Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 95
Fig. 2-68 A 1-year-old male mixed
breed dog was examined after thoracic
trauma. The dog had evidence
of mild respiratory distress. In this
lateral radiograph, the collapsed caudal
lung lobes are outlined by air
within the pleural space (open
arrows). The cardiac silhouette is
separated from the sternum. The
esophagus can be seen in the caudal
thorax (solid straight arrow). There is
a thin, soft tissue–dense linear structure
present in the caudal ventral
thorax (curved arrows). This represents
a portion of the caudal mediastinum,
which is outlined by the
pleural air. Diagnosis: Pneumothorax.
cumstances, intrapleural pressure may exceed atmospheric pressure (Fig. 2-69). Severe lung
lobe collapse, flattening, or caudal displacement of the diaphragm into the abdomen and
tenting of the diaphragm at its costal attachments may be evident. The thorax may be
widened, or barrel chested, and the mediastinum may shift away from the side in which the
air has accumulated if the mediastinum is intact. This emergency condition should be recognized
and treated immediately.
Pneumothorax may be mimicked by overexposure of the radiograph, overlying skin
folds, overinflation of the lung, or hypovolemia. If this is suspected, a high-intensity light
should be used to determine if the pulmonary vessels and airways extend to the thoracic
wall. Lung lobe margins remain almost parallel to the thoracic wall when the lungs collapse
due to pneumothorax. Any apparent margins that are not parallel are probably artifacts.
Skin folds often can be traced beyond the thoracic wall. Separation of the heart from the
sternum on a recumbent lateral view is a sign of mediastinal shift and is not pathognomonic
for pneumothorax. Full inflation of the right middle lung lobe in a normally deepchested
breed or overinflation of the lung can produce this same heart-sternum separation.
Anytime radiographic changes suggest the presence of pleural air on one radiograph but
the diagnosis cannot be supported by another view, the diagnosis is suspect. A horizontal
beam view may be diagnostic in these situations.
MEDIASTINAL ABNORMALITIES
Abnormalities of the mediastinum that may be detected radiographically include changes
in size, shape, and position, and alterations in density. The mediastinum is divided into cranial
(precardiac), middle (cardiac), and caudal (postcardiac) portions. The trachea is the
only structure in the cranial mediastinum that can be identified consistently on thoracic
radiographs. It therefore serves as a landmark or reference point for evaluation of cranial
mediastinal lesions, especially masses.
The size of the mediastinum varies among individuals due to the accumulation of fat
and the presence of the thymus in young animals. This normal variation must be
considered before an abnormality is diagnosed. In a fat animal, a widened mediastinum
that has a smooth margin and does not displace or compress the trachea is probably normal.
However, a widened mediastinum in a thin dog probably has clinical significance.