Pelvic Floor Muscles and Nerves in Relation to the Ano-rectum, and Their Preservation during Low Anterior Resection

Part I: Salient Pelvic Floor Muscles

1. Levator Ani Complex

2. External Anal Sphincter (EAS)

3. Internal Anal Sphincter (IAS)

4. Obturator Internus

Part II: Salient Pelvic Nerves

A. Sympathetic Innervation

• Formed by convergence of the right and left lumbar sympathetic trunks and the aortic plexus, at the level of the aortic bifurcation and anterior to the sacral promontory (L5/S1)
• Divides into the right and left hypogastric nerves which descend into the pelvis along either side of the rectum (posterior to its fascia)
• Injury here causes loss of ejaculation (failure of seminal emission) in males and altered bladder neck function

• Descend from the superior hypogastric plexus along the posterolateral aspects of the rectum
• Lie on the posterior surface of the mesorectum - at risk during posterior rectal mobilisation if dissection strays into an incorrect plane
• Join the pelvic plexuses laterally

B. Parasympathetic Innervation

• Arise from S2, S3, and S4 sacral nerve roots
• Exit the sacral foramina and travel anterolaterally to join the pelvic (inferior hypogastric) plexus
• They are the erection nerves - injury causes erectile dysfunction in males

C. The Pelvic (Inferior Hypogastric) Plexus

• A paired autonomic plexus located on the lateral pelvic sidewall, at the level of the lateral stalks/lateral ligaments of the rectum
• Receives both sympathetic (from hypogastric nerves) and parasympathetic (from nervi erigentes) inputs
• Sends branches to bladder, prostate/seminal vesicles, urethra, uterus/vagina, and rectum
• Contains the neurovascular bundle of Walsh (the cavernous nerves) which run alongside Denonvilliers' fascia anterolaterally
• Injury leads to bladder dysfunction (retention or incontinence) and sexual dysfunction

D. Pudendal Nerve (S2, S3, S4)

• Exits the pelvis through the greater sciatic foramen, wraps around the ischial spine, and re-enters through the lesser sciatic foramen into Alcock's canal
• Gives the inferior rectal nerve (to external anal sphincter and perianal skin), perineal nerve, and dorsal nerve of the penis/clitoris
• Injury causes faecal incontinence, urinary incontinence, and impaired perineal sensation

Part III: Measures to Prevent Nerve Injury during Low Anterior Resection (LAR)

1. High Ligation of the Inferior Mesenteric Artery - Identifying the Superior Hypogastric Plexus

• During sigmoid/left colon mobilisation, the superior hypogastric plexus lies just inferior to the aortic bifurcation and the sacral promontory, slightly to the left of the midline
• The IMA is ligated below the left colic artery origin where possible, or just at its origin from the aorta
• Dissection is kept in the correct retroperitoneal plane to avoid sweeping the plexus anteriorly; the plexus should be left adherent to the posterior presacral fascia

2. TME in the "Holy Plane" - Preserving Hypogastric Nerves

• Posterior rectal dissection is performed in the avascular plane between the mesorectal fascia (visceral layer) and the presacral fascia (parietal layer)
• This plane passes anterior to the hypogastric nerves, keeping them against the presacral fascia/sacrum
• Waldeyer's recto-sacral fascia is divided at the level of S4 as the dissection reaches the pelvic floor, taking care to remain anterior to the sacral fascia
• Sharp dissection rather than blunt avulsion prevents nerve traction injury

3. Lateral Dissection - Sparing the Pelvic Plexus

• The lateral ligaments (which contain the middle rectal artery and part of the pelvic plexus) are divided close to the rectal wall to minimise plexus injury
• The neurovascular bundles course on the lateral surface of the mesorectal fascia; the dissection stays medial to them
• Avoiding excessive traction or diathermy near the lateral stalks is important

4. Anterior Dissection - Preserving Nerves of Denonvilliers

• Anteriorly, the dissection enters the plane between the anterior mesorectal fascia and Denonvilliers' fascia (for posterior tumours) or between Denonvilliers' fascia and the prostate/seminal vesicles (for anterior tumours)
• The cavernous nerves (from pelvic plexus) run posterolaterally to the prostate in the neurovascular bundle - staying in the correct plane of Denonvilliers' fascia prevents erectile nerve injury
• In females, the rectovaginal septum is entered with care to protect the pelvic plexus branches coursing along the vaginal wall

5. Avoiding Excessive Lateral Traction

• Medialisation of the rectum with excessive tension can stretch and avulse the pelvic plexuses and nervi erigentes - gentle retraction is essential

6. Nerve Stimulation (Intraoperative Neuromonitoring)

• Intraoperative nerve stimulation with recording of bladder/urethral sphincter responses can help identify the pelvic plexus and nervi erigentes, especially in difficult re-operative or post-radiotherapy cases

7. Correct Patient Positioning and Lighting

• The steep Trendelenburg position and adequate lighting/laparoscopic magnification (particularly with robotic/laparoscopic approaches) enhance visualisation of the nerve structures; robotic surgery may reduce nerve injury by virtue of improved pelvic visibility

8. Avoiding Diathermy Near Nerves

• The nervi erigentes are at greatest risk near the sacral foramina; diathermy or clips here can cause thermal injury even when the anatomical plane seems correct

Summary