Is a distended colon distal to the level of the rectum seen on Computed Tomography Angiography (CTAP) always due to constipation?

Distended Colon on CTAP: Constipation vs. Other Causes

A distended colon distal to the level of the rectum seen on CT angiography (CTAP) is highly suggestive of constipation but is not diagnostic by itself, as other conditions such as bowel obstruction, colonic pseudo-obstruction, or colonic volvulus can present with similar imaging findings.

Diagnostic Considerations

Constipation Features on Imaging

• Distended colon with fecal material is the hallmark finding
• Absence of a transition point (unlike mechanical obstruction)
• Rectal distension with stool is typically present
• No significant bowel wall thickening (unless stercoral colitis has developed)

Key Differential Diagnoses

1. Mechanical Obstruction:

   • Look for a clear transition point between dilated and non-dilated bowel
   • May see "coffee bean" sign in sigmoid volvulus 1
   • Often shows proximal bowel distention with collapsed distal segments

2. Colonic Pseudo-obstruction:

   • Massive colonic dilatation without mechanical obstruction
   • Similar appearance to constipation but often more acute onset
   • Cecum typically most distended (>9cm is concerning for perforation risk)

3. Stercoral Colitis:

   • Complication of severe fecal impaction
   • Shows colonic dilatation with impacted feces
   • Mural thickening and pericolic fat stranding 2
   • Risk of perforation if not promptly treated

Diagnostic Algorithm

1. Evaluate for mechanical obstruction signs:

   • Presence of transition point
   • "Bird's beak" sign (in volvulus) 1
   • Abrupt change from dilated to non-dilated bowel

2. Assess fecal load:

   • Significant fecal loading throughout colon suggests constipation
   • Fecal loading can be quantified on CT and correlates with transit time 3

3. Examine bowel wall:

   • Normal wall thickness favors constipation
   • Thickened wall with fat stranding suggests inflammation (stercoral colitis)

4. Look for complications:

   • Perforation (free air)
   • Ischemia (pneumatosis, portal venous gas)

Clinical Correlation

The imaging finding of a distended colon should be correlated with:

• Duration of symptoms (acute vs. chronic)
• Presence of abdominal pain (severe pain suggests obstruction or ischemia)
• Bowel movement patterns (infrequent defecation suggests constipation)
• Recent medication changes (opioids, anticholinergics can cause constipation) 1

Management Implications

If constipation is confirmed:

• Initial management includes osmotic or stimulant laxatives 1
• Assess for fecal impaction requiring disimpaction 1
• For refractory cases, consider defecography to evaluate for defecatory disorders 1

Pitfalls to Avoid

• Don't miss mechanical obstruction: Look carefully for transition points that may require surgical intervention
• Don't overlook stercoral colitis: This complication of severe constipation can lead to perforation if not treated promptly 2
• Don't assume all colonic distention is benign: Toxic megacolon and ischemic colitis can be life-threatening

Remember that while a distended colon with fecal loading on CTAP strongly suggests constipation, the diagnosis should be confirmed by clinical history and physical examination to rule out more serious conditions requiring urgent intervention.