Peritoneum, Retroperitoneum, and Mesentery

Anatomy

The abdominal cavity is divided into both intraperitoneal and retroperitoneal spaces. The peritoneal cavity is lined by visceral and parietal peritoneum (a thin mesothelial membrane) and lies within the abdominal cavity, with two potential spaces, the greater and lesser sacs. The lesser sac communicates with the greater sac via the epiploic ^∗

∗ From epipluo (Greek root) “to float upon” as in the omentum that floats upon abdominal contents (omentum in Latin).

foramen. The mesentery represents a double layer of visceral peritoneum that encloses the small bowel, transverse, and sigmoid colon. These mesenteries are fixed to the posterior abdominal wall, with small bowel mesentery extending from the lower right abdomen to the upper left and the sigmoid mesentery also passing obliquely within the pelvis. The remaining ascending and descending colon are covered by peritoneum only on their anterior surface and are predominantly retroperitoneum, as is most of the duodenum. The greater curvature of the stomach, another double-layered peritoneum, gives rise to the greater omentum, which drapes over the transverse colon before turning back on itself to enfold the transverse colon and attach to the posterior abdominal wall. The lesser omentum passes from the lesser curvature of the stomach to cover the proximal duodenum and attaches to the inferior hepatic margin. The peritoneum contains a number of recesses formed by the folds of peritoneum and peritoneal ligaments.

The retroperitoneum lies behind the peritoneum within the abdominal cavity and is covered by parietal peritoneum and bordered posteriorly by the abdominal wall. Various organs and anatomical structures lie within this space, including the kidneys, ureters, bladder, adrenal glands, aorta, inferior vena cava, lower esophagus, duodenum (except for first part), pancreas, ascending and descending colon, and rectum.

Hernias (see Chapter 1 , Chapter 2 , Chapter 3 , Chapter 4 , Chapter 5 )

Abdominal hernias are defined as internal or external. Internal hernias are secondary to defects within the peritoneal mesentery, either congenital or acquired (usually after surgery). External hernias occur far more frequently, the most common being inguinal hernia followed by femoral hernia. There are, however, numerous other external abdominal hernias (see Chapter 4 ).

The significance of both internal and external hernias is that small, and to a lesser extent large, bowel can pass through the hernia, which may lead to bowel obstruction, particularly if the afferent and efferent loops of bowel pass through a narrow or tight hiatus. Usually, when the hiatus is wide mouthed, the bowel is free to “slide” in and out of the hernial orifice, but as this orifice becomes narrower, the bowel may become fixed, at which point it is known as being incarcerated. The proximal (or afferent) loop may or may not become obstructed by external compression at this stage. As bowel contents pass into the herniated loop, however, the herniated bowel becomes distended, further constricting the afferent loop at its entry point, and proximal bowel dilatation develops. Small (or large) bowel obstruction can then present with distention of the proximal bowel and collapse of the distal bowel ( Fig. 10-1 ). The bowel is now at risk of ischemia at the hiatal margin as the tight orifice further constricts the arterial and venous blood supply. Ultimately, if hernias are left untreated, bowel infarction (see Chapter 4 ) and sometimes death occur. All external hernias are at risk of this dynamic.

Bochdalek Hernia

Bochdalek ^∗

∗ Vincent Bochdalek (1801-1883), Bohemian anatomist.

hernia accounts for approximately 95% of congenital diaphragmatic hernias and is situated in a posterolateral position, mostly on the left. Sometimes the hiatus is large enough for stomach, small or large bowel, and rarely the spleen to freely enter the chest, which can cause compression of the left lung and mediastinal displacement. Many patients are asymptomatic, however, and the hernia is detected at CT performed for incidental reasons ( Fig. 10-2 ).

Morgagni Hernia

Morgagni ^†

† Giovanni Morgagni (1682-1771), Italian anatomist.

hernia is uncommon, comprising approximately 2% of congenital diaphragmatic hernias. It is situated anteriorly and occurs when the colon and omentum (less commonly stomach, small bowel, and liver) herniate through the foramen of Morgagni, situated adjacent to the sternal xiphoid process. Most are asymptomatic, but the heart may be compressed by the herniated bowel ( Fig. 10-3 ).

Diaphragmatic Eventration

Diaphragmatic eventration is not actually a hernia, but rather a congenital elevation of one side of an intact diaphragm, creating a space above the normally situated diaphragm that becomes filled by the bowel. It is quite common, usually small ( Fig. 10-4 ) but occasionally large ( Fig. 10-5 ), and asymptomatic in most cases. In the newborn, however, it can cause respiratory distress.

Diaphragmatic Rupture

Diaphragmatic rupture is a tear of the diaphragm and is usually traumatic. Given that abdominal pressure is higher than chest pressure, the bowel often herniates through the diaphragmatic rent, which may sometimes be sufficient to cause symptomatic compression of the lungs or heart. The diagnosis is usually made by multiplanar CT or MRI ( Figs. 10-6 and 10-7 ), although oral contrast material injected through a nasogastric tube should demonstrate herniated bowel contents in the chest. Most traumatic diaphragmatic hernias require surgical repair.

Pneumoperitoneum

Although pneumoperitoneum is sometimes referred to as “free air,” the gas is generally not “air,” but rather carbon dioxide inserted from laparoscopic procedures or bowel gas that is present in an extraluminal location. The gas may resemble air, however, for a short while after open laparotomy procedures. The presence of pneumoperitoneum may therefore be a benign postsurgical finding or may represent more sinister causes secondary to bowel ischemia or perforation ( Box 10-1 ). The detection of pneumoperitoneum is important and sometimes critical because it may be the only imaging sign of bowel perforation. Its detection, however, can be challenging (especially on plain radiograph) but is more likely when the x-ray beam is tangential to the location of the gas, which rises to the most nondependent part of the abdomen. Therefore, on an upright view the gas is most likely to be identified under the diaphragm ( Fig. 10-8 ), but on an anteroposterior view the gas may not be appreciated because the x-ray beam does not pass tangential to it. Pneumoperitoneum in the lateral or sagittal plane is better appreciated with computed tomography (CT), which demonstrates the extraluminal gas against the anterior abdominal wall ( Fig. 10-9 ). Smaller volumes of pneumoperitoneum can be quite subtle to detect on plain radiograph ( Fig. 10-10 ), and a lateral chest view may be required before small volumes of gas are identified ( Fig. 10-11 ). The most sensitive plain radiograph procedure is the left decubitus (right side up) view of the right upper quadrant, where as little as 1 mL of extraluminal gas can be detected between the liver margin and diaphragm. This procedure is rarely performed, primarily because CT is a far more sensitive tool for the detection of pneumoperitoneum, especially for small volumes of gas. However, even larger volumes of extraluminal gas may be missed unless viewed using lung window contrast settings, since the gas might otherwise be confused with intraluminal gas ( Fig. 10-12 ). Other plain radiograph findings can be demonstrated with larger volumes of gas. These include the visualization of the falciform ligament ( Fig. 10-13 ), which is also better visualized by CT ( Fig. 10-14 ) or recognized as a “football” sign, representing a large ovoid lucency in the center of the abdomen on supine radiographs. Gas can also sometimes be visualized in the Morison ^∗

∗ James Rutherford Morison (1853-1939), British surgeon.

pouch or may outline the lateral umbilical ligaments, but more often both sides of the bowel wall are outlined, usually referred to as the Rigler ^†

† Leo George Rigler (1896-1979), American radiologist.

sign (less commonly, double-wall sign). This sign is generally identified on the supine view, and large volumes of extraluminal gas are usually present ( Fig. 10-15 ). The sign may be quite subtle or obvious ( Fig. 10-16 ).

Peritoneal Disease

As a potential space, the peritoneal cavity can fill with fluid (ascites) and is susceptible to a number of inflammatory conditions. There are also a number of rare primary neoplastic lesions involving the mesentery, although metastatic deposits from intraabdominal malignancies are more common, particularly given its larger surface area.