Page 304 - Feline diagnostic imaging

18.3 Pneumothoraa 309
to identify the vascular structures and follow them to their Right‐sided heart failure, perhaps from cardiomyopathy or
origin. Finally, if the carina is dorsally deviated, even if the congenital right heart disease, mediastinal mass, with
cardiac silhouette is not visualized on either view, cardio- compression of the cranial vena cava, pericardial effusion,
genic causes of pleural effusion are more likely. heartworm disease, venous thrombosis, or congenital or
In one study, radiographs revealed the underlying cause acquired thoracic duct anomaly have all been implicated
of pleural effusion (mediastinal mass, pulmonary neopla- [2,6–12]. In most cases, however, an exact cause cannot be
sia, pneumonia, foreign body, evidence of trauma, and car- determined, resulting in a diagnosis of idiopathic chylotho-
diac abnormality) in only 37% of cases. In the remaining rax. Pleural effusion, with or without a visible underlying
cases, only pleural effusion was noted radiographically [2]. etiology, is present, and can be either bilateral or unilateral.
The type of fluid (transudate, modified transudate, exu- Ultrasound‐guided mesenteric lymph node injection with
date, hemorrhage, or chyle) cannot be determined radio- iohexol, followed by thoracic and abdominal CT, has been
graphically, but the images should be carefully evaluated to used to identify and evaluate the thoracic duct [13].
try to identify any underlying abnormalities that might
result in pleural effusion.
Rib tumors such as osteosarcoma and chondrosarcoma 18.2 Fibrosing Pleuritis
can create extrapleural masses which can result in pleural
effusion. Diaphragmatic hernias can also result in pleural Fibrosing pleuritis is a potential serious sequela to chronic
effusion. Cranial displacement of abdominal viscera or the chylothorax, although it can develop from any prolonged
appearance of gas‐filled intestinal loops cranial to the dia- exudative type of fluid (chylothorax, pyothorax, FIP, or
phragm are consistent with hernia. Underlying pneumonia hemothorax) [6,10,11,14]. A thick fibrous layer forms on
or pulmonary masses may not be seen with larger volumes the pleural surface, resulting in progressive atelectasis/
of pleural effusion, but might be better visualized after compression of the lung and preventing complete lung
thoracocentesis. The same is true for mediastinal masses expansion. The lung lobes take on a very rounded shape, or
(Figures 18.1, 18.5, and 18.6). Cardiac enlargement, with can appear as small nodules when severely atelectatic. The
secondary congestive heart failure, should also be consid- lobes do not change in shape or inflate, even after removal
ered as a cause of pleural effusion (Figure 18.3). Thoracic of pleural fluid (Figures 18.7 and 18.8). The only effective
ultrasound can be very helpful in determining a cause, as treatment is decortication (removal of fibrous layer), but
well as guiding thoracocentesis. this is effective only in the early stages [6,10]. Pneumothorax
is common with this procedure.
18.1.3 Pyothorax

Pyothorax is the accumulation of septic purulent fluid within 18.3 Pneumothorax
the pleural space. This infection is likely caused by aspiration
of oropharyngeal flora with subsequent infection of the lower Air or gas within the pleural space creates a pneumotho-
respiratory tract, followed by direct extension of infection to rax. Radiographic signs include a widened, radiolucent
the pleural space. This parapneumonic spread appears to be pleural space, and separation of heart and sternum on lat-
more likely than bacterial contamination secondary to bite eral views (due to relocation of the heart to the more
wounds of the thoracic wall. Because of the exudative nature, dependent midthoracic wall) (Figure 18.9). Radiolucent air
pleural fluid from a pyothorax may not pass through medias- will surround the dorsocaudal lung lobes when air is in the
tinal fenestrations, remaining localized to one pleural space. nondependent hemithorax. Radiolucent pleural fissure
Radiographic changes of concurrent pneumonia may also be lines are not seen, likely due to compression of the pleural
present on thoracic images (Figure 18.6) [2,4,5]. space as lung lobes collapse. Atelectasis will occur to vary-
ing degrees, resulting in increased pulmonary opacity. In
general, lateral projections demonstrate a pneumothorax
18.1.4 Chylothorax
more readily than ventrodorsal or dorsoventral images.
Chylothorax is the accumulation of chyle in the pleural Pneumothorax is most often due to trauma, with pulmo-
space, and can be secondary to a variety of disease pro- nary contusion and rupture of the pleura. Secondary spon-
cesses. Although a ruptured thoracic duct was initially taneous pneumothorax may be due to rupture of bullae or
thought to be the most common cause, this actually appears blebs, or could be due to underlying pulmonary disease. It
to be a rare occurrence. Anything that can cause high right‐ is important to examine the radiographs or CT images
sided venous pressures or restriction of thoracic duct carefully in these cases to try to determine the cause of the
lymph into the venous system can result in chylothorax. spontaneous pneumothorax. Feline asthma has been