Localization and Spread of Disease

CHAPTER 6 Localization and Spread of Disease



Imaging to investigate problems of the abdomen and pelvis is often focused to a specific organ or body system. This can be problematic because diseases are often not confined to a specific organ, and pathologic processes can present distant from their origins. A thorough approach to abdominal and pelvic disease processes necessitates a basic understanding of how disease is contained and spread within the abdomen and pelvis. Such knowledge allows one to diagnose disease distant from its site of origin and predict the primary site of abnormality. This chapter considers how diseases are spread throughout the abdomen and pelvis, and examines relevant anatomic relations, the classic concepts of intraperitoneal and extraperitoneal, and the unifying concept of the subperitoneal space.




MESENTERIES AND LIGAMENTS OF THE ABDOMEN AND PELVIS



Ventral and Dorsal Mesenteries


The primitive mesentery is divided by the primitive gut into the ventral and dorsal mesenteries. The ventral mesentery regresses except for the portion that is associated with the foregut (ventral mesogastrium). The rapid growth of the liver within the ventral mesentery divides this mesentery into the lesser omentum (gastrohepatic ligament) and falciform ligament. The visceral peritoneum forms the liver capsule as it encases the liver, except for the surface embedded within the septum transversum (the bare area). Here, the peritoneum is reflected as the coronary ligament. The reflections of the coronary ligament again oppose each other as they attach to the parietal peritoneum anteriorly as the falciform ligament and laterally as the triangular ligaments. The falciform ligament extends from fissure for the ligamentum venosum to the anterior abdominal wall (Fig. 6-2). The falciform ligament provides a potential conduit for transmission of inflammation or hemorrhage from remote organs such as the pancreas (via the gastrohepatic ligament) to the anterior abdominal wall.



The dorsal mesentery extends in continuity from the intraabdominal portion of the esophagus to the rectum. This mesentery, in addition to giving support to the gut, serves as a conduit for the blood vessels, lymphatics, and nerves of the body organs. The spleen and dorsal portion of the pancreas appear by the fifth gestational week between the folds of the dorsal mesogastrium, which contains the splenic artery and vein, accompanying nerves, and lymphatics. The mesentery of the pancreas fuses with the posterior parietal peritoneum, leaving the spleen suspended by the splenorenal ligament and gastrosplenic ligament (Fig. 6-3). This is followed by posterior fusion of the splenorenal ligament. Inflammation or neoplasm may extend between the stomach and spleen via the gastrosplenic ligament. It is important to recognize that the pancreas is positioned beneath the posterior parietal peritoneum. The pancreas is centrally located within the subperitoneal space and in continuity with the abdominal organs via their mesenteric attachments. Neoplasms and inflammation can spread from the pancreas directly to the spleen via the splenorenal ligament, to the colon via the transverse mesocolon, to the liver via the hepatoduodenal ligament, and into the small bowel mesentery along the superior mesenteric vessels (Fig. 6-4). It is important to remember that the derivatives of the dorsal and ventral mesenteries remain in continuity after development, and that the individual named mesenteries are identified by their contained vessels (Tables 6-1 and 6-2).




Table 6-1 Ventral Mesentery Derivatives















Name Landmarks
Gastrohepatic ligament Left gastric vessels, right gastric vessels
Hepatoduodenal ligament Portal vein, hepatic artery, bile duct
Falciform ligament Ligamentum teres

Table 6-2 Dorsal Mesentery Derivatives





















Name Landmarks
Gastrosplenic ligament Short gastric vessels, left gastroepiploic vessels
Splenorenal ligament Splenic artery and vein
Gastrocolic ligament Right and left gastroepiploic vessels, gastrocolic trunk
Transverse mesocolon Middle colic vessels
Greater omentum Epiploic vessels





Ligamentous Support of the Colon and Small Bowel


The transverse mesocolon is derived from the dorsal mesocolon and is in continuity with the root of the small-bowel mesentery (Fig. 6-7). The phrenicocolic ligament is the left lateral extension of the root of the transverse mesocolon, and the duodenocolic ligament is the right lateral extension. The posterior reflections of the small intestine mesentery extend from the region below the transverse mesocolon in the left upper abdomen to the right lower abdomen, providing continuity between the left upper and right lower abdomen (Fig. 6-8). The dorsal mesocolon undergoes extensive posterior fusion after reentry to the abdominal cavity during development. The ascending and descending portions of the dorsal mesocolon lie in their lateral positions and fuse with the parietal peritoneum, as does the mesorectum. For imaging purposes, the ascending and descending colon and rectum are considered extraperitoneal structures. The appendix cecum, transverse mesocolon, and sigmoid mesocolon persist (Fig. 6-9). Therefore, the transverse and sigmoid portions of the colon are considered to be intraperitoneal. However, it is important to realize that the entire mesentery of the colon and rectum remains in continuity regardless of fused portions.







EXTRAPERITONEAL SPACES


Spread and containment of extraperitoneal disease is related to the anatomy of the extraperitoneal spaces. The localization of a process within an extraperitoneal compartment also allows for development of a differential diagnosis related to organs within that space. The three classic extraperitoneal spaces are demarcated by fascial planes that divide the extraperitoneum into compartments with specific contents and relations. The anterior and posterior renal fascias (collectively known as Gerota’s fascia, although the posterior renal fascia was first described by Zuckerkandl) segment the extraperitoneum into the anterior pararenal, posterior pararenal, and perirenal spaces (Fig. 6-10). The anterior and posterior pararenal spaces combine below the iliac crest and continue inferiorly as the extraperitoneal space of the pelvis, providing anatomic continuity.




Anterior Pararenal Space


The anterior pararenal space lies between the posterior parietal peritoneum and the anterior renal fascia (Figs. 6-11 and 6-12). This space contains the pancreas, extraperitoneal portion of the duodenum, and the ascending and descending colon. Its cephalad extent is the bare area of the liver (on the right) and its caudad extent is the level of the iliac fossae. The anterior pararenal space is continuous as the subperitoneal space of the mesenteries of the upper abdomen, transverse mesocolon, and small bowel. In this manner, disease processes may spread bidirectionally between the organs of the upper abdomen, pancreas, small bowel, and colon. The anterior pararenal space is constrained laterally by the lateroconal fascia formed by fusion of the layers of the anterior and posterior renal fascia. Anatomically, the anterior pararenal space is compartmentalized into colonic and pancreaticoduodenal compartments by fusion of the primitive mesenteries. This process of fusion results in creation of the retropancreaticoduodenal and retromesenteric interfascial planes that can serve as conduits for spread of fluid within the extraperitoneum.







Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Localization and Spread of Disease
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