The peritoneal spaces of the abdomen

margin of psoas and the crura of the diaphragm. They receive lymph from the structures supplied by these branches, including the posterior abdominal wall, the diaphragm, the kidneys and adrenals and the gonads. On the right, nodes lie anterior to and on both sides of the inferior vena cava. Nodes lying between the aorta and inferior vena cava are referred to as aortocaval nodes. A node lying between the IVC and the portal vein is frequently identified and is referred to as the portocaval node.

Lymph from the legs drains via the deep inguinal and external and common iliac nodes to the para-aortic nodes. Lymph from the pelvis drains via internal and common iliac nodes to the para-aortic nodes.

Efferent vessels from the para-aortic nodes unite to form the right and left lumbar trunks.

Retroaortic nodes

These nodes drain no area in particular and are really only posteriorly placed nodes of the lateral group. They are worthy of mention only because very little else lies between the aorta and the vertebral bodies, and a mass posterior to the aorta is therefore likely to have arisen in lymph nodes.

Cisterna chyli

This thin-walled sac, 6 mm wide and 6 cm long, is really the dilated proximal end of the thoracic duct before it passes through the aortic hiatus of the diaphragm. It lies in front of the bodies of L1 and L2 to the right crus of the aorta. The cisterna chyli receives the intestinal trunk, the right and left lumbar trunks and paired descending intercostal trunks.

Radiological features of the abdominal lymphatics

CT and MRI

The 'bean-shaped' appearance of the normal node may be appreciated, w i th its fatty hilum, although the fatty hilum is not always evident. The long axis is parallel to the direction of drainage. Pathologically enlarged nodes tend to be more rounded in shape, and lose their fatty hilum. Normal nodes vary in length from millimetres to several centimetres but are narrow in diameter. In the abdomen the normal cross-sectional diameter is variable, but should be less than 10 mm. In imaging, nodes are measured in their short axis. On MRI spatial resolution is less than w i th CT, but greater contrast resolution allows easier differentiation from nearby vessels, bowel loops and other retroperitoneal structures.

Lymphography Contrast is introduced into lymph vessels of the feet to visualize the lymph channels and nodes of the posterior abdominal wall. Lymph channels are visible w i t h in hours and should clear in less than 24 hours. Lymph nodes are best seen after 24 hours, when contrast has accumulated within them and the channels are clear.

Normal lymph nodes have a fine reticular appearance on lymphography. Normal lymph nodes are no longer than 3.5 cm, w i th their long axis in the line of the lymph vessels; however, some abnormal lymph nodes may measure less than this.

Lymphography by this method shows the deep inguinal nodes, the external iliac nodes and the common iliac nodes. The para-aortic nodes are seen as right and left lumbar chains of nodes along the tip of the transverse processes of the lumbar vertebrae to L2 level, where they drain to the cisterna chyli. In about 60% of cases a middle chain arises from the right chain. Fine, tortuous transverse vessels link the chains. Gaps in the right chain are frequent and associated w i th bypass vessels, but there are no gaps in the left chain. The upper left lumbar nodes are particularly numerous and are called the 'upper lumbar clump'.

On an AP view, the distance between the lateral border of a vertebral body and the most lateral lymph node should not be greater than 2 cm. On a lateral view the distance between the anterior border of T12 — L2 vertebrae and the lymph nodes or cisterna chyli should be less than 3 cm.

Lymphography is associated w i th a risk of respiratory compromise due to pulmonary oily contrast emboli. This procedure is contraindicated in patients w i th poor respiratory reserve.

THE PERITONEAL SPACES OF THE ABDOMEN

(Figs 5.52-5.54; see Fig. 5.55)

Development and terminology of the mesentery and peritoneal spaces

Most abdominal ligaments and mesenteries are formed from remnants of the ventral and dorsal mesenteries, which suspend the primitive gut (Fig. 5.52). In the abdomen a peritoneal ligament is formed by two folds of peritoneum that enclose a structure w i t h in the peritoneal cavity, and is usually named for the two structures it joins, e.g. gastrohepatic, splenorenal etc. An omentum is a ligament that joins the stomach to another structure. A mesentery comprises two folds of peritoneum that attach a loop of bowel to the retroperitoneum. In the embryo the mesenteries divide the coelomic cavity into right and left halves. In the upper abdomen the mesentery suspends the developing stomach and gut in the middle, w i th the developing liver in the ventral mesentery and the developing spleen in the dorsal mesentery. As development progresses, the organs migrate in an anticlockwise fashion and the liver comes to lie on the right, w i th the spleen on the left, dragging the mesenteries into the position they occupy at maturity (Fig. 5.53). The right coelomic cavity above the transverse mesocolon becomes the perihepatic space and lesser sac, and the left coelomic cavity becomes the left subphrenic space.

Fig. 5.52 Embryological development of peritoneal spaces: (a) fetus at 5th week; (b) fetus at 10th week; (c) maturity.

The dorsal mesentery gives rise to ligaments in the upper abdomen where it is reflected between structures: • The gastrophrenic ligament is the reflection from the stomach to the left hemidiaphragm. • The gastrosplenic ligament is the reflection of the mesentery from stomach to spleen. • The splenorenal ligament is the reflection from the left kidney to the spleen. • The phrenicocolic ligament is the reflection from the splenic flexure to the left diaphragm. • The gastrocolic ligament is the reflection from the stomach to the splenic flexure.

The dorsal mesentery also gives rise to: • The transverse mesocolon; • The small bowel mesentery; • The sigmoid mesocolon.

The ventral mesentery gives rise to: • The falciform ligament, which is the peritoneal reflection from the anterior surface of the liver to the anterior abdominal wall. The falciform ligament has the obliterated umbilical vein running w i t h in it, which is known as the ligamentum teres. The leaves of the falciform ligament are continuous w i th the fissure for the ligamentum venosum; • The gastrohepatic ligament, which is the reflection of the mesentery from the lesser curve of the stomach to the porta of the liver (and which is the upper part of the lesser omentum); • The hepaticoduodenal ligament, which is the reflection of the peritoneum from the first part of the duodenum to the porta of the liver and also part of the lesser omentum.

The lower free edge of this is the free edge of the lesser omentum and forms the entrance to the lesser sac.

The ventral mesentery regresses below the level of the transverse mesocolon.

The lesser sac and associated ligaments (Figs 5.53 and 5.54) The lesser sac forms as a result of rotation of the viscera in fetal life and is the remnant of the primitive right coelomic cavity. The posterior wall of the lesser sac is formed by the peritoneum over the pancreas and the left adrenal and upper pole of the left kidney, whereas its anterior wall is formed by the peritoneum over the posterior wall of the stomach and the lesser omentum. It is limited laterally by the spleen and its attached gastrosplenic and splenorenal ligaments. The lesser sac is partially divided by the fold of peritoneum over the left gastric artery, the pancreatogastric fold.

Fig. 5.53 Peritoneal spaces and ligaments: transverse section.

Medially the lesser sac communicates w i th the general cavity of the peritoneum via the epiploic foramen (of Winslow).

Boundaries of the epiploic foramen • Posteriorly: the IVC; • Anteriorly: the free edge of the lesser omentum containing the portal vein, hepatic artery and common bile duct; • Superiorly: the caudate process of the liver; and • Inferiorly: the first part of the duodenum.

Recesses of the lesser sac • The superior recess surrounds the medial aspect of the caudate lobe and is separated from the splenic recess by the gastropancreatic f o ld (a peritoneal fold covering the proximal left gastric artery as it runs superiorly).

This fold runs from the retroperitoneum to the posterior aspect of the fundus of the stomach and partially divides the superior recess from the splenic recess. • The splenic recess extends across the midline to the splenic hilum. • The inferior recess lies between the stomach and the pancreas and transverse mesocolon.

Ligaments of the lesser sac The lesser sac is bounded by remnants of both the dorsal and ventral mesenteries. Anteriorly, the lesser sac is bounded by the lesser omentum. This is a combination of gastrohepatic and hepaticoduodenal ligaments. • The gastrohepatic ligament arises in the fissure for the ligamentum venosum and connects the medial aspect of the liver to the lesser curve of the stomach. It contains the left gastric artery, left gastric vein and lymph nodes. • The hepticoduodenal ligament forms the inferior (free) edge of the lesser omentum and contains the portal vein, common duct and hepatic artery. • The gastrosplenic ligament is a remnant of the dorsal mesentery; it connects the splenic hilum to the greater curve of the stomach and contains the short gastric vessels. • The splenorenal ligament, also a remnant of the dorsal mesentery, forms part of the lateral border of the lesser sac, running from the posterior aspect of the spleen to the anterior pararenal space overlying the left kidney.

It encloses the tail of pancreas and the distal pancreatic and proximal splenic arteries. • The gastrocolic ligament, another dorsal remnant, connects the greater curve of the stomach w i th the superior aspect of the transverse colon and forms part of the anterior border of the lesser sac. It contains the gastroepiploic vessels and also forms the superior aspect of the greater omentum.

Fig. 5.54 Peritoneal attachments to posterior abdominal wall and sagittal sections to show peritoneal spaces.

Radiological points of interest

In abdominal ascites, when the lesser sac also contains fluid, a malignant process is more likely. Isolated fluid collections in the lesser sac are more likely to occur secondary to local pathology, such as pancreatitis or a perforated duodenal or gastric ulcer. Gastric pathology may spread along the gastrocolic ligament to involve the superior aspect of the transverse colon. Pathology of the pancreatic tail may spread along the splenorenal ligament to the anterior pararenal space, or along the gastrosplenic ligament. The leaves of the falciform ligament are continuous with the fissure for the ligamentum venosum, and this forms a potential pathway for the spread of disease from the porta to the falciform ligament.

Subphrenic and subhepatic spaces and associated ligaments

The peritoneum over the liver is continuous w i th that on the abdominal surface of the diaphragm except for a bare area posterosuperiorly. The peritoneal reflections that form the boundaries of the bare area are called ligaments: • The left triangular ligament on the left; and • The superior and inferior coronary ligaments on the right, w i th the right triangular ligament where these meet laterally.

Fig. 5.54 (a) This section passes through the right lobe of liver and right kidney. Note the position of the bare area of the liver, where fluid cannot abut the liver in cases of free intra-abdominal fluid. The right subphrenic space communicates with the right sub-hepatic space around the front of the liver, (b) This section passes through the liver, stomach, colon with its greater and lesser omenta, and the retroperitoneal duodenum and pancreas. Note that the lesser omentum runs up into the fissure for the ligamentum venosum in the liver. The free lower edge of this contains the vessels running to and from the porta of the liver: hepatic artery portal vein and bile duct, (c) Section through left lobe of liver, stomach with lesser sac behind and colon. Pancreas, duodenum and left kidney are retroperitoneal. Note that the lesser sac extends up to the left subphrenic space, (d) Section through spleen. Note splenorenal and gastrosplenic ligaments.

These peritoneal reflections form the boundaries of the subphrenic and subhepatic spaces, which are common sites of intraperitoneal collections.

The subphrenic space lies anterosuperiorly between the diaphragm and the liver. It is incompletely divided by the falciform ligament into right and left parts.

On the right side the triangular ligament divides the subphrenic space into two distinct compartments: • The subphrenic space lies between the right hemidiaphragm and the right lobe of liver. It is limited posteriorly by the right superior reflection of the coronary ligament and the right triangular ligament. • The subhepatic space lies posteroinferiorly between the liver and the abdominal viscera. It is limited superiorly by the inferior reflection of the coronary ligament and the right triangular ligament. It communicates anteriorly and laterally w i th the subphrenic space, and laterally w i th the right paracolic gutter. The posterior part of the subhepatic space between the liver and the kidney is called the hepatorenal space (Morrison's pouch). This is the deepest point of the abdominal cavity in a supine patient and is therefore a common site of collection of pus. The anterior part of the right subhepatic space is located just posterior to the porta hepatis and communicates w i th the lesser sac via the foramen of Winslow.

On the left side there is one large combined subphrenic space comprising three parts: • The immediate subphrenic space lies between the diaphragm and the fundus of the stomach and includes the area anterior to the lateral segment of the left lobe of liver (segments II and III). The gastrophrenic ligament runs through this space, suspending the stomach from the dome of the diaphragm. • The subhepatic space is also known as the gastrohepatic recess and is situated between the stomach and left lobe of liver. • The perisplenic space surrounds the spleen and is partially separated from the left paracolic gutter by the phrenicocolic ligament. This is a major suspensory ligament of the spleen and prevents the passage of fluid from the left paracolic gutter to the left subphrenic space.

The bare area of the liver lies between the reflections of the right and left coronary ligaments and is devoid of peritoneum. Intra-abdominal f l u id cannot extend medial to the attachments of the right coronary ligament or between the bare area of the liver and the diaphragm. The bare area of the liver is in continuity w i th the right anterior pararenal space, and pathologic processes can spread via this route.

Inframesocolic mesenteries and spaces

The transverse mesocolon, small bowel mesentery and sigmoid mesentery are all remnants of the dorsal mesocolon. Below the mesocolon (the mesentery of the transverse colon) the infracolic space is divided into right and left parts by the root of the mesentery of the small bowel. The attachment of this mesentery runs in an oblique line from the duodenal flexure upper left to lower right (see Fig. 5.55). Right and left paracolic spaces (gutters) are found lateral to the ascending and descending colon. The infracolic and paracolic spaces are continuous w i th the rectovesical and rectouterine pouches of the pelvis (see Chapter 6).

Radiological features of the peritoneal spaces

Plain films of the abdomen The peritoneum is not visible in the normal abdomen, but in pneumoperitoneum gas may outline the subphrenic space, the falciform ligament, the umbilical ligaments (see section on the abdominal wall) or the lesser sac.

Ultrasound This is a good method of visualizing f l u id collections in the right subphrenic and subhepatic spaces. The left subphrenic space between the left lobe of the liver or the spleen and the diaphragm can also be visualized. Gas in the stomach may obscure part of this space. Pelvic ultrasound or transrectal or transvaginal ultrasound may show fluid collections in the pelvic part of the peritoneal cavity.

Fig. 5.55 Axial CT of abdomen. Subject with extensive abdominal fluid. Note - the vessels running in the thin mesenteric folds, outlined by fat; and the retroperitoneal location of the ascending and descending colon, the ureters, the aorta and inferior vena cava.