25.2 Pediatric GI & Noncardiac Chest

Pediatric abdominal and chest sonography rewards patience, warm gel, and high-frequency linear transducers. Small body habitus and minimal fat make children excellent ultrasound subjects, and several pediatric emergencies have pathognomonic sonographic signs you must recognize on the exam.

Hypertrophic Pyloric Stenosis

Hypertrophic pyloric stenosis (HPS) is progressive hypertrophy of the circular pyloric muscle that obstructs the gastric outlet. The classic patient is a 2-6-week-old infant (often a firstborn male; males are affected about 4:1) with projectile, nonbilious vomiting — nonbilious because the obstruction is proximal to the ampulla of Vater. Chronic vomiting of hydrochloric acid produces a hypochloremic metabolic alkalosis, and a palpable "olive" mass may be felt in the epigastrium.

Technique: place the infant supine or in the right lateral decubitus position (fluid then fills the antrum and the pylorus elevates toward the gallbladder). Using a high-frequency linear transducer (7.5 MHz or higher), locate the pylorus between the fluid-distended stomach and the duodenal cap, usually just medial to the gallbladder. Offer a small sugar-water or electrolyte feed if the stomach is collapsed.

Diagnostic measurements:

• Pyloric muscle thickness (single wall, hypoechoic muscle layer on a transverse view) of 3 mm or greater is abnormal; many labs use 4 mm for added specificity
• Pyloric channel length of 15-16 mm or greater on the longitudinal view is abnormal
• Secondary signs: a fluid-distended, hyperperistaltic stomach with visible to-and-fro peristaltic waves and failure of fluid to pass through the canal into the duodenum

Do not confuse HPS with pylorospasm: in spasm the canal may appear elongated transiently, but it relaxes during the exam, measurements normalize below 3 mm, and fluid eventually passes. If measurements are borderline, re-scan after a short interval or after feeding.

Intussusception

Intussusception is telescoping of one bowel segment into the adjacent distal segment, most commonly ileocolic (ileum into the ascending colon, about 90% of cases). The peak age is 6 months to 2 years (roughly 3 months to 3 years). Children present with intermittent colicky pain, vomiting, and late currant-jelly stools (blood and mucus). A sausage-shaped right-sided mass may be palpable.

Sonographic signs:

• Target or donut sign on transverse imaging: concentric alternating rings — echogenic mucosa and mesenteric fat dragged in between hypoechoic bowel wall layers
• Pseudokidney (or sandwich) sign on longitudinal imaging: layered hypoechoic walls around an echogenic center mimicking a kidney
• A mass diameter greater than about 3 cm supports the diagnosis; a smaller target may be benign transient small-bowel intussusception
• Trapped peritoneal fluid between the intussusceptum and intussuscipiens predicts harder reduction; absent color Doppler flow in the bowel wall raises concern for ischemia

In neonates and children older than about 3 years, or with recurrent episodes, look for a pathologic lead point — a Meckel diverticulum, polyp, duplication cyst, or lymphoma. Treatment is pneumatic (air) or hydrostatic (contrast or saline) enema reduction, which can be performed under fluoroscopic or sonographic guidance; recurrence occurs in roughly 10% of cases.

Appendicitis in Children

Ultrasound is the preferred first test for pediatric right lower quadrant pain because it avoids radiation. Use the graded compression technique: a high-frequency linear transducer applies slow, sustained pressure to displace gas-filled bowel over the point of maximal tenderness.

Findings of acute appendicitis:

• A noncompressible, blind-ending tubular structure in the right lower quadrant with an outer diameter greater than 6 mm
• A target appearance in cross-section with a wall thickness of 2 mm or more per layer
• An appendicolith: an echogenic focus with clean posterior acoustic shadowing
• Surrounding echogenic (inflamed) mesenteric fat, periappendiceal fluid, and hyperemia on color Doppler
• Focal tenderness directly over the appendix — the sonographic McBurney sign
• With perforation: loss of the wall's layered architecture, a phlegmon, or a walled-off complex abscess containing debris or gas

A normal appendix may be seen as a compressible tube 6 mm or less; failure to visualize it does not exclude appendicitis, and CT may follow an equivocal scan.

Midgut Malrotation and Volvulus

In normal mesenteric anatomy, the superior mesenteric vein (SMV) lies to the right of the superior mesenteric artery (SMA) on transverse epigastric imaging. In malrotation, this relationship is reversed or abnormal — the SMV sits to the left of, or directly anterior to, the SMA. Malrotation predisposes to midgut volvulus, a surgical emergency in which the bowel twists around the narrow mesenteric pedicle and obstructs its own blood supply. The classic presentation is bilious vomiting in a neonate.

The sonographic hallmark of volvulus is the whirlpool sign: on transverse imaging with color Doppler, the SMV and mesentery are seen spiraling clockwise around the SMA. Additional findings include a dilated, fluid-filled duodenum proximal to the obstruction. Any suspected volvulus warrants immediate surgical consultation; an upper gastrointestinal series remains the confirmatory study, but ultrasound often raises the alarm first.

Noncardiac Chest Sonography

Ultrasound evaluates the pleural space, peripheral lung, and diaphragm without radiation:

• Pleural effusion: an anechoic or complex collection above the diaphragm, best seen in the costophrenic angles with the child upright or decubitus. Compare the echogenic pattern with the liver and spleen; ultrasound distinguishes simple effusion from septated, loculated fluid or empyema and marks safe sites for thoracentesis. Note that the mirror-image artifact can duplicate the liver above the diaphragm and must not be mistaken for a supradiaphragmatic mass
• Lung consolidation: consolidated (hepatized) lung loses its normal air artifact and appears as a tissue-like, liver-echogenicity region, often with air bronchograms — bright branching air pockets coursing through it — and fluid bronchograms in obstructive pneumonia
• Diaphragmatic motion: real-time and M-mode imaging through the liver or spleen window documents excursion with respiration. Paradoxical motion (the paralyzed hemidiaphragm rising with inspiration, the sniff test) indicates phrenic nerve palsy or eventration
• Lung ultrasound basics: in aerated lung, the pleural line produces A-lines — horizontal, equidistant reverberation artifacts below the pleural line. B-lines are vertical, laser-like comet-tail artifacts arising from the pleural line and erasing A-lines; multiple B-lines indicate interstitial fluid (edema, pneumonitis, or respiratory distress syndrome in neonates)

Abdominal Wall Masses in Children

Ultrasound quickly triages pediatric abdominal wall findings: umbilical hernias show bowel or omentum gliding through a fascial defect and usually close spontaneously; epigastric hernias contain preperitoneal fat in the linea alba; inguinal hernias may contain bowel or, in girls, the ovary. Differentiate hernias from hematomas, lymphadenopathy, and embryologic remnants such as urachal cysts along the midline between the umbilicus and bladder. Color Doppler and dynamic maneuvers (crying, Valsalva) demonstrate vascularity, reducibility, and the fascial defect.