What are the signs of ileus on an abdominal x-ray?

Signs of Ileus on Abdominal X-Ray

On plain abdominal x-ray, ileus classically presents with multiple air-fluid levels, diffuse dilatation of both small and large bowel loops, and absence of a clear transition point—distinguishing it from mechanical obstruction. 1

Classic Radiographic Findings

Primary Features

• Multiple air-fluid levels throughout the abdomen, representing stagnant intestinal contents 1
• Diffuse bowel dilatation affecting both small bowel (>2.5 cm) and colon, without a discrete transition point 2, 3
• Generalized or localized ileus pattern (sentinel loop), though this finding is non-specific 1
• Gas distributed throughout small and large bowel, unlike mechanical obstruction where gas is typically absent distal to the obstruction 1

Secondary Features

• Colon cut-off sign may be present but is non-specific 1
• Renal halo sign occasionally observed but unreliable for diagnosis 1
• Absence of pneumoperitoneum (free air), which would suggest perforation rather than simple ileus 1

Distinguishing Ileus from Mechanical Obstruction

The key differentiating feature is that ileus shows gas throughout the entire bowel without a clear transition point, while mechanical obstruction demonstrates a triad of multiple air-fluid levels, distended small bowel loops proximal to obstruction, and absence of gas in the colon distal to the blockage. 1

Critical Distinctions

• Mechanical small bowel obstruction shows dilated loops proximal to obstruction with collapsed bowel distally and absent colonic gas 1
• Ileus demonstrates more uniform dilatation without a discrete cutoff point 2, 3
• Bowel wall thickness is typically thin in ileus, whereas thickened walls suggest ischemia or strangulation 4

Important Limitations and Pitfalls

Diagnostic Accuracy Issues

• Plain x-rays have only approximately 70% sensitivity for detecting bowel obstruction and are even less reliable for ileus 1
• The reported plain abdominal x-ray findings in ileus are unreliable and non-specific, making them insufficient for definitive diagnosis 1
• Early ileus may show minimal findings on plain films, as it takes up to 6 hours to develop classical radiographic features 3

When Plain Films Are Insufficient

• CT scanning is the preferred imaging modality when there is diagnostic uncertainty, with approximately 90% accuracy in predicting the need for surgery 1
• Ultrasound can detect dilated bowel loops (>2.5 cm) with decreased or absent peristalsis and has 90% sensitivity and 96% specificity for bowel obstruction, though it is operator-dependent 5
• The absence of massive fluid collection and meteorism on plain films actually makes ultrasound examination more optimal for early diagnosis 3

Clinical Context Matters

Associated Findings to Evaluate

• Absence of peritoneal signs on physical examination supports ileus over ischemia or perforation 1
• Laboratory values including elevated CRP >75 and WBC >10,000/mm³ may suggest peritonitis or strangulation, though sensitivity is low 1
• Electrolyte abnormalities, particularly hypokalemia, are frequently found and contribute to ileus 1

High-Risk Scenarios Requiring Advanced Imaging

• Suspected strangulation or ischemia requires CT scan, as plain films cannot detect early signs 1
• Postoperative patients where clinical examination may be obscured 1
• Elderly or unconscious patients where history and physical examination are unreliable 1