound to the trachea has occurred, gas inclusions within cervical soft tissues, pneumomediastinum and subcutaneous emphysema will be evident radiographically. Similar to an oesophageal neoplasm, oesophageal enlargement for other reasons (megaoesophagus, oesophageal foreign body) may also result in cough.

Cardiovascular Causes

In addition to heartworm disease and heart base masses listed above, many heart diseases have the potential to result in cough either by physical compression of the caudal trachea and mainstem bronchi by an enlarged cardiac chamber (such as seen with left atrial enlargement in dogs with mitral endocardiosis and regurgitation) and/or secondary to congestive left heart failure resulting in cardiogenic pulmonary oedema. Radiographically, cardiomegaly is recognized by enlargement of one or more of the cardiac chambers.

In case of left atrial enlargement which commonly results in cough in canine patients, the caudal margin of the cardiac silhouette is straight, there is loss of the caudal cardiac waist, the trachea is displaced dorsally, and on the VD or DV view there is widening of the tracheal bifurcation (“bow legged cowboy sign”). In congestive left heart failure most dogs exhibit a caudal dorsal distributed (perihilar) unstructured interstitial to alveolar pattern and possibly visible enlargement of the pulmonary veins compared to the corresponding pulmonary arteries. In Doberman Pinschers with dilated cardiomyopathy and resultant congestive heart failure and in dogs with acute rupture of the chordae tendineae, distribution of pulmonary oedema is often random rather than confined to the caudal dorsal lung fields.

References

1.  Thrall OE. Textbook of Veterinary Diagnostic Radiology. 6th ed. St Louis, MO: Elsevier Saunders; 2013.

2.  Ettinger SJ. Coughing. Chapter 46. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 5th ed. Philadelphia, PA: WB Saunders Co; 2000.

3.  Suter PF. Thoracic Radiography. Wettswil: Selbstverlag PF Suter; 1984.