Normal Distance of the Rectum from the Anal Verge
The rectum extends from the anal verge to 15 cm proximally, as measured by rigid sigmoidoscopy, with the distal 10-20 cm of this segment typically traversed during flexible sigmoidoscopy. 1
Anatomical Definition and Measurement
The rectum is definitively defined as extending up to 15 cm from the anal margin, with tumors beyond this distance classified as colonic rather than rectal. 1, 2 This measurement should be performed using rigid sigmoidoscopy, which provides the most accurate and reproducible distance measurement from the anal verge. 1
Key Anatomical Landmarks
The rectum can be further subdivided into three clinically relevant segments based on distance from the anal verge:
- Low rectum: 0-5 cm from the anal verge 1, 2
- Middle rectum: >5-10 cm from the anal verge 1, 2
- High rectum: >10-15 cm from the anal verge 1, 2
The anorectal ring (formed by the palpable upper border of the anal sphincter and puborectalis muscles) marks the inferior boundary of the rectum at approximately 3-5 cm from the anal verge. 3
Clinical Measurement Considerations
Flexible Sigmoidoscopy Measurements
During flexible sigmoidoscopy, the rectum is usually traversed by the distal 10-20 cm of the endoscope. 1 The procedure report should document the depth of insertion in centimeters from the anal verge, ideally with a straightened endoscope to ensure accuracy. 1
Important Caveats
Anatomical variation is substantial: Research demonstrates that the actual sigmoid take-off (junction of mesorectum and mesocolon) occurs at a mean distance of 12.6 cm from the anal verge (range 9.4-19.0 cm). 4 At the commonly used 15 cm cutoff, the bowel segment is actually sigmoid colon rather than rectum in 84% of patients. 4 This highlights that rigid anatomical measurements may not always correspond to the true rectosigmoid junction, though the 15 cm definition remains the clinical standard for classification purposes. 1, 2
Gender Differences
The surgical anal canal (anorectal ring to anal verge) averages 4.4 cm in men (range 3.2-5.3 cm) and 4.0 cm in women (range 3.0-5.0 cm), which affects the starting point for rectal measurements. 5
Alternative Anatomical Definitions
The National Comprehensive Cancer Network defines the rectum as the area below the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI. 1, 3 However, at the sacral promontory landmark, the bowel segment is actually sigmoid colon in 28% of patients, demonstrating the limitations of bony landmarks. 4
Clinical Implications
For ulcerative proctitis, the International Organization for the Study of Inflammatory Bowel Diseases defines disease extent as a maximum of 15 cm from the anal verge. 1 This same 15 cm threshold is used universally across colorectal pathology for defining rectal versus colonic disease. 1, 2