Normal Caecum Size on Radiography and CT: <9 cm
The caecum is the most proximal and widest segment of the large bowel, located in the right iliac fossa, and is particularly prone to distension due to its thin wall and large luminal diameter. Accurate measurement of caecal calibre is essential in the assessment of large bowel obstruction, pseudo-obstruction, and toxic megacolon, where dilation beyond accepted thresholds carries a risk of perforation.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Anteroposterior | <9 cm |
Clinical Significance
A caecal anteroposterior diameter of 9 cm or greater is considered abnormal and warrants urgent clinical correlation. The caecum is the segment most vulnerable to ischaemia and perforation during colonic distension because tension in the bowel wall increases proportionally with luminal diameter (LaPlace’s law). Risk of perforation rises substantially once the diameter exceeds 9–12 cm, particularly if distension is prolonged.
Common causes of caecal dilatation include:
- Large bowel obstruction — mechanical causes such as carcinoma, volvulus, or stricture
- Ogilvie syndrome (acute colonic pseudo-obstruction) — functional dilation without mechanical obstruction
- Toxic megacolon — associated with inflammatory bowel disease or Clostridioides difficile colitis
- Caecal volvulus — axial twist leading to closed-loop obstruction
- Ileus — postoperative or metabolic aetiology
A key pitfall is mistaking a normal, gas-filled caecum in a mobile position (caecal bascule) for pathological dilation. Clinical context, serial imaging, and assessment of accompanying wall thickening or pneumatosis are essential to guide management.
Reference: Horton KM, Corl FM, Fishman EK. CT evaluation of the colon: inflammatory disease. Radiographics. 20 (2): 399-418.
Imaging Notes
On plain abdominal radiography, the caecal diameter is measured in the anteroposterior dimension on a supine projection. The measurement is taken at the widest visible point of the caecal shadow, perpendicular to the bowel wall. Bowel gas pattern, haustral fold integrity, and the distribution of dilation should be assessed simultaneously.
On CT, the anteroposterior diameter is measured in the axial plane at the point of maximum distension, from inner wall to inner wall. CT offers superior anatomical detail, allowing evaluation of the caecal wall for thickening, pneumatosis, pericolic fat stranding, or free perforation — findings that directly influence surgical decision-making. Multiplanar reconstructions are useful when the caecum lies in an oblique orientation.