What is the most appropriate initial investigation for a 7-month-old baby presenting with abdominal distension, palpable mass, irritability, and vomiting?

Abdominal Ultrasound is the Most Appropriate Initial Investigation

For this 7-month-old infant presenting with classic signs of intussusception (intermittent colicky pain with leg drawing, vomiting, palpable right upper quadrant mass, and lethargy), abdominal ultrasound is the definitive first-line imaging study. 1

Clinical Presentation Strongly Suggests Intussusception

This infant demonstrates the pathognomonic triad of intussusception:

• Intermittent crampy abdominal pain manifested as episodes of inconsolable crying with drawing up of legs, followed by pain-free intervals 1
• Vomiting that has progressed over 4 hours, indicating worsening obstruction 1
• Palpable mass in the right upper quadrant on examination 1
• Lethargy and ill appearance suggesting disease progression 1
• Fever (38.6°C) and tachycardia indicating systemic involvement 1

The progression from initial vomiting to the current presentation suggests worsening obstruction, which is concerning for intussusception 1.

Why Ultrasound is the Correct Answer

Ultrasound is the imaging modality of choice for suspected intussusception in infants and young children because it:

• Directly visualizes the intussusception with high sensitivity and specificity 2, 3
• Avoids ionizing radiation in this young patient 2, 3
• Can be performed rapidly in the emergency setting 2
• May be therapeutic if followed by ultrasound-guided or fluoroscopy-guided reduction 3
• Provides real-time assessment of bowel viability and complications 2

The American College of Radiology guidelines emphasize that ultrasound is highly effective for diagnosing acute abdominal pathologies in pediatric patients, particularly intussusception 2, 3.

Why the Other Options Are Incorrect

Plain Abdominal X-ray (Option C)

While plain radiographs may show nonspecific findings like bowel obstruction or a soft tissue mass, they:

• Cannot definitively diagnose intussusception 4, 3
• Have low sensitivity for this condition 3
• Delay definitive diagnosis when ultrasound is readily available 3
• Are primarily useful for detecting free air or other complications, not for initial diagnosis of intussusception 4

Contrast Barium Enema (Option A)

Contrast enema is NOT appropriate for initial diagnosis in this clinical scenario because:

• Ultrasound must confirm the diagnosis first before considering therapeutic enema 3
• Performing enema without ultrasound confirmation risks complications if the diagnosis is incorrect 1
• The ACR guidelines specify that contrast enema is reserved for therapeutic reduction after ultrasound diagnosis, not initial diagnostic evaluation 3
• In cases with peritonitis or perforation (suggested by fever and lethargy), contrast enema is contraindicated 1

Abdominal CT Scan (Option D)

CT is inappropriate as initial imaging because:

• Exposes the infant to unnecessary ionizing radiation 3, 5
• Ultrasound provides superior diagnostic information for intussusception without radiation 2, 3
• CT offers no advantage over ultrasound for this specific diagnosis 3
• The radiation risk is particularly concerning in young children with increased cancer susceptibility 5

Critical Management Considerations

This is a surgical emergency requiring immediate pediatric surgical consultation once intussusception is confirmed 1. The presence of fever, lethargy, and a palpable mass suggests:

• Possible bowel compromise requiring urgent intervention 1
• Risk of perforation if diagnosis and treatment are delayed 1
• Need for IV fluid resuscitation given the clinical presentation 1

Common pitfall: Delaying ultrasound to obtain plain films first wastes valuable time when the clinical presentation is classic for intussusception 3. The "currant jelly" stool (bloody mucoid stool) is a late finding and its absence does not exclude the diagnosis 1.