Normal adult Basset Hound thorax. The cause for this patient's clinical signs is not apparent on the radiographs.
The Basset Hound is an example of a "shallow-chested" breed. This thoracic conformation may give the false impression of cardiomegaly due to 1) the relatively small tracheospinal angle and 2) the "square" or "rounded" appearance of the heart, both of which are normal for this breed. In addition, the costochondral junctions of the Basset Hound indent the thoracic wall. This causes undulating bands of soft tissue opacity along the lateral margins of the thorax on the dorsoventral view. These bands can be mistaken for pleural effusion by the unwary. We know this patient does not have pleural effusion because no pleural fissure lines are present.
There is relatively uniform widening of the cranial and caudal mediastinum on the dorsoventral view. On the lateral view, the heart and ventral diaphragm are poorly visualized due to silhouetting with soft tissue opacity material within the mediastinum. A pleural fissure line is present overlying the cardiac silhouette on the lateral view, and the caudal lung lobes are slightly separated from the thoracic wall by soft tissue opacity material on the DV view. No other abnormalities are detected. Given the history, the most likely differential is mediastinal hemorrhage and mild bilateral pleural hemorrhage secondary to rodenticide toxicity.
The normal mediastinum should be no more than twice the width of the spine on the DV view. The relative uniformity of the widening in this case makes the presence of mediastinal fluid more likely than the presence of a mass. Without the history of possible rodenticide toxicity, another differential diagnosis would be the presence of pus within the mediastinum, possibly secondary to a perforating esophageal foreign body.
Thoracic radiographic findings in dogs with anticoagulant rodenticide toxicity have been described. Findings include increased mediastinal soft tissue opacity with or without tracheal narrowing, variable amounts of pleural effusion, and patchy pulmonary infiltrates. (1) The patient in this case had mediastinal fluid without tracheal narrowing and mild pleural effusion, but no pulmonary infiltrates. Remember that the mediastinum extends from the spine to the sternum, so it is superimposed on the lung fields on the lateral view. This can give the false impression of pulmonary changes. Luckily, evaluation of the lung fields is still possible on the DV view.
1) Berry CR, Gallaway A, Thrall DE, Carlisle C. Thoracic radiographic features of anticoagulant rodenticide toxicity in 14 dogs. Vet Radiol Ultrasound 34(6):391-396, 1993.