Inflammatory lung disorders causing coughing and respiratory distress are one of the most common clinical presentations of dogs and cats. When radiographs show complex or rapidly changing patterns, CT in particular is the next step in identifying the nature and location of the diseased lung. Differentiating between diffuse and focal disease, and primarily lung or airway associated disorders is key in planning further diagnostics, medical treatment, and surgery if necessary. This excerpt from section 4.6, Inflammatory lung disorders, shows you some of the imaging criteria associated with these diseases. This is one of the longest chapters in the Atlas of Small Animal CT & MRI, with 36 figures!
Idiopathic interstitial pneumonia
A number of inflammatory interstitial lung disorders in people fall under the broad category of idiopathic interstitial pneumonia. Although the array of speculated causes vary, these disorders have characteristic constellations of CT features that are adequate for specific diagnosis, and most eventually lead to end‐stage pulmonary fibrosis. These entities have not been well described in dogs and cats, but we do occasionally encounter patients with histologically confirmed interstitial pneumonitis without an apparent underlying infectious or other noninfectious exogenous cause.
Eosinophilic bronchopneumopathy
Canine eosinophilic bronchopneumopathy is thought to be immune‐mediated and the result of a hypersensitivity to aeroallergens, although infectious and other immune mediated causes have also been proposed as initiators in some instances. Average age of onset is 4–6 years, and both large‐ and small‐breed dogs are affected. Females are at over twice the risk for developing the disease. Although there is not a clear consensus regarding features, our experience suggests three clinical manifestations. Some dogs present with a predominantly bronchitic manifestation, with CT features of bronchial wall thickening and evidence of intraluminal bronchial exudates. Other dogs have findings more similar to bronchopneumonia with mixed interstitial and alveolar infiltrates. Less commonly, the disorder manifests as pulmonary granulomas that appear as focal, multifocal, or regional irregularly margined nodules or masses.
Viral pneumonia
Although CT appearance of viral pneumonias has been described in people and include ground‐glass, unstructured interstitial, and centrilobular nodular interstitial patterns, CT features have not been characterized in companion animals because conventional radiography would likely be used as an initial imaging diagnostic test. Based on reported radiographic features of viral pneumonia in dogs and cats, one might expect to see interstitial, ground glass opacities regionally or diffusely distributed, with a predilection for the caudodorsal lung fields.
Aspiration pneumonia
Aspiration pneumonia, as the name implies, results from aspiration of gastric or other fluids that cause a chemical pneumonitis. Gastric fluid is acidic and is therefore particularly damaging. Swallowing and esophageal disorders and gastric reflux are common predisposing factors leading to aspiration. Aspiration pneumonia typically occurs in the dependent regions of affected lobes but may have an atypical distribution if aspiration occurs with the patient in lateral or dorsal recumbency, as can occur while under anesthesia. Bacterial contamination will lead to lobar pneumonia.
Bacterial bronchopneumonia and related disorders
In people, bacterial pneumonia is subdivided into bronchopneumonia and lobar pneumonia, reflecting the initial location and subsequent progression of the inflammatory process. Bronchopneumonia arises through accumulation of exudates in terminal bronchioles and respects septal boundaries. Lobar pneumonia results from alveolar flooding of inflammatory exudates and has a greater propensity to spread. Given the differences in subgross anatomy between people and companion animals, this distinction may not be relevant in cats and dogs. CT features of bacterial bronchopneumonia include mixed interstitial and alveolar infiltrates in dependent regions of involved lung lobes. In many instances, complete lobar consolidation occurs with air bronchograms surrounded by uniformly soft‐tissue attenuating alveolar infiltrates. Lung volume loss can occur but is usually mild and insufficient to explain the increased lung attenuation. Mycoplasma pneumonia has been reported in both cats and dogs and appears to include airway collapse and bronchitis as part of the clinical manifestation in addition to consolidating pneumonia. Pleuropneumonia can lead to pleural thickening and regional pleural effusion, and necrotizing bronchopneumonia can result in pneumothorax.
Foreign‐body‐induced bronchopneumonia is common in some parts of the world where ingested or inhaled plant awns migrate down the bronchial tree, lodging in small‐caliber distal airways and initiating a bacterial bronchopneumonia. CT features depend on acuity and severity of pneumonia but often appear as a multifocal consolidating pneumonia. Alveolar infiltrates are often not in the dependent regions of the lung, and middle and caudal lobes are preferentially affected, presumably because of the migratory path of the awns. Foreign bodies often seed bacteria that can also lead to more fulminant granulomatous pneumonia.
Homework
Compare some of the images in your collection with the examples in this chapter to hone your interpretation skills. The Atlas of CT & MRI is available through Amazon, and has over 700 figures showing case examples to further your learning. Check it out if this article was helpful!
See you next time,
Allison Zwingenberger
Co-Author, Atlas of Small Animal CT and MRI
P.S.
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