Section 6.1: Vascular Pathology

Vascular Pathology in the Abdomen

Vascular sonography of the abdomen requires a comprehensive understanding of arterial and venous pathology, hemodynamics, and Doppler instrumentation. Candidates must be fluent in diagnostic criteria, waveform analysis, and clinical indications for major abdominal vascular diseases.

Abdominal Aortic Aneurysm (AAA)

An abdominal aortic aneurysm (AAA) is a localized dilation of the abdominal aorta. The standard definition of an AAA is a focal arterial segment measuring 3.0 cm or greater in outer-to-outer diameter, or a segment that is at least 50% larger than the adjacent normal segment. The normal abdominal aorta typically tapers from its proximal segment near the diaphragm (approximately 2.0 to 2.5 cm) to its bifurcation at the common iliac arteries (approximately 1.5 to 2.0 cm). Diffuse, mild dilation of the aorta without a focal aneurysm is termed ectasia.

Aneurysms are categorized morphologically as fusiform or saccular. Fusiform aneurysms are the most common, presenting as a symmetric, spindle-shaped dilation of all three walls of the vessel (intima, media, and adventitia). Saccular aneurysms are focal, spherical outpouchings that are often asymmetric and localized to one side of the vessel, frequently resulting from trauma, surgery, or infection (mycotic aneurysms). The majority of AAAs are infrarenal, located distal to the origin of the renal arteries.

Sonographers must measure the aorta from the outer wall to the outer wall (outer-to-outer) in the anterior-posterior (AP) and transverse planes, ensuring the measurement beam is strictly perpendicular to the long axis of the aorta to avoid overestimating the diameter. If mural thrombus is present—which is common in chronic aneurysms—the measurement must include the thrombus, not just the patent lumen. The surgical intervention threshold is a diameter greater than 5.5 cm, as the risk of catastrophic rupture increases exponentially beyond this size. Patients with aneurysms measuring 3.0 to 5.4 cm are monitored with serial ultrasound examinations at regular intervals (typically 6 to 12 months).

Aortic Dissection

Aortic dissection is a life-threatening vascular emergency characterized by a tear in the intima (the innermost layer of the vessel wall). This tear allows blood to force its way under systemic pressure into the media, separating the intima from the adventitia and creating a false lumen. Dissections are classified using the Stanford system: Type A involves the ascending aorta, while Type B involves only the descending aorta (distal to the left subclavian artery).

The classic sonographic hallmark of aortic dissection is the visualization of a thin, linear, echogenic structure—the intimal flap—within the aortic lumen. This flap fluctuates and moves dynamically with the cardiac cycle. On spectral and color Doppler, the true lumen and false lumen demonstrate distinct flow patterns. The true lumen typically exhibits normal forward flow, whereas the false lumen may demonstrate sluggish, turbulent, or reversed flow, and is highly prone to thrombus formation. Complications of dissection include rupture, branch vessel occlusion (leading to renal or bowel ischemia), and pericardial tamponade.

Portal Hypertension and Doppler Changes

Portal hypertension is characterized by an abnormal increase in pressure within the portal venous system (hepatic venous pressure gradient > 10 mmHg). The etiology of portal hypertension is categorized as prehepatic (e.g., portal vein thrombosis), intrahepatic (e.g., liver cirrhosis, which is the most common cause in Western countries), or posthepatic (e.g., Budd-Chiari syndrome or right-sided heart failure).

Sonographically, portal hypertension presents with several key findings:

1. Splenomegaly: Spleen length exceeding 13 cm in the long axis.
2. Ascites: Free fluid within the peritoneal cavities.
3. Portal Vein Dilation: Portal vein diameter exceeding 13 mm at the level of the porta hepatis during quiet respiration.
4. Altered Portal Flow: Normal portal venous flow is hepatopetal (toward the liver) and exhibits a continuous, slightly wavy (phasic) velocity profile. In portal hypertension, portal vein velocity decreases (often below 15 cm/s). As resistance increases, flow may become biphasic and eventually hepatofugal (reversed, flowing away from the liver).
5. Portosystemic Collaterals (Shunts): As portal blood seeks alternative routes to return to the systemic circulation, embryonic pathways open. The most specific sonographic sign is the recanalization of the ligamentum teres (recanalized umbilical vein), which is seen in the left lobe of the liver as a patent, circular vessel with hepatofugal flow heading toward the anterior abdominal wall (which clinically manifests as caput medusae). Other common shunts include dilated left gastric (coronary) veins (>5 mm) and splenorenal shunts.

Budd-Chiari Syndrome

Budd-Chiari syndrome is a rare disorder caused by the obstruction of hepatic venous outflow. The occlusion can occur anywhere from the small hepatic venules to the junction of the inferior vena cava (IVC) and the right atrium. It is commonly associated with hypercoagulable states, polycythemia vera, pregnancy, oral contraceptive use, and tumor invasion (e.g., hepatocellular carcinoma or renal cell carcinoma).

Acutely, the liver appears enlarged and hypoechoic due to passive congestion. Chronically, caudate lobe hypertrophy develops. Because the caudate lobe has independent venous drainage directly into the IVC via emissary veins, it is spared from the congestion and undergoes compensatory enlargement, while the right and left lobes atrophy. Sonographically, the three main hepatic veins (right, middle, and left) appear narrowed, echogenic, or completely non-visualized. Doppler analysis demonstrates absent, monophasic, or reversed flow in the hepatic veins, replacing the normal triphasic, pulsatile waveform that reflects right atrial activity. Flow in the IVC may also be sluggish or reversed.

Renal Artery Stenosis (RAS)

Renal artery stenosis is a major, correctable cause of renovascular hypertension. It is typically caused by atherosclerosis in older males (typically near the ostium) or fibromuscular dysplasia (FMD) in younger females (typically in the mid-to-distal renal artery).

The ultrasound evaluation of RAS utilizes both direct (main renal artery) and indirect (intrarenal segmental arteries) scanning techniques:

• Direct Criteria (Renal Artery):
  • Peak Systolic Velocity (PSV): A PSV greater than 180 cm/s in the main renal artery indicates a hemodynamically significant stenosis (>60% diameter reduction).
  • Renal-to-Aortic Ratio (RAR): Calculated by dividing the highest PSV in the renal artery by the PSV of the abdominal aorta at the level of the celiac axis or SMA. An RAR greater than 3.5 is diagnostic for significant stenosis.
• Indirect Criteria (Intrarenal Segmental/Lobar Arteries):
  • Acceleration Time (AT): The time from end-diastole to the peak systolic peak. An AT greater than 0.07 seconds (70 milliseconds) indicates distal stenosis.
  • Tardus-Parvus Waveform: A waveform characterized by a delayed systolic upstroke (tardus) and a low-amplitude, rounded peak (parvus).
  • Loss of the Early Systolic Peak (ESP): The normal sharp notch during early systole disappears in the presence of proximal stenosis.

Summary of Abdominal Vascular Pathology Diagnostic Criteria