Supine X-Ray in Small Bowel Obstruction (SBO)

Key Radiological Features on Supine AXR

1. Dilated Small Bowel Loops

• Calibre >3 cm (up to 3.5-5 cm) is considered dilated
• Loops lie centrally in the abdomen (vs large bowel which is peripheral)
• Multiple loops arranged in a ladder or step-ladder pattern

2. Valvulae Conniventes (Plicae Circulares)

• The hallmark of small bowel on X-ray
• These folds cross the entire width of the lumen (unlike haustra of the colon, which are incomplete)
• Regularly spaced, giving a "concertina" or "stack of coins" effect - most prominent in jejunum
• The ileum is largely featureless - smooth walls without prominent folds

3. Absent or Minimal Colonic Gas

• Gas is absent from the colon distal to the obstruction - this is an important differentiating feature
• Contrast to paralytic ileus, where both small and large bowel are dilated with gas throughout

4. String of Beads Sign

• Seen when dilated small bowel loops are almost completely fluid-filled (very little gas)
• Residual gas is trapped between valvulae conniventes, forming a line of small gas bubbles
• Virtually diagnostic of SBO when present
• Supine film may appear deceptively normal in fluid-filled SBO (the loops appear as soft-tissue density with no gas to outline them)

5. Normal-Appearing Film (Pitfall)

• In a proportion of SBO cases, the supine AXR is normal or equivocally abnormal
• This occurs when dilated loops are predominantly fluid-filled, with no gas to be seen
• A normal plain film does not exclude SBO - CT is mandatory if clinical suspicion persists

Summary Table: SBO vs LBO on Supine AXR

Important Additional Signs

• Hernia: Always inspect below the inguinal ligament on the supine film. A gas-filled loop passing into the inguinal region suggests a strangulated hernia as the cause (especially in women aged >60).
• Pneumobilia (Gallstone Ileus triad - Rigler's triad): SBO + pneumobilia + ectopic gallstone - present in ~38% of cases of gallstone ileus.
• Soft-tissue mass: May indicate intussusception. A "target sign" (concentric rings) is seen if the intussusception is orientated end-on.
• Fluid levels: More prominent on erect film; the number of fluid levels is proportional to the degree and site of obstruction (more levels = more distal obstruction). On supine alone, fluid levels are not visible.

Limitation of Supine AXR

• Identifying the transition point (dilated proximal loops + collapsed distal loops)
• Determining the aetiology (adhesions, hernia, mass, volvulus, intussusception)
• Detecting complications - strangulation, ischaemia, perforation, closed-loop obstruction

"A normal plain radiograph does not exclude an obstruction - if there is persistent concern, further imaging is indicated; CT is the modality of choice." - Bailey & Love's Surgery, 28th Ed.

How to Check Air-Fluid Levels in X-Ray for SBO

First, the Critical Point: Supine vs Erect

The Erect AXR - Step by Step

1. Air-fluid levels - sharp horizontal white lines at the gas-fluid interface within each bowel loop
2. Gas above (dark on X-ray) + fluid below (grey/white on X-ray) within the same loop
3. The levels are at different heights in different loops - this is the stepladder pattern, and is the classic SBO appearance

Specific Criteria for SBO on Erect AXR (Box 91.2, Sabiston)

"The most specific finding for SBO is the triad of: dilated small bowel loops (>3 cm), air-fluid levels on upright films, and absence of gas in the colon." - Schwartz's Principles of Surgery, 11th Ed.

What the "Stepladder Pattern" Means

• Multiple bowel loops, each with their own air-fluid level, but the levels are at progressively different heights across the film
• Looks like a staircase or ladder going across the abdomen
• The loops are arranged transversely, centrally, with their valvulae conniventes visible
• This pattern occurs because each loop has trapped a different amount of fluid/gas and they do not all empty into each other

Fluid Levels - What They Tell You

• Fluid levels appear later than gas shadows (gas accumulates first, then fluid catches up and separates)
• The number of fluid levels is directly proportional to:
  • The degree of obstruction (more complete = more levels)
  • The site of obstruction (more distal = more levels, as more loops are involved proximal to it)
• When fluid levels are very prominent and numerous, the obstruction is advanced
• Two fluid levels are normal in adults (one at duodenal cap, one at terminal ileum) - don't over-call

Normal AXR Does Not Exclude SBO

• If dilated loops are predominantly fluid-filled (no gas), the supine film may look completely normal - described as a "gasless abdomen" or "pseudotumor sign"
• Even the erect film may show no air-fluid levels if loops contain only fluid
• Plain films miss SBO in up to 20-33% of cases
• CT is mandatory when clinical suspicion remains despite a normal or equivocal X-ray

Three-View Approach for Suspected SBO

1. Supine AXR - assess bowel gas pattern, loop size, valvulae conniventes, absent colonic gas
2. Erect AXR - look for air-fluid levels, stepladder pattern, free air under diaphragm
3. Erect CXR - look for free air under diaphragm (perforation)

"Free air in the peritoneal cavity, reflecting perforation of a viscus, is best diagnosed on upright or lateral views, and is easily missed on the supine view." - Mulholland & Greenfield's Surgery

Air-Fluid Levels in SBO - Bailey & Love's 28th Edition

The Direct Answer: ERECT Radiograph

"Small bowel fluid levels may be seen on an erect abdominal radiograph"

"Fluid levels seen radiologically appear later than gas shadows as it takes time for gas and fluid to separate. These are most prominent on an erect abdominal radiograph or cross-sectional imaging."

What the Two Views Show (Bailey & Love)

• On supine: you see the gas pattern and valvulae conniventes (the "concertina/ladder" folds of jejunum)
• No fluid levels are visible - gas and fluid have not separated from the viewing angle

• On erect: fluid settles to the bottom, gas rises - you see a sharp horizontal white line (the interface) with dark gas above it
• Multiple such levels in different loops = the stepladder pattern

Bailey & Love's Key Points on Fluid Levels (p. 1405)

Summary (Bailey & Love's, verbatim logic)

• Supine AXR → shows dilated loops, valvulae conniventes, absent colonic gas
• Erect AXR → shows fluid levels (gas above, fluid below = stepladder pattern)
• Fluid levels appear after gas shadows, so an early SBO may show gas but no fluid levels yet
• A normal plain film does not exclude SBO

Why Fluid Levels Are Seen in X-Ray of SBO

Part 1: Why Fluid Accumulates in the Obstructed Bowel

Gas (Dark component on X-ray)

"There is a significant overgrowth of both aerobic and anaerobic organisms, resulting in considerable gas production. Following the reabsorption of oxygen and carbon dioxide, the majority is made up of nitrogen (90%) and hydrogen sulphide."

• Bailey & Love's, p. 1397

Fluid (White/grey component on X-ray)

"Fluid is made up of the various digestive juices:

• Saliva: 500 mL/24h
• Bile: 500 mL/24h
• Pancreatic secretions: 500 mL/24h
• Gastric secretions: 1 litre/24h

This accumulates in the gut lumen as absorption by the obstructed gut is retarded."

• Bailey & Love's, p. 1397

• Reduced oral intake
• Defective intestinal absorption
• Losses from vomiting
• Sequestration in the bowel lumen (the "third space")
• Transudation into the peritoneal cavity

Part 2: Why This Creates a Visible Fluid Level on X-Ray

• Gas and fluid physically separate within each distended loop
• At the interface, there is an abrupt change from fluid density (white) below to gas density (black) above
• The X-ray beam passes horizontally across this sharp horizontal interface
• This creates the characteristic bright white horizontal line = the fluid level

• Gas and fluid are still both present, but the interface is now parallel to the X-ray beam (both beam and interface are horizontal)
• The beam cannot "cut across" the interface → no fluid level is visible
• You only see the gas outline of the loops

Part 3: Why Fluid Levels Appear LATER Than Gas Shadows (Bailey & Love)

"Fluid levels seen radiologically appear later than gas shadows as it takes time for gas and fluid to separate."

• Bailey & Love's Summary Box 78.9, p. 1405

• Early SBO → mostly gas pattern visible on supine, no fluid levels yet on erect
• Established SBO → gas and fluid have separated → fluid levels now visible on erect
• Advanced/paralytic SBO → fluid levels become more conspicuous and more numerous

"During the obstructive process, fluid levels become more conspicuous and more numerous when paralysis has occurred. When fluid levels are pronounced, the obstruction is advanced."

• Bailey & Love's, p. 1405

Summary Diagram (concept)

ERECT AXR - single obstructed loop:

    ┌───────────────┐
    │  GAS (dark)   │  ← nitrogen, hydrogen sulphide, swallowed air
    ├───────────────┤  ← FLUID LEVEL (sharp white horizontal line)
    │ FLUID (white) │  ← pooled digestive secretions (up to 2.5L/day)
    └───────────────┘

Multiple loops → multiple levels at different heights = STEPLADDER PATTERN