12 Abdominal Veins

This chapter deals with the normal and variant anatomy of the abdominal venous system and the main types of pathology detectable by ultrasound. Because of their embryology, the inferior vena cava and its tributaries and the mesenteric and extrahepatic portal venous systems are discussed in the same chapter. Diseases of the intrahepatic portal vein branches and portal hypertension are described in Chapter 15.5.8.

12.2 Anatomy, Variants, and Collaterals

Angiogenesis begins in the second half of the third week of embryonic development with two longitudinal dorsal aggregations of cells that give rise to the aortae and also a capillary plexus from which the heart and primary venous system are derived.²⁰ At the center of embryonic development is the sinus venosus with the large, paired embryonic veins:

•The umbilical veins arise from the chorionic villi and deliver oxygen-rich maternal blood to the embryo. The right umbilical vein is obliterated while the left vein persists until birth. Afterwards it persists as an obliterated tube, the round ligament of the liver.

•The vitelline veins drain the yolk sac. Just before entering the sinus venosus, they form a capillary plexus receptive to the ingrowth of hepatocytes. It develops into the sinusoidal plexus of the liver. The lower stepladder-like anastomoses of the paired vitelline veins, which form a venous ring around the duodenum, develop into the portal vein through a process of fusion and partial regression. The distal portion of the right vitelline vein persists as the superior mesenteric vein, while the proximal portions of both vitelline veins persist as the right and left hepatic veins.

•The anterior and posterior cardinal veins collect blood from the embryo body with the primitive kidneys and merge to form a common trunk. The subcardinal veins develop medially to form a new venous system, anastomosing between the embryonic kidneys, and with further development they form some of the venous drainage of the posterior cardinal veins (Fig. 12.2).

•Finally, the supracardinal veins develop and anastomose with the subcardinal veins. As development proceeds, the posterior cardinal veins are obliterated and the lower left supracardinal veins regress. The right supracardinal vein persists and becomes the postrenal segment of the inferior vena cava, whose other segments develop from the right supracardinal vein and from anastomoses between the supracardinal and subcardinal veins.^{18 ,​ 20}

This complex embryonic development from multiple segments accounts for the many possible variants and abnormalities, which are discussed in the sections below.

12.2.1 Inferior Vena Cava, Lumbar and Pelvic Veins

The inferior vena cava is formed at the L4–L5 level by the confluence of the common iliac veins (Fig. 12.2). It drains blood from the pelvic viscera and lower extremities and first runs a short distance posteriorly before ascending on the right side of the aorta. At the L2–L3 level it turns anteriorly and runs a short intrathoracic course before entering the right atrium. Its caliber varies with its degree of fullness and with respirations. A diameter greater than 2 cm is not unusual. Its lumen narrows on inspiration and may temporarily collapse. The inferior vena cava is normally constricted or impressed by the common iliac artery anteriorly, the right renal artery posteriorly, by the liver, and by degenerative changes in the spinal column.

The system of lumbar veins and the azygos and hemiazygos veins can rarely be imaged sonographically. Computed tomography (CT) scans of the upper abdomen display the azygos and hemiazygos veins as small tubular structures located posterior to the diaphragm crura (Fig. 12.3). The vertebral plexuses are drained by a stepladder-like array of segmental veins, some opening directly into the inferior vena cava and others into the ascending lumbar vein (Fig. 12.9).

In the classification of Chuang,²⁰ congenital anomalies of the inferior vena cava are classified into four main types (Table 12.1) in accordance with their embryology:

•Type A: The retrocaval ureter results from persistence of the posterior cardinal vein. It may lead to obstruction of the right pelvicalyceal system. On intravenous pyelography, the ureter is shifted toward the midline and shows posterior displacement in the lateral projection.

•Type B: Persistence of the right supracardinal vein with regression of the left supracardinal vein results in a normal right-sided inferior vena cava.

•Type C: Mirror-image regression of the right supracardinal vein leads to a left-sided inferior vena cava. It has an incidence of 0.2% to 0.5%.

•Type BC: Persistence of both supracardinal veins leads to a double inferior vena cava (Fig. 12.4).

Other variants such as a persistent left posterior cardinal vein (type D) with a left-sided retrocaval ureter or a combination of types D and A are extremely rare and have no clinical importance.

Agenesis of the hepatic or prerenal segment leads to interruption of the inferior vena cava with drainage through an alternative pathway such as the azygos or hemiazygos system.

Other variants in the pelvic region may be found at the level of the caudal anastomosis of the original posterior cardinal veins. Variants of termination and course mainly involve duplications of the common iliac vein or internal iliac vein, variant contralateral terminations, and dysplasia of the left common iliac vein with lumbar collateralization.

12.2.2 Renal Veins

The renal veins develop from anastomoses between the subcardinal and supracardinal veins (see above). A circumaortic venous ring develops into which two renal veins open initially, later reducing to one renal vein per kidney in most individuals. The right renal vein is 2 to 4 cm long and runs obliquely downward to enter the inferior vena cava at the L1 level. The left renal vein is 4 to 11 cm long. It runs a more horizontal course, curving between the aorta and superior mesenteric artery (SMA) to enter the vena cava.

The intrarenal veins have much the same distribution pattern as the arteries: A deep system drains the cortex via the interlobular veins, which drain centrally to the arcuate veins and from there to the interlobar veins. Larger-caliber veins are formed by anastomoses at the level of the papillae and calyces, some uniting anterior to and some posterior to the renal pelvis to form the renal vein. There is also a superficial venous system that collects blood from the cortex via cortical veins and drains to the arcuate veins.

Variants of the renal veins are common and have major practical relevance. The most common anomalies are retroaortic renal vein and persistent periaortic venous ring. Their reported incidence is as high as 8.7%.⁵⁸ Other variants are multiplicity and variant terminations, including renal vein termination in the common iliac vein. It is important for these variants to be looked for and identified during preoperative staging, especially in patients with renal tumors, and before the surgical treatment of an aortic aneurysm.

12.2.3 Gonadal Veins

The ovarian veins and testicular veins collect blood from the gonads. The ovarian veins receive blood from the plexus around the uterus and from the ovaries. The testicular veins receive their blood from the pampiniform plexus. Their further retroperitoneal course is the same in males and females: The veins cross over the ureters in the middle third of the iliopsoas muscle and open into the renal vein on the left side and usually open directly into the inferior vena cava on the right side. Ultrasound can define at least portions of the veins anterior or anterolateral to the psoas muscle.

Variants of the gonadal veins are also common and have major practical relevance. In most cases the right gonadal vein enters the inferior vena cava just below the renal vein; in approximately 30% of cases, it terminates at the same level as the renal vein. On the left side, the testicular vein or ovarian vein usually drains into the left renal vein. In a rare variant (1%) it opens directly into the inferior vena cava. Approximately 30% of cases show duplication of the gonadal veins as well as accessory vessels, which also have a variable termination.³⁷

12.2.4 Portal Venous System and Mesenteric Venous System

The portal venous system drains venous blood from the digestive tract including the gallbladder, pancreas, and spleen via the portal vein to the liver. The main tributaries of the portal vein are the splenic vein, superior and inferior mesenteric veins, short gastric veins, left gastroepiploic vein, pyloric vein, and left gastric vein.

Portal Vein

The portal vein is formed behind the pancreatic isthmus by the confluence of the superior mesenteric vein and splenic vein (Fig. 12.5). Its extrahepatic portion is 8 to 10 cm long and runs laterally and obliquely to the porta hepatis, passing between the common bile duct and hepatic artery in the hepatoduodenal ligament. On entering the liver, it divides into right and left main branches that undergo additional divisions, delivering blood to the sinusoids along with blood from accompanying terminal arterial branches as described in Chapter 15.

Variants of the portal vein trunk have been described in rare cases. They result from the complex embryology of the venous system and may arise at various stages of portal vein development (Chapter 15). A preduodenal portal vein is generally noted as an incidental finding during surgery. Other possible anomalies are duplication of the portal vein, strictures, and obstructive valves. Atresia and hypoplasia have been considered rarities and are sometimes associated with a congenital portocaval shunt (CPCS). This anomaly may be intra- or extrahepatic. In a complete CPCS, the splenic vein and mesenteric veins may open separately or by a common trunk directly into the vena cava or into the splenic vein.^{40 ,​ 53 ,​ 66}

Splenic Vein

The splenic vein is formed by the union of two to six branches at the splenic hilum. It has an average length of 15 cm and runs below and just posterior to the splenic artery (Fig. 12.6a). The body and tail of the pancreas are anterior to it.

The superior mesenteric vein drains blood from the jejunum, ileum, ascending colon, transverse colon, and pancreaticoduodenal arcades (Fig. 12.5, Fig. 12.6b; see also Video 11.1 in Chapter 11).

The vasa recta and mesenteric arcades are formed by the union of intramural veins. They in turn form segmental branches that run parallel to the homonymous arteries. The middle colic vein anastomoses with the left colic branch of the inferior mesenteric vein. Draining veins from the jejunum and ileum open into their mesenteric vein from the left side, veins from the ascending and transverse colon from the right side.

Nonmesenteric Branches

Other, nonmesenteric branches are the right gastroepiploic vein and the veins draining the pancreas.

Blood from the anterior portion of the pancreas is drained by the anterior superior pancreaticoduodenal vein, which unites with the gastro-omental vein to form the gastropancreatic vein. That vessel receives the right colic vein and then runs from the right side, anterior to the pancreas, as the gastropancreaticocolic trunk, which opens into the superior mesenteric vein (Fig. 12.5). This provides an important collateral pathway in patients with a distal occlusion of the superior mesenteric vein.

The anterior inferior pancreaticoduodenal vein usually opens directly or via the jejunal veins into the superior mesenteric vein. The posterior inferior pancreaticoduodenal vein is not consistently present. The posterior portions of the pancreas are drained chiefly by the posterior superior pancreaticoduodenal vein (PSPDV), which runs from the lateral inferior border of the pancreatic head over the posterior surface of the gland to the superior border of the pancreatic head. There it usually runs behind the common bile duct to the portal vein, terminating approximately 1 to 2 cm above and to the right of the confluence. Its average diameter at that level is 2.8 mm, and the vessel is detectable sonographically in over 50% of cases.⁶²

Hepatic Veins

The intrahepatic veins normally unite to form three main veins that open directly into the inferior vena cava with no significant extrahepatic course. Angiography with selective retrograde catheterization can define branches up to the fourth-order level (Fig. 12.7).

The principal variants include absence of the middle hepatic vein. This eliminates double drainage of the four central hepatic segments. More than three main veins are present in 30% of cases.⁸

In addition to the principal veins, there are a variable number of accessory veins located mainly in the area where the liver attaches to the diaphragm. The most important accessory veins are the right and left inferior phrenic veins, the dorsal hepatic veins, the caudate lobe veins, and collaterals to the right suprarenal vein. The presence of the caudate lobe veins explains how patients with Budd-Chiari syndrome can survive.^{37 ,​ 49} Details on the intrahepatic divisions of the portal vein and the hepatic veins are described in the chapter on the liver (see Chapter 15).

12.3 Examination Technique

12.3.1 Transducer

A linear-array or curved-array transducer with a transmission frequency of 2 to 5 MHz is recommended for imaging the inferior vena cava and its tributaries.

12.3.2 Protocol

First the B-mode image should be optimized and analyzed. For this purpose, the liver is imaged in an oblique subcostal scan, and the gain and time gain compensation (TGC) are adjusted until the echo pattern in all segments is as homogeneous as possible and each of the hepatic veins can be traced to the second-order branch level.

Next the inferior vena cava is imaged in longitudinal and transverse scans from the anterior side (Fig. 12.8), then in coronal scans from the right side. The vena cava can be traced to the iliac level by applying gentle transducer pressure. Then, by rotating the caudal end of the transducer, the operator can define the common and external iliac veins as far as the inguinal ligament in the same pass (Fig. 12.22). The lumbar veins can also be visualized in thin patients (Fig. 12.9). If bloating is present, it may be helpful in select cases to start the examination at the inguinal level and then proceed cephalad while applying gentle probe pressure.

The renal veins are best evaluated in transverse scans, proceeding downward from the level of the celiac trunk and SMA. The left renal vein can be identified between the aorta and SMA and then traced to the inferior vena cava (Fig. 12.10). For imaging the right kidney, the transducer should be rotated slightly caudad on the right side. The kidneys can also be imaged from the lateral side in coronal scans that are angled slightly anteriorly. The full length of the right renal vein can usually be visualized with this technique. It saves time to activate color Doppler right away when scanning the renal veins.

The transducer should be suitable for near-field imaging in examinations of the confluence, splenic vein, superior and inferior mesenteric veins, and possible superficial portal venous collaterals. Very little transducer pressure should be applied as it might fully compress the splenic and mesenteric veins in thin patients, causing them to escape detection.

The portal vein can be identified anterior to the liver, starting from the porta hepatis. Often it is much easier to scan from the right side through an intercostal window. This permits a better evaluation of portal vein flow owing to the smaller angle between the transducer and vessel axis. Starting from the confluence, the splenic vein is imaged from the anterior side in transverse scans, supplemented next by lateral scans of the spleen from the left side.

The superior mesenteric vein is best demonstrated in transverse scans proceeding downward from its junction with the confluence. It is located to the right of the homonymous artery, has a larger lumen, and is easily identified on color Doppler by its continuous flow. Doppler spectra usually show a flat spectrum with flow directed toward the liver.

12.3.3 Velocity Measurements

The mean flow velocities in the abdominal veins are lower than in the abdominal arteries, so the velocity scale should be set to intermediate values. The color gain and threshold should be adjusted to a level that just eliminates color artifacts while the transducer is stationary. Color flow signals should be displayed with no extraluminal color bleed. If a vessel contains no flow, the selected velocity scale and Doppler angle should be checked. If the vessel axis is perpendicular to the transducer, as in the case of the vena cava, color filling can be obtained only by varying the scan angle or using a sector-shaped beam. In contrast to the wedge-shaped standoff pad used for peripheral vascular imaging, it is advantageous to use an abdominal transducer that allows beam steering.

In systems with adjustable wall filters, the filter setting for portal vein imaging should not exceed 100 Hz, otherwise slow flow could go undetected. Other problems can result from an incorrect focal position. The focus should be at level with or just distal to the vessel under investigation. If peristalsis is strong enough to create a coarse color mosaic that overwrites vessel walls, it is often helpful to administer an antispasmodic (e.g., buscopan).

When color Doppler is activated, the vena cava shows homogeneous color fill in early apnea (Fig. 12.8) with flow directed toward the heart (antegrade). On rapid inspiration, flow increases due to the suction effect of the lower intrathoracic pressure and becomes turbulent. It becomes laminar again in apnea. Turbulent areas, which appear as a color mosaic, occur physiologically in the terminal portions of the hepatic and renal veins.

Doppler spectra sampled from the center of the inferior vena cava during the cardiac cycle show two brief, antegrade flow peaks at the start of ventricular systole and when the AV valves open (see Chapter 15). Atrial contraction is accompanied by a cessation of flow or transient retrograde flow, appearing on color duplex as a brief color reversal. This is a physiologic phenomenon that also occurs in the right renal vein and hepatic veins.

The distal inferior vena cava and the lumen of the common and external iliac veins are often obscured by scattering artifacts in the B-mode image. But when scanning is done through the “water path” of the distended bladder, the iliac veins can be identified as a color-filled vascular band even under poor scanning conditions. It should be noted, however, that a greatly distended bladder may compress both external iliac veins and mimic an occlusion. Emptying the bladder in this case will show patent iliac veins with a normal flow pattern. With newer scanners, it is often possible to identify the gonadal veins and even the lumbar veins in some cases (Fig. 12.9).

12.4.2 Renal Veins

The renal veins can usually be traced from the hilum or from the inferior vena cava, and their flow is directed toward the heart (Fig. 12.10). The left renal vein usually shows continuous flow. In thin patients it is not unusual to find aliasing between the aorta and SMA with increased flow like that found in a stenosis.

More pronounced cases may involve an anterior nutcracker syndrome, which can cause a very high-grade stenosis or functional occlusion of the renal vein. The prestenotic venous segment is dilated, flow is absent or greatly diminished, and collateral drainage may occur through enlarged gonadal veins with retrograde flow (Fig. 12.30).

12.4.3 Hepatic Veins

The hepatic veins are best demonstrated in a subcostal oblique scan and in longitudinal scans through the left and right hepatic lobes (Fig. 12.11). In the absence of liver disease, the hepatic veins arch smoothly from the periphery to the inferior vena cava, entering that vessel at an angle less than 45 degrees. Their borders are smooth and sharp, and their lumen is normally echo-free.

In CDS the hepatic veins can be visualized as far as second- and third-order branches. Their flow normally shows cardiac modulation with a brief cessation or reversal of flow during arterial systole. When power Doppler is added, the veins supplying the caudate lobe can usually be defined (Fig. 12.11b).

12.4.4 Portal and Mesenteric Venous System

CDS shows complete, homogeneous filling of the portal vein with color pixels (Fig. 12.18). Flow in the vein is directed toward the liver. Respiratory and cardiac modulations are often detectable only by spectral analysis. Velocity measurements show a normal range of 10 to 25 cm/s (Table 15.4), but the peak velocity may rise above 40 cm/s after a meal. When scanned from the anterior side, the portal vein can be traced along its full length in more than three-fourths of cases. An intercostal scan from the right side can almost always demonstrate its full length.

Splenic Vein and Mesenteric Veins

The most important portal vein tributaries that can be visualized with ultrasound are the splenic vein and superior mesenteric vein. Both vessels can be assessed in over 80% of patients. The inferior mesenteric vein is often obscured in varying degrees by overlying air.

When examined by CDS, these vessels and the portal vein normally show continuous, usually laminar flow directed toward the liver (Fig. 12.12). The splenic vein shows a color reversal because of its curved course. Flow is not detected in segments that run parallel to the transducer (Chapter 2). Turbulence is usually seen at the confluence with the superior mesenteric vein and has no pathologic significance. The distal third of the splenic vein may be obscured by gas in anterior scans but can generally be imaged in a lateral scan together with the splenic artery at the hilum of the spleen (Fig. 12.12). The superior mesenteric vein is also perfused by continuous antegrade flow, showing a moderate increase in flow and volume during expiration. Even light transducer pressure is sufficient to compress it.

With newer scanners, the jejunal and iliac veins and gastropancreaticoduodenal trunk can be identified in the B-mode image. The first two to four jejunal veins and the ileocolic vein can be demonstrated with CDS in over 60% to 72% of cases.⁴¹

Additional second- and third-order branches can be visualized with power Doppler. In some cases, the superior mesenteric vein may show string-of-beads dilatations at the level of its tributaries (Fig. 12.13). This is caused by inflow from the jejunal branches that cross behind the SMA and open into the superior mesenteric vein. The inferior mesenteric vein can be identified by its course above the SMA.

The posterior superior pancreaticoduodenal vein appears as an elongated structure with slow blood flow in the pancreatic head or at its posterior border. In most cases it is distinguishable from the arterial pancreatic arcades only by Doppler spectral analysis.

Given the complex embryogenesis of the venous system, a variety of variants and malformations may occur (Fig. 12.14). The spectrum of findings ranges from partial aplasia to duplication of the inferior vena cava (Fig. 12.4). Generally, these anomalies do not cause complaints, but it may be vitally important to detect them prior to major abdominal surgery (e.g., of the aorta) and before vena cava filter placement. Frequently, anomalies such as a double vena cava are already detectable in the B-mode image. Their hemodynamics can be evaluated with CDS (Fig. 12.15).

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