Thoracic Radiography in Canine Respiratory Disease: Interpreting Lung Patterns

Thoracic radiography provides important information when evaluating dogs with respiratory disease. Its usefulness extends beyond the pulmonary parenchyma because thoracic radiographs also provide information about cardiac size, great vessels, chest wall and pleural space. For a coughing dog, this makes radiographic assessment valuable in distinguishing cardiac from respiratory causes. 

What to Assess on Thoracic Radiographs 

Thoracic radiography is considered a good diagnostic tool for lower airway disease in dogs¹. Radiographs provide information about cardiac size, gross chamber abnormalities, great vessels, chest wall, pleural space, and pulmonary structures, particularly the bronchi, interstitial spaces, and alveolar tissues¹. 

They can also help differentiate primary cardiac disease from respiratory problems in coughing dogs¹. 

However, radiography has less utility for soft-tissue pathology and requires appropriate expertise. A radiographic pattern should therefore be interpreted alongside the clinical findings rather than treated as a standalone diagnosis. 

Recognizing the Alveolar Pattern 

The major lung patterns described include alveolar, bronchial, vascular, and interstitial patterns, with the interstitial pattern further classified as nodular or diffuse². 

An alveolar pattern is commonly associated with bacterial and aspiration pneumonia. Other differential diagnoses include pulmonary contusion, pulmonary thromboembolism, neoplasia, fungal pneumonia, and systemic coagulopathy^1,3. 

Therefore, an alveolar pattern can narrow the differential list but does not establish a single cause. 

Recognizing the Bronchial Pattern 

A bronchial pattern may appear as doughnuts, tram lines, or train tracks. It occurs with canine chronic bronchitis, allergic bronchitis, and canine infectious tracheobronchitis, and is less commonly associated with bacterial and parasitic infection^1,2. 

In a dog with chronic cough, this pattern can therefore support consideration of chronic airway disease, but the clinical history remains important for prioritizing the differential diagnoses. 

Interstitial and Nodular Patterns¹ 

Reticular interstitial patterns may be associated with viral, bacterial, and mycotic infections, toxoplasmosis, idiopathic interstitial pneumonia, and neoplasia. 

Nodular patterns have been associated with mycotic infections, tumors, and abscesses. 

The overlap between these conditions illustrates why radiographic pattern recognition should be used to organize the differential diagnosis rather than to make an isolated etiological diagnosis. 

Practical Clinical Insight 

A systematic radiographic assessment can help determine whether the dominant abnormality is alveolar, bronchial, vascular or interstitial. The next diagnostic step can then be selected according to the clinical context. 

When radiography does not sufficiently distinguish the underlying cause, the source describes additional approaches including bronchoscopy, transtracheal wash, bronchoalveolar lavage and transthoracic FNAC. 

Bronchoscopy can be used to assess structural abnormalities such as tracheobronchial collapse, intraluminal masses, and strictures, as well as inflammatory conditions including pneumonia and chronic bronchitis. 

References 

1. Kaur J, Singh S. Respiratory diseases and their diagnosis in dogs: A review. Indian Journal of Animal Research. 2022. https://arccjournals.com/journal/indian-journal-of-animal-research/B-4994 

2. Spasov K, Kunovska M, Dimov D. Lung patterns in the dog–normal and pathological. Tradition and Modernity in Veterinary Medicine. 2018 Jun 25;3(1):7-14. https://scij-tmvm.com/index.php/pub/article/download/55/55 

3. Dear JD. Bacterial pneumonia in dogs and cats: an update. The Veterinary Clinics of North America. Small Animal Practice. 2019 Dec 5;50(2):447. https://pmc.ncbi.nlm.nih.gov/articles/PMC7114575/pdf/main.pdf