By Steven Tsai, DVM, DACVR
Imaging of the mediastinum and pleura in dogs and cats can be challenging, as the majority of structures are soft tissue opaque and silhouette into a fairly nondescript part of the thorax. The heart and lungs typically dominate the clinical and radiographic picture, with the rest of the mediastinum and pleural space being an afterthought. Knowing the normal anatomy of the mediastinum goes a long way towards identifying and understanding common mediastinal abnormalities.
Mediastinum: Normal Anatomy
The mediastinum is the median partition of the thoracic cavity, covered by the mediastinal part of the parietal pleura and containing all the thoracic viscera and structures except the lungs. It is loosely subdivided into cranial (cranial to the heart), middle (the heart and everything immediately dorsal and ventral to the heart), and caudal (caudal to the heart). The cranial mediastinum contains the trachea, esophagus, thymus, sternal and cranial mediastinal lymph nodes, various blood vessels, and nerves. The middle mediastinum contains the heart, the main pulmonary artery and its principal branches, the aorta, the azygos vein, thoracic duct, carina and mainstem bronchi, tracheobronchial lymph nodes, esophagus, phrenic nerves, vagus nerve, and the thymus. The caudal mediastinum contains the aorta, azygos vein, thoracic duct, esophagus, vagus nerve, phrenic nerves, and the caudal vena cava. On a lateral view, the ventral margin of the cranial mediastinum is typically a straight line coursing roughly parallel with the vertebral column. On a DV or VD view, the cranial mediastinum is roughly twice the width of the thoracic vertebrae. These are both considered loose rules of thumb rather than hard limits. In a normal thorax on a DV or VD view, the tracheal deviates slightly to the right of midline in the cranial mediastinum. This is exacerbated in brachycephalic dogs.
Mediastinum: Common Abnormalities
Pneumomediastinum – With the exception of the tracheal lumen, there is normally no free gas within the mediastinum. Pneumomediastinum can occur with a tracheal tear or other airway leakage, esophageal tear, or from gas tracking caudally from the neck secondary to cervical trauma. A difficult jugular venipuncture can sometimes be enough to cause a secondary pneumomediastinum. Pneumomediastinum can lead to pneumothorax and pneumoretroperitoneum, and typically the reverse does not occur. The radiographic signs of pneumomediastinum primarily involve increased visibility of normal mediastinal anatomy, including being able to distinctly visualize the dorsal and ventral walls of the esophagus and trachea, or increased visibility of individual great vessels such as the cranial vena cava, major branches of the aortic arch, and the azygos vein.
Mediastinal mass lesions – Mediastinal mass lesions most commonly arise from the thymic remnant, mediastinal lymph nodes, or the heart base. Common thymic masses include thymoma, thymic lymphoma, or thymic carcinoma. Thymic mass lesions are typically centered immediately cranial to the heart, slightly to the left of midline. Heart base mass lesions are typically not radiographically visible unless they are quite large. Enlargement of the cranial mediastinal lymph node is typically seen as a ventral bulging of the margin of the cranial mediastinum. Enlargement of the sternal lymph node is typically seen as a broad-based soft tissue opacity dorsal to the sternebrae (second and third sternebrae in dogs, third and fourth sternebrae in cats). Tracheobronchial lymphadenopathy can be very challenging to diagnose as the radiographic sign is usually a vague increased soft tissue opacity in the perihilar region. Tracheobronchial lymphadenopathy is commonly mistaken for cardiogenic pulmonary edema. Mediastinal cysts or branchial cysts are benign embryonic remnants which are radiographically indistinguishable from a sternal lymph node or thymic mass.
Esophageal disorders – The most common esophageal disorders include gastric hiatal hernias, esophageal foreign bodies, and megaesophagus. Gastric hiatal hernias are typically sliding and incidental, especially common in brachycephalic breeds. Occasionally, hiatal hernias may be permanent or result in clinically significant regurgitation and require surgical correction. Esophageal foreign bodies are most commonly located immediately cranial to the carina or immediately cranial to the diaphragm, as these are the two locations with the greatest resistance to aborad movement. Esophageal dilation cranial to the foreign body may or may not be present. Megaesophagus is defined as dilation of the esophagus (with no denotation of underlying cause) and may be transient or permanent. A diagnosis of esophageal dysmotility should never be made on the basis of a single static radiographic study. Transient megaesophagus is commonly seen with esophagitis secondary to vomiting or regurgitation. A diagnosis of esophageal dysmotility should only be made after documentation of persistent megaesophagus over multiple points in time.
Tracheal disorders – Tracheal collapse secondary to tracheomalacia is probably the most commonly encountered structural tracheal abnormality seen in general practice. This condition most commonly affects miniature and toy breed dogs and is characterized by a honking cough which is easily elicited with tracheal palpation. Diagnosing tracheal collapse with static radiographs is quite difficult as this is a dynamic problem, although it can be strongly suspected with clinical history and demonstration of tracheal narrowing on lateral radiographs. It is important to differentiate between a redundant dorsal tracheal membrane (caused by superimposition with the trachealis muscle) and true tracheal narrowing. Tracheal hypoplasia is sometimes seen in brachycephalic dogs as part of a general brachycephalic airway syndrome. Tracheal hypoplasia is diagnosed based on measurement of the ratio between the tracheal lumen diameter at the level of the first rib and the thoracic inlet (measuring the distance between the cranioventral aspect of the first thoracic vertebra in the cranial aspect of the sternum). Normal dogs have a ratio >0.21, while English Bulldogs may have a ratio as low as 0.11 without clinical symptoms. Tracheal mass lesions are rare and typically characterized by a focal tracheal narrowing which may or may not visibly be associated with a mass lesion.
Pleural Space: Normal Anatomy
The pleural lining is a single layer of mesothelial cells overlying a thin layer of vessels and lymphatics. The parietal pleura covers the inner surface of the thoracic body wall as well as the mediastinum. Visceral pleura covers the surface of the lungs. The left and right pleural cavities typically communicate through several fenestrations in dogs and cats, which is why pleural effusion and pneumothorax are frequently bilateral even if the underlying etiology is unilateral. The gap between the parietal and visceral pleura is the pleural space, a potential space which does not normally enclose any viscera. Normally, approximately 2-3 mL of fluid is present within the pleural space as a lubricant, and homeostasis of this pleural fluid is maintained by a balance between constant fluid production and subsequent lymphatic drainage (in humans, 5-10 liters of fluid pass through the pleural space every 24 hours). The pleural space maintains a slight negative pressure which helps to keep the lungs expanded at rest. The pleura is largely not visible radiographically, although occasionally the angle of the x-ray beam can pass directly in plane with a portion of the pleura, creating a very thin pleural fissure line. Thin pleural fissure lines can also be seen in older pets due to incidental pleural thickening. Pleural fissure lines are typically seen extending from the periphery at the level of the anatomic interlobar fissures between the cranial, middle, and caudal pulmonary lobes/subsegments.
Pleural Space: Common Abnormalities
Pleural effusion – Mild pleural effusion can sometimes be difficult to differentiate from incidental pleural thickening. Typically, pleural effusion will result in fissure lines which widen subtly (or not so subtly) towards the periphery. On lateral views, ventral pleural effusion will frequently obscure the cranioventral margin of the heart. This should be differentiated from pseudo-effusion, which can result from accumulation of fat ventral to the heart. In this case, the cardiac margin should still be visible through the opacity. Also note that in cats, a triangular soft tissue opacity can be seen at the caudodorsal margin of the thorax, mimicking pleural effusion. This is caused by the cranial attachment of the psoas minor muscle, which extends farther cranial in cats than in dogs, and can be seen at the T12-T13 level. The different types of pleural effusion (transudate, exudate/pyothorax, chylous, hemorrhage, malignant) cannot be reliably differentiated radiographically, and diagnostic thoracocentesis is generally recommended. Chronic pleural effusion (especially exudative or chylous) can result in severe pleural thickening causing restrictive pleuritis. This is typically seen as abnormal rounding of the lung margins and an inability of the lungs to return to a normal size/shape even after resolution of the pleural effusion.
Pneumothorax – Free air within the pleural space is never normal and can result from external trauma or internal pathology. Compromise of the airway or pulmonary parenchyma can result from pneumonia, chronic pulmonary disease, or neoplasia. The most common cause of spontaneous pneumothorax in dogs is rupture of a pulmonary bulla (or bleb); the most common cause in cats is chronic inflammatory airway disease (asthma). CT of the thorax with spontaneous pneumothorax is typically unrewarding, as the underlying pulmonary disease is usually multifocal or diffuse, and the active air leak cannot be identified. The radiographic sign of pneumothorax is a hyperlucent area between the lung and the body wall, in which no pulmonary vessels/parenchyma is visible. With a diagnostic quality radiograph, the margin of the lung is often visible retracted from the body wall or diaphragm. It is important to differentiate from a pseudo-pneumothorax which can be seen in some dogs due to body conformation. In deep chested dogs on a lateral view, there is often normal separation between the heart and the sternum, but lung parenchyma should be visible in the gap. On a VD view in some dogs, skin folds along the long axis of the torso may mimic a lung margin retracted from the body wall. Tension pneumothorax is important to recognize radiographically as this is a surgical emergency. In this condition, air is actively leaking into one pleural cavity at a high enough rate to cause a gradually increasing intrathoracic pressure. This results in significant unilateral pulmonary atelectasis (the collapsed lungs may be rounded and mimic mass lesions) with a concomitant mediastinal shift away from the affected side (due to unilateral increased pressure).
Extrapleural masses – An abnormality in the pleural margin can sometimes be seen with extrathoracic mass lesions, creating a so-called “extrapleural sign.” This is a smooth, broad-based, convex bulge from the thoracic wall into the pleural cavity. This can be seen with mass lesions arising from the ribs or soft tissue structures of the thoracic wall.