Page 392 - Small Animal Internal Medicine, 6th Edition

364 PART II Respiratory System Disorders

than 3 g/dL of protein and more than 1000 nucleated cells/µL, DIAGNOSTIC TESTS FOR THE PLEURAL
with a distribution similar to that of peripheral blood. Over CAVITY AND MEDIASTINUM
VetBooks.ir time the numbers of neutrophils and macrophages increase. RADIOGRAPHY
Hemorrhagic effusions (except those obtained immediately
Pleural Cavity
after bleeding into the thorax) are readily distinguished from
the recovery of peripheral blood through traumatic thora- The pleura surrounds each lung lobe and lines the thoracic
cocentesis by several features: hemorrhagic effusions show cavity. It is not normally visible radiographically, and indi-
erythrophagocytosis and an inflammatory response on cyto- vidual lung lobes cannot be distinguished. Abnormalities of
logic evaluation; hemorrhagic effusions do not clot; and the the pleura and pleural cavity include pleural thickening,
packed cell volume of hemorrhagic effusions is lower than pleural effusion, and pneumothorax. The mediastinum in
that of peripheral blood. the dog and cat is not an effective barrier between the left
Hypovolemia and anemia may contribute to clinical signs and right sides of the thorax, and effusion or pneumothorax
in patients with hemothorax (see Chapter 25). Hemothorax therefore is usually bilateral.
can result from trauma, systemic bleeding disorders, neopla-
sia, and lung lobe torsion. Rarely, septic exudates are grossly Pleural Thickening
hemorrhagic (see Fig. 23.1, F) and are distinguished cyto- Pleural thickening results in a thin, fluid-dense line between
logically. Respiratory distress caused by hemothorax may be lung lobes, where the pleura is perpendicular to the X-ray
the only clinical sign in animals with some bleeding disor- beam. These lines arc from the periphery toward the hilar
ders, including rodenticide intoxication. An activated clot- region and are known as pleural fissure lines. The lines can
ting time and platelet count should be performed early in the occur as a result of prior pleural disease and subsequent
evaluation of these animals, followed by more specific clot- fibrosis, mild active pleuritis, or low-volume pleural effusion.
ting tests (i.e., prothrombin time and partial thromboplastin They can be an incidental finding in older dogs. Infiltration
time). Hemangiosarcoma of the heart or lungs is a common of the pleura with neoplastic cells generally results in effu-
neoplastic cause of hemorrhagic effusion, but malignant cells sion rather than thickening.
are rarely identified cytologically. Neoplastic effusions are
discussed further in the next section. Pleural Effusion
Pleural effusion is visible radiographically after about 50 to
EFFUSION CAUSED BY NEOPLASIA 100 mL has accumulated in the pleural cavity, depending on
Neoplasia within the thoracic cavity can result in most types the size of the animal. A mild effusion assumes the appear-
of effusions (modified transudates, exudates, chylous effu- ance of pleural fissure lines and can be confused with pleural
sion, or hemorrhagic effusion). Neoplasms may involve any thickening. As fluid accumulates, the lung lobes retract and
of the intrathoracic structures, including lungs, mediastinal the lung lobe borders become rounded. Rounding of the
tissues, pleura, heart, and lymph nodes. In some cases, neo- caudodorsal angles of the caudal lung lobes is especially
plastic cells exfoliate from the tumor into the effusion, and noticeable. The fluid silhouettes the heart and the diaphragm,
an early diagnosis can be made through fluid cytology. This obscuring their borders. The lungs float on top of the fluid,
is often possible in patients with mediastinal lymphoma. displacing the trachea dorsally and causing the illusion of a
Unfortunately, other than in cases of lymphoma, it can be mediastinal mass or cardiomegaly (Fig. 23.2, A). As more
difficult or impossible to establish a definitive diagnosis of fluid accumulates, the lung parenchyma appears abnormally
neoplasia on the basis of cytologic findings in the pleural dense as a result of incomplete expansion. Collapsed lobes
fluid alone. Inflammation can result in considerable hyper- should be examined carefully for evidence of torsion (see
plastic changes in mesothelial cells, which are easily con- Chapter 20). Pockets of fluid accumulation or unilateral effu-
fused with neoplastic cells. A cytologic diagnosis of neoplasia sion indicate the possibility of concurrent pleural adhesions
other than lymphoma should be made with extreme caution. (see Fig. 23.2, B).
In most cases, neoplastic cells are not present in the fluid, Critical radiographic evaluation of intrathoracic struc-
or a cytologic diagnosis cannot be made. Thoracic radiog- tures, including lungs, heart, diaphragm, and mediastinum,
raphy and ultrasonography should be performed to evalu- cannot be performed in animals with pleural effusion until
ate the thorax for evidence of neoplasia. Ultrasonography the fluid has been removed. Interpretation of radiographs
can be used to differentiate localized accumulations of fluid obtained in the presence of fluid is prone to error. An excep-
from soft tissue masses. If soft tissue masses are detected, tion to this rule is the finding of gas-filled intestinal loops in
aspirates or biopsy specimens are obtained for cytologic or the thorax, which is diagnostic of diaphragmatic hernia.
histopathologic evaluation. A definitive diagnosis cannot be Both left and right lateral views should be evaluated, in addi-
made on the basis of radiographic findings or ultrasound tion to a ventrodorsal view, to improve the sensitivity of
images alone. detecting masses.
Diffuse neoplastic infiltration of the pleura and some
masses cannot be seen with these imaging techniques. Pneumothorax
Repeated CT, thoracoscopy, or surgical exploration may be Pneumothorax is the presence of air in the pleural space.
necessary in such cases. Air opacity without vessels or airways can be seen between