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18

Pleura
Martha M. Larson

Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, USA

18.1 Pleural Effusion
the patient is placed in sternal recumbency (dorsoventral
view), the heart is located in a more dependent position, and
18.1.1 Diagnosis of Pleural Effusion
is poorly visualized due to border effacement from the sur-
Pleural effusion is the abnormal accumulation of fluid in rounding pleural fluid. A widened, radiopaque pleural space
the pleural space that may result from conditions that and fissure lines may still be visible, but can be less promi-
increase systemic and pleural capillary hydrostatic pres- nent than noted on the ventrodorsal view. Pleural fluid, as
sure, decrease plasma oncotic pressure, increase capillary noted in ventral recumbency, creates a more generalized
permeability, or cause lymphatic obstruction or dysfunc- increase in opacity. However, pulmonary vessels are still
tion [1,2]. Right‐sided congestive heart failure, hypoalbu- well visualized, as the abnormal fluid is in the pleural space
minemia, inflammation, trauma, coagulopathies, infectious, and not in the lung parenchyma.
or neoplastic processes can all result in pleural effusion [1]. On lateral projections, along with fissure lines, pleural
In a large study of cats with pleural effusion, the most com- fluid accumulates dorsal to the sternum, forming a scal-
mon underlying conditions were chylothorax, pyothorax, loped appearance with the retracted ventral lung margins.
intrathoracic neoplasia, hypertrophic cardiomyopathy, and The ventral border of the heart silhouettes with the pleural
feline infectious peritonitis (FIP) [2]. fluid when small/moderate volume is present, but may be
In patients in respiratory distress due to pleural effusion, completely effaced with large volumes. Tracheal elevation,
an ultrasound examination or a dorsoventral projection is often associated with cardiomegaly, can occur with pleural
the safest method of evaluation prior to thoracocentesis. A effusion alone [3]. Fat frequently accumulates in the retros-
ventrodorsal image in a cat with a large volume of pleural ternal location, and should not be mistaken for pleural
fluid will further compromise the remaining aerated lung fluid (fat is radiolucent to the cardiac silhouette, allowing
and the patient. If a large volume of fluid is present, tapping visualization of the cardiac margins).
the chest prior to additional imaging will improve the With fluid accumulation in the pleural space, the lung
respiratory status of the patient and will assist with visuali- lobes retract away from the chest wall in all views as the
zation of thoracic structures. lobes collapse to varying degrees. Fluid typically crosses
Radiographic signs of pleural effusion are variable, and the mediastinum freely, to accumulate in both hemithora-
depend on fluid volume and patient position (Figures 18.1–18.3). ces. Exudative fluid, such as a pyothorax, may not cross as
On ventrodorsal projections, a widened, radiopaque pleural freely, and can be localized to one side. Chylothorax has
space is noted, along with rounded costophrenic angles and also been noted to localize unilaterally. Fluid may be pock-
more prominent fissure lines (wedge‐shaped linear soft tis- eted, or inflammation may obstruct or close mediastinal
sue opacities between lung lobes, widening toward the chest fenestrations [2,4,5]. Unilateral pleural effusion can be
wall). Fissure lines are only visible when the X‐ray beam large in volume, resulting in a mediastinal shift toward the
strikes them end‐on so pleural fluid volume may be underes- unaffected side (Figure 18.4). Ultrasound or computed
timated on radiographs. The heart is better visualized when tomography (CT) may be necessary in these cases to deter-
the patient is in dorsal recumbency, as the heart is located mine if the unilateral opacity is due to pleural fluid, severe
(along with the lungs) in a nondependent position. When lung consolidation, or a large mass.

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.