Diaphragm

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CHAPTER 58 Diaphragm

The diaphragm is a curved musculofibrous sheet that separates the thoracic from the abdominal cavity (Figs 58.1, 58.2). Its mainly convex upper surface faces the thorax, and its concave inferior surface is directed towards the abdomen. The positions of the domes or cupolae of the diaphragm are extremely variable because they depend on body build and the phase of ventilation. Thus the diaphragm will be higher in short, fat people than in tall, thin people, and overinflation of the lung, as occurs for example in emphysema, causes marked depression of the diaphragm. Usually, after forced expiration the right cupola is level anteriorly with the fourth costal cartilage and therefore the right nipple, whereas the left cupola lies approximately one rib lower. With maximal inspiration, the cupola will descend as much as 10 cm, and on a plain chest radiograph the right dome coincides with the tip of the sixth rib. In the supine position, the diaphragm will be higher than in the erect position, and when the body is lying on one side, the dependent half of the diaphragm will be considerably higher than the uppermost one.

ATTACHMENTS AND COMPONENTS

The muscle fibres of the diaphragm arise from the highly oblique circumference of the thoracic outlet: the attachments are low posteriorly and laterally, but high anteriorly. Although it is a continuous sheet, the muscle can be considered in three parts, sternal, costal and lumbar, which are based on the regions of peripheral attachment. The sternal part arises by two fleshy slips from the back of the xiphoid process, and is not always present. The costal part arises from the internal surfaces of the lower six costal cartilages and their adjoining ribs on each side, and interdigitates with transversus abdominis (see Fig. 54.14). The lumbar part arises from two aponeurotic arches, the medial and lateral arcuate ligaments (sometimes termed lumbocostal arches) and from the lumbar vertebrae by two pillars or crura.

The lateral arcuate ligament is a thickened band in the fascia that covers quadratus lumborum, and it arches across the upper part of that muscle. It is attached medially to the front of the transverse process of the first lumbar vertebra, and laterally to the lower margin of the twelfth rib near its midpoint. The medial arcuate ligament is a tendinous arch in the fascia that covers the upper part of psoas major. Medially, it is continuous with the lateral tendinous margin of the corresponding crus, and is thus attached to the side of the body of the first or second lumbar vertebra. Laterally, it is fixed to the front of the transverse process of the first lumbar vertebra.

The crura are tendinous at their attachments, and blend with the anterior longitudinal ligament of the vertebral column. The right crus is broader and longer than the left, and arises from the anterolateral surfaces of the bodies and intervertebral discs of the upper three lumbar vertebrae. The left crus arises from the corresponding parts of the upper two lumbar vertebrae. The medial tendinous margins of the crura meet in the midline to form an arch, the median arcuate ligament, which crosses the front of the aorta at the level of the thoracolumbar disc; it is often poorly defined.

From these circumferential attachments, the fibres of the diaphragm converge into a central tendon. Fibres from the xiphoid process are short, run almost horizontally and are occasionally aponeurotic. Fibres from the medial and lateral arcuate ligaments, and more especially those from the ribs and their cartilages, are longer. They rise almost vertically at first and then curve towards their central attachment. Fibres from the crura diverge, and the most lateral become even more lateral as they ascend to the central tendon. Medial fibres of the right crus embrace the oesophagus where it passes through the diaphragm, the more superficial fibres ascend on the left, and deeper fibres cover the right margin. Sometimes, a fleshy fasciculus from the medial side of the left crus crosses the aorta and runs obliquely through the fibres of the right crus towards the vena caval opening, but this fasciculus does not continue upwards around the oesophageal passage on the right side.

The central tendon of the diaphragm is a thin but strong aponeurosis of closely interwoven fibres situated near the centre of the muscle, but closer to the front of the thorax, so that the posterior muscular fibres are longer. In the centre it lies immediately below the pericardium, with which it is partially blended. Its shape is trifoliate. The middle, or anterior, leaf has the form of an equilateral triangle with the apex directed towards the xiphoid process. The right and left folia are tongue-shaped and curve laterally and backwards, the left being a little narrower. The central area of the tendon consists of four well-marked diagonal bands fanning out from a thick central node where compressed tendinous strands decussate in front of the oesophagus and to the left of the vena cava.

APERTURES

A number of structures pass between the thorax and abdomen via apertures in the diaphragm. There are three large openings, for the aorta, oesophagus and inferior vena cava, and a number of smaller ones (Fig. 58.2).

The aortic aperture is the lowest and most posterior of the large openings, and is found at the level of the lower border of the twelfth thoracic vertebra and the thoracolumbar intervertebral disc, slightly to the left of the midline. It is an osseo-aponeurotic opening defined by the diaphragmatic crura laterally, the vertebral column posteriorly and the diaphragm anteriorly. Strictly speaking, it lies behind the diaphragm and its median arcuate ligament (when present). Occasionally, some tendinous fibres from the medial parts of the crura also pass behind the aorta, converting the opening into a fibrous ring. The aortic opening transmits the aorta, thoracic duct, lymphatic trunks from the lower posterior thoracic wall and, sometimes, the azygos and hemiazygos veins.

The oesophageal aperture is located at the level of the tenth thoracic vertebra, above, in front and a little to the left of, the aortic opening. It transmits the oesophagus, gastric nerves, oesophageal branches of the left gastric vessels and some lymphatic vessels. The elliptical opening has a slightly oblique long axis, and is bounded by muscle fibres that originate in the medial part of the right crus and cross the midline, forming a ‘chimney’ approximately 2.5 cm long, which accommodates the terminal portions of the oesophagus. The outermost fibres run in a craniocaudal direction, and the innermost fibres are arranged circumferentially. There is no direct continuity between the oesophageal wall and the muscle around the oesophageal opening. The fascia on the inferior surface of the diaphragm is continuous with the transversalis fascia and is rich in elastic fibres. It extends upwards into the opening as a flattened cone to blend with the wall of the oesophagus 2–3 cm above the oesophago-gastric (squamocolumnar) junction. Some of its elastic fibres penetrate to the submucosa of the oesophagus. This peri-oesophageal areolar tissue is referred to as the phreno-oesophageal ligament. It connects the oesophagus flexibly to the diaphragm, permitting some freedom of movement during swallowing and ventilation while at the same time limiting upward displacement of the oesophagus.

The vena caval aperture, the highest of the three large openings, lies at about the level of the disc between the eighth and ninth thoracic vertebrae. It is quadrilateral, and located at the junction of the right leaf with the central area of the tendon, and so its margins are aponeurotic. It is traversed by the inferior vena cava, which adheres to the margin of the opening, and by some branches of the right phrenic nerve.

There are two lesser apertures in each crus: one transmits the greater, and the other the lesser, splanchnic nerve. The ganglionated sympathetic trunks usually enter the abdominal cavity behind the diaphragm, deep to the medial arcuate ligament. Openings for minute veins frequently occur in the central tendon.

On each side of the diaphragm there are small areas where the muscle fibres are replaced by areolar tissue. One, between the sternal and costal parts, contains the superior epigastric branch of the internal thoracic artery and some lymph vessels from the abdominal wall and convex surface of the liver. The other, between the costal part and the fibres that spring from the lateral arcuate ligament, is less constant; when it is present, the posterosuperior surface of the kidney is separated from the pleura only by areolar tissue.