CHAPTER 65 Abdominal oesophagus and stomach
ABDOMINAL OESOPHAGUS
The abdominal oesophagus is effectively tethered to the diaphragm by connective tissue (Fig. 65.1); the phreno-oesophageal ligament. This is formed of two thickened bands of elastin-rich connective tissue; the inferior phreno-oesophageal ligament is effectively an extension of the transversalis fascia extending beneath the parietal peritoneum as it is reflected from the diaphragm onto the abdominal oesophagus. The fibres are only loosely attached to the adventitial tissues and a variable amount of fat often lies beneath it, between the oesophageal wall and the crural sling. This oesophageal fat pad tends to act to tether the oesophagus to the fibres of the crura but tends to regress with age. On the thoracic side of the diaphragm the superior phreno-oesophgeal ligament is similarly formed from an extension of the subpleural endothoracic fascia. It is denser than its inferior counterpart with more elastin present and is tethered much more firmly through the muscle fibres of the oesophageal wall into the submucosal tissues. It may well act to restore lower oesophageal position after the movement engendered by the peristalsis of swallowing (Kwok et al 1999). Anteriorly, the subperitoneal connective tissue is particularly dense and blends with both the outer layer of the oesophageal wall and the apex of the crural fibres of the diaphragm. On the posterior aspect the peritoneal reflection is extremely short since the crura lie steeply angled, and the posterior oesophageal wall has a much shorter ‘effective length’ than the anterior. This short reflection of peritoneum is sometimes referred to as the gastrophrenic ligament and, via the peritoneum over the oesophagus continues directly onto the posterior surface of the stomach. It covers the oesophageal branches of the left gastric vessels and the coeliac branches of the posterior vagus and can thus be said to form an extremely short, wide mesentery to the abdominal oesophagus. In all but the thinnest individuals a pad of adipose tissue is found beneath the peritoneum covering the anterior surface of the lower abdominal oesophagus and the adjacent gastric wall. It is a useful surgical marker for the external location of the gastro-oesophageal junction.
Fig. 65.1 The anatomical structures around the abdominal oesophagus.
(Reprinted from Netter Anatomy Illustration Collection, © Elsevier Inc. All Rights Reserved.)
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE
STOMACH
PARTS OF THE STOMACH
For descriptive purposes, the stomach is divided by arbitrary lines drawn on its external surface into a fundus, body, pyloric antrum and pylorus (Fig. 65.2). The internal appearance and microstructure of these regions varies to some degree. The fundus is dome shaped and projects above and to the left of the cardiac orifice to lie in contact with the left dome of the diaphragm. It lies above a line drawn horizontally from the incisura cardiaca to the greater curvature. The body extends from the fundus to the incisura angularis, which is a constant external notch at the lower end of the lesser curvature. A line drawn from the incisura angularis to an indentation on the greater curvature defines the lower boundary of the body. The pyloric antrum extends from this line to the sulcus intermedius, where the stomach narrows to become the pyloric canal (1–2 cm long), which terminates at the pyloric orifice.
GASTRIC RELATIONS
Gastric curvatures
Lesser curvature
The lesser curvature extends between the cardiac and pyloric orifices and forms the medial (posterior and superior) border of the stomach. It descends from the medial side of the oesophagus in front of the decussating fibres of the right crus of the diaphragm, curves downwards and to the right and lies anterior to the superior border of the pancreas (Fig. 65.3). It ends at the pylorus just to the right of the midline. In the most dependent part there is typically a notch, the incisura angularis, whose position and appearance vary with gastric distension. The lesser omentum is attached to the lesser curvature and contains the right and left gastric vessels.
Gastric surfaces
When the stomach is empty and contracted, the two surfaces tend to lie facing almost superiorly and inferiorly, but with increasing degrees of distension they come to face progressively more anteriorly and posteriorly (Fig. 65.4).
Anterior (superior) surface
The lateral part of the anterior surface is posterior to the left costal margin and in contact with the diaphragm, which separates it from the left pleura, the base of the left lung, the pericardium and the left sixth to ninth ribs (Fig. 65.5). It lies posterior to the costal attachments of the upper fibres of transversus abdominis, which separate it from the seventh to ninth costal cartilages. The upper and left part of this surface curves posterolaterally and is in contact with the gastric surface of the spleen. The right half of the anterior surface is related to the left and quadrate lobes of the liver and the anterior abdominal wall. When the stomach is empty, the transverse colon may lie adjacent to the anterior surface. The entire anterior (superior) surface is covered by peritoneum.
Posterior (inferior) surface
The posterior surface lies anterior to the left crus and lower fibres of the diaphragm, the left inferior phrenic vessels, the left suprarenal gland, the superior pole of the left kidney, the splenic artery, the anterior pancreatic surface, the splenic flexure of the colon and the upper layer of the transverse mesocolon (Fig. 65.3). Together these form the shallow stomach bed: they are separated from the stomach by the lesser sac (over which the stomach slides as it distends). The upper left part of the surface curves anterolaterally and lies in contact with the gastric surface of the spleen. The greater omentum and the transverse mesocolon separate the stomach from the duodenojejunal flexure and ileum. The posterior surface is covered by peritoneum, except near the cardiac orifice, where a small, triangular area contacts the left diaphragmatic crus and sometimes the left suprarenal gland. The left gastric vessels reach the lesser curvature at the right extremity of this bare area in the left gastropancreatic fold. The gastrophrenic ligament passes from the lateral aspect of this bare area to the inferior surface of the diaphragm.
GASTRIC ORIFICES
Cardiac orifice and gastro-oesophageal junction
The opening from the oesophagus into the stomach is the cardiac orifice (Fig. 65.1). It is typically situated to the left of the midline behind the seventh costal cartilage at the level of the eleventh thoracic vertebra. It is typically 10 cm from the anterior abdominal wall and 40 cm from the incisor teeth. The abdominal oesophagus is continuous with the cardiac orifice; the right side is continuous with the lesser curvature, the left side with the greater curvature. There is no specific anatomically discernible cardiac sphincter related to the orifice.
Internally, the transition between oesophagus and stomach is difficult to define because mucosa of gastric fundal pattern extends a variable distance up into the abdominal oesophagus. It usually forms a ‘zig-zag’ squamo-columnar epithelial junction with the oesophageal epithelium above this Z line: for histological and endoscopic purposes, this is often referred to as the gastro-oesophageal junction. A sling of longitudinal and oblique gastric muscle fibres forms a loop on the superior, left, side of the gastro-oesophageal junction between the oesophagus and the lesser curvature, and this is taken as the external boundary of this junction. Bands of thickened circular smooth muscle in the upper wall of the greater curvature and the distal oesophagus are sometimes confusingly referred to as ‘clasp’ or ‘sphincter’ fibres.
Lower oesophageal sphincter and gastro-oesophageal reflux
At rest there is a gastro-oeosophageal pressure gradient due to the presence of negative intra-thoracic pressure (transferred to the thoracic oesophagus) and positive intra-abdominal pressure (transferred to the stomach and augmented by any contraction of the stomach wall itself). Several anatomical and physiological factors normally prevent gastro-oesophageal reflux. Minor factors include the folds of gastric mucosa present in the gastro-oesophageal junction, the mucosal rosette, which contribute to the formation of a fluid- and gas-tight seal and also help to ensure that even low levels of tone within the lower oesophageal wall muscles may occlude the lumen of the junction against low pressures of gastric gas; the angle of the cardiac orifice, which is formed, in part, by the pull of the long oblique fibres of the inner layer of the gastric smooth muscle and may help to form a type of ‘flap valve’; and the length of the abdominal oesophagus, which is buttressed externally by pads of adipose connective tissue at and below the level of the diaphragmatic hiatus. However, the major anti-reflux mechanisms are the tonic contractions of the specialized smooth muscle of the wall lower oesophageal and the encircling fibres of the right diaphragmatic crus, which, together, exert a radial pressure that can be measured by electromyography or manometric testing (Paterson 2001, Mittal 2006), and form an effective high pressure zone (HPZ). At and just below the level of the entry of the abdominal oesophagus into the stomach, the circular fibres of the intermediate layer of the muscularis externa lying over the upper lesser curvature are particularly pronounced and sometimes referred to as ‘clasp’ fibres and exert fairly constant myogenic tone (Fig. 65.6). Since the oesophagus passes obliquely into the stomach, with increasing gastric distension the tone in the clasp fibres rises and they may act to draw the anterior and posterior surfaces together, increasing the tone at the gastro-oesophageal junction, contributing to the HPZ. These anatomical and physiological features are together referred to as the lower oesophageal sphincter (LOS). If reflux is to be prevented, the pressure within the HPZ must exceed the difference between the pressures on either side of the junction. The oesophageal part of the LOS is controlled by the intramural plexuses of the enteric nervous system via the neural release of nitric oxide which relaxes the smooth muscle of the LOS. Tone is reduced in advance of the oesophageal peristaltic wave during swallowing and raised again after the food bolus has passed. During inspiration, the greater negative intrathoracic pressure which increases the gastro-oesophageal pressure gradient is offset by increased pressure in the HPZ due to contraction of the peri-oesophageal fibres of the right diaphragmatic crus. Activation of the crural diaphragm slightly before the costal fibres ensures that this increase precedes the increase in the gradient.
Fig. 65.6 Detail of the arrangements of the muscle layers of the oesophageal and gastric walls.
(Reprinted from Netter Anatomy Illustration Collection, © Elsevier Inc. All Rights Reserved.)
