April 17, 2009
Signalment: Eight-year-old male castrated standard poodle
History: Two-month history of decreased appetite and weight loss.
1. What are the primary findings?
2. What do you think is the primary reason for the clinical signs?
Radiographic interpretation: A soft tissue mass effect is causing a silhouette sign with the cranial border of the heart. The mass runs in a oblique right-cranial to left-caudal direction, indicating that it is in the ventral mediastinum.
The mass is seen better on the right lateral recumbent view due to the aerated lung highlighting the mass (recumbent atelectasis partially silhouettes the mass on the left lateral view). The heart and lungs are normal.
The mass in the cranial ventral mediastinum is most consistent with a neoplasm. Thymoma or lymphoma are the most common mediastinal neoplasia. Ultrasound of the cranial mediastinum with guided aspiration is often used to help differentiate these possibilities.
Normal anatomy: The mediastinum is the central portion of the pleural space and is composed of the left and right pleural sacs. Mediastinal disease usually involves the space between the mediastinal pleural layers. It extends from the thoracic inlet to the diaphragm and is primarily in the median plane of the thorax.
In some animals the mediastinum is fenestrated. It communicates with the fascial planes of the thoracic inlet as well as the retroperitoneal space. The main mediastinal organs that are identified radiographically are the heart, aorta, trachea and caudal vena cava.
Many other structures are located in the mediastinum but are not normally identified. The ventral aspect of the mediastinum is narrow and oblique to the median plane. The midportion of the media- stinum is wider as it contains a variety of structures and then it is narrow dorsally again. The dorsal mediastinum is parallel to the median plane.
The cranial mediastinum is visualized on the ventrodorsal view and normally should not be wider than twice the width of the vertebrae except for the brachycephalic breeds, which may accumulate fat in this location.
Typically, if fat is causing widening of the cranial mediastinum, the parallel lines of the dorsal mediastinum on the VD view remain parallel to the spine. Masses in this area often will cause the lines of the mediastinum to become convex. Ultrasound may be used to better evaluate this area.
In young animals, the thymus may be visualized extending to the left and is called the “sail” sign. On the lateral view, the cranial waist of the heart is composed of structures within the mediastinum.
The parallel lines cranially represent the dorsal mediastinum. The oblique line cranially is the ventral mediastinum. The thickness of these structures is variable depending upon the amount of fat present.
Mediastinal abnormalities: Four conditions commonly affect the mediastinum:
A mediastinal shift occurs when a loss of lung volume allows the mediastinum to shift toward the loss of volume. This is typical of lung atelectasis. A shift also can occur in the presence of tension pneumothorax. This shift is usually identified on the VD view.
Masses often affect the mediastinum, most commonly seen cranially. Common radiographic signs include dorsal displacement of the trachea, caudal displacement of the heart and a loss of visualization of the cranial waist of the heart.
Fluid also can accumulate in the mediastinum due to infection, inflammation, trauma or neoplasia. Large volumes of fluid can mimic masses radiographically, and ultrasound is often useful to further differentiate these two possibilities.
Pneumomediastinum is often detected due to increased visualization of mediastinal structures. Often, the brachycephalic trunk is noted.
Pneumomediastinum can lead to pneumoretroperitoneum or subcutaneous emphysema. Common causes include trauma to the lung, subcutaneous gas in the neck extending into the thoracic inlet, tracheal rupture, esophageal perforation, retroperitoneal gas or gas producing organisms.
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