The mesorectum is the fatty tissue surrounding the rectum within the pelvis. This structure contains the blood vessels and the lymphatic system serving the lower portion of the large intestine. Its importance stems from the fact that rectal tumors frequently spread into this surrounding fat, making the mesorectum the central focus for surgical intervention. The complete removal of this structure has fundamentally changed the prognosis for patients with rectal malignancies.
The Anatomy of the Mesorectum
The mesorectum is the perirectal fat and its contents, extending from the rectosigmoid junction down to the pelvic floor muscles. This tissue is encased by a thin, fibrous covering known as the mesorectal fascia, which separates the rectum from surrounding pelvic structures.
The mesorectum is primarily adipose tissue, cushioning the rectum within the bony pelvis. Embedded within this fat are the superior rectal artery and vein, along with their branches, which supply and drain the organ. The mesorectum also contains the lymphatic drainage network of the rectum, including numerous lymph nodes and vessels.
These lymph nodes are concentrated mainly along the superior rectal vessels. The mesorectal fascia acts as the outer boundary, adhering to the presacral fascia covering the sacrum. This adherence creates a natural dissection plane, often called the “holy plane,” which surgeons follow to remove the mesorectum intact.
The Role in Rectal Cancer Spread
The mesorectum is the primary pathway for the local spread of rectal cancer. Tumors grow through the muscular layers of the rectal wall and directly into the perirectal fat. This local invasion within the mesorectum is a factor in determining the cancer stage and the risk of the disease returning.
Because the mesorectum contains lymph nodes, cancer cells can travel through the lymphatic vessels and establish new tumor sites. Microscopic spread, such as small tumor nodules or lymphatic vessel invasion, is common. If these tumor deposits or involved lymph nodes are not completely removed, the risk of local recurrence increases substantially.
The circumferential resection margin (CRM) is the shortest distance between the tumor edge and the cut surface of the mesorectum specimen. If the tumor comes within one millimeter of the mesorectal fascia, the CRM is considered positive, which elevates the risk of local recurrence. Distal rectal tumors are at a higher risk of involving the mesorectal fascia because the mesorectum naturally tapers in this area.
Total Mesorectal Excision (TME)
Total Mesorectal Excision (TME) is the definitive surgical procedure for treating mid- and low-rectal cancers. TME revolutionized rectal cancer treatment by focusing on the removal of the entire mesorectum as a single, intact specimen. This technique led to a dramatic reduction in local recurrence rates.
The goal of TME is to perform a sharp, meticulous dissection along the avascular plane between the mesorectal fascia and the parietal pelvic fascia. This ensures the tumor, its entire lymphatic drainage system, and all surrounding fat are removed “en bloc,” or as one unit. Complete removal of this tumor-bearing tissue minimizes the chance of leaving behind microscopic cancer cells that could lead to local recurrence.
The quality of the surgical specimen predicts patient outcome, and pathologists grade the TME specimen after removal. A “complete” specimen is smooth and intact, with the mesorectal fascia fully preserved and showing no tears. An incomplete specimen, which has defects exposing the muscular wall, is associated with a significantly higher risk of local recurrence, similar to a positive CRM.
To facilitate this complex dissection, surgeons often perform a high ligation of the inferior mesenteric artery and divide the colon at the descending and sigmoid junction. The procedure requires careful, circumferential dissection down to the pelvic floor muscles. TME can be performed using open surgery, laparoscopy, or robotic techniques, all aiming for a complete, intact mesorectal envelope.
Nerve Preservation and Functional Outcomes
A major challenge during TME is the close proximity of the autonomic nerves, which control bladder, sexual, and bowel function, to the mesorectal fascia. The dissection must be performed precisely within the “holy plane” to preserve these nerve plexuses. The autonomic nervous system in the pelvis includes the sympathetic nerves of the superior hypogastric plexus and the parasympathetic nerves of the inferior hypogastric plexus.
Damage to these nerves during the procedure can result in long-term functional consequences that significantly impact a patient’s quality of life. Injury to sympathetic fibers can lead to retrograde ejaculation in men and reduced intensity of orgasm in women. Damage to parasympathetic fibers is associated with difficulty emptying the bladder (voiding dysfunction) and erectile dysfunction in men.
Sexual dysfunction rates in men can range widely after TME without dedicated nerve preservation. Voiding dysfunction, including urinary incontinence or difficulty, can also affect patients. Surgeons employ a nerve-sparing technique, using sharp dissection and high-magnification views, often provided by robotic or laparoscopic systems, to carefully identify and protect these structures. Successful nerve preservation balances the need for complete cancer removal with the goal of maintaining the patient’s normal bodily functions.