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.
Fig. 58.1 A, Abdominal aspect of the diaphragm. Note the apertures for the oesophagus and inferior vena cava and the aortic hiatus. The medial part of the right crus frequently consists of three components and extends further distally than the left crus. B, Trunk sectioned at the level of the tenth thoracic vertebra.
(From Sobotta 2006.)
Fig. 58.2 Thoracic aspect of the diaphragm. Note that the fibres descend from their relatively ‘high’ anterior sternocostal attachments steeply and obliquely to their complex ‘low’ posterior attachments. The phrenic nerves are demonstrated on the diaphragmatic surfaces.
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 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.
RELATIONS
The upper surface of the diaphragm is related to three serous membranes. On each side, the pleura separates it from the base of the corresponding lung, and the pericardium is interposed between the middle folium of the central tendon and the heart. This latter area, which is almost flat, is referred to as the cardiac plateau and extends more to the left than the right. In anteroposterior view, the superior profile of the diaphragm rises on either side of the cardiac plateau to a smooth convex dome or cupola, the cupola on the right being higher and slightly broader than that on the left. Most of the inferior surface is covered by peritoneum. The right side is accurately moulded over the convex surface of the right lobe of the liver, the right kidney and right suprarenal gland. The left side conforms to the left lobe of the liver, the fundus of the stomach, the spleen, the left kidney and the left suprarenal (adrenal) gland. In view of these differences in the profile and anatomical relationships of the right and left sides of the diaphragm, the side should always be specified in clinical descriptions. At full inspiration, the right hemidiaphragm is found at the anterior sixth rib on a posteroanterior chest radiograph; the left hemidiaphragm is 1.5–2.5 cm lower than the right (see Fig. 55.16). Unilateral paralysis may be seen as a raised hemidiaphragm on a chest radiograph, but this sign should not be relied upon uncritically in clinical practice.
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 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.
Reflux of gastric contents into the oesophagus, with risk of inhalation into the lungs, is normally prevented by a physiological antireflux barrier located at the gastro-oesophageal junction. The major components of this barrier are the specialized smooth muscle of the wall of the lower oesophagus and the encircling fibres of the crural diaphragm. See also Chapter 65.
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