Veins of the posterior abdominal wall

Ultrasound

The IVC can be identified where it is not obscured by intestinal gas. Its hepatic and proximal courses are readily seen w i th ultrasound. It can be seen posterior to the portal vein at the epiploic foramen. The hepatic veins can be seen draining to the IVC before it enters the right atrium. The IVC curves ventrally just before piercing the diaphragm, unlike the aorta, which passes posterior to the diaphragm.

Computed tomography The IVC is seen on the right of the aorta. It is a transverse oval in cross-section but its shape varies from slit-like on inspiration to circular on expiration. The right adrenal gland is seen to be partly posterior to the IVC. Some liver tissue may also be found posterior to it in its upper course. The right aortic branches that pass behind the IVC are usually visible.

The union of the common iliac veins to form the IVC can be identified, and the right common iliac vein is seen to be posterior to the left common iliac artery. In fact, all the pelvic veins are dorsal to the corresponding arteries. The IVC and its tributaries can be non-invasively visualized by a few coronal and sagittal sections without the need for contrast.

VEINS OF THE POSTERIOR ABDOMINAL WALL

(Fig. 5.51) There is a rich venous anastomosis in the lumbar area between lumbar, sacral, intercostal veins and inferior vena cava, via segmental lumbar veins which drain directly into the cava and the ascending lumbar and azygos systems. This system usually has a connection w i th the left renal vein.

The internal and external vertebral venous plexuses drain via segmental lumbar veins into the inferior vena cava. The lumbar veins are joined sequentially on both sides of the vertebral column by an ascending lumbar vein, which also extends inferiorly to the lateral sacral and iliolumbar veins. (The iliolumbar vein also drains into the common iliac vein.) The ascending lumbar vein ascends on the vertebral bodies at the root of the transverse processes deep to the psoas muscle and extends superiorly to the azygos system.

Magnetic resonance imaging

The azygos vein is a right-sided structure and may arise in one of three ways: • It may be simply the continuation of the ascending lumbar vein on the right side entering the thorax w i th the aorta or by piercing the right crus of diaphragm. • It may arise from the left renal vein or from the IVC at the level of the renal veins. • It may begin as the continuation of the subcostal vein.

Similarly the hemiazygos vein may arise in different ways: • It may be the continuation of the left ascending lumbar vein and enter the thorax to the left of aorta or by piercing the left crus. • It may arise from the posterior aspect of the left renal vein. • It may be the continuation of the left subcostal vein.

In congenital absence of the IVC, or following obstruction of the IVC, the ascending lumbar and azygos veins drain blood to the SVC and are enlarged. The role of this system in the spread of malignant and other disease from the pelvis to the spine is controversial. It is said to account for the propensity of prostate cancer to spread to the sacrum and lumbar vertebrae, and for infections arising in the pelvis to seed the intervertebral discs.

Radiological features of the veins of the posterior abdominal wall

CT and MRI

The ascending lumbar veins, the azygos and hemiazygos veins, may be visible deep to psoas distally and are identified on either side of the aorta proximally as they pass between the crura of the diaphragm. These veins are much more easily seen if enlarged, as occurs in obstruction of the IVC or SVC.

Lymphatic drainage of the abdomen

Lymph channels in the abdomen travel w i th the arteries. Most lymph drains to nodes around the abdominal aorta. These are arranged into four groups: • Preaortic • Right para-aortic • Left para-aortic; and • Retroaortic.

A ll of these drain to the cisterna chyli, which is an elongated lymph channel that continues in the thorax as the thoracic duct.

Preaortic nodes

These are arranged around the anterior branches of the aorta: the coeliac trunk and the superior and inferior mesenteric arteries. They drain lymph in the areas supplied by these arteries, that is, the gastrointestinal tract, the liver, gallbladder, pancreas and spleen. Lymph from these viscera passes through visceral nodes, such as the nodes in the porta hepatis and nodes near the intestinal wall, and along the course of the arteries before reaching the preaortic nodes. Lymph from the preaortic nodes passes in the right and left intestinal trunks to the cisterna chyli.

Coeliac nodes

These receive gastric, hepatic and pancreaticosplenic nodes.

The gastric nodes have three groups: left gastric, gastroepiploic and pyloric. • The left gastric nodes lie in the lesser curve and drain the lower oesophagus as well as the stomach. These nodes are usually smaller than the para-aortic group; over 8 mm is considered abnormal. The gastroepiploic group lie low on the greater curve, near the pylorus, and are related to the right gastroepiploic artery. These drain to the pyloric group - a group of approximately five nodes which lie in the bifurcation of the gastroduodenal artery at the junction of the first and second part of the duodenum. • The hepatic nodes are related to the hepatic artery at the porta hepatis in the lesser omentum (gastrohepatic ligament). They are variable in size and number and drain liver, gallbladder and bile ducts, as well as stomach, duodenum and pancreas. • The pancreaticosplenic nodes run w i th the splenic artery and lie above and behind the pancreas and in the gastrosplenic ligament. These drain pancreas, spleen and stomach.

Superior and inferior mesenteric nodes

The superior and inferior mesenteric nodes drain the bowel from the duodenojejunal flexure to the anal canal. Lymph drainage from the small bowel is to mesenteric nodes, from the terminal ileum and colon to ileocolic nodes, and from the rectum to pararectal nodes. From here, lymph drains along the arterial supply to more proximally located nodes along the arterial supply and to nodes at the origins of the superior and inferior mesenteric arteries. A ll nodes lie on the mesenteric side of the bowel, and lymphatic channels run in the mesentery. Perirectal nodes may lie anywhere around the rectum within the perirectal fat. They drain to a superior rectal group w i th the superior rectal artery and thence to to an inferior mesenteric group. They also drain to the internal and common iliac nodes. The lower part of the anal canal drains to external iliac nodes.

Para-aortic nodes

These nodes lie on either side of the aorta in relation to its lateral paired branches. They lie anterior to the medial