RADIOLOGICAL EXAMINATION OF THE MEDIASTINUM AND DIAPHRAGM
Anatomic Division and Contents
The mediastinum is the space lying between the right and left pleurae in and near the median sagittal plane of the chest. It extends from the posterior aspect of the sternum to the anterior surface of the thoracic vertebrae and contains all of the thoracic viscera except the lungs. It is divided into three major parts: anterior, middle, and posterior . The superior mediastinum has also been described and lies between the manubrium sterni and the upper four thoracic vertebrae. It contains structures of the anterior, middle, and posterior mediastinum that extend into it from below and above as well as those that traverse it and the mediastinum below it. Therefore, the designations anterior, middle, and posterior mediastinum are most frequently used to include the length of the thorax without specific designation of superior, which contains the aortic arch and its branches as well as the brachiocephalic veins, upper half of the superior vena cava, trachea, esophagus, thoracic duct, thymus, lymph nodes, and various nerves, some of which traverse the length of the mediastinum. If the three compartments are used to include their portion of the superior mediastinum, the anterior mediastinum is bound above by the thoracic inlet, laterally by the pleura, anteriorly by the sternum, and posteriorly by the pericardium and great vessels. It contains loose areolar tissue, lymph nodes, some lymphatic vessels that ascend from the convex surface of the liver, thymus, thyroid, parathyroids, and internal mammary arteries and veins. It is termed “prevascular space”. The middle mediastinum, the “vascular space,” contains the heart and pericardium, the ascending and transverse arch of the aorta, the superior vena cava and the azygos vein that empties into it, the brachiocephalic arteries and veins, the phrenic nerves, the upper vagus nerves, the trachea and its bifurcation, the main bronchi , the pulmonary artery and its two branches, the pulmonary veins, and adjacent lymph nodes (see Fig. 1). It is bound in front by the anterior mediastinum and posteriorly by the posterior mediastinum. The posterior mediastinum, the “postvascular space,” lies behind the heart and pericardium and extends from the level of the thoracic inlet to the twelfth thoracic vertebra. It contains the thoracic portion of the descending aorta, esophagus, thoracic duct, azygos and hemiazygos veins, lymph nodes, sympathetic chains, and inferior vagus nerves. There is some difference of opinion regarding the limits of the mediastinal divisions and the contents of each, but the preceding description is used extensively.
Lymph Nodes
The mediastinal nodes are divided into two major groups: anterior mediastinal and paratracheobronchial. The anterior mediastinal (prevascular) nodes lie anterior to the superior vena cava and the right innominate vein on the right side. On the left, they lie anterior to the aorta and carotid artery. One or two nodes lie anterior to the ligamentum arteriosum, the ductus node(s) in the region of the aortopulmonary window, which is a concave area between the inferior aspect of the transverse aortic arch and the pulmonary artery. When these nodes are enlarged, the concavity is effaced or a convexity is produced. The anterior group of nodes cannot be reached by mediastinoscopy, but a biopsy can be done via an anterior parasternal approach.
The paratracheobronchial nodes can be subdivided into paratracheal, bifurcation, and pulmonary root nodes. The right paratracheal nodes lie behind the superior vena cava and innominate vein, anterolateral to the trachea. The lowermost node in this chain, the azygos node, lies medial to the azygos arch. The drainage to the azygos node comes not only from the right lung and other nodes on the right but also from the left lung and nodes, so it is very important clinically. A biopsy of these nodes can be done by mediastinoscopy. The left paratracheal nodes are more posterior than those on the right, and they lie posterolateral to the trachea and posterior to the left subclavian artery and the adjacent portion of the aortic arch. The subcarinal (bifurcation) nodes lie below the tracheal bifurcation, and a few small nodes extend along the undersurface of the main bronchi. There may also be a few small nodes in the pretracheal and posttracheal areas at the level of the carina. The pulmonary root nodes, now termed hilar nodes, extend in a variable manner along the central portions of the major bronchi and pulmonary vessels. They can be divided into anterior, posterior, superior and inferior groups according to their relationship to the bronchi and vessels. A few inconstant nodes may be found more peripherally, the interlobar and the lobar or intrapulmonary nodes. The interlobar group lies at the division of the lobar bronchi and the lobar group at the division of the segmental bronchi. Radiographic differentiation of the pulmonary root (hilar) nodes from the more central interlobar nodes is virtually impossible.
In addition to the node groups described in the mediastinum, there are three more peripheral groups termed parietal or extrapleural. The posterior group is variable and lies along the intercostal vessels and nerves and along the thoracic vertebral surfaces. The paravertebral nodes are more numerous along the lower half of the thoracic spine. The internal mammary nodes lie adjacent to the internal mammary arteries and posterior to the ribs and interspaces. They are more numerous superiorly. The diaphragmatic nodes lie anterior to the pericardium, usually lateral to the midline on both sides.
As indicated, a biopsy can be done of the right paratracheal nodes but not of the anterior (prevascular) nodes at mediastinoscopy. The left paratracheal nodes are infrequently involved by lung tumors, but right paratracheal nodes are involved by right-sided and occasionally by left-sided lung tumors. Tumors of the left lung tend to metastasize to the anterior (prevascular) nodes such as the aortopulmonary window (ductus) nodes, which are visible on chest radiographs when enlarged. Subcarinal nodes are more commonly involved by lower lobe than by upper lobe tumors. Normal nodes are small and cannot be identified on chest radiographs. Some small normal nodes are sometimes observed on CT, however.
Other Roentgen Features of the Mediastinum
The trachea and main bronchi are usually visible in a chest roentgenogram of good quality. These structures lie within the mediastinum, and the trachea is situated in the midline except for very slight deviation to the right at the level of the aortic arch. In infants, moderate tracheal deviation away from the side of the aortic arch (usually to the right) is common, but this is not usually observed in children after the age of 5 years. In older persons the trachea may curve slightly to the left above the arch and then to the right as it passes the arch. As a rule, the angle formed by the bronchi with the sagittal plane is equal bilaterally until age 15 or so. Then the tracheal deviation to the right causes an increase in the angle on the left, the normal adult configuration. The sum of the two angles, the subcarinal angle, averages about 60°, but there is a wide variation in normal subjects. The trachea extends from the level of the sixth cervical vertebra downward to the level of the fifth thoracic vertebra or slightly lower, where it divides into the right and left main bronchi. It is identified on the roentgenogram as a band of radiolucency in the midline that extends from the lower cervical region downward to the point of bifurcation.
The main bronchi are somewhat smaller in diameter. The right main bronchus continues downward more vertically than the left in the adult and divides into two main branches. On the right the first branch is the upper lobe bronchus, which curves sharply upward above the right pulmonary artery (the eparterial bronchus). The continuation downward is termed the hyparterial or bronchus intermedius, which continues as the right lower lobe bronchus. The middle lobe bronchus arises from and marks the lower end of the bronchus intermedius. It extends downward and laterally from its point of origin. On the left side the main bronchus is somewhat longer, deviates laterally more than on the right, and forms a greater portion of the subcarinal angle. In addition to the lateral angulation, it curves outward in its distal portion and divides into a lower lobe bronchus and a left upper lobe bronchus that courses horizontally for a short distance before dividing. A continuation of the left main bronchus downward and laterally forms the lower lobe bronchus. It is usually possible to outline the main bronchi and portions of the upper and lower lobe bronchi in the normal patient. These structures have an appearance similar to that of the trachea, namely, a band of radiolucency, but are smaller in diameter.
In the frontal projection of the chest, the mediastinum along with the sternum and thoracic spine forms the dense central shadow observed on the normal roentgenogram. On the right side the superior border is formed by the brachiocephalic artery or vein, below which lies the superior vena cava. The ascending aortic arch is usually not border forming, but in cardiac diseases or aortic diseases that produce aortic dilatation it may form the right border (usually convex) for a short distance. Immediately below the ascending aortic arch is the hilum. The smooth convex border of the right atrium forms the lower right mediastinal border. On the left side the left subclavian artery forms the superior border of the mediastinum. Below this, the rounded convexity of the aortic arch is outlined. The aortopulmonary window is a local concavity or notch between the aortic arch and the pulmonary artery. The pulmonary artery and the hilum of the left lung lie immediately below the aortic arch, and the left ventricle forms most of the left lower mediastinal border, although a short segment of the pulmonary outflow tract may be visible below the hilum.
In infants the thymus is often a large structure that lies in the superior portion of the anterior mediastinum. When visible, it produces widening of the mediastinum superiorly; this widening is often asymmetrical. The thymus then forms the lateral border of the upper mediastinum on both sides. At times, the inferior aspect of the enlarged thymus forms an acute angle on one or both sides. It is not unusual to note some lobulation of the thymus. When such superior mediastinal widening is present, it may be necessary to obtain a lateral projection to prove that the shadow is in the anterior mediastinum and thus represents the thymus. Moderate widening of the superior mediastinal shadow is not considered abnormal in infancy; this portion of the mediastinum usually assumes its normal width during the first year of life.
The left superior intercostal vein arises from the second, third, and fourth intercostal veins posteriorly, courses downward in the lateral vertebral gutter, and turns anteriorly at the level of the aortic arch. It empties into the posterior aspect of the left innominate vein. In 75% of patients, this vein communicates with the accessory hemiazygos vein at its lower end. The course of this vein around the aortic arch is somewhat variable, and when it is lateral, inferolateral, or superolateral to the aorta, a small protuberance, the aortic nipple, is observed. This represents the vein, seen in cross section, as it courses around the aorta (1.4%).
The lungs approach the midline in the anterior mediastinum. As a result, the air in the lung on either side defines a vertical linear shadow called the anterior junction line described earlier. It extends from a point near the level of the sternal angle superiorly to a point 3 or
The lungs also outline a pleural interface in the paraspinal area on both sides. This is usually 2 to
In the lateral view of the chest the anatomic divisions of the mediastinum are well demonstrated (Fig. 2). The superior mediastinum lies above the horizontal line and is included because of many references to it in the literature. The anterior mediastinum is seen as an area of relative radiolucency between the sternum and the heart. It is roughly triangular in shape, with the apex pointing downward. The large thymus is noted as an area of opacity in the anterior mediastinum in infants. The middle mediastinum is clearly defined in a lateral roentgenogram, since it contains the heart and aorta. The posterior mediastinum is the area lying between the heart and the spine. It is visualized as a radiolucency approximating that of aerated lungs in normal subjects because it contains no opaque structures such as the heart and great vessels, which occupy the middle mediastinum. The trachea is visible in the lateral roentgenogram of the chest as a radiolucent structure that angles slightly posteriorly as it extends into the chest. The posterior tracheal stripe (wall), about 2 to
Computed Tomography of the
The images of the mediastinum obtained by CT depict cross-sectional anatomy that is quite different from the anatomy depicted in sagittal and coronal planes on chest radiographs. Individual structures can be recognized and the site of an enlargement can be determined. At the level of the sternoclavicular junction, which is at about the thoracic inlet, five vessels can usually be identified right (from anterior to posterior): brachiocephalic vein, brachiocephalic artery (usually medial to vein); left (from anterior to posterior): left brachiocephalic vein, left carotid artery, and left subclavian artery. The left brachiocephalic vein may be seen longitudinally as it crosses the midline. At a level slightly above the sternoclavicular junction, the paired jugular veins, carotid arteries, and subclavian arteries may be visible. The trachea and esophagus are recognized at both levels. The veins are usually anterior to the arteries and the carotids anterior to the subclavian. At the level of the aortic arch, only the superior vena cava and aorta are seen in addition to the trachea and esophagus. When the aorta is dilated and tortuous, the left brachiocephalic vein may lie anterior to the arch. At the level of the aortopulmonary window, immediately below the aortic arch and just cephalad to the carina, the ascending and descending aorta, the superior vena cava as well as the trachea and esophagus are seen in cross section. In about this area, the azygos arch may be included as the vein courses anteriorly from the prevertebral area to enter the superior vena cava. At a level just below the carina, the aorta, superior vena cava, and esophagus are seen along with the left pulmonary artery, the left main bronchus, the right upper lobe bronchus, the right upper lobe artery (truncus anterior) just anterior to the bronchus, and the prevertebral portion of the azygos vein. A short segment of the distal azygos vein may also be seen as it enters the superior vena cava. Just below this level, the right pulmonary artery is seen as it bifurcates from the main pulmonary artery and courses from anterior to posterior in an oblique position behind the ascending aorta and superior vena cava. The descending aorta, bronchus intermedius, and left upper lobe bronchus are usually visible at this level. The hilar vessels and bronchi are discussed in greater detail in paragraphs to follow.
At a level about a centimeter below the left upper lobe bronchus, the left and right atria, the aortic root, descending aorta, and main pulmonary artery are visible along with the esophagus and azygos vein. One or more pulmonary veins may be observed draining into the left atrium. The esophagus is just posterior to the left atrium at this level. Lower, at the level of the ventricles, the anterior pericardium is usually identified as a thin, dense, retrosternal structure with fat anterior and posterior to it. The inferior vena cava lies lateral to a narrow venous structure extending forward to the right atrium, the coronary sinus. The esophagus and descending aorta are also seen on this section. The septal groove is often identified anteriorly. When a bolus of intravenous contrast medium opacities the ventricles, the septum is visible. Below the heart, the diaphragmatic crura are observed, with lung anterior to them and the esophagus, descending aorta, azygos and hemiazygos veins, and thoracic duct noted in the retrocrural space. This marks the lower extent of the posterior mediastinum. The superior aspect of the liver is observed on the right and sometimes on the left, where stomach and spleen may also be visible.
Intravenous contrast enhancement can be accomplished by dynamic scanning, which consists of injecting a bolus of contrast material followed quickly by a series of rapid sequence scans at the same or different levels. This method is used to identify suspected vascular structures or lesions. Continuous drip of additional contrast material may be added as more scans are obtained. In many departments, CT of the mediastinum is usually performed after and during infusion of intravenous contrast material in doses ranging from 125 to 150 ml of 60% contrast agent.
THE MEDIASTINUM
Because a number of structures lie within the mediastinum and tumors metastasize there, several diagnostic possibilities arise when abnormality is seen. CT has proved to be the most productive study when a mediastinal abnormality is observed on plain films.
Because several imaging sequences are usually required, MRI is more timeconsuming and expensive; therefore, it is ofteecessary to limit the clinical use of MRI in mediastinal disease. Invasion of vascular structures is often demonstrated better on MRI than on CT; the same is true of subtle invasion of other mediastinal structures. The ability to produce coronal and sagittal images is an advantage of MRI which is of definite value in some situations, such as superior pulmonary sulcus (Pancoast) tumor where vascular and CNS invasion is more clearly defined. The ultimate place of these two modalities in relation to various abnormalities in the mediastinum and thorax has yet to be determined.
INFLAMMATORY DISEASES
Acute Mediastinitis
Acute inflammations of the mediastinum, although rare, usually arise following injury of the esophagus caused by ingestion of sharp foreign bodies or instrumentation of the esophagus. When esophageal rupture is suspected, it can be confirmed quickly by the demonstration of extravasation of ingested contrast material into the mediastinum or pleural space. Occasionally inflammation extends into the mediastinum from infections involving the sternum, spine, anterior chest wall, and mediastinal lymph nodes, and rarely from infections originating in the anterior neck or the subdiaphragmatic area. These mediastinal infections may result in the formation of abscesses, which may rupture into the esophagus, tracheobronchial tree, or pleural space. These ruptures may cause a rapid change in the mediastinal contour as well as a sudden clinical change. When antibiotics are given early following an esophageal injury, abscess formation is uncommon, so the incidence of mediastinal abscess is now relatively low. The chest radiographic findings include a diffuse increase in opacity and widening of the mediastinum to both sides of the midline in the region of involvement. If the process extends downward from a retropharyngeal abscess, roentgenograms of the neck obtained in lateral projection often show the soft-tissue mass displacing the pharynx and trachea anteriorly. When the infection has resulted from esophageal injury it is not uncommon to observe a small amount of mediastinal emphysema manifested by streaks of radiolucency. If an abscess becomes chronic, it may be large and clearly defined so its appearance may simulate that of mediastinal tumor. It is not unusual to have enough pleural reaction to produce effusion. The suspicion of acute mediastinitis arises on the basis of correlation of roentgen findings with the clinical history. CT can then be used to define more clearly the mediastinal abnormalities. The findings are varied. In some, there is diffuse involvement leading to loss of tissue planes and mediastinal widening, local or diffuse. In others, a local mediastinal abscess is present. There may also be gas in the soft tissues if the esophagus is ruptured or if the infection is caused by gas-forming organisms. In patients who have had surgery with sternotomy, soft tissue planes are effaced so benign postoperative changes may not be differentiated from acute mediastinitis. In more subacute or chronic situations, osteomyelitis of the sternum may be present, usually after sternotomy. A rapid accumulation of blood, chyle, or edema fluid may also cause acute mediastinal widening. The history and clinical findings are usually helpful in differentiating these conditions.
Chronic Mediastinitis
Granulomatous Mediastinitis
Tuberculosis, histoplasmosis, and rarely actinomycosis may produce a chronic granulomatous infection in the mediastinum. Other organisms are less commonly the cause. The involvement is usually in the upper mediastinum at and above the level of the hila and usually anterior to the trachea. Slight mediastinal widening is often the only finding. The radiographic findings are not characteristic and may be very minor, consisting of slight mediastinal widening, which is usually more evident on the right. There may or may not be calcified nodes associated with the chronic disease, which often is asymptomatic. Occasionally diffuse mediastinal inflammation caused by histoplasmosis results in enough fibrosis to obstruct the superior vena cava, and in these patients roentgen findings may be minimal. There may be associated pulmonary involvement. Enlargement of mediastinal nodes is common early in the disease. These nodes may decrease in size and disappear in the chronic phase, however. In other patients they may calcify and may gradually increase in size. If they are adjacent to the superior vena cava, they may produce caval obstruction many years after the initial infection.
Chronic Fibrous (Sclerosing) Mediastinitis Occasionally, signs of superior caval obstruction will appear in a patient with no history of pulmonary, esophageal, tracheal, or pharyngeal disease. These signs evidently may be caused by a number of conditions, most commonly histoplasmosis, but there may be other infectious causes as well as an idiopathic process similar to that noted in retroperitoneal fibrosis. No cardia may occasionally cause mediastinitis with caval obstruction, and rarely sarcoidosis is responsible. The location of the disease is the same as in chronic granulomatous mediastinitis namely, the upper anterior half of the mediastinum. Roentgen findings consist of slight mediastinal widening, often slightly more in the right paratracheal area than elsewhere. Hilar enlargement may or may not be present, sometimes with evidence of calcified nodes. Some tracheal narrowing may also be present. Venography can be used to demonstrate venous obstruction, but CT with intravenous contrast can also be used. MRI is better at assessing vascular patency without the use of contrast agents. As indicated above, CT or MRI are sometimes of value, particularly when a well defined granulomatous mass is observed, which may be amenable to surgery. When the fibrosis is diffuse and CT shows obliteration of fat planes, narrowing or obstruction of vessels or proximal airways, or narrowing of the esophagus, surgery may have little to offer. When the mediastinal fibrosis is a part of systematic fibrosis, which often begins in the retroperitoneum, the process is often predominantly posterior. Granulomatous histoplasmosis may also be posterior. Calcifications are present in some patients. The clinical findings must be correlated with radiographic findings to determine diagnosis. If tissue diagnosis is needed, mediastinoscopy or surgical exploration may be necessary.
Inflammatory Diseases of the Mediastinal Lymph Nodes
Iearly all infectious diseases of the lungs and bronchi there is histopathologic involvement of the mediastinal nodes, but most of these diseases do not usually cause enough enlargement to be of roentgen importance. There are chronic pulmonary inflammatory diseases, however, in which there is sufficient enlargement of hilar and mediastinal nodes to produce recognizable opacity on chest radiographs.
Acute Nonspecific Lymphadenopathy Some enlargement of the hilum shadow is often present in pneumonia, but is so minimal it usually escapes detection or is obscured by the pulmonary infection. Lung abscess is frequently accompanied by hilar and/or mediastinal adenopathy. Rarely, infectious mono-nucleosis is associated with mediastinal adenopathy. Occasionally an infected node may undergo suppuration, leading to acute mediastinitis or mediastinal abscess.
Chronic Inflammations
Occasionally, lymph nodes are enlarged in patients with chronic suppurative disease of the bronchi and in some patients with mucoviscidosis, leading to recognizable hilar enlargement which is often poorly defined and is associated with the accentuation of basal markings and patchy pneumonia often found in bronchiectasis. A number of fungi are capable of producing pulmonary disease and they also involve the hilar nodes, resulting in enlargement. Adenopathy is most commonly found in the acute phase of coccidiodomycosis and histoplasmosis and may be present in other infectious diseases, such as actinomycosis and blastomycosis. In these diseases the pulmonary infection produces changes that have been described in the previous chapters relating to these infections. This involvement of hilar and mediastinal nodes often leads to calcification in the nodes, which requires 1 or 2 years to develop. It is most commonly observed in histoplasmosis.
Primary tuberculosis is almost always associated with mediastinal lymphadenopathy, particularly in children. The enlargement of hilar or paratracheal nodes may or may not be associated with a visible parenchymal lesion. Characteristically, in primary tuberculosis a single group of nodes is involved. If more than one group is affected, one group is usually considerably larger than the other. The roentgen findings are those of a somewhat lobulated node enlargement; the outline of the hilum may be moderately fuzzy and indistinct. A visible parenchymal lesion may be present. When the acute phase is over, the borders of the hilum become more distinct and nodes gradually decrease in size. Calcification is ofteoted after a year or more.
All of these chronic inflammatory diseases that involve the lungs, as well as the mediastinal nodes, must be differentiated from one another by appropriate skin tests and bacteriologic studies. Node enlargement is noted in a variety of noninfectious diseases of the lungs, including some of the pneumoconioses. In these patients the pulmonary lesions are usually predominant. Massive mediastinal lymph-node enlargement has been reported occasionally in patients without other disease. The large nodes simulate mediastinal tumors or malignant lymphomas. Biopsy reveals a nonspecific chronic lymphadenitis. Sarcoidosis is usually associated with lymphadenopathy sufficient to produce recognizable enlargement of hilar and mediastinal lymph nodes at some time during the course of the disease.
Other Causes of Mediastinal Node Enlargement
Giant mediastinal lymph node hyperplasia (Castle-man’s disease) is an asymptomatic, idiopathic, massive adenopathy usually appearing as a solitary mass most commonly in the middle or posterior mediastinum. The mass becomes extremely large but does not calcify. Benign lymphoid hyperplasia may be associated with abnormalities of the immune system such as hypergammaglobulinemia. Angioimmunoblastic (immuno-blastic) lymphadenopathy is a hyperimmune disorder that may resemble Hodgkins disease. It causes enlargement of mediastinal nodes similar to that noted in Hodgkin s disease. It occurs in men more often than in women, usually in those older than 50 years of age. Roentgenographic findings are similar to those in Hodgkin s disease. The three disorders just described must be differentiated from disseminated malignant disease by laboratory methods and by biopsy.
MEDIASTINAL TUMORS AND ALLIED LESIONS
There are a number of structures within and extending through the mediastinum. Tumors, cysts, and other masses may arise from any of them. There is a correlation between location within the mediastinum and the type of mass. Table 1 lists the causes of mass lesions located in each mediastinal compartment. When large, any of the masses may occupy portions of two or even three compartments. The conditions causing lymph node enlargement, although predominantly in the middle mediastinum, may also involve the anterior mediastinum. Hematoma may occur in any of the compartments.
In addition to routine roentgenograms in frontal and lateral projections, it may be useful to obtain oblique projections. Esophagrams or vascular studies may be needed in some cases. CT is very useful to determine the location, site, extent, and relative density of mediastinal masses and is indicated unless some obvious cause such as esophageal hiatus hernia is present. Its utility in locating obscure lesions and in determining relative tissue density and extent of mediastinal tumors as well as in the investigation of abnormal plain film findings has been demonstrated, as previously indicated. When the mass is localized in relation to other mediastinal structures and its characteristics are determined, it is often possible to be reasonably certain of the diagnosis. In other instances, biopsy is necessary to make the histologic diagnosis. MRI has also been used in the study of mediastinal masses, as mentioned earlier.
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Radiographic Considerations in Lymphoma. As indicated, the lymphomas commonly involve the hilar and mediastinal lymph nodes and often cause massive enlargement of them. The involvement is characteristically bilateral and the nodes affected produce mass shadows corresponding to their location. The roentgen findings vary widely with the amount and distribution of the enlargement. Often there is a single large mass projecting to both sides of the superior mediastinum with bilateral hilar enlargement. In other patients, individual nodes are outlined, resulting in a more lobulated appearance. The latter is somewhat more common in Hodgkin’s disease, while the single massive enlargement is often found ion Hodgkin’s lymphoma. Ion Hodgkin’s lymphoma, intrathoracic disease is present in 40% to 50% at the time of diagnosis, compared to 85% in Hodgkin’s disease. Non-Hodgkin’s lymphoma occurs on pleural and pericardial surfaces, in contrast to node enlargement in Hodgkin’s disease. In the lateral view the masses usually appear in the anterior and middle mediastinum. When unilateral, the trachea may be displaced but this is not commonly noted in lymphoma. Tracheal compression may be observed, particularly in children, when adjacent masses are large. Occasionally there are stringy opacities extending outward into the pulmonary parenchyma, indicating involvement of the lungs by direct extension. This results in some blurring of the hilar or mediastinal mass along its outer borders. Pulmonary parenchymal involvement is more frequent in Hodgkin’s disease (11.6%) than ion-Hodgkin’s lymphoma (3.7%). In addition to direct extension, pulmonary involvement takes the form of pulmonary nodules, which may be unilateral or bilateral, may cavitate, and may be clearly outlined or poorly defined.
When the chest radiographs (PA and lateral) are negative, in many instances no further chest studies may be indicated. When mediastinal adenopathy is noted, CT with contrast and in some instances MRI may be necessary, particularly when intravenous cotrast is contraindicated.
Metastases
Lymph node metastases are often a part of generalized lymphangitic spread to the lungs, resulting in enlargement of mediastinal nodes plus strands of density radiating outward from the hilum into the lung. Pleural involvement resulting in pleural effusion is commonly associated with this type of disease. Enlarged mediastinal nodes may also be associated with the multiple nodular hematogenous type of metastases. Occasionally, enlarged nodes may be the only roentgen manifestation of metastatic disease in the chest. Small cell tumor of the lung may appear as a mediastinal mass, whereas the primary lesion in the lung may not be visible. In the latter instance, differentiation between metastasis and lymphoma cannot be made solely on the basis of roentgen findings. When enlarged mediastinal nodes are observed and suspected to represent malignancy, a definite diagnosis must be made. A history of primary tumor elsewhere favors metastasis, while the presence of lymph node enlargement elsewhere favors lymphoma. If nodes are not available elsewhere for biopsy, it is necessary to biopsy the mass of nodes, using percutaneous or transbronchial thin-needle aspiration, mediastinoscopy, mediastinotomy, or thoracotomy as the situation warrants.
Neurogenic Tumors
Ganglioneuroma and neurofibroma are the two most common tumors of neurogenic origin found in the mediastinum. The neurofibromas arise from intercostal nerves and occur most commonly in the posterior mediastinum, where they produce a round density that may reach a large size. Some neurofibromas arise in the region of a spinal foramen and extend into the spinal canal, producing the so called dumbbell type of tumor. These neurofibromas often cause pressure erosion of the adjacent pedicles and vertebral body. Oblique views are usually necessary to determine the presence or absence of bony involvement, and when bony involvement is manifest the diagnosis of neurogenic tumor can be made with great accuracy. Ganglioneuroma arises in the sympathetic ganglia of the thoracic region and also produces a posterior mediastinal tumor that may become very large. Occasionally the thoracic neurofibroma may be a manifestation of multiple neurofibromatosis and is then associated with numerous subcutaneous neurofibromas. There are some slight differences that aid in the differentiation of the neurogenic tumors. The neuroflbroma tends to have a narrow mediastinal base, as seen in the frontal projection, whereas the ganglioneuroma tends to have a broad mediastinal base. The angle between the tumor and the mediastinum tends to be acute in the neurofibroraa and obtuse in the ganglioneuroma. As a general rule the neurofibroma or neurilemoma tends to be a round mass, while the ganglioneuroma is more elongated, (i.e., its vertical diameter is greater than the transverse or sagittal diameter).
The neurogenic tumors are usually fairly smooth and clearly defined, but occasionally they may be lobulated. They are usually benign, but occasionally a neurofibrosarcoma is found in the same location in the mediastinum as that in which a benigeurogenic tumor may arise. Rarely a neurogenic tumor may arise from the vagus nerve and present as a middle mediastinal mass. Bilateral neurofibroma of the vagus has been reported in patients with neurofibromatosis. Rarely, neuroblastoma may involve the mediastinum. This tumor, which occurs during childhood, tends to metastasize widely and grow rapidly. When primary in the mediastinum, it often contains some punctate calcification, and there may be rib or vertebral erosion. In contrast, metastatic neuroblastoma does not contain calcium, does not erode bone, and often appears as multiple mediastinal masses. Pheochromocytoma also may occasionally originate in the posterior mediastinum. It has no distinguishing roentgen features. Paraganglioma (chemodectoma) is another rare tumor of neurogenic origin; it may arise anywhere in the mediastinum.
Pleural fluid is occasionally present in patients with neurogenic tumors and does not necessarily indicate malignancy. Calcification is rarely seen in these tumors, except in primary neuroblastoma of the mediastinum.
Teratoid Tumors
Benign Lesions
Mediastinal teratoma is an anterior mediastinal lesion that grows very slowly and may reach great size before producing symptoms. When it contains only ectodermal derivatives it may be called a “dermoid cyst.” Because histologic examination usually reveals derivatives of the other germ layers, the term “teratoma” or “teratoid cyst” is more accurate. The tumor is usually found in young adults. When benign, it is usually a well encapsulated cystic tumor that is multilocular and smooth but may be somewhat lobulated. Calcification is often present in the wall, but it is uncommon to find bone and poorly formed teeth within the mass.
The roentgen findings are those of an anterior mediastinal mass, the bulk of which extends to one side. Calcification in the wall and within it can be more clearly demonstrated on well-penetrated roentgenograms or CT. It may be round, oval, or lobulated and is usually clearly defined laterally while the medial wall blends with mediastinal structures. Very often there is fat in the teratoid tumor or cyst which may be detected on CT scan. When poorly formed teeth or fat or both are detected on CT, the diagnosis is virtually certain. The tumor elements must be present at birth, but because the mass often grows slowly it is usually found in young adults. Rarely, massive benign teratoma is present at birth.
Malignant Lesions
A malignant teratoma may also occupy the anterior mediastinum, but this lesion grows rapidly and usually causes death within a year of discovery. It produces symptoms and is discovered because of them’, unlike the benign teratoma that is often discovered on routine roentgen study of the chest. The chest radiographic and CT findings are those of an anterior mediastinal mass that is often less clearly defined and more lobulated than benign teratoma. Extension into the lung as well as into mediastinal structures blurs the margins. Pleural extension may result in effusion. The prognosis is poor in such a malignant teratoma.
Seminoma is another malignant tumor that may arise in the mediastinum from aberrant germ cells. This rare tumor is found in young men in the third and fourth decades. The radiographic study reveals a lobulated anterior mediastinal mass, often extending to both sides of the midline. This tumor may or may not cause symptoms of dyspnea, cough, and substernal pain. The prognosis is generally good with proper treatment.
Endodermal sinus (yolk sac) tumor is a rare cause of an anterior mediastinal mass. It occurs in young males who are usually symptomatic when the mass is discovered. The prognosis is poor. There are no distinctive radiographic signs, except that the patient with a large, often lobulated, anterior mediastinal mass has symptoms of fever, chest pain, and sometimes cough and dyspnea.
Mediastinal Cysts
The mediastinal cysts usually appear as rounded or oval mass lesions that are smooth and clearly defined. They tend to change slightly in shape with alteration in position and with respiration. CT or ultrasound are helpful in delineating the cystic nature of these masses. In addition to the congenital cysts in which low attenuation contents are usually present, it must be remembered that areas of low attenuation may be present in areas of hemorrhage or necrosis in bronchogenic neoplasms, enlarged neoplastic nodes, in areas of cystic degeneration in neurogenic tumors, and in lymphangiomas and meningoceles.
Bronchogenic Cyst
Cysts of bronchogenic origin are lined with ciliated columnar epithelium and are usually asymptomatic. They commonly occur in the middle mediastinum near the trachea and in the region of the carina but may be found anywhere in the mediastinum. They range considerably in size, are clearly defined, and are not lobulated. They may compress or displace the bronchi and/or trachea in children, resulting in symptoms. In adults they do not usually compress or displace these structures and are ordinarily asymptomatic. Rarely, “milk of calcium” is observed within the cyst. CT is useful in outlining the cyst. Ordinarily the contents are near water density, so the diagnosis can be made. Occasionally there is greater density, so tumor cannot be excluded on CT. As in other benign mediastinal cysts, the contour may change with positional changes. A somewhat pointed appearance has been reported on CT and may be helpful in the diagnosis when high attenuation cysts are present.
Gastroenteric Cyst (Duplication) Gastroenteric cysts probably represent small local duplications of the intestinal tract. They contain secretory cells and may grow to a large size early in life. They produce symptoms because of pressure on mediastinal structures and are often discovered during infancy, usually before the child is 2 years old. They appear as large rounded or oval opacities in the posterior mediastinum near the esophagus and usually extend to one side of the midline. Sonography may establish the cystic nature of the mass, but CT may be necessary. They are related to neurenteric cysts but have no connection to the neural canal. Many are associated with vertebral anomalies and other abnormalities such as meningocele, intestinal malrotation, and esophageal atresia; congenital heart disease may also be present. They are also related to bronchopulmonary foregut malformations but usually have no connection with the gastrointestinal tract. Partial pericardial defect may accompany the type that is more closely related to bronchopulmonary foregut malformation.
Neurenteric Cyst
A neurenteric cyst is a rare mediastinal cyst that appears to be formed from a remnant of the neurenteric canal that forms an evanescent communication from the gut through the dorsal midline structures to the dorsal surface of the embryothe neural canal. The lesion consists of a mediastinal cystic mass that may be continuous with a duplication or giant diverticulum of the intestinal tract, and may be associated with a diaphragmatic hernia. There is also a defect in the anterior aspect of the spine and faulty vertebral development such as a butterfly vertebra. A fibrous stalk connects the cyst to the meninges in the spinal canal. These cysts may connect with the gut by way of a tubular structure. Therefore, various combinations of closed cysts or communicating cysts with the gut and/or neural canal may be found, at times with associated congenital anomalies. They do not appear to be related to bronchial or pericardial cysts. CT is very useful, be cause defects in the vertebra can be readily detected. MRI is useful when the spinal canal is separated by the anomaly. Occasionally a communication with the intraabdominal gastrointestinal tract is demonstrated during an upper GI study.
Pericardial Cyst
Pericardial cysts usually arise near and are attached to the parietal pericardium. They are lined by flat cells that may be endothelial or mesothelial in origin. They contain clear fluid and are sometimes termed “clear water” or “simple” cysts. When a small cyst of this type is found in the region of the cardiohepatic angle, it may extend into the primary interlobar fissure. The interlobar portion is then tear-drop or pear-shaped. The usual location of the oval mass adjacent to the heart and attached to the pericardium is helpful in making the diagnosis. The pericardial cyst is often basal and may simulate a diaphragmatic tumor or a foramen of Morgagni hernia. CT can usually define the mass as a cyst. However, these cysts may contain thick viscous material and have high CT numbers. Ultrasonography may then be of value in demonstrating that the mass is cystic. If so, cyst puncture and aspiration may be performed for confirmation. In some instances, sonography followed by cyst puncture is sufficient.
Mediastinal Pancreatic Pseudocyst
Rarely a pancreatic pseudocyst will migrate or extend into the posterior mediastinum through the aortic or esophageal hiatus. Usually thoracic and abdominal pain and dyspnea along with a mass in the lower posterior mediastinum are present. There are a few reports of very large pseudocysts extending into the superior aspect of the mediastinum. The esophagus and stomach may be displaced by the mass.
Cystic Hygroma (Lymphangioma)
Lymphangioma or cystic hygroma may extend into the superior mediastinum from the neck and produce a mass that can be visualized radiographically. The presence of the spongy mass in the neck plus apparent continuation of it into the superior mediastinum usually permits the diagnosis. This lesion is soft and pliable, and alteration in contour may be visible in different phases of respiration on films or at fluoroscopy. Anterior mediastinal lymphangioma may also occur in the absence of a cervical mass. The mass is soft and spongy, so it does not ordinarily displace or compress the trachea. It molds to the mediastinal contours and surrounds vessels but does not invade or displace them. They are not calcified. These characteristics should lead to the diagnosis on CT when such a low attentuation mass is present.
Intrathoracic Thyroid
Intrathoracic thyroid is a relatively common tumor, noted in the anterior mediastinum, which is usually connected by an isthmus of tissue to the thyroid gland in the neck. Rarely there is no connection. This isthmus is often wide enough to be recognized roentgenographically so that the connection between the intrathoracic mass and the thyroid is demonstrated. Even if the tumor is bilateral, it is usually eccentric enough to produce tracheal deviation and often compression; this becomes a significant finding for the diagnosis of intrathoracic goiter. The lateral view often shows posterior displacement of the trachea as well. It is not uncommon to observe calcification within the mass. The intrathoracic thyroid often moves on swallowing but may be fixed. It may be posterior in position and produce a mass in the posterior mediastinum behind the trachea, almost always on the right side. The mass of thyroid tissue may also extend far downward into the inferior aspect of the anterior mediastinum. We have observed one patient in whom the goiter encircled the esophagus in a manner that simulated esophageal leiomyoma. The mass may reach great size without producing symptoms. When the mediastinal thyroid is posterior in position, there is usually some displacement of the esophagus as well as the trachea. This is helpful in making the diagnosis. Brachiocephalic vessels may be displaced or compressed. Rarely esophageal compression produces dysphagia and occasionally tracheal compression results in respiratory distress. If calcification is minimal, it may be observed only on CT. When the reaction is positive, scans using radioactive iodine (131I) are diagnostic; but there may be no function, so a negative scan does not exclude the possibility of intrathoracic thyroid.
Parathyroid Adenoma
Parathyroid adenoma is usually found in the anterior aspect of the superior mediastinum or in the anterior mediastinum. It is usually relatively small, eccentric, and presents on either side. There is rarely any calcification in the tumor. There is nothing diagnostic about the appearance of the mass as visualized on the roentgenogram. When renal lithiasis or bone lesions of hyperparathyroidism are present in association with an anterior mediastinal mass, the diagnosis of parathyroid adenoma causing hyperparathyroidism can be made. CT scanning plays a major role in the examination of these patients with hypercalcemia, even though no mass is visible on the chest radiograph. Small tumors can be detected and localized to reduce surgical time in exploring for parathyroids, particularly in the posterior mediastinum. CT is also indicated in patients who have had surgery without change in their hyperparathyroid-ism. In these patients, the parathyroid is often ectopic and may lie in the posterior superior mediastinum near the groove between the trachea and esophagus. Parathyroid adenoma may also occur in the neck. High resolution ultrasound may be very useful in detecting and localizing cervical tumors.
Lesions of the Thymus
The normal thymus, a bilobed pyramidal structure, lies in a retrosternal position largely behind the manubrium, but in children, in whom it is often large, it may extend well down into the retrosternal space anterior to the heart. The retrosternal line formed by the anterior mediastinal pleural reflections is seen above the heart on the lateral view of the thorax. There is usually some soft tissue between the sternum and this line, which ranges from 2 or
Benign Thymic Lesions. Benign thymomas are located in the anterior mediastinum at the level of the junction of the heart and great vessels. They grow slowly and may become very large. They account for about 15% of mediastinal tumors. They occur in adults with an average age of 50 years. At times the tumor is in the midline and may be difficult or impossible to visualize in the frontal projection, but is readily visible in oblique or lateral views. Mottled calcification is occasionally noted within it. Myasthenia gravis is found in about 30% of patients with thymoma. Of patients with myasthenia gravis, 10% to 15% have thymic tumors, some of which are malignant and invasive. Patients with myasthenia gravis should be examined for thymic tumor. In addition to PA and lateral views, a shallow (20%) oblique projection may be helpful in patients younger than 21 years. CT is indicated only when there is a questionable lesion on chest radiography or when clinical symptoms and signs suggest thymic abnormality. CT should remain the procedure of choice wheo obvious abnormality is found on chest radiography. However, it cannot differentiate the various causes of thymic enlargement in many instances. There is no way to be certain on roentgen examination that a thymic mass is benign or malignant.
Thymic Cyst. Thymomas may be cystic and contain calcium which outlines the wall and suggests the cystic nature of the tumor. Thymic cyst location is similar to that of the solid type of thymoma, but it is much less common. The cystic elements should be readily apparent on CT. Rarely they may extend into the neck.
Thymolipoma. A thymolipoma is an uncommon benign fatty tumor that originates in the atrophic thymus. It usually becomes very large and is asymptomatic. The presence of this tumor may be suggested when a large, fatty, anterior mediastinal mass is observed in an asymptomatic adult. There is no association with myasthenia gravis. The fatty nature of the mass is readily detected on CT.
Malignant Thymic Lesions. Early in its development, the malignant thymic tumor resembles a benign thymoma in appearance and location. It grows rapidly, however, and is invasive, so the margins become blurred. It often becomes very large, extends to both sides of the midline, and then resembles lymphoma. It spreads by direct invasion, and sometimes there may be implantation of tumor on the adjacent pleura or pericardium. The tumor may be carcinomatous or sarcomatous. CT is useful in defining the extent of tumor, and sometimes can detect invasion of adjacent structures. MRI may be necessary when findings of invasion are equivocal on CT.
Miscellaneous Masses
Benign tumors such as lipoma, fibroma, chondroma, and hemangioma occur rarely within the mediastinum. There is nothing characteristic about the chest radiographic appearance of a lipoma or fibroma, but lipoma can usually be recognized on CT. If necessary, thin-needle aspiration biopsy can be used to confirm the diagnosis if the tumor is accessible. Other fat containing masses that must be differentiated include liposarcoma, benign teratoma, thymolipoma, steroid induced fat, and benign lipomatosis. The chondroma may contain a considerable amount of irregular calcification that helps to identify it. Hemangiomas are often poorly defined, widespread masses that may contain small rounded calcifications that represent phleboliths. When the latter are present, the diagnosis can be made with a reasonable degree of certainty. Most hemangiomas occur in the anterior mediastinum. They tend to be defined clearly in the anteroposterior projection but are often difficult to outline in the lateral projection.
Fibrosarcoma and other sarcomas arising in the mediastinal soft tissues are extremely rare. Soft-tissue sarcoma may arise posteriorly and can present as a paravertebral mass; however, there is nothing characteristic about the radiographic appearance.
Mediastinal hematoma may also produce a mediastinal mass. Usually there is a history of trauma or of a recent surgical procedure, so a presumptive diagnosis can be made. Rapid changes in the size of the mass are characteristic when they occur. Rarely in patients with a mediastinal hematoma caused by an aortic rupture, the blood may dissect along bronchovascular sheaths to resemble pulmonary edema. This is a very unlikely complication, however. Cardiac disease and lesions of the great vessels within the mediastinum too.
THE DIAPHRAGM
The diaphragm is the muscle of respiration that separates the thorax from the abdominal cavity. The muscle fibers of the diaphragm originate from the xiphoid process and from the eighth to twelfth ribs and insert into the central tendon. Posteriorly, the diaphragmatic crura arise from the upper lumbar vertebral bodies, originating as low as the third lumbar body. The central tendon lies somewhat anterior to the middle thorax and roughly parallels the anterior chest wall. The muscle fibers anteriorly are relatively short and posteriorlaterally are two to three times longer, so the major muscle mass is in the posterior aspect of the diaphragm. There are a number of openings in it through which structures such as the esophagus and aorta pass to enter the abdomen. The superior surface of the diaphragm is readily seen in the roentgenogram of the normal chest because it is clearly outlined by the radiolucent lung above it. Its lower margin is often visible on the left, at least in part, because there is usually some gas in the fundus of the stomach and gas or fecal material in the splenic flexure of the colon that define a portion of its undersurface. On the right side, the liver is of comparable density, so the undersurface of the diaphragm cannot be seen unless pneumoperitoneum is present. The right hemidiaphragm is usually one interspace higher than the left. In full inspiration the dome of the right leaf of the diaphragm is at the approximate level of the tenth rib posteriorly, while the left is at the level of the eleventh rib posteriorly. Using the anterior ribs as landmarks, the dome of the right hemidiaphragm lies between the level of the fifth rib and the level of the sixth interspace measured on the standard 6-foot roentgenogram obtained with the patient in moderately deep inspiration. The dome tends to be higher in hypersthenic and obese patients and may be lower in asthenic subjects. The height of the diaphragm appears to be related to the position of the apex of the heart rather than to the position of the liver (i.e., the low hemidiaphragm is on the side of the cardiac apex); this is observed in patients with dextrocardia and normal abdominal situs.
CHANGES IN THE DIAPHRAGM WITH AGE
The esophageal hiatus gradually enlarges with advancing age. Diaphragmatic defects (areas of thinning) increase from zero in the third decade to 56% in patients in the seventh and eighth decades. The most severe defects consist of an area of discontinuity of the diaphragm associated with herniation of omental fat into the thorax. Defects are found largely postero-medially. Emphysema is a factor in the development of the defects, which have been found in 84% of patients with emphysema. The defects are more common in females, if the presence of emphysema is excluded. The status of skeletal muscle and obesity do not appear to be factors. Diaphragmatic muscle generally does not change in thickness with aging.
MOTION OF THE DIAPHRAGM
Asynchronous motion is common, and there is often more excursion on the left than on the right. Average range of motion is 3 to
FUNCTIONAL DISTURBANCES OF THE DIAPHRAGM
The most common disturbance of the diaphragm is hiccough (singultus). This consists of sudden diaphragmatic contraction associated with closure of the glottis. It is either local in origin, caused by irritation of the diaphragm, or central, in which case it may be produced by encephalitis, uremia, or brain tumor. Occasionally it is hysterical in origin. Attacks are usually very short, but occasionally paroxysms may last for months or years. Radiography is of value only if it may serve to identify an irritating lesion producing the contraction. Fluoroscopy can be used to determine the severity of the contraction and to determine whether one or both hemidiaphragms are involved.
Tonic contraction or splinting of the diaphragm often results when basal pleuritis, subphrenic abscess, or trauma has produced diaphragmatic injury. In any of these instances there is elevation of the diaphragm, which may be bilateral but is usually unilateral, and may be confined to one portion of a hemidiaphragm. Chest roentgenography outlines the height of the diaphragm and fluoroscopy will reveal the amount and location of the limitation of motion.
PARALYSIS AND PARESIS OF THE DIAPHRAGM
When the diaphragm is paralyzed, it is elevated, because intraabdominal pressure is greater than intrathoracic pressure. The amount of elevation varies considerably and paresis or paralysis may be unilateral or bilateral. When paralysis is complete on one side, paradoxical motion is usually visible at fluoroscopy. This means that during inspiration the paralyzed hemidiaphragm rises while the normal hemidiaphragm descends; during expiration the normal hemidiaphragm rises and the paralyzed one descends. This paradoxical motion can be accentuated by having the patient sniff. This causes a rapid but shallow inspiration. In paresis of the diaphragm there may or may not be elevation visible on chest roentgenograms, but a lag in contraction of the involved hemidiaphragm is readily visible fluoroscopically and this is also accentuated by having the patient sniff. Normally there may be a slight diflerence in motion on the two sides. In our experience, the left hemidiaphragm moves slightly more rapidly than the right on deep, rapid inspiration. When there is much difficulty in determining relative diaphragmatic motion on the two sides, fluoroscopy with the patient in the lateral decubitus position is helpful, because motion on the dependent side is augmented and comparison is easier with greater excursion.
EVENTRATION OF THE DIAPHRAGM
Eventration of the diaphragm is the term used to describe an abnormal elevation of the diaphragm. It is thought to result from a deficiency of muscular development that may be general or local. The local form is occasionally bilateral, but bilateral general eventration is very rare. Local eventration is nearly always on the right side anteromedially. There is some difference of opinion as to the cause of eventration. Some observers believe there is a deficiency of nervous as well as of muscular tissue. The diagnosis is based on observation of the elevated diaphragm on a chest roentgenogram. There is no disease or tumor visible to produce the elevation, and at fluoroscopy the involved hemidiaphragm is usually observed to move. The movement may be normal or diminished and paradoxical motion can be observed on rapid inspiration in some instances. It is often difficult to differentiate from hernia and may be impossible to differentiate from phrenic paralysis or paresis. Congenital eventration, manifested in childhood, is usually on the right, in contrast to eventration developing in adults, which is usually on the left and in males. When eventration is left-sided, there is often a long fluid level in the gastric fundus when the patient is in the upright position. The afferent and efferent limbs of the stomach or colon are widely separated in eventration, but are together or nearly so in congenital or acquired hernia, being constricted at the hernial opening. Ultrasound can be used to identify the intact diaphragm in these patients with eventration.
Localized Eventration
Local weakness of the diaphragm with upward protrusion of the liver is the most common manifestation of localized eventration. It usually occurs on the ante-romedial aspect of the right hemidiaphragm, through which a portion of the right lobe of the liver bulges. This has been termed the “anteromedial hump” of the liver. The smoothly rounded appearance of the bulge is usually characteristic, but, if the bulge simulates tumor, liver scan or CT will make differentiation possible. In our experience, primary or metastatic hepatic tumor does not cause this type of local elevation. Local eventrations may occur elsewhere, particularly posteriorly, where upward displacement of the kidney may produce a rounded mass density simulating tumor.This probability should be considered when the mass is comparable in size to the upper pole of the kidney. When suspected, intravenous urography can be used to outline the kidney and determine its relationship to the diaphragm. However, ultrasound of the kidney is diagnostic in most instances.
DIAPHRAGMATIC DISPLACEMENTS
In addition to the elevation of the diaphragm noted in eventration, paralysis, and paresis, a number of intrathoracic and intraabdominal conditions may result in elevation of the diaphragm. On the right side, tumors and cysts of the liver, subphrenic abscess, and right renal tumors may elevate the diaphragm generally or locally. On the left side the causes include enlargement of the spleen, left renal tumors, and dilatation or tumors of the stomach and of the splenic flexure of the colon. Ascites, obesity, large intraabdominal tumors, and pregnancy may result in bilateral elevation. Intrathoracic diseases that decrease pulmonary volume cause elevation on the involved side. These include pulmonary fibrosis, chronic pleural disease, and atelectasis. The elevation may be relatively uniform or rather irregular. The amount depends on the severity of the lesion producing it. If the disease causing it is bilateral, elevation is bilateral.
Irregularity of the diaphragm superiorly is often secondary to previous pulmonary inflammatory disease. This is termed “tenting” or “adhesive tenting” and is often associated with basal pulmonary fibrosis and obliteration of the costophrenic sulcus. The diaphragm is displaced downward by lesions that produce an increase in thoracic volume, such as large intrathoracic neoplasms, massive pleural effusion, pulmonary emphysema, and tension pneumothorax. Massive pleural effusion may cause inversion of the left hemidiaphragm, displacing the kidney, spleen, and stomach downward. This inversion may produce a pseudomass in the left upper abdominal quadrant which disappears when thoracentesis and removal of pleural fluid are accomplished. Less commonly, the right hemidiaphragm is inverted by massive pleural effusion. Ultrasound can usually identify the diaphragm in this condition, since it cannot be detected on chest radiography.
DIAPHRAGMATIC TUMORS
Primary diaphragmatic tumors are rare and may be benign or malignant. The most common benign tumor is lipoma, but numerous other benign tumors have been reported; they include fibroma, chondroma, neurofibroma, angiofibroma, and angioma as well as congenital cysts such as cystic teratoma. Benign cystic teratoma is extremely rare, but can be diagnosed on CT because it contains fat, calcium and/or teeth, and soft tissue quite similar to ovarian cystic teratomas. Because malignancy cannot be excluded by any imaging method, these cystic masses should be removed surgically. The malignant tumors are all sarcomas; fibrosarcomais the most common but others such as fibromyxosarcoma, fibroangioendothelioma, undifferentiated sarcoma, myosarcoma, hemangioendothelioma, hemangiopericytoma, leiomyosarcoma, or mesenchymoma may occur. Malignant tumors predominate in a ratio of three to two over benign tumors arising in the diaphragm. These tumors produce basal masses that usually project above the normal rounded opacity produced by the diaphragm. They may be smooth or lobulated and vary considerably in size. When the tumor is on the left, it may project downward to encroach on the gastric air bubble. The limits of the tumor may then be outlined inferiorly as the tumor projects into the stomach and superiorly as it projects above the diaphragm. Conventional roentgen studies serve only to identify a mass. CT is used to determine the site and extent of the mass, and if a lipoma is present, the diagnosis can be made. Because the other cell types produce no characteristic findings, biopsy or surgical exploration is usually indicated when a tumor is found in the diaphragm.
DIAPHRAGMATIC HERNIAS
Esophageal Hiatal Hernia
Herniation of all or part of the stomach through the esophageal hiatus into the thorax produces a mass shadow at the left medial base that is often visible on the frontal roentgenogram. Not infrequently, gas and fluid within the thoracic portion of the stomach make the diagnosis apparent on plain film roentgenograms; if not, the lesion can be readily identified by means of a barium swallow. This examination also serves to identify the occasional diverticulum of the esophagus in this region and to differentiate the esophageal lesions from pulmonary cyst or abscess as well as from diaphragmatic tumor.
Morgagni Hernia
Foramen of Morgagni hernia is a rare diaphragmatic hernia that may result in a basal mass shadow, usually in the region of the cardiohepatic angle because it occurs mainly on the right. It is most often observed in obese adults. This type of hernia through the retrosternal foramen of Morgagni (space of Larrey) on either side of the midline is usually small and often contains omentum. Theoretically a hernial sac should be present, but is not always demonstrated at surgery. Occasionally a portion of bowel may lie within the hernial sac. In the latter instance, it may be possible to make the diagnosis on routine roentgenographic study of the chest, but a CT or sonographic study is usually required. If omentum is herniated, the fat can be identified. Other studies include barium enema, which\may show upward angulation of the mid transverse colon when the hernial sac contains omen tum. The pyloric end of the stomach and proximal duodenum may also be displaced upward toward the diaphragm.
Rarely, the liver may herniate through the foramen of Morgagni into the thorax in infants and young children. This is usually accompanied by partial obstruction of the inferior vena cava. Inferior vena cavography demonstrates the kinking and partial obstruction of the vena cava. When Morgagni hernia presents in infancy, it is usually accompanied by one or more anomalies. In one study, 13 of 17 patients had significant congenital defects including cardiac defects such as dextrocardia, ventricular septal defect; anomalous
pulmonary venous return, trisomy 21, and large omphaloceles. Liver, colon, and small bowel are often found in the hernia in infants, in contrast to the adult presentation where omental fat is often the only structure in the hernia and there are no associated congenital anomalies.
Bochdalek Hernia
Normally the pleuroperitoneal hiatus or foramen is posterolateral in position, but in congenital hernias arising in this area, the foramen may be very large, with absence of much of the involved hemidiaphragm. In such instances, most of the abdominal viscera may be in the thorax, leading to severe ipsilateral pulmonary hypoplasia. The presenting complaint may then be neonatal respiratory distress. These hernias occur predominantly on the left side (two to one). In contrast to the foramen of Morgagni hernias, true pleuroperitoneal hiatal hernias do not have a hernial sac. This is because the abdominal contents enter the thorax before the space between the septum transversum and the pleuroperitoneal membrane is closed. Herniation through the pleuroperitoneal foramen of Bochdalek is often large and loops of bowel can be visualized and identified, so differential diagnosis is not difficult. When the hernia is smaller and does not contain gas-filled bowel, the diagnosis is more difficult to make. The use of CT to examine the thorax and upper abdomen has revealed a higher incidence of Bochdalek hernias than was previously reported. An incidence of 6% was reported in a CT study of 940 patients. The size of the diaphragmatic defect did not correlate with the size of the hernia. Most hernias contained fat and were frequently incidental findings on abdominal or chest scans of adults. Large Bochdalek hernias usually present at birth and produce respiratory distress and unilateral pulmonary hypoplasia. In some patients, however, no symptoms may appear for several months. Iewborns with respiratory distress in the first 12 hours of life; mortality rate is about 50%. High risk factors includp: right-sided hernia; stomach in thorax; pneumothorax; non aeration of the ipsilateral lung; and less than 50% aeration of the contralateral lung. The roentgen findings vary with the amount of herniation. Gas-filled bowel is recognized within the thorax when the defect is on the left. The liver may also extend into the thorax when the defect is on the right. The remaining diaphragm is often visible. CT can be used to identify the diaphragm and its relation to the intrathoracic mass. Ultrasound also is sometimes useful in identifying the diaphragm in these patients, and real time sonography may be particularly valuable as a bedside technique in critically ill patients.
Traumatic Hernia of the Diaphragm
Although gunshot wounds and knife wounds may lacerate the diaphragm, immediate surgery is usually necessary, so the defect is identified, at times before any radiographic studies are done. Traumatic rupture of the diaphragm usually results from severe blunt injury to the abdomen and less commonly to the thorax. In severely traumatized patients, the combination of rib fractures and pelvic or vertebral fractures should raisethe possibility of diaphragmatic rupture especially in patients wearing seatbelts. About one third of diaphragmatic ruptures present immediately after the injury and are diagnosed then. The diaphragm may be ruptured completely with defects in the parietal pleura and peritoneum so there is no hernial sac. In other instances, either the pleura or peritoneum may form a sac. The left hemidiaphragm is involved in about 90% of patients who survive their injury. The rupture usually involves the posterior central portion of the diaphragm, but there may be avulsion of the diaphragm from the ribs. The stomach is the organ most often herniated, but colon, small bowel, spleen, liver, or gallbladder may also herniate. The radiographic signs in this condition vary with the extent of the rupture and depend on upward displacement of abdominal content. At times, no abnormality is observed. There may be elevation of the hemidiaphragm or apparent elevation, since herniated viscera\may parallel the diaphragm on both frontal and lateral projections. This pseudoelevation may change in shape with change in position, however. In these instances it is strongly suggestive of rupture with hernia. Apparent normal diaphragmatic motion does not exclude the possibility of hernia. Recognizable bowel shadows may be visible in the thorax and sometimes in the pericardial sac. Administration of barium by mouth and by rectum to identify the relationship of the gastrointestinal tract to the diaphragm usually confirms the diagnosis. Both loops of gut are kept close together by the diaphragm surrounding the rupture an hourglass appearance so ordinarily there is no problem in differentiation from eventration. Because there may not be immediate herniation of abdominal viscera into the thorax following trauma, serial films are sometimes useful in the posttraumatic period and should be obtained if there is any suspicion of diphragmatic rupture. Other signs which suggest rupture include ipsilateral pleural effusion, mediastinal shift to the opposite side, and poor definition of the involved hemidiaphragm.
Because the defects do not heal spontaneously, traumatic rupture of the diaphragm may be followed by a traumatic hernia sometimes years after the traumatic event. The roentgen findings are similar to those in the immediate type of posttraumatic herniation. Oblique and lateral views often permit localization of the defect. There may be partial or complete obstruction of the involved gut, either with or without vascular compromise in both immediate and delayed herniation. Hemothorax or hydrothorax may also be present and raises the possibility of strangulation. The combination of a high left hemidiaphragm and splenic flexure obstruction in a patient with history of trauma is very suggestive. Right-sided rupture, which may be more common than reported figures would indicate, also may cause diaphragmatic elevation with partial or total herniation of the liver. A high index of suspicion when diaphragmatic abnormality is observed in a patient who has suffered thoracoabdominal trauma is necessary to suggest, then confirm, the diagnosis. Immediate surgery is necessary in many instances.
In patients with suspected diaphragmatic rupture, chest x-ray is more useful than is generally reported, particularly if a follow-up film is obtained 6 to 12 hours after the initial radiograph. CT has been disappointing. MRI may be of more value because coronal and sagittal images can be obtained. Sonography is of great value in outlining diaphragmatic defects, but its use may be limited in some instances by interstitial emphysema of the overlying soft tissues and in others by severe pain in the area.
Epicardial Fat Pads
Localized fat deposits are often present at the cardiac apex and in the cardiohepatic angle. Those at the apex are usually readily identified. When the amount of fat is unusually great it can produce a mass in the cardiohepatic angle that simulates foramen of Morgagni hernia or diaphragmatic tumor. Deposits of fat are usually of less density than are the adjacent heart and diaphragm, but this difference is small and not entirely reliable because the omentum, frequently present in foramen of Morgagni hernia, is of similar opacity. It is probable that a small foramen of Morgagni hernia is sometimes the cause of the shadow, but this is usually asymptomatic and the differentiation is then of no clinical importance. The fat pads tend to occur in obese patients, and when a fat pad is present in the cardiohepatic angle, there is usually a fat pad at the cardiac apex. The association of these opacities is of some diagnostic importance. CT can be used to identify the fat in these masses, if necessary.
ACCESSORY DIAPHRAGM
Accessory diaphragm (venolobar syndrome, duplication of the diaphragm), is very rare and occurs on the right side. It consists of a sheet of fibrous and muscular tissue, which represents a partial duplication, extending from the anterior aspect of the normal diaphragm upward and posteriorly to insert along the fifth to seventh ribs. It parallels the major fissure and may extend into it to separate the lower lobe from the upper and middle lobes. It is usually attached to the pericardium medially and has a medial hiatus. Pulmonary anomalies associated with accessory diaphragm include partial fissure anomalies, aplasia or hypoplasia of a lobe, partial diversion of the lower lobe by the anomalous diaphragm, and anomalous pulmonary vascular supply including lower lobe venous drainage into the inferior vena cava, the scimitar syndrome, and anomalous arterial supply to the lower lobe from the aorta. This is the reason this complex of anomalies is sometimes called the “venolobar syndrome.”
Roentgen findings include a shift of the mediastinum to the involved side because of hypoplasia and lack of clarity of the mediastinum on the same side with hazy opacity of the central lung. On the lateral view the accessory diaphragm may be visible; it resembles the major fissure but extends to the diaphragm and is more anterior in position than the normal fissure. Bronchography may show the lobar hypoplasia and angiography may demonstrate the anomalous arterial supply arid-venous drainage that often accompany this anomaly.
CYSTS OF THE DIAPHRAGM
Intradiaphragmatic cysts usually represent extralobar sequestration in which aberrant lung tissue is enclosed within the diaphragm. The left hemidiaphragm is involved in about 90% of cases. Occasionally, a coelomic cyst may be found in the diaphragm. Roentgen findings of a diaphragmatic mass are similar to those produced by benign diaphragmatic tumors. Sonography and CT are useful to demonstrate the cystic nature of the diaphragmatic mass.
