Tuesday, September 2, 2008

Gaster Anatomy II

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE


ARTERIES

The arterial supply to the stomach comes predominantly from the coeliac axis although intramural anastomoses exist with vessels of other origins at the two ends of the stomach (Figs 8, 9). The left gastric artery arises directly from the coeliac axis. The splenic artery gives origin to the short gastric arteries as well as the left gastroepiploic artery and may occasionally give origin to a posterior gastric artery. The hepatic artery gives origin to the right gastric artery and the gastroduodenal artery, which in turn gives origin to the right gastroepiploic artery.

Left gastric artery


The left gastric artery is the smallest branch of the coeliac axis. It ascends to the left of the midline and crosses the left crus of the diaphragm beneath the peritoneum of the upper posterior wall of the lesser sac. Here it lies adjacent to the left inferior phrenic artery and medial or anterior to the left suprarenal gland. It runs forwards into the superior portion of the lesser omentum adjacent to the superior end of the lesser curvature. It turns anteroinferiorly to run along the lesser curvature between the two peritoneal leaves of the lesser omentum. At the highest point of its course, it gives off an oesophageal branch. In its course along the lesser curvature, it gives off multiple branches that run onto the anterior and posterior surfaces of the stomach and anastomose with the right gastric artery in the region of the incisura angularis.

The left gastric artery may arise from the common hepatic artery or its branches. The most common variant is an origin from the left hepatic artery, when the left gastric artery passes between the peritoneal layers of the superior lesser omentum to reach the lesser curvature of the stomach. Other variants include a common origin with the common hepatic artery. An aberrant left hepatic artery can occasionally arise from the left gastric artery: identification of an aberrant origin may be of importance during surgical mobilization of the upper stomach.

Short gastric arteries

The short gastric arteries are variable in number, commonly between five and seven, and arise from the splenic artery, its divisions, or from the proximal left gastroepiploic artery. They pass between layers of the gastrosplenic ligament to supply the cardiac orifice and gastric fundus, and anastomose with branches of the left gastric and left gastroepiploic arteries. An accessory left gastric artery may arise with these vessels from the distal splenic artery.

Left gastroepiploic artery

The left gastroepiploic artery arises from the splenic artery as its largest branch near the splenic hilum. It runs anteroinferiorly between the layers of the gastrosplenic ligament and into the upper gastrocolic omentum. It lies between the layers of peritoneum close to the greater curvature, running inferiorly to anastomose with the right gastro epiploic artery. It gives off gastric branches to the fundus of the stomach through the gastrosplenic ligament and to the body of the stomach through the gastrocolic omentum. These are necessarily longer than the gastric branches of the right gastroepiploic artery and may be 8-10 cm long. Epiploic (omental) branches arise along the course of the vessel and descend between the layers of the gastrocolic omentum into the greater omentum. A particularly large epiploic branch commonly originates close to the origin of the left gastroepiploic artery, descends in the lateral portion of the greater omentum and provides a large arterial supply to the lateral half of the omentum.

Posterior gastric artery

Variant:
A distinct posterior gastric artery may occur. When present, it arises from the splenic artery in its middle section posterior to the body of the stomach. It ascends behind the peritoneum of the lesser sac towards the fundus. It reaches the posterior surface of the stomach in the gastrophrenic fold.

Right gastric artery

The right gastric artery arises from the hepatic artery as it passes forwards from the posterior wall of the lesser sac into the lower border of the lesser omentum above the first part of the duodenum. The right gastric artery then runs between the peritoneal layers of the lesser omentum just above the medial end of the lesser curvature. It passes superiorly along the lesser curvature, giving off multiple branches onto the anterior and posterior surfaces of the stomach, and anastomoses with the left gastric artery.

The origin of the right gastric artery is often variant. The most common alternative origins are from the common hepatic, left hepatic, gastroduodenal or supraduodenal arteries.

Figure 8 Arterial supply of the stomach
Gastroduodenal artery

The gastroduodenal artery arises from the common hepatic artery posterior and superior to the first part of the duodenum. It gives origin to the right gastroepiploic and superior pancreaticoduodenal arteries at the lower border of the first part of the duodenum.
Right gastroepiploic artery

The right gastroepiploic artery originates from the gastroduodenal artery behind the first part of the duodenum, anterior to the head of the pancreas. It passes inferiorly towards the midline between the layers of the gastrocolic omentum. It lies inferior to the pylorus and then runs laterally along the greater curvature. It ends by anastomosing with the left gastroepiploic artery. It is adjacent to the pylorus but, more distally, lies c.2 cm from the greater curvature of the stomach. Gastric branches ascend onto the anterior and posterior surfaces of the antrum and lower body of the stomach while epiploic branches descend into the greater omentum. It also contributes to the supply of the inferior aspect of the first part of the duodenum.

Arterial anastomoses of the stomach

There is an anastomosis between the oesophageal arteries originating from the thoracic aorta and the vessels supplying the fundus in the region of the cardiac orifice. At the pyloric orifice the extensive network of vessels supplying the duodenum allows for some anastomosis between vessels of superior mesenteric artery origin and the pyloric vessels. The major named vessels supplying the stomach form extensive arterial anastomoses both on the serosal surface and around the curvatures. The right and left gastroepiploic arteries and the left and right gastric arteries anastomose freely with each other along the greater and lesser curvatures respectively. Anastomoses also form between the short gastric and left gastric arteries in the region of the fundus, and between the right gastric and right gastroepiploic arteries in the region of the antrum. In addition to the extensive serosal anastomoses, networks form within the stomach wall at intramuscular, submucosal and mucosal levels. A true plexus of small arteries and arterioles is present within the submucosa: it supplies the mucosa and shows considerable regional variation both in the gastric wall and in the proximal duodenum. The rich arterial supply to the stomach ensures that the high mucosal blood flow required for physiological functioning is maintained even if one or more vessels become occluded. As a consequence, the stomach exhibits considerable resistance to ischaemia even when multiple arterial supplies are lost.

The pyloric arteries are rami of the right gastric and right gastroepiploic arteries and pierce the duodenum distal to the sphincter around its entire circumference. They pass through the muscular layer to the submucosa where they divide into two or three rami, which turn back into the pyloric canal beneath the mucosa and run to the end of the pyloric antrum . They supply the entire mucosa of the pyloric canal. Branches of these pyloric submucosal arteries may anastomose close to their origin with the duodenal submucosal arteries. Their terminal rami also anastomose with gastric arteries from the prepyloric antrum. The pyloric sphincter is supplied by the gastric and pyloric arteries via rami that leave their parent vessels in the subserosal and submucosal levels to penetrate the sphincter.

Dieu la Foy lesions

Abnormalities of the intramural vascularity of the stomach are a rare cause of bleeding from the upper gastrointestinal tract. So-called 'Dieu la Foy' lesions commonly occur in the proximal body or fundus. When not actively bleeding, they appear as small, raised, red dots marking the mucosal surface of the proximal body or fundus. They were originally thought to be small arteriovenous malformations of the submucosal plexus. It is now considered that such lesions are caused by a larger than normal penetrating arterial vessel running through the muscular coat of the stomach into the submucosa before branching into the submucosal plexus. Although not a pathological abnormality, the vessel has a greater than normal calibre for arteries at this level. The pulsatile flow, combined with its proximity to the overlying mucosa, may then lead to focal ulceration and rupture of the vessel following minor trauma, leading to profuse intraluminal bleeding.

VEINS

The stomach veins drain ultimately into the portal vein. A rich submucosal and intramural network of veins gives rise to veins that usually accompany the corresponding named arteries. They drain either into the splenic or superior mesenteric veins although some pass directly into the portal vein.

Short gastric veins

Four or five short gastric veins drain the gastric fundus and the upper part of the greater curvature. They drain into the splenic vein or one of its large tributaries.

Left gastroepiploic vein
The left gastroepiploic vein drains both anterior and posterior gastric surfaces and the adjacent greater omentum. It runs superolaterally along the greater curvature, between the layers of the gastrocolic omentum. It receives multiple tributaries from the anterior and posterior surfaces of the body of the stomach and the greater omentum, and drains into the splenic vein within the gastrosplenic ligament.

Right gastroepiploic vein
The right gastroepiploic vein drains the greater omentum, distal body and antrum of the stomach. It passes medially, inferior to the greater curvature, in the upper portion of the gastrocolic omentum. Just proximal to the pyloric constriction it passes posteriorly to drain into the superior mesenteric vein below the neck of the pancreas. It may receive the superior pancreaticoduodenal vein close to its entry into the superior mesenteric vein.

Left gastric vein
The left gastric vein drains the upper body and fundus of the stomach. It ascends along the lesser curvature to the oesophageal opening where it receives several lower oesophageal veins. It then curves posteriorly and medially behind the posterior peritoneal surface of the lesser sac. It drains into the portal vein directly at the level of the upper border of the first part of the duodenum.

Right gastric vein
The right gastric vein is typically small and runs along the medial end of the lesser curvature. It passes under the peritoneum as it is reflected from the posterior aspect of the pylorus and first part of the duodenum onto the posterior wall of the lesser sac. It drains directly into the portal vein at the level of the first part of the duodenum. It receives the prepyloric vein as it ascends anterior to the pylorus at the level of the pyloric opening.

Posterior gastric veins
Distinct posterior gastric veins may occur. When present, they accompany the posterior gastric artery from the middle of the posterior surface of the stomach. They drain into the splenic vein and may occur as multiple small vessels.

Gastric varices
Variceal dilatation of the submucosal veins of the stomach may occur in the presence of portal hypertension. The anastomosis between portal and systemic venous circulations occurs around the lower oesophagus and upper stomach. Submucosal veins close to the cardiac orifice may become involved in the pathological flow of blood from the stomach and other upper abdominal viscera into the oesophageal veins. Gastric varices present less commonly in clinical practice than oesophageal varices. Occasionally gastric varices exist without the presence of oesophageal varices. In these circumstances, it may be that the effective 'point of meeting' between portal and systemic venous systems is lower than usual and occurs in the upper stomach rather than the lower oesophagus.

LYMPHATIC DRAINAGE

The stomach has a rich network of lymphatics that connect with lymphatics draining the other visceral organs of the upper abdomen. At the gastro-oesophageal junction the lymphatics are continuous with those draining the lower oesophagus. In the region of the pylorus they are continuous with those draining the duodenum. In the main, they follow the course of the arteries supplying the stomach, however many separate node groups are now recognized (Fig. 11). The relationship of separate node groups to the regions of the stomach and the vascular territories supplied is of great importance during resection of the stomach, particularly for malignancy. Pancreatic and hepatic lymphatics play a considerable role in draining areas of the stomach during disease.

Figure 9 Lymph node stations of A, the stomach and B, upper abdominal viscera.

INNERVATION

The stomach is innervated by sympathetic and parasympathetic fibres. The sympathetic supply originates from the fifth to twelfth thoracic spinal segments and is mainly distributed to the stomach via the greater and lesser splanchnic nerves and the coeliac plexus. Periarterial plexuses form along the arteries and supply the stomach from the coeliac axis. Additional innervation comes from fibres of the hepatic plexus, which pass to the upper body and fundus via the upper limit of the lesser omentum. Some innervation is also provided via direct branches from the greater splanchnic nerves.

The parasympathetic supply is from the vagus nerves (Fig. 10). Usually one or two rami branch on the anterior and posterior aspects of the gastro-oesophageal junction. The anterior nerves are mostly from the left vagus and the posterior from the right vagus, both emerging from the oesophageal plexus.

The anterior nerves supply filaments to the cardiac orifice and divide near the oesophageal end of the lesser curvature into gastric, pyloric and hepatic branches. Gastric branches (between four and ten) radiate on the anterior surface of the body and fundus. The greater anterior gastric nerve is the major gastric branch and lies in the lesser omentum near the lesser curvature. Pyloric branches (generally two) originate below the cardiac orifice. The smaller of the two nerves runs between the peritoneal layers of the lesser omentum almost horizontally towards its free edge and turns down on the left side of the hepatic artery to reach the pylorus. The larger nerve usually arises from the greater anterior gastric nerve during its course over the anterior surface of the stomach and runs inferomedially to the pyloric antrum. Hepatic branches (one or two) originate from the pyloric branches and run superiorly to contribute to the hepatic plexus.

The posterior nerves produce two main groups of branches, gastric and coeliac. Gastric branches originate behind the cardiac orifice and upper body of the stomach. They radiate over the posterior surface of the body and fundus and extend to the antrum but do not reach the pyloric sphincter. The largest is termed the greater posterior gastric nerve and runs posteriorly along the lesser curvature, giving branches to the coeliac plexus. Coeliac branches are often larger than the gastric branches. They run beneath the peritoneum, deep to the posterior wall of the lesser sac, at the upper limit of the lesser omentum to reach the coeliac plexus. Hepatic branches (one or two) are often small and originate from the coeliac branches. No true plexus occurs on either the anterior or posterior gastric surfaces, but plexuses are present in the submucosa and between the layers of the muscularis externa.

The gastric sympathetic nerves are vasoconstrictor to the gastric vasculature and inhibitory to gastric musculature. The sympathetic supply to the pylorus is motor, and brings about pyloric constriction. The sympathetic supply also conducts afferent impulses that mediate sensations, including pain. The parasympathetic gastric supply is secretomotor to the gastric mucosa and motor to the gastric musculature. It is also responsible for coordinated relaxation of the pyloric sphincter during gastric emptying.

Coeliac plexus

The coeliac plexus is the largest major autonomic plexus, sited at the level of the twelfth thoracic and first lumbar vertebrae. It is a dense network uniting two large coeliac ganglia and surrounds the coeliac artery and the root of the superior mesenteric artery (Fig. 13). It is posterior to the stomach and lesser sac, anterior to the crura of the diaphragm and the commencement of the abdominal aorta, and lies between the suprarenal glands. The plexus and ganglia are joined by greater and lesser splanchnic nerves and branches from the vagus and phrenic nerves. The plexus extends as numerous secondary plexuses along adjacent arteries.

The coeliac ganglia are irregular masses on each side of the coeliac trunk adjacent to the suprarenal glands. They lie anterior to the crura of the diaphragm. The right ganglion is posterior to the inferior vena cava, the left ganglion posterior to the origin of the splenic artery. The ipsilateral greater splanchnic nerve joins the upper part of each ganglion. The lower part of each ganglion forms a distinct subdivision usually termed the aorticorenal ganglion. This receives the ipsilateral lesser splanchnic nerve and gives origin to the majority of the renal plexus. It most commonly lies anterior to the origin of the renal artery. The coeliac plexus is connected to or gives rise to the phrenic, splenic, hepatic, superior mesenteric, suprarenal, renal and gonadal plexuses.

Phrenic plexus

The phrenic plexus lies around the inferior phrenic arteries on the crura of the diaphragm. It arises as a superior extension of the coeliac ganglion and often receives one or two sensory branches from the phrenic nerve. The left phrenic plexus is usually larger than the right. On the left it supplies branches to the left suprarenal gland and the cardiac orifice of the stomach. The right phrenic plexus joins the phrenic nerve, forming a small phrenic ganglion. This distributes branches to the inferior vena cava, suprarenal gland and hepatic plexus.

Figure 10 Distribution of the vagal nerves to the stomach.

REFERRED PAIN

The majority of the sensation of pain arising from the stomach is poorly localized. In common with other structures of foregut origin, it is referred to the central epigastrium. Pain arising from the region of the gastro-oesophageal junction may involve innervation from the oesophagus and is commonly referred to the lower retrosternal and subxiphoid areas.

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