Rectum and Hypogastric Nerves

Pl 363-9.

 

Rectus (Lat) = straight. Hardly so in human.  12cm(males) or 13cm(fems) long.  Ie ~15 cm from anal verge given 3cm of anus.
Has no mesentery.

Begins at rectosigmoid jx: gradual distinction from level of 3rd part of sacrum.; ~6cm distal to sacral promontory.

-           based on peritoneal attachments: sigmoid mesocolon ends; muscle coat become sphincters

-           Rectum has a complete longitudinal muscle coat (taeniae coalexce), no sacculations and no app epiploicae

 

3 lateral curves which correspond to rectal valves (of Houston - ?role to do with flatus discrimination) Pl 365

-           upper and lower curves are convex right; middle curve is convex left

-           they are produced by circular muscle layer (c.f. circular folds of small bowel).

-           Middle is largest, projects in from right wall at level where peritoneum reflects to form the rectovesical/rectouterine pouch; ie 8cm from anal orifice.

 

Lowest part of rectum is slightly dilated as the rectal ampulla.

Initially slopes gradually forwards, then turns down and back abruptly at anal canal, 2-3cm in front of tip of coccyx

Anorectal jx is slung forward by puborectalis, which merges with the top of the external sphincter

-           the posterior wall makes a right-angle bend at anorectal junction

-           the angle widens during defecation

Divided into thirds from POV of surcal planning; e.g. <6, 6-10, 11-15 cm from verge.

 

Mesorectum

The rectum has no mesentery, but the visceral pelvic fascia around it is called mesorectum by surgeons.

-           the pararectal nodes are in this, so it is removed in rectal excision for Ca.

Peritoneum covers lateral / posterior surfacesÕ upper third, and to middle third anteriorly.

-           reflected forwards to cover bladder or upper vagina to form  the rectovesical/uterine pouch

 

Rectovesical pouch  (363)

Lowest part of the peritoneal cavity (7.5cm from anal margin in males, 5.5 in females)

-           normal contain coils of intestine.

Above anteriorly: tips of seminal vesicles, upper bladder base.

Below anteriorly: rest of seminal vesicles, bladder base, prostate, ends of ureters and ductus deferens. 


Rectovesical fascia
(of Denonvilliers) intervenes between rectum and these structures

-           Adherent to mesorectal fat rather than wall of bladder, prostate, vagina(?) Essentially is between.
Divided opinion about which side to resect on; but seems oncologically sounder to remove it in rectal excision for Ca. 
--> In reality it depends; remove if anterior tumour; close; bulky; else no need.

-           fascia connects above to floor of rectovesical pouch, below to apex of prostate / peroneal body.
*beware parasympathetic nerves at lateral border of Denonvilliers, which eventually run anterior to fascia at posterolateral border of the apex and base of prostate. Most common site of injury --> impotence.


Rectouterine pouch (of Douglas) (363)

Anteriorly: upper vagina (fornix, with uterine cervix)

Below: vagina and thin rectovaginal fascia.

Posterolateral: lower three pieces of sacrum, coccyx, piriformis, levator ani, coccygeus, anterior rami of lower three sacral & coccygeal nerves, sympathetic trunk, pelvic splanchnic nerves, rectal vessels.


WaldeyerÕs fascia

Endopelvic fascia; separates rectum from sacrum.  Two layered; one on rectum, one back near sacrum; connected ahead of 4th sacral vertebral by a short dense bank known as the rectosacral fascia
  - these dense bands
may be important in preventing prolapse.

Passes off sacrum to pelvic floor.

Nerves run within or behind the fascia

Want a bloodless dissection in the retrorectal space, sharp (don't avulse the fibrous band or can get presacral vein bleeding.  The 'Holy Plane'.
Retroperitoneal tissue around middle rectal vessels are lateral ligaments but these are often small.

Has to be breached during APR; best done from above since posterior dissection can injure nerves; avoid simultaneous dissection; do abdomen to pelvic floor first.
Rectourethralis muscle is small slips of longitudinal muscle that pass to perineal body and sphincter urethrae.

Lateral Ligaments
Misnomer as not real ligaments; band like structure with entry of middle rectal vessels and conveying sympathetics downwards.
Embedded in fat and fibrous tissue from pelvic side wall to mesorectum.
Beware diathermy injury to splanchnic nerves when achieving hemostasis of the lateral ligament; do not ligate the ligament.

Blood supply of rectum (369)

Mainly SRA (continuation of IMA after pelvic brim) which crosses anterior to L common iliac vessels medial to L ureter and divides at S3 into two branches that sink into muscular wall, supplying its whole thickness. 

-           anastomoses with inferior rectal artery vessels in the submucosa; excellent and reliable; hence can ligate superior rectal without much concern for the rectal stump.

Middle rectal arteries (from internal iliacs) approach along lateral ligaments, across pelvic floor. 

-           often small or absent; can safely mobilize rectum to pelvic floor without concern for vascularity.

Inferior rectal arteries (from internal pudendal) penetrate walls of anal canal; branches run up within walls to supply lower rectum.  IRA can supply rectum at least to level of peritoneal reflection

Median sacral may make an unimportant contribution to the posterior wall.

Veins correspond with arteries but anastomose freely forming plexuses submucosally (internal rectal plexus) and outside (external rectal plexus) the muscle wall. 

-           middle rectal vein may be very small and insignificant

-           superior and inferior are main veins: follow arteries making a porto-systemic anastomosis.

 

Lymphatics

Firstly ¨ perirectal nodes (on surface of rectum) ¨ follow arteries; internal iliac for M&IRA and pre-aortic for SRA.  Believed that lateral extension occurs only when proximal route is obstructed.

-           some nodes follow median sacral artery to nodes in the sacral hollow.

Nerves

Sympathetics: from hypogastric plexus and by fibres that accompany inferior mesenteric and superior rectal arteries

Parasympathetic: (S2-3 or S3-4) from pelvic splanchnics (motor to rectal muscle)

Pain accompanies both, as in the bladder; distension goes via parasympathetics.

Examination

Posteriorly: sacrum and coccyx, ischial spines at the sides.  Anorectal ring is a shelf-like projection.

Anteriorly: Prostate or cervix (uterosacral ligaments and sometimes ovaries laterally)

Development

Lower hindgut opens into (endodermal) cloaca, closed off by cloacal membrane which lies in the (ectodermal) proctodeum. 

-           anal membrane breaks down probably where the pectinate line is.

-           Anal valves may represent membrane remnants

Endodermal / ectodermal origins explain difference in blood / nerve / lymph supplies.

Surgical Anatomy

Rectum and most of mesorectum are removed, with sigmoid colon and mesocolon.

Lower part of anal canal is attached to mobilised descending colon.

= anterior resection.

-           plane of dissection is anterior to presacral fascia; rectovesical/vaginal fascia is removed.

-           ureter and main neurovascular structures on the lateral pelvic wall are preserved.

-           Care is taken to safeguard superior & inferior hypogastric plexi.

-           IMA is divided close to its origin to ensure all lymph nodes are taken.

Complete excision of rectum and anus = abdominoperineal resection AP

-           peroneal approach includes dividing the pelvic floor, making elliptical incisions either side of the anus to enter the ischioanal fossae.

-           Coccyx is dislocated or excised and rectosacral fascia divided from below.

-           Plane between rectum and prostate is defined, and puborectalis and rectourethralis divided to free the bowel.

-           a terminal colostomy is made in the LIF.


Hypogastric Nerves  (plate 380 in 2nd)


Superior splanchnic branches
Arise from lumbar splanchnics esp L1-3; L3-4 ganglia join looping common iliac
--> Superior hypogastric plexus
intermingling web of nerves in front of the L5 body; just below aortic bifurcation

* At risk in front of aorta and behind IMA in dissection of the IMA; mesentery; initiation of TME plane.

Exits as L and R Hypogastric nerves
= connect superior and inferior hypogastric plexi

May be a bundle rather than discrete nerves
Descend on the posterolateral pelvic wall, just underneath parietal endopelvic fascia

1-2mm, lie 1cm lateral to midline and 2cm medial to ureters

Small unimportant rectal branches connect to mesorectum.

Some surgeons will deliberately identify them to preserve them during surgery.

Enter Inferior hypogastric plexus
Connects with 1) L+R hypogastrics; 2) sacral splanchnic root supply from S3,S4 and sometimes S2 ganglia; 3) pelvic PNS splanchnic nerves (S2-4; nervi erigentes)

A rhomboid-shaped flat plexus lying just beheath parietal layer of endopelvic fascia lateral to the mid rectum.
--> project forward to pelvic viscera


Pelvic Splanchnics (Nervi Erigentes)
Arise s2-4, pierce parietal endopelvic to join inf plexus
Lower third of rectum.
Most fibres continue anteriorly to genitourinary organs.


Sympathetics --> control sphincters, ejaculation
Parasympathetics --> SM walls, erection.

Mixed plexus --> impotence (fem = vaginal dryness), urinary retention or both.

* Possible injury sites
1. high ligation and dissection of IMA (pre-aortic sympathetics)
2. mobilization of sigmoid mesocolon (superior hypogastric plexus; sns)
3. initial post-rectal mobilisation (peritoneal breach; hypogastric nerves; sns)
4. post. breach of mesorectal plane (inf. plexus + pelvic splanchnics; pns+sns)
5. lateral denonvillier's fascia at anterior rectum / off SV's (terminal pelvic plexus)