RADIOGRAPHIC ANATOMY

The mediastinum is formed by the reflection of the parietal pleura and the space and structures between them. The two mediastinal pleural layers are part of the right and left pleural sacs. Each pleural sac is composed of mediastinal, diaphragmatic, costal and pulmonary pleura. These pleural components are continuous. There is controversy regarding whether the mediastinal pleura is fenestrated. In most dogs and cats mediastinum is fenestrated, allowing free communication between the two pleural sacs.

The mediastinum extends from the thoracic inlet to the diaphragm. The mediastinum is usually divided into three sections: a cranial portion cranial to the heart, a middle portion that includes the heart, and a caudal portion caudal to the heart. The mediastinum may also be divided into dorsal and ventral portions by a dorsal plane through the tracheal bifurcation. The organs present in the mediastinum are: heart, trachea, major blood vessels, oesophagus, lymph nodes, vagus and phrenic nerves, thoracic duct and in young animals the thymus. Normally, only the heart, trachea, caudal vena cava, aorta, and occasionally the oesophagus are visible on survey radiographs. The normal thymus may also be seen before involution in young dogs and cats (usually younger than 4 to 6 months of age). The normal thymus has a "sail sign" shape on the ventrodorsal view and may obscure the right side of the heart on the lateral view due to silhouette sign.

The mediastinum communicates cranially with the fascial planes of the neck via the thoracic inlet. Caudally the mediastinum communicates with the retroperitoneal space through the aortic hiatus. These communications provide the means for the spread of mediastinal disease to the neck and abdomen and vice versa.

The normal width of the cranial mediastinum varies among dog and cats breeds. On the ventrodorsal or dorsoventral projections, the mediastinum is superimposed on the spine, and the width of the mediastinum is usually less than approximately two times the width of the spine. In obese patients, the cranial mediastinum may be widened by fat accumulation, and the resultant opacity can be confused with a mediastinal mass.

There are three mediastinal reflections (Figs 1, 2, and 3) two of which are frequently identified in normal thoracic radiographs: (1) the cranioventral mediastinal reflection; (2) the caudoventral mediastinal reflection; and (3) the vena caval mediastinal reflection or the plica vena cava.

The cranioventral mediastinal reflection is caused by extension of the right cranial lung lobe across the midline, pushing the mediastinum to the left. It is usually visible on the ventrodorsal or dorsoventral projection as a curvilinear soft tissue opacity extending from T1 or T2 to the cranial left border of the heart (approximately the level of the pulmonary artery segment). The thymus lies in this reflection. On the lateral projection, the cranioventral mediastinal reflection and the margin of the right cranial lobe may frequently be identified immediately cranial to the heart.

The caudoventral mediastinal reflection is caused by extension of the right accessory lung lobe across the midline and is only seen on some ventrodorsal or dorsoventral radiographs. It is seen as a relatively straight, soft tissue opaque line extending from the left apex of the heart caudally to approximately the middle of the left diaphragm.

The caudal vena caval mediastinal reflection, or plica vena cava, is not visible.

Figures adapted from PF Suter and JA Gomez

RADIOGRAPHIC ABNORMALITIES OF THE MEDIASTINUM

Radiographic abnormalities of the mediastinum are related to:

1.  Positional abnormalities (mediastinal shift)

2.  Mediastinal effusion

3.  Mediastinal masses

4.  Abnormal visualization of mediastinal structures (pneumomediastinum)

1. Mediastinal Shift

A mediastinal shift occurs when the mediastinum is displaced by pressure differences between the right and left pleural cavities.

Mediastinal shift is diagnosed by evaluating the position of the heart, trachea, aorta and caudal vena cava or the mediastinal reflections on dorsoventral or ventrodorsal views.

When a mediastinal shift is seen on a radiograph, we must:

 Examine for a technical artifact, because rotation of the animal will mimic mediastinal shift

 Check for underlying lung disease (atelectasis or bronchial obstruction)

 Look for signs of a previous or present pleural disease

 Look for concomitant changes of the diaphragm, the spine or the thoracic wall.

The conditions that may cause mediastinal shift are:

1.  Unilateral or occasionally bilateral pneumothorax

2.  Unilateral pleural effusion

3.  Diaphragmatic hernia

4.  Bronchial obstruction

5.  Emphysema

6.  Positional atelectasis

7.  Pleural adhesions of the heart or lung to the thoracic wall.

2. Mediastinal effusion

Free mediastinal fluid is usually of soft-tissue opacity; therefore, it may appear radiographically as a mediastinal mass or as cardiomegaly if it collects around the heart, or both.

Mediastinal fluid may results in reverse fissures lines as fluid dissects into the interlobar fissures from hilar region. The reserve fissures line are wide centrally and narrow peripherally.

The more common causes of mediastinal fluid are:

1.  Feline infectious peritonitis

2.  Trauma

3.  Coagulopathy

4.  Oesophageal perforation

5.  Mediastinal masses

3. Mediastinal Masses

Mediastinal mass lesions are common, and the radiographic appearance of many mediastinal masses is similar. The ventrodorsal or dorsoventral views are more useful than the lateral view in deciding whether an abnormal mass is located in the mediastinum versus lung. Mediastinal location should be considered, if (1) the mass lies on or adjacent to the midline; (2) the mass is in a position consistent with one of the three previously described mediastinal reflections; (3) the mass deviates a mediastinal structure. Lung masses should not be confused with a mass of mediastinal origin because they usually lie in a position lateral to the mediastinum.

The differentiation of mediastinal masses is based on their specific location in one of five regions:

1.  Cranioventral

2.  Craniodorsal

3.  Hilar and perihilar

4.  Caudodorsal

5.  Caudoventral

Cranial mediastinal masses often cause elevation of the trachea. Elevation of the trachea, however may also be produced when there is a large volume of fluid in the pleural space but no mediastinal mass is present, due to the lungs are floating in the pleural fluid. Tracheal elevation is not seen when only a small volume of fluid is present in the pleural space unless a mediastinal mass is also present. The location of a mass lesion within the mediastinum is helpful in formulating a differential diagnosis. Causes of mediastinal masses are listed in Table 1.

Table 1. Typical Location of Mediastinal Masses

Mediastinal lymphadenopathy is one of the most common causes of a mediastinal mass, usually involving the cranial mediastinal, tracheobronchial and/or sternal lymph nodes. It is an extension of a disease process from the lung, heart, spine, oesophagus, sternum or chest wall, as a local manifestation of a generalized disease (e.g., lymphosarcoma) or as a congenital anomaly (e.g., branchial cyst).

The cranial mediastinal lymph nodes lie along the cranial vena cava and brachiocephalic, left subclavian, and costocervical arteries, and are located in the cranial mediastinum, just ventral to the trachea. Enlargement of the cranial mediastinal lymph nodes results in a visible mass in the cranial mediastinum that often elevates the trachea dorsally and to the right of the midline.

The tracheobronchial lymph nodes are known as the right, left and middle tracheobronchial lymph nodes. The right and left nodes lie on the lateral side of their respective bronchus and also contact with the trachea. The right node is ventral to the azygos vein, the left ventral to the aorta. The middle tracheobronchial lymph node is the largest of the group. It is in form of a V and lies in the angle formed by the origin of the primary bronchi from the trachea. Enlargement of the tracheobronchial lymph nodes is commonly seen with generalized lymphoreticular neoplasms or inflammatory lung disease and results in visualization of a soft-tissue opacity in the region of the tracheal bifurcation on the lateral view. Enlargement of the tracheobronchial lymph nodes is usually more apparent on the lateral than on the ventrodorsal or dorsoventral projections. Tracheobronchial lymph node enlargement usually displaced the main stem bronchi ventrally at the carina, and there is often narrowing of the trachea and/or main stem bronchus.

The sternal lymph node is usually represented by a single node on each side in the dog and a single node in the cat. In the dog, there is occasionally only a single median node. The sternal node lies in the ventral mediastinum, immediately cranial to the transverses thoracis muscle and medial to the second costal cartilage or second interchondral space. Afferent vessels receive tributaries from ribs, sternum, serous membranes, thymus, adjacent muscles, and mammary glands. Sternal lymphadenopathy appears as an isolated soft-tissue opacity dorsal to the region of the second to fourth sternebra and is best seen on the lateral projection.

4. Pneumomediastinum

Pneumomediastinum is free gas in the mediastinum. Mediastinal air provides excellent radiographic contrast, thereby resulting in enhanced visualization of mediastinal organs (oesophagus, azygos vein, the external wall of the trachea, the cranial vena cava, aorta, and/or the brachycephalic artery). The heart may be displaced from the sternum in a recumbent lateral view as in pneumothorax but lungs margins will be seen extending to the periphery of thorax. Pneumomediastinum is not readily seen on ventrodorsal or dorsoventral radiographs.

Pneumomediastinum may progress to pneumothorax if mediastinal pressure results in tearing of mediastinal pleura, thus establishing communication between the mediastinum and the pleural space. Due to communication of the mediastinum with the neck and retroperitoneal space, pneumomediastinum may result in subcutaneous emphysema or pneumoretroperitoneum.

The pneumomediastinum may be caused by a variety of conditions:

1.  Extension of gas into the mediastinum from the deep fascial planes of the neck. This occurs with deep open wounds of the neck or mouth; it also may result from tracheal or oesophageal rupture.

2.  Air escaping into the lung interstitium from sites of alveolar rupture can diffuse in a retrograde direction in loose connective tissue adjacent to bronchi and vessels into the mediastinum. This situation occurs commonly after trauma and also occurs after iatrogenic pulmonary hyperinflation during anesthesia or resuscitation.

3.  A tear in the wall of the trachea may occur as a result of trauma or erosion associated with neoplasia or inflammatory disease. If the tear is intrathoracic, air enters the mediastinum directly. If the tear is in the neck, air may dissect along the trachea through the thoracic inlet into mediastinum

4.  Air escaping from the oesopheal rupture may occur.

5.  Air ascending from the abdomen, particularly the retroperitoneal space into the mediastinum may occur.

References

1.  Suter P.F., Gómez J.A.Diseases of the thorax- Radiographic Diagnosis. Iowa State University Press, Ames. 1987.Pp 23-30.

2.  Thrall D.E. The Mediastinum. In Textbook of Veterinary Diagnostic Radiology Ed D.E. Thrall. 3 Ed W.B. Saunders Co. Philadelphia.