Section 6.2: Gastrointestinal Pathology

Gastrointestinal Pathology in the Abdomen

Ultrasound of the gastrointestinal (GI) tract requires high-resolution linear transducers (typically 5 to 12 MHz or higher) and graded compression technique to displace overlying bowel gas. Key clinical conditions covered in the ARDMS Abdomen exam include infantile hypertrophic pyloric stenosis, intussusception, acute appendicitis, and diverticulitis.

Hypertrophic Pyloric Stenosis (HPS)

Hypertrophic pyloric stenosis (HPS) is an acquired hypertrophic narrowing of the pyloric canal, which prevents gastric emptying. It characteristically affects male infants (male-to-female ratio of 4:1) between the ages of 2 and 12 weeks. The hallmark clinical presentation is progressive, non-bilious projectile vomiting, and a palpable olive-shaped mass in the epigastric region (pylorus).

For the sonographic examination, the infant is placed in a supine and right lateral decubitus position (the latter allows fluid to gather in the antrum/pylorus and displaces gas). A high-frequency linear transducer is used to scan the epigastric region just to the right of the midline. In the transverse plane (relative to the body), the pylorus appears as a target or doughnut sign, with a hypoechoic hypertrophied muscle ring surrounding an echogenic central mucosa. In the longitudinal plane, the pylorus appears as a cervix-like structure adjacent to the gastric antrum.

Diagnostic criteria for HPS are:

1. Pyloric Muscle Thickness: Measured as a single wall of the hypoechoic muscle layer in the transverse view. A thickness of 3.0 mm or greater (some protocols use >3.0 mm) is diagnostic.
2. Pyloric Channel Length: Measured from the antral end to the duodenal end of the canal. A channel length of 15 mm or greater (some protocols use >15 mm) is diagnostic.
3. Pyloric Channel Diameter: A total cross-sectional diameter of greater than 10 mm.
4. Dynamic Observations: The pylorus remains closed and elongated throughout the examination. No passage of fluid or gastric contents through the channel is visualized, and hyperperistalsis of the stomach is observed as it attempts to force fluid past the obstruction.

Intussusception

Intussusception occurs when a segment of bowel (the intussusceptum) invaginates or prolapses into the lumen of an adjacent distal segment of bowel (the intussuscipiens). This is the most common cause of intestinal obstruction in children, typically occurring between the ages of 6 months and 2 years. The classic clinical triad consists of colicky abdominal pain (with drawing up of the legs), a palpable sausage-shaped abdominal mass, and "red currant jelly" stools (containing blood and mucus from ischemic mucosa). The vast majority of pediatric intussusceptions are ileocolic.

Sonographically, intussusception is easily identified using a high-frequency linear probe:

• Transverse Plane: It appears as a "target" or "doughnut" sign, consisting of multiple concentric rings of bowel wall. The outer loop is the intussuscipiens, and the inner loop is the intussusceptum.
• Longitudinal Plane: It appears as a "pseudokidney" or "sandwich" sign, where the layers of invaginated bowel and associated mesentery resemble the kidney parenchyma and renal sinus.
• Doppler Assessment: Color Doppler should be applied to the intussusception to assess bowel viability. Decreased or absent blood flow indicates bowel ischemia and potential necrosis, which is a contraindication for hydrostatic or pneumatic reduction (air enema) and requires urgent surgical intervention. Free peritoneal fluid is also commonly seen and should be documented.

Acute Appendicitis

Acute appendicitis is the most common cause of emergent abdominal surgery in the world. It results from obstruction of the appendiceal lumen, typically by a fecalith (appendiolith), lymphoid hyperplasia, or foreign body, leading to bacterial overgrowth, distention, and ischemia. Patients classically present with periumbilical pain that migrates to the right lower quadrant (McBurney's point), fever, leukocytosis, and rebound tenderness.

The examination requires the graded compression technique using a high-frequency linear transducer. Graded compression applies gentle, steady pressure to displace normal bowel gas and compress the cecum, allowing the appendix to be visualized. A normal appendix is compressible, has a diameter of less than 6 mm, and is often difficult to visualize.

Diagnostic criteria for acute appendicitis include:

1. Blind-Ended Loop: A non-compressible, blind-ended tube arising from the cecum.
2. Diameter: An outer-to-outer diameter of greater than 6.0 mm.
3. Wall Thickness: Wall thickness greater than 2 mm.
4. Appendiolith (Fecalith): An echogenic intraluminal focus demonstrating posterior acoustic shadowing.
5. Hyperemia: Color Doppler shows marked hypervascularity of the appendiceal wall (hyperemic wall or "ring of fire").
6. Periappendiceal Changes: Echogenic, inflamed surrounding fat (known as the "thyroid-in-the-belly" sign or fat thyroid sign) and localized fluid collections.

Diverticulitis

Diverticulitis is the inflammation of one or more diverticula, which are small outpouchings of the colonic wall. It typically affects older adults and is most common in the sigmoid colon (left lower quadrant). Symptoms include left lower quadrant pain (often called the "left-sided appendicitis"), fever, and altered bowel habits.

Sonographic findings of acute diverticulitis include:

• Segmental Wall Thickening: Concentric thickening of the colonic wall greater than 4.0 mm over a diseased segment.
• Inflamed Diverticulum: An outpouching of the bowel wall containing an echogenic focus (feces or gas) with posterior acoustic shadowing.
• Echogenic Fat: Prominent, echogenic, and non-compressible surrounding pericolic fat representing inflammatory changes.
• Color Doppler: Hyperemia within the thickened colonic wall and surrounding fat.
• Complications: Abscess formation (complex fluid collection with internal echoes or gas), fistula tract, or free intraperitoneal air from perforation (causing bright echogenic lines with dirty shadowing/ring-down artifacts along the peritoneal line).

Summary of Gastrointestinal Pathology