The Diaphragm

The diaphragm is the musculocutaneous partition between the thoracic and abdominal cavities. Embryologically, the diaphragm is formed by the septum transversum ventrally and by the mesentery of the foregut and two pleuroperitoneal folds dorsally.

Movement of the diaphragm provides approximately 75% of the change in intrathoracic volume during quiet respiration.¹ The diaphragm also acts as a mechanical partition between the thorax and the abdomen. Lymph vessels from the abdomen penetrate the diaphragm and drain into the thoracic lymph nodes and vessels. Thus inflammatory or neoplastic abdominal disease may spread to the mediastinum and pleural space. Lymph flow is unidirectional with the final destination being the thoracic trunks.²

The diaphragm consists of a tendinous center and three thin peripheral muscles: the pars lumbalis, the pars costalis, and the pars sternalis. The pars lumbalis consists of the right and left crura, which attach to the cranial ventral border of L4 and the body of L3. The attachment area on these vertebrae occasionally has a concave indistinct ventral margin that may be mistaken for bone lysis (Fig. 29-1). The pars costalis attaches in an oblique direction to the thirteenth through eighth ribs, and the pars sternalis attaches to the xiphoid cartilage.³ The diaphragm is convex and extends into the thorax from its attachments, creating the phrenicocostalis and phrenicolumbalis recesses.

Fig. 29-1 Lateral view of the lumbar spine of a normal dog. Compared with L2 and L5, note the indistinct ventral cortex of L3 and L4 caused by the diaphragm attachment site. This can be misinterpreted as lysis if this normal appearance is not understood. *L2,* Second lumbar vertebra.

There are three openings through the diaphragm: (1) the dorsally located aortic hiatus encloses the aorta, azygos and hemiazygos veins, and the lumbar cistern of the thoracic duct; (2) the centrally located esophageal hiatus encloses the esophagus and vagus nerve trunks; and (3) the caudal vena cava foramen is located at the junction of the muscular and tendinous portions of the diaphragm.

Normal Radiographic Anatomy

Radiographically, only a small portion of the diaphragm can be seen on any one view. Radiographic visualization of the diaphragm depends on adjacent structures being of different opacity. Most of the thoracic surface is visible because of the adjacent gas-filled lungs. Parts of the thoracic surface are not visualized where the lungs are not in contact with the diaphragm—the phrenicocostalis and phrenicolumbalis recesses. A large portion of the abdominal diaphragmatic surface is not seen because it silhouettes the adjacent liver. The ventral abdominal diaphragmatic surface is visible on the lateral view when fat is present within the falciform ligament. The dorsal aspect of the left diaphragmatic crus and the gastric wall appear as one linear structure when gas is present in the gastric cardia.

Diaphragmatic structures that may be visualized distinctly radiographically are the right and left crura, the intercrural cleft, and the cupula (body) (Figs. 29-2 to 29-5). Associated structures that may also be seen are the caudal vena cava and the caudal ventral mediastinum. On the lateral view, the right crus of the diaphragm blends with the caudal vena caval border, and the gastric fundus may be seen adjacent to the abdominal surface of the left crus. The intercrural cleft is a shorter, convex, opaque line caudal and ventral to the crura (see Figs. 29-2 and 29-3). The cupula is the most cranial convex portion of the diaphragm on both the lateral and the dorsoventral or ventrodorsal views. Also on these views, the thoracic surface of the diaphragm may be visualized as one, two, or three convex projections into the thoracic cavity (see Figs. 29-4 and 29-5).

Fig. 29-2 Left lateral radiograph of the diaphragmatic region of a normal dog.

Several normal variations of diaphragmatic position and shape may be seen radiographically. Factors that cause this variable appearance are real and apparent. Real factors consist of breed, age, obesity, respiration, and gravity. Apparent factors are x-ray beam centering and animal positioning during radiographic examination. Many combinations are possible when all permutations of these variables are considered.⁴ Most of these variables are not radiographically significant; however, some must be recognized and understood. Changes most apparent radiographically are the position, shape, and visualization of the cupula and crura. The relative position of the crura depends mostly on position and size of the animal and primary x-ray beam centering.

The most dependent crus is usually displaced cranially when an animal is in lateral recumbency. In right lateral recumbency, the crura appear to be parallel (see Fig. 29-3); in left lateral recumbency, they sometimes appear to cross. The crura also appear to be more extensively separated, by up to 2.5 vertebral lengths, if the animal is rotated slightly or if the x-ray beam is centered over the mid or cranial thorax.⁴

The radiographic appearance of the diaphragm in ventrodorsal or dorsoventral projections varies with x-ray beam centering. The diaphragm may appear as two or three separate domed-shaped structures (see Fig. 29-4) or as a single dome-shaped structure (see Fig. 29-5). The three structures represent the cupula and two crura. A single domed diaphragm may be seen on a ventrodorsal view when the x-ray beam is centered on the mid-abdomen or on a dorsoventral view when the x-ray beam is centered mid-thorax. Two or three separate domed structures are seen when the animal is in the ventrodorsal position and the x-ray beam is centered mid-thorax or on a dorsoventral view with the x-ray beam is centered mid-abdomen.⁴

The diaphragmatic position and shape vary with inspiration, expiration, and intraabdominal pressure. The normal intersection point of the diaphragm and spine is between T11 and T13 but may vary between T9 and L1. The diaphragm changes position with normal respiration from one half to two vertebral lengths. On extreme inspiration, the diaphragm changes position and shape. On a lateral thoracic view made in extreme inspiration, the diaphragm is oriented more vertically; the shape changes from convex to straight. The diaphragm is displaced cranially by increased intraabdominal pressure, which may be caused by obesity, ascites, gastric or intestinal distention, abdominal pain, or abdominal masses.

Separate diaphragmatic structures are not seen as distinctly in the cat, probably because of the relatively small thoracic size (Fig. 29-6). On extreme inspiration, particularly if the animal is in respiratory distress, small symmetric muscle projections are noted from the thoracic diaphragmatic surface in the ventrodorsal or dorsoventral view (Fig. 29-7). This has been referred to as tenting of the diaphragm and is discussed in Chapter 33 in reference to pulmonary hyperinflation.

Fig. 29-6 Radiographs of the diaphragmatic region of a normal cat. A, Left lateral view. B, Right lateral view. C, Ventrodorsal view. The right *(R)* and left *(L)* diaphragmatic crura are almost superimposed on both lateral views with little change in position. The body has a convex shape projecting into the thorax. In C, the diaphragm projects as a single convex opacity into the caudal aspect of the thorax *(white arrows).*

Radiographic Signs of Diaphragmatic Disease

The signs directly associated with the diaphragm are not as numerous and specific as those in many other organs. Radiographic changes observed most frequently with diaphragmatic disease include general or focal loss of the thoracic diaphragmatic surface outline and changes in diaphragmatic shape and position (Table 29-1).

Table • 29-1

Radiographic Signs of Diaphragmatic Disease

RADIOGRAPHIC SIGNS	CAUSES
General loss of diaphragmatic thoracic surface outline	Bilateral pleural fluid
General loss of diaphragmatic thoracic surface outline	Generalized pulmonary disease in caudal lung lobes
Localized or partial loss of the thoracic surface outline	Thoracic masses adjacent to the diaphragm
	Diaphragmatic hernias
	Focal pulmonary disease in caudal lung lobes
Shape changes	Thoracic masses adjacent to the diaphragm
	Hiatal hernias
	Small diaphragmatic hernias
	Pleural reaction on the diaphragmatic surface
	Neoplasia arising from the diaphragm
	Hemiparalysis of the diaphragm
	Unilateral tension pneumothorax
Position Changes
Cranial displacement	Obesity
	Peritoneal fluid
	Abdominal pain
	Abdominal masses or organ enlargement; liver enlargement and masses frequently cause cranial displacement
	Generalized diaphragmatic paralysis
	Cranial displacement of the cupula caused by a diaphragmatic defect with the peritoneum and pleura intact
Caudal displacement	Severe respiratory distress—ventilation or perfusion problems
	Tension pneumothorax
	Caudal displacement of the cupula caused by contact with the heart

The thoracic diaphragmatic surface outline will not be visualized radiographically if soft tissue or fluid contacts the surface. Changes in diaphragm shape occur most frequently on the cupula; they are often normal and are frequently caused by contact with the heart (Fig. 29-8) or position of the animal during radiographic examination. The shape and position may also appear altered in some large-breed dogs, with the body appearing more convex and extending to a more cranial position in the thorax. This may be the result of a flaccid tendinous membrane, or it may be associated with a peritoneopleural hernia, which often produces no clinical signs.

Fig. 29-8 Ventrodorsal view of the diaphragmatic region of a normal dog. A cardiac impression *(white arrows)* is present on the diaphragmatic body (cupula). The left diaphragmatic crus *(black arrows)* is visible. The right crus cannot be visualized as a separate structure.

Thoracic masses or lung disease adjacent to the diaphragm, hiatal and small traumatic diaphragmatic hernias, masses originating from the diaphragm, and chronic pleural inflammatory reactions are the most frequent pathologic causes associated with diaphragmatic shape changes. An asymmetric diaphragmatic shape may occur with unilateral tension pneumothorax or hemiparalysis. Suspected hemiparalysis should be confirmed by observing diaphragmatic movement during fluoroscopy.

Positional changes consist of cranial and caudal displacement. Because the position of the diaphragm changes during the respiratory cycle, minor changes are difficult to diagnose and in most instances are not clinically significant. Severe positional changes may be significant and indicative of thoracic or abdominal disease. Cranial diaphragmatic displacement is usually associated with abdominal disease (see Table 29-1) or generalized diaphragmatic paralysis, which should be confirmed by fluoroscopic observation. Caudal diaphragmatic displacement is usually associated with severe respiratory disease (Fig. 29-9). The caudally positioned diaphragm is an attempt by the animal to increase the level of systemic oxygenation, which may be low because of ventilation or perfusion deficiencies in the lungs. Bilateral tension pneumothorax may also cause a caudally displaced diaphragm from increased pleural pressure.

Fig. 29-9 Lateral views of the diaphragmatic region of a normal cat on expiration **(A)** and on extreme inspiration **(B).** The entire diaphragm is displaced caudally with inspiration and has a flatter contour compared with expiration.

Although many of the radiographic signs of diaphragmatic disease are not specific, their cause should be determined. In some instances, ultrasonography or additional radiographic studies, such as positional views with a horizontally directed x-ray beam and contrast medium studies, may be indicated to determine the cause of the radiographic signs.

Diaphragmatic Diseases

The most frequently observed diaphragmatic diseases in the dog and cat are hernias, which may be traumatic and congenitally predisposed. Motor or innervation disturbances occur less frequently.

Diaphragmatic Hernias

A diaphragmatic hernia is a protrusion of abdominal viscera through the diaphragm into the thorax. Diaphragmatic hernias that may be recognized radiographically include traumatic, peritoneopericardial, hiatal, peritoneopleural, and those secondary to congenital diaphragmatic defects.

Abdominal trauma is the most common cause of diaphragmatic hernia. A high momentary increase in abdominal pressure when the glottis is open produces a high pleuroperitoneal pressure gradient that may result in a diaphragmatic hernia. The high pleuroperitoneal gradient may produce a rent in the muscular portion of the diaphragm, or it may force abdominal viscera through congenitally weak or defective areas. Clinical signs that may be observed with diaphragmatic hernias include dyspnea, pain, vomiting, regurgitation, muffled heart sounds, and a weak femoral pulse.5,6 Some diaphragmatic hernias may not cause clinical signs and are detected incidentally.

Radiography plays an important role in confirming a diagnosis of diaphragmatic hernia and may provide information about location, extent, contents, and secondary complications associated with the hernia.7–11 If a diagnosis cannot be confirmed from survey radiographs (Box 29-1), ultrasonography and/or other imaging procedures may be performed to provide additional diagnostic information. Other radiographic procedures consist of administration of oral barium sulfate, positional radiographic views, removal of pleural fluid and repeat radiography of the thorax, and positive contrast-medium peritoneography.

Box • 29-1 Radiographic Signs Associated with Traumatic Diaphragmatic Hernia

Abdominal Viscera within the Thorax