Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

92 Small Animal Radiolo g y and Ultrasono graphy
FIG. 2-65 A 6-year-old neutered
female domestic short-haired cat
had difficulty breathing and tachypnea.
An ultrasonographic examination
revealed a large volume of
pleural effusion (PE). There was
severe collapse of some lung lobes
(L) that appears as a thin, triangular
structure. Diagnosis: Chylothorax.
Pleural thickening and outlining of pleural interlobar fissures due to pleural fibrosis or calcification
may be observed in older animals. This will be similar to the changes that occur with
small amounts of pleural fluid. Thickened pleural fissures, however, are linear while pleural
fluid accumulations are triangular, with the peripheral portion of the density usually wider
than the central portion. A change in the animal’s position without an appropriate change in
the appearance of these lines will identify the fibrotic or calcific nature of these densities.
Free pleural fluid will displace thoracic viscera only because of gravitational effects. In
the presence of large amounts of fluid, the heart will move toward the most dependent portion
of the thoracic cavity. This displacement causes the trachea to appear elevated on the
lateral view, which may create the false impression of cardiomegaly. 128 Displacement of the
cardiac silhouette in a direction against the effect of gravity, compression of the tracheal
lumen, displacement of the carina caudally, or a localized elevation of the trachea indicate
that a mass is present within the fluid. Loculated or encapsulated fluid may mimic this effect.
Sonography will demonstrate reliably even small amounts of pleural fluid if the transducer
is placed on the most dependent portion of the chest or over an area of trapped
fluid. 26,28,29 Atelectasis of the lungs may be noted because the lungs will collapse into triangle-shaped,
thin structures (Fig. 2-65). The presence of fibrin tags on pleural surfaces
indicates chronicity of the fluid accumulation. If the fluid contains a lot of floating debris,
pyothorax may be present. A pleural mass that was not identified radiographically due to
the lack of density difference between the fluid and the mass may be identified (Fig. 2-66).
Lung masses, atelectasis, or pneumonia also may be identified.
The causes of hydrothorax are numerous and include right heart failure, neoplasia,
hypoproteinemia, infection, traumatic rupture of vascular or lymphatic structures, and
inflammation. * The nature of the fluid cannot be determined using ultrasonography,
although highly echogenic fluid suggests the presence of a highly cellular fluid or one with
a high protein level. Identification of intrathoracic abnormalities such as an enlarged right
heart, pericardial fluid or mass, pleural mass, diaphragmatic hernia, or lung lobe torsion
will provide important clues to the diagnosis. Sonography is very helpful in identifying
opportune sites for thoracocentesis.
P L E U R A L A I R
Air within the pleural space is referred to as pneumothorax. Some minor differences can
be observed when comparing right with left lateral recumbent radiographs and
*References 26, 77, 86-88, 121, 129-151.