Normal Rectosigmoid Index Size on Barium Enema
The rectosigmoid index (RSI) is a radiographic ratio comparing the maximum diameter of the rectum to the maximum diameter of the sigmoid colon on barium enema. In normal bowel anatomy, the rectum is wider than the sigmoid, producing an index greater than 1. This measurement is a key tool in the early radiologic diagnosis of Hirschsprung’s disease, where the normal rectal caliber relationship is reversed.
Normal Reference Values
| Measurement |
|---|
| >1 |
Clinical Significance
Under normal circumstances, the rectal ampulla distends more readily than the sigmoid colon, yielding an RSI >1. In Hirschsprung’s disease (aganglionic megacolon), the aganglionic distal segment — most commonly involving the rectum — remains narrow and spastic, while the proximal sigmoid becomes dilated. This produces a pathologically inverted RSI of <1, with the sigmoid diameter exceeding that of the rectum, and is a classic fluoroscopic finding on contrast enema.
The RSI is most reliable when evaluated in conjunction with the transition zone, delayed evacuation films, and clinical presentation. False negatives can occur in total colonic aganglionosis, where the rectosigmoid ratio may appear normal or only subtly abnormal. False positives may be seen with rectal preparation or recent enemas, which can artificially distend the rectum.
- Hirschsprung’s disease — RSI <1 due to distal aganglionic narrowing
- Functional constipation / megarectum — RSI >1, rectum markedly distended
- Total colonic aganglionosis — RSI may be equivocal; transition zone absent
- Colonic pseudo-obstruction — diffuse dilation, RSI unreliable
- Post-operative or post-enema state — may cause spurious rectal distension
Reference: Pochaczevsky R, Leonidas JC. The “recto-sigmoid index”. A measurement for the early diagnosis of Hirschsprung’s disease. Am J Roentgenol Radium Ther Nucl Med. 1975;123(4):770-7.
Imaging Notes
The RSI is measured on barium enema (contrast enema), ideally performed without prior bowel preparation in neonates and infants to preserve the natural caliber differential. The maximum transverse diameter of the rectum and the maximum transverse diameter of the sigmoid colon are each measured on the same frontal projection. The RSI is calculated as: rectal diameter divided by sigmoid diameter. A lateral view may supplement assessment of the transition zone.
In neonates, a water-soluble contrast agent is often preferred over barium to reduce aspiration and perforation risk. Images should be obtained during gentle retrograde filling without excessive distension pressure, as overdistension may mask the aganglionic narrowing and obscure the transition zone. Delayed 24-hour films evaluating contrast retention further support the diagnosis.
