Editing Colon and rectum (section)

== '''Gross anatomy''' ==

=== '''Caecum''' ===

*** Saccular beginning of the colon
*** Average diameter 7.5cm and length of 10cm
*** No mesentery - usually completely covered by peritoneum, and is therefore considered an intra-peritoneal structure
*** Variably connected to the posterior abdominal wall by a peritoneal reflection
*** Variable mobility - hyper-mobile caecums can predispose to volvulus or bascule (intermittent anterior and superior folding of the caecum, associated with obstructive symptoms)
*** Thin wall compared to rest of colon, and large diameter, and therefore the site most likely to perforate (Laplace's law)
**** Acute dilation to >12cm is associated with risk of ischaemic necrosis and perforation and requires prompt treatment
*** Terminal ileum empties into the caecum along the posteromedial border through the ileocaecal valve
**** Two semilunar mucosal lips that fuse, regulating ileal emptying by preventing backward reflux of colonic contents
**** 20% incompetent
**** Can produce a closed loop obstruction if competent and LBO is present
**** Angulated entry maintained by superior and inferior ileocaecal folds

=== '''Vermiform appendix''' ===

*** Blind-ending tubular structure arising from posteromedial caecum approximately 3cm from ICV
**** 5-35cm in length, average length 8-10cm
**** Average diameter of 5mm
*** Position varies depending on fetal development:
**** Retrocaecal 65%
**** Pelvic 31%
**** Sub-caecal 2.3%
**** Pre-ileal 1%
**** Retro-ileal 0.4%
**** Base can be found by identifying the point of convergence of the taeniae coli
*** Locating the appendix
**** Follow the anterior taenia of the caecum to the convergence with the other two taeniae
**** Identify the bloodless fold of Treves extending from the antimesenteric border of TI to the base of the appendix, or the anterior surface of the mesoappendix, or to both areas. It contains no sizeable blood vessels. The TI is the only part of the ileum with a fold on the antimesenteric aspect.
*** Agenesis, duplication and triplication of the appendix have been reported
*** Mesoappendix is attached to caecum and proximal appendix, and contains the appendicular artery (branch of ileocolic) from SMA
**** Not a true mesentery, more of a peritoneal fold
**** There is also often a peritoneal fold in front of the appendix - bloodless fold of Treves

**

=== '''Right/ascending colon''' ===

*** Extends from ileocaecal junction and extends upward towards the hepatic flexure. Approximately 15cm in length.
*** Anterior and lateral surfaces covered in peritoneum (considered intra-peritoneal) and the posterior surface is fixed against the retroperitoneum by the fascia of Toldt
*** Can occasionally be bound down by an extension of peritoneum across the front of it, known as Jackson's veil
*** Either abnormal mobility or abnormal fixation both predispose to caecal volvulus
*** Best mobilised along the lateral peritoneal reflection by incising the white line of Toldt - this represents the fusion line of the peritoneum with the posterior fascia of the same name
**** Must be aware of the proximity of D2 near the hepatic flexure, which can be inadvertently injured here
*** Important posterior relations:
**** Quadratus lumborum/psoas muscles
**** Ureter
**** Upper part - lower pole of right kidney laterally, D2/D3 medially
*** Anteriorly it is overlain by liver/GB

=== '''Transverse colon''' ===

*** Approximately 45cm in length
*** Suspended between the hepatic and splenic flexures, which are fixed structures
*** Completely covered by visceral peritoneum
*** Connected to the posterior abdominal wall by the transverse mesocolon - made up of four layers (same as greater omentum)
*** Can be dissected free of greater omentum with relatively little bleeding due to embryonic tissue planes - if getting bleeding, likely in transverse mesocolon - easiest to establish correct plane at either flexure and work medially
*** 'U' shape in coronal plane which can extend as far inferiorly as the pelvis in some patients
*** Blood supply from middle colic artery
***

=== '''Splenic flexure''' ===

*** Where the transverse colon flexes downward
*** Usually higher and deeper than the hepatic flexure
*** Suspended by four mainly avascular ligaments:
**** Phrenicocolic ligament to the diaphragm (this is continuous with gastrosplenic ligament, but more laterally)
**** Splenocolic ligament to the lower pole of the spleen
**** Renocolic ligament to Gerota fascia
**** Pancreaticocolic ligament to the tail of the pancreas
*** It can be released without dividing any major blood vessels if one is in the correct plane
**** Commonly done by first dissecting along the white line of Toldt from below, then entering the lesser sac by lifting the omentum above the transverse colon, then taking down the splenic flexure with minimal traction


**

=== '''Descending colon''' ===

*** Begins at the splenic flexure where the intestine loses its mesentery, and extends downwards on the left side of the abdomen, until it transitions into the sigmoid colon
*** About 25cm long
*** Smaller in diameter than the ascending colon
*** Similar to right colon in terms of peritoneal attachments and dissection strategy
*** Posterior relations:
**** Lower pole of left kidney
**** Quadratus lumborum and psoas
**** Ureter
**** Gonadal vessels

=== '''Sigmoid colon''' ===

*** Begins at or below the level of the iliac crest, where the colon becomes completely intra-peritoneal
*** Covered on both sides with peritoneum and has a mesentery
**** Attached by the sigmoid mesocolon to the posterior abdominal wall and pelvis in an inverted V, creating the inter-sigmoid fossa
***** Left limb passes downwards along left edge of vertebral column
***** Right limb passes downwards towards the midline and terminates at around S3 level
**** This mesenteric fold is a good surgical landmark for the underlying left ureter and bifurcation of left CIA, as the ureter crosses over the iliac vessels at the apex
*** Thicker and more mobile compared to the descending colon
*** Varies in length from 15-50cm (average 38cm)
*** Ends at the rectosigmoid junction (see below)

**

=== '''Rectum''' ===

*** Begins at the rectosigmoid junction and ends at the anus - total length 15-20cm
**** '''Proximal landmarks of the rectum'''
***** Colonic taeniae confluence to form a complete longitudinal muscle layer (blending of lateral and anti-mesenteric taeniae to form a flat anterior muscular band)
***** Cessation of haustra
***** Colon loses its mesentery
***** Usually between the level of the sacral promontory and S3 (Jamieson's says at S3 level, 6cm distal to sacral promontory)
***** Can also be measured by stretching the rectum against the sacral promontory, then using a rigid sigmoidoscope to go 15cm from anal verge
**** '''Distal border''' defined differently by anatomists and surgeons
***** Anatomists - dentate (pectinate) line
***** '''Surgeons''' - proximal border of the anal sphincter complex at the level of levator ani, which is about 2cm above the dentate line. The muscle coat of the rectum becomes continuous with the sphincter mechanism here.
*** Divide into thirds, based on peritoneal covering:
**** '''Upper third''' - anteriorly and laterally - to about 10cm above dentate line
**** '''Middle third''' - only covered anteriorly, as the peritoneal covering is reflected onto bladder/uterine body - 5-10cm above dentate line. Widest part, lies on anterior sacrum, separated from it by the mesocolon.
**** '''Lower third''' - totally extra-peritoneal, no covering, from 1-5cm above the dentate line. Turns forward prior to penetrating levator ani, then turns backwards to penetrate the pelvic floor and ending at the anal canal.
*** Wide, easily distensible reservoir
*** No haustra, appendices epiploicae or taeniae coli
*** Surrounded by perirectal fat
*** '''Three curvatures''' corresponding to three intraluminal folds: left superior, right middle and left inferior folds ('valves of Houston')
**** Proximal and distal valves fold to the right and the middle valve folds to the left
**** These valves are lost after full surgical mobilisation of the rectum, which provides an extra 5cm of length
*** '''Relations'''
**** Anterior peritoneal reflection
***** Rectovesicular pouch (men) - 7-9cm from anal verge
***** Pouch of Douglas (women) - 5-7.5cm from anal verge
***** Metastases can accumulate here and be felt on DRE ("Blumer shelf")
**** Mesorectum
***** The visceral mesentery of the rectum - the direct continuation of the sigmoid mesocolon
***** Thick posteriorly, thinner along the sides, and very thin anteriorly
***** Thins out in the lower third, and virtually absent over the last 1cm
***** Contained within the '''mesorectal fascia''' (a.k.a. fascia propria or pelvic visceral fascia), which is fused posteriorly with the presacral fascia, and there is a bloodless plane between the two fasciae
****** Dissection in this plane is relatively bloodless, and allows consistent excision of lymphatics
****** Total mesorectal excision (TME) occurs in this plane
**** '''Presacral fascia''' (of Waldeyer)
***** Separated from rectum by thick posterior mesorectum, then thin layer of investing fascia (mesorectal fascia), then a bloodless plane, then presacral fascia
***** Mobilisation and dissection of the rectum posteriorly for cancer should proceed between the presacral fascia and mesorectal fascia
***** Separates the rectum and mesorectum from the sacrum and various nerves found posteriorly
***** '''Presacral space'''/retrorectal space
****** The space between the posterior wall of the rectum and the anterior surface of the sacrum
****** Boundaries
******* Anterior - mesorectum and mesorectal fascia (depending on the context, this could instead be considered the posterior wall of rectum - e.g. radiologically)
******* Posterior - sacrum
******* Superior - peritoneal reflection
******* Inferior - retro-sacral fascia
******* Lateral - lateral ligaments (endopelvic fascia), ureters, internal iliac vessels
****** Contents
******* Loose connective tissue
******* Lymphatics
******* Middle sacral vessels
******* Superior rectal vessels
******* Branches of SNS and PNS
******* Presacral venous plexus, lying on the anterior sacrum and coccyx. These veins drain into the sacral foramina. Dissecting deep to the presacral fascia can lead to severe bleeding here, as the vessels tend to withdraw into the sacral foramina.
**** Rectosacral fascia
***** A thick condensation of endopelvic fascia connecting the presacral fascia (3-5cm above the anorectal junction) to the mesorectal fascia at the level of S2-4
***** Note some authors describe this as Waldeyer's fascia too, but this might be erroneous
***** Divides the presacral space into superior and inferior compartments
***** Two layers - visceral and parietal
***** The '''<nowiki/>'holy plane'''' continues on from above between the visceral and parietal layers, which is the ideal posterior plane for TME and should be bloodless. Careful not to get behind the fascia entirely.
***** May be important in preventing rectal prolapse
***** Careful not to avulse, will get troublesome bleeding
***** Division will be necessary to complete the abdominal phase of TME in an APR
**** Endopelvic fascia
***** The lateral attachments of the pelvic organs to the pelvic side walls
***** Endopelvic fascia must be breached in an APR to get from pelvic floor to TME, however it is better to do this from above, to avoid risking getting behind the presacral fascia inadvertently.
**** Lateral stalks/ligaments of rectum
***** No ligaments really exist, but this is the complex of middle rectal vessels, splanchnic nerves entering the mesorectum, and their accompanying connective tissue
***** Extends from pelvic sidewall to the mesorectum
***** Common site of damage to inferior hypogastric plexus or splanchnic nerves, when diathermy is used to control bleeding from here
**** '''Denonvilliers fascia'''
***** Located anterior to the rectum
***** May be hard to find a clear ligament between mesorectal fascia and posterior vagina in women
***** A well-developed membranous layer that is an extension of the inferior peritoneal reflection and extends to the perineal body
***** Separates the rectum from anterior structures
***** Considered as the anterior border of a TME - some controversy as to which side is better to dissect along, perhaps oncologically sounder to dissect anteriorly, but perhaps higher risk of injury
***** Cavernous nerves run in neurovascular bundles anterior to the fascia at the posterolateral border of the apex and base of the prostate - probably where most parasympathetic nerve damage happens during ULAR
**** Relations to other organs
***** Men:
****** Anteriorly: bladder, ureters, vas deferens, seminal vesicles, prostate
****** Posterior: sacrum, sacral vessels, sacral nerve roots
***** Women
****** Intraperitoneal: uterus, tubes, ovaries, upper part of posterior vaginal wall
****** Extraperitoneal: uterine cervix, posterior vaginal wall
****** Posterior: sacrum, sacral vessels, sacral nerve roots

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** '''Anal canal'''
*** See separate topic