Intussusception
The most likely diagnosis is intussusception. This 7-month-old infant presents with the classic constellation of findings that define this pediatric emergency: intermittent colicky abdominal pain (manifested as irritability with leg drawing), vomiting that has progressed to suggest obstruction, lethargy, abdominal distension, and a palpable right upper quadrant mass 1.
Clinical Reasoning
Age and Presentation Pattern
- Intussusception is the most common cause of intestinal obstruction in infants and young children, with peak incidence in the first five years of life 2, 3.
- The 7-month age falls squarely within the typical presentation window for this condition 3.
Cardinal Features Present
This patient demonstrates multiple key diagnostic features:
- Intermittent crampy abdominal pain: The episodes of inconsolable crying with leg drawing followed by pain-free intervals is pathognomonic for intussusception 1.
- Vomiting: Present in 84.4% of intussusception cases and represents the most common symptom 3. The progression to vomiting suggests worsening obstruction 1.
- Palpable abdominal mass: Found in 56.3% of cases and is a highly specific finding 3. The right upper quadrant location is consistent with ileocolic intussusception 4.
- Lethargy: This is an increasingly recognized cardinal symptom of intussusception and may indicate significant illness even before other classic features fully develop 5.
- Fever and ill appearance: The temperature of 38.6°C and overall toxic appearance suggest possible bowel compromise 6.
Why Not the Other Diagnoses
Gastroenteritis would not explain:
- The palpable abdominal mass 3
- The intermittent colicky pattern with pain-free intervals 1
- The degree of lethargy and toxic appearance 5
Appendicitis is:
- Extremely rare at 7 months of age
- Would not produce a right upper quadrant mass
- Does not present with intermittent colicky pain 2
Pancreatitis is:
- Exceptionally rare in infants
- Would not cause a palpable abdominal mass
- Does not present with intermittent colicky episodes 2
Critical Clinical Pitfall
The complete classic tetrad (vomiting, abdominal pain, abdominal mass, and rectal bleeding) is present together in only 7.5% of patients 3. The absence of bloody "currant jelly" stools at this point does not exclude intussusception—this is often a late finding indicating mucosal damage 1. A high index of suspicion is essential when any combination of these features is present 3, 6.
Immediate Management Priority
- This infant requires urgent abdominal ultrasonography, which has 98.1% sensitivity for detecting the classic target lesion 3, 6.
- Intravenous access, fluid resuscitation, and pediatric surgery notification should occur immediately before any contrast enema attempt 6.
- Bilious vomiting in any infant mandates immediate evaluation for surgical conditions including intussusception 1.
The combination of age, intermittent colicky pain pattern, palpable mass, and systemic signs makes intussusception the overwhelming diagnostic probability requiring emergent intervention to prevent bowel necrosis and perforation 2, 5.