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findings that lead to suspected lung disease. To further characterize the type of lung disease, pulmonary patterns are often the intuitive choice. The concept
of pulmonary pattern recognition was developed to
aid in generating the most likely differential diagnosis based on the assumption that different disease types affect different compartments within the lungs. There
is, however, a large degree of overlap between the radiographic pulmonary patterns in many types of lung disease and focusing on pulmonary patterns only is not the most successful way of interpreting radiographs. The distribution of the abnormal lung patterns as well as other radiographic findings are often more helpful than the pulmonary patterns alone.
Alveolar pulmonary pattern is characterized by marked increased pulmonary opacity, loss of visibility of vascular structures in the affected segment, presence of air bronchograms and if only on lung lobe is affected, presence of a lobar sign. Pneumonia is one of the most common causes of alveolar pulmonary pattern besides heart failure. Contrary to heart failure, alveolar patterns with pneumonia are typically ventrally distributed
with a preference to the cranial and right middle lung lobes. Ventral alveolar patterns can also be caused by atelectasis which is differentiated from acute pneumonia by evidence of volume loss of the affected lung lobe causing a midline shift to the same side. Pleural effusion is rarely seen with pneumonia on dogs and presence of effusion should point to a different disease process such as trauma, neoplasia or primary pleural space disease with secondary involvement of the lungs.
Bronchial pulmonary patterns can be difficult to recognize as they do not result in overall increased pulmonary opacity but there is also a tendency to over- interpret the normal structure of the lung parenchyma as bronchial pattern. Signs of advanced bronchial disease include bronchiectasis which is characterized by lack
of normal tapering of the bronchial lumen towards the periphery of a bronchus. Presence of bronchiectasis
is often a sign that there is chronicity to the bronchial disease. Chondromalacia leading to airway collapse
can be recognized by paying close attention to airway diameter between radiographic projection particularly
if they were obtained during slightly different phases
of respiration. In a normal dog there should be minimal variance between airway diameter in in- vs. expiratory radiographs.
Interstitial lung disease is characterized by diffuse increase in pulmonary opacity without loss of visibility of vascular structures or bronchial walls. Interstitial pattern is the least useful of the pulmonary patterns. Most disease processes have an interstitial component, and as outlined above when describing the pathogenesis of cardiogenic edema, many disease processes start out as or resolve as interstitial pattern.
Overlapping disease patterns
There are cases where it is very difficult if not impossible to decide of the radiographic findings are most likely due to cardiac or pulmonary disease. One of the
more difficult diseases to recognize radiographically
are pulmonary infiltrates secondary to pulmonary hypertension. Dogs with moderate to severe pulmonary hypertension can present with diffuse patchy alveolar infiltrates consistent with non-cardiogenic edema. The clinical presentation may include acute dyspnea and syncope and often heart murmurs suggestive of valvular insufficiency are present. The clinical and radiographic findings may lead to an initial misdiagnosis of congestive heart failure or pneumonia whereas the dogs improve both clinically and radiographically once therapy with sildenafil is instituted.
Atypical appearance of heart disease can occur if there is acute chorda tendinea rupture, or in cases of severe cardiomegaly, left atrial rupture with subsequent acute pericardial effusion. Pulmonary thromboembolism is another disease with a large variation in radiographic appearance, ranging from normal appearing lungs to hyperlucency or patchy alveolar pattern. These dogs typically present with marked dyspnea.
References
1. J Vet Cardiol 2015, 17(3): 182-91.
2. J Vet Intern Med. 2008, 21 (2): 258-264.
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