Page 393 - Small Animal Internal Medicine, 6th Edition

CHAPTER 23 Clinical Manifestations and Diagnostic Tests of Pleural Cavity and Mediastinal Disease 365

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A

FIG 23.3
Lateral view of a dog with pneumothorax and
pneumomediastinum. The pneumothorax is mild and
demonstrated by elevation of the heart above the sternum.
When the contrast of the original radiographs was
decreased, retraction of lung borders could also be seen. It
is possible to visualize the outer wall of the trachea and
major blood vessels in the anterior mediastinum because of
the pneumomediastinum. A chest tube placed to stabilize
the dog’s condition is also visible (arrows).

(indicating trauma) or cavitary lesions. For accurate evalua-
tion of the pulmonary parenchyma, the air must be removed
and the lungs allowed to expand. Cavitary lesions are not
always apparent radiographically. Further evaluation for
cavitary lesions in patients with spontaneous pneumothorax
includes CT.

Mediastinum
The cranial and caudal mediastinum contains the heart
and great vessels, esophagus, lymph nodes, and associated
support structures. Radiographic abnormalities involving
the mediastinum include pneumomediastinum, altera-
B tions in size (e.g., mass lesions), displacement, and abnor-
malities involving structures within the mediastinum (e.g.,
FIG 23.2 megaesophagus).
(A) Lateral thoracic view of a cat with pleural effusion. See Pneumomediastinum is the accumulation of air within
text. (B) Ventrodorsal view showing that the effusion is the mediastinum. If pneumomediastinum is present, the
unilateral.
outer wall of the trachea and other cranial mediastinal
structures, such as the esophagus, major branches of the
aortic arch, and the cranial vena cava, are contrasted against
the lung lobes and the chest wall on radiographs. It may the air (see Fig. 23.3). These structures are not normally
be necessary to carefully scrutinize the radiographs with visible.
adjustments in contrast to detect mild pneumothorax. As a Abnormal soft tissue opacities can occur in the cranial
greater volume of air accumulates in the pleural space, the mediastinum, although concurrent pleural effusion often
lung parenchyma becomes more dense because of incom- obscures mass lesions. Localized lesions can represent neo-
plete expansion, facilitating the radiographic diagnosis. plasia, abscesses, granulomas, or cysts. Less discrete disease
The heart is generally elevated above the sternum, with air can cause general widening of the mediastinum that is seen
opacity apparent between these two structures (Fig. 23.3). to exceed the width of the vertebra on ventrodorsal views.
Radiographs should be examined carefully for evidence of Exudates, edema, hemorrhage, tumor infiltration, and fat
possible causes of the pneumothorax, such as rib fractures can cause a widened mediastinum. Megaesophagus can