Chapter 32 Peritoneal Cavity and Gastrointestinal Tract

Introduction

In order to understand metastatic disease in the abdomen and pelvis, one must appreciate the remarkable complexity of the peritoneum. By definition, the serosal peritoneum is a membrane that covers the lining of the abdominal and pelvic cavity and reflects over the viscera to form ligaments, mesenteries, and omenta. These reflections support the organs, not only structurally but also as conduits for the blood vessels, lymphatics, and nerves.

The network of connections formed by the peritoneal reflections serves to provide continuity between the abdominal walls and the organs therein. In addition, it also bridges the retroperitoneum with the peritoneum. These connections not only serve a physiologic and life-sustaining role but also act as a pathway for spread of disease.¹ These processes include inflammation, infection, trauma, and importantly, tumor.

Primary tumors of the peritoneum are rare.² Most commonly, primary tumor elsewhere metastasize to the peritoneum. Patients present with vague and nonspecific symptoms. Therefore, imaging plays a key role in the diagnosis of peritoneal disease. Because of the spaces and compartments formed by the peritoneal reflections, there are predictable patterns of disease spread that can be anticipated with certain tumors. Common tumors that spread and metastasize to the peritoneum include stomach, colon, ovarian, and pancreatic cancer and lymphoma.

Peritoneal disease could not be assessed in a direct manner radiographically before the advent of cross-sectional imaging.³ With improving technologies such as multidetector row computed tomography (CT), positron-emission tomography/computed tomography (PET/CT), magnetic resonance imaging (MRI), and ultrasound, peritoneal metastasis can now be readily evaluated and followed on scans for surveillance.

Anatomy

Embryology

To fully understand the peritoneum, a basic understanding of its embryologic development is necessary.⁴^–⁷ However, to explain the complex and detailed embryologic development of the peritoneum is not the purpose of this chapter. Briefly, the primitive gut is suspended within the peritoneal cavity by a dorsal and ventral primitive mesentery, which divides the peritoneum into a right and a left cavity. Unlike the ventral mesentery, the dorsal mesentery does not stop its attachment at the stomach, but continues to connect the primitive gut to the posterior abdominal wall inferiorly. The absence of the ventral mesentery in the lower abdomen allows for communication between the left and the right abdominal cavity.

The liver develops within the ventral mesentery. The pancreas and spleen develop within the dorsal mesentery. The ventral mesentery anterior to the liver and attaching it to the anterior abdominal wall later becomes the falciform ligament, containing the umbilical vessels. The ventral mesentery between the liver and the stomach will develop into the gastrohepatic and hepatoduodenal ligaments.

Further growth, organ development, elongation, cavitation, and rotation form the adult peritoneum (Figures 32-1 and 32-2). This includes the formation of the lesser sac, which is isolated from the remainder of the peritoneum or greater sac, except at a small opening called the foramen of Winslow.

Figure 32-1 Embryologic rotation of the peritoneal structures to the eventual adult position. The liver (L) will grow and fill the right side of the peritoneal cavity. The spleen (Sp) moves similarly to the left. GHL, gastrohepatic ligament; GSL, gastrosplenic ligament; K, kidney; S, stomach; SRL, splenorenal ligament.

(Illustration courtesy of David Bier.)

Figure 32-2 Peritoneal structures in the adult, position and relationships.

(Illustration courtesy of David Bier.)

A single layer of mesothelial cells forms the peritoneum. It is separated from the submesothelial layer of connective tissue by a basal lamina. The submesothelial layer of connective tissue consists of collagen, elastic fibers, fibroblast-like cells, arteries, veins, and lymphatics.⁶ The submesothelial layer or subperitoneal space is a virtual space that allows continuity between the mesenteries, the ligaments, the abdominal wall, and the retroperitoneum, as are described further.

Individual mesothelial cells are flat, contain abundant cytoplasm, and have long slender microvilli. This is especially true for cells that cover the viscera. The microvilli enables the cells to entrap hyaluronite, which aids to lubricate organ motion and movement. The peritoneum normally also contains a small amount of sterile fluid for lubrication, bacterial defense, and fluid transport.

Visceral and Parietal Peritoneum

The peritoneum is classified as either visceral or parietal.⁸ The abdominal and pelvic walls are lined by the parietal peritoneum. This includes the anterior surface of the retroperitoneum, the abdominal wall, and the undersurface of the hemidiaphragm. Conversely, the visceral peritoneum covers the intraperitoneal organs or viscera and forms the omenta and mesenteries.

In males, the greater peritoneal cavity is a closed continuous cavity. Conversely, in females, it is discontinuous at the ostia of the oviducts, providing a communication between the peritoneal cavity and the lower pelvis, which is extraperitoneal.⁶

Double folds of the peritoneum result in formation of ligaments and mesenteries. They suspend and form the supporting structure for the peritoneal organs. For example, the mesentery suspends the small bowel within the peritoneal cavity. The mesentery serves also to carry the arteries, lymphatics, and nerves. The omenta are formed from a double fold of the visceral peritoneum that extends from the stomach (Figures 32-3 and 32-4). The peritoneal cavity is divided into various interconnecting compartments by the ligaments and mesenteries.

Figure 32-3 The peritoneal structures in midsagittal view, diagrammatic representation.

(Illustration courtesy of David Bier.)

Figure 32-4 The peritoneal structures in oblique view. Diagrammatic representation and relationship of the stomach (S), pancreas (P), and transverse colon (TC) and their attachments.

(Illustration courtesy of David Bier.)

Supramesocolic and Inframesocolic Space

The peritoneal cavity is divided into a supramesocolic and an inframesocolic space by the transverse mesocolon ⁹ (Figure 32-5). The supramesocolic space is further divided into the right and left supramesocolic space by the falciform ligament. The left supramesocolic space is then further subdivided into the anterior and posterior perihepatic spaces. The right supramesocolic space also is subdivided into the anterior perihepatic space and a posterior compartment known as the lesser sac. The right and left supramesocolic space communicates via the foramen of Winslow, allowing communication between the lesser sac and the remainder of the peritoneal cavity or greater sac. There is continuity between the right paracolic gutter and the right supramesocolic space. However, the phrenicocolic ligament acts as a barrier between the left paracolic gutter and the left supramesocolic space.

Figure 32-5 A and B, Coronal view of the peritoneal spaces and attachments, including the supramesocolic and inframesocolic spaces. Note that there is no communication between the left supramesocolic and the left paracolic spaces, owing to the phrenocolic ligament.

(Illustration courtesy of David Bier.)

Important supporting ligaments in the supramesocolic space include the gastrohepatic ligament, hepatoduodenal ligament, gastrocolic ligament, gastrosplenic ligament, and splenorenal ligament (Figures 32-6 to 32-8). The ligaments are anatomically connected and continuous and their location and relationship can be identified by certain landmarks, mostly vasculature (Table 32-1).

Figure 32-6 Computed tomography (CT) with intravenous contrast. The transverse mesocolon is delineated by the middle colic artery and its branches (arrow). Note the solid enhancing mass in the right kidney from renal cell carcinoma (line).

Figure 32-7 CT with intravenous contrast. The gastrosplenic ligament is delineated by the short gastric artery (arrow) and left gastroepiploic vessels (not shown). The gastrosplenic ligament extends from the greature curvature of the fundus and upper body of the stomach to the splenic hilum.

Figure 32-8 CT with intravenous contrast. The hepatoduodenal ligament contains the portal vein (arrow), hepatic artery, extrahepatic bile duct, and nodal stations. It connects the lesser curvature of the stomach to the hepatic hilum.

Table 32-1 Supramesocolic Ligaments: Their Organ Relationship and Landmarks

LIGAMENT	ORGAN RELATIONSHIP	LANDMARKS
Gastrohepatic ligament	Lesser curvature of the stomach to left hepatic lobe	Left gastric vessels and left gastric nodal station
Hepatoduodenal ligament	Lesser curvature of the stomach to the hepatic hilum	Portal vein, hepatic artery, extrahepatic bile duct, and nodal stations
Gastrocolic ligament/supracolic omentum	Greater curvature of the stomach to the body of the transverse colon	Perigastric branches of the left gastroepiploic vessels with anastomosis to the right gastroepiploic vessels
Greater omentum	Transverse colon extending as an apron anterior to the small bowel	Epliploic vessels and branches of the gastroepiploic vessels
Gastrosplenic ligament	Continuous and to the left of the gastrocolic ligament, from the greater curvature of the fundus and upper body of the stomach to the splenic hilum	Short gastric vessels and left gastroepiploic vessels
Splenorenal ligament	Continuity between the spleen and the tail of the pancreas	Distal splenic artery or proximal splenic vein

The inframesocolic compartment is divided into a right and a left inframesocolic space by the obliquely oriented small bowel mesentery. The ascending colon provides the lateral border of the right inframesocolic space. The inframesocolic compartment consists of the root of the mesentery, jejunal mesentery, ileal mesentery, ascending mesocolon, descending mesocolon, sigmoid mesentery, and the pelvic floor and peritoneal folds (Table 32-2).

Table 32-2 Inframesocolic Compartment, Organ Relationship, and Landmarks

LIGAMENT	ORGAN RELATIONSHIP	VASCULAR LANDMARKS
Root of mesentery	From the horizontal portion of the duodenum to the right iliac fossa	SMA, SMV, and ileocolic artery and vein
Ileal mesentery	From the root of the mesentery to the ileum	Ileal artery and veins
Ascending mesocolon	Root of the mesentery to the ascending colon	Right colic artery and vein, cecal artery and vein
Jejunal mesentery	From the base of the mesentery to the jejunum	Jejunal artery and vein
Descending mesocolon	Base of the transverse mesocolon along the tail of the pancreas to the descending colon	Left colic artery and vein
Sigmoid mesocolon	Root of the sigmoid mesocolon	Sigmoid arteries, superior hemorrhoidal artery and vein

SMA, superior mesenteric artery; SMV, superior mesenteric vein.

Paravesicular Spaces

Peritoneal folds or reflections in the pelvis also result in potential spaces and compartments ¹⁰^–¹² (Figures 32-9 and 32-10). The urinary bladder, obliterated umbilical arteries, and inferior epigastric vessels indent upon the parietal peritoneum to form the anterior and posterior paravesicular spaces.