Colon secretes K and reabsorbs Na and water (mostly in right colon and cecum)

  4 layers – mucosa (columnar epithelium) → submucosa → muscularis propria → serosa

•  Muscularis mucosa – small interwoven inner muscle layer just below mucosa but above basement membrane

•  Muscularis propria – circular layer of muscle

  Ascending, descending, and sigmoid colon are all retroperitoneal

•  Peritoneum covers anterior upper and middle ⅓ of the rectum

  Plicae semilunares – transverse bands that form haustra

  Taenia coli – 3 bands that run longitudinally along colon. At rectosigmoid junction, the taeniae become broad and completely encircle the bowel.

  Vascular supply

•  Ascending and ⅔ of transverse colon supplied by SMA (ileocolic, right and middle colic arteries)

•  ⅓ transverse, descending colon, sigmoid colon, and upper portion of the rectum supplied by IMA (left colic, sigmoid branches, superior rectal artery)

•  Marginal artery – runs along colon margin, connecting SMA to IMA (provides collateral flow)

•  Arc of Riolan – short direct connection between SMA and IMA

•  80% of blood flow goes to mucosa and submucosa

  Venous drainage follows arterial except IMV, which goes to the splenic vein

•  Splenic vein joins the SMV to form the portal vein behind the pancreas

  Superior rectal artery – branch of IMA

  Middle rectal artery – branch of internal iliac (the lateral stalks during low anterior resection [LAR] or abdominoperineal resection [APR] contain the middle rectal arteries)

  Inferior rectal artery – branch of internal pudendal (which is a branch of internal iliac)

  Superior and middle rectal veins drain into the IMV and eventually the portal vein

  Inferior rectal veins drain into the internal iliac veins and eventually the caval system

  Superior and middle rectum – drain to IMA nodal lymphatics

  Lower rectum – drains primarily to IMA nodes, also to internal iliac nodes

  Bowel wall contains mucosal and submucosal lymphatics

  Watershed areas

•  Splenic flexure (Griffith’s point) – SMA and IMA junction

•  Rectum (Sudak’s point) – superior rectal and middle rectal junction

•  Colon more sensitive to ischemia than small bowel secondary to ↓ collaterals

  External sphincter (puborectalis muscle) – under CNS (voluntary) control

•  Inferior rectal branch of internal pudendal nerve

•  Is the continuation of the levator ani muscle (striated muscle)

  Internal sphincter – involuntary control

•  Is the continuation of the muscularis propria (smooth muscle)

•  Is normally contracted

  Meissner’s plexus – inner nerve plexus

  Auerbach’s plexus – outer nerve plexus

  Pelvic splanchnic nerves – parasympathetic

  Lumbar and sacral plexus – sympathetic

  From anal verge – anal canal 0–5 cm, rectum 5–15 cm, rectosigmoid junction 15–18 cm

  Levator ani – marks the transition between anal canal and rectum

  Crypts of Lieberkühn – mucus-secreting goblet cells

  Colonic inertia – slow transit time; patients may need subtotal colectomy

  Short-chain fatty acids – main nutrient of colonocytes

  Stump pouchitis (diversion or disuse proctitis) – Tx: short-chain fatty acids

  Infectious pouchitis – Tx: metronidazole (Flagyl)

  Denonvilliers fascia (anterior) – rectovesicular fascia in men; rectovaginal fascia in women

  Waldeyer’s fascia (posterior) – rectosacral fascia

POLYPS

  Hyperplastic polyps – most common polyp; no cancer risk

  Tubular adenoma – most common (75%) intestinal neoplastic polyp

•  These are generally pedunculated

  Villous adenoma – most likely to produce symptoms

•  These are generally sessile and larger than tubular adenomas

•  50% of villous adenomas have cancer

  > 2 cm, sessile, or villous lesions have ↑ cancer risk

  Polyps have left-side predominance

  Most pedunculated polyps can be removed endoscopically

  If not able to get all of the polyp (which usually occurs with sessile polyps) → need segmental resection

  High-grade dysplasia – basement membrane is intact (carcinoma in situ)

  Intramucosal cancer – into muscularis mucosa (carcinoma in situ → still has not gone through basement membrane)

  Invasive cancer – into submucosa (T1)

  Screening – at 50 for normal risk, at 40 (or 10 years before youngest case) for intermediate risk (eg family history of colon CA)

  Screening options – 1) colonoscopy every 10 years; or 2) high-sensitivity fecal occult blood testing every 3 years and flexible sigmoidoscopy every 5 years; or 3) high-sensitivity fecal occult blood testing annually

•  Double contrast barium enema or CT colonography every 5 years may be alternatives to above

•  False-positive guaiac – beef, vitamin C, iron, cimetidine

•  No colonoscopy with recent MI, splenomegaly, pregnancy (if fluoroscopy planned)

  Polypectomy shows T1 lesion – polypectomy is adequate if margins are clear (2 mm), is well differentiated, and has no vascular/lymphatic invasion; otherwise, need formal colon resection

  Extensive low rectal villous adenomas with atypia – Tx: transanal excision (can try mucosectomy) as much of the polyp as possible

•  No APR unless cancer is present

  Pathology shows T1 lesion after transanal excision of rectal polyp → transanal excision is adequate if margins are clear (2 mm), it is well differentiated, and it has no vascular/lymphatic invasion

  Pathology shows T2 lesion after transanal excision of rectal polyp → patient needs APR or LAR

COLORECTAL CANCER

  2nd leading cause of CA death

  Symptoms: anemia, constipation, and bleeding

  Red meat and fat → O₂ radicals are thought to have a role

  Colon CA has had an association with Clostridium septicum infection

  Colon CA – main gene mutations are APC, DCC, p53, and k-ras

  Sigmoid colon – most common site of primary

  Disease spread

•  Spreads to nodes first

•  Nodal status – most important prognostic factor

•  Liver – #1 site of metastases; lung – #2 site of metastases

•  Portal vein → liver metastases; iliac vein → lung metastases

•  Liver metastases – if resectable and leaves adequate liver function, patients have 35% 5-year survival (5-YS) rate

•  Lung metastases – 25% 5-YS rate in selected patients after resection

•  Isolated liver or lung metastases should be resected

•  5% get drop metastases to ovaries

•  Rectal CA – can metastasize to spine directly via Batson’s plexus (venous)

•  Colon CA typically does not go to bone

•  Colorectal CA growing into adjacent organs can be resected en bloc with a portion of the adjacent organ (ie partial bladder resection)

  Lymphocytic penetration – patients have an improved prognosis

  Mucoepidermoid – worst prognosis

  Rectal ultrasound – good at assessing depth of invasion (sphincter involvement), recurrence, and presence of enlarged nodes

  Need total colonoscopy to rule out synchronous lesions in patients with colorectal CA

  Goals of resection

•  En bloc resection, adequate margins, and regional adenectomy

•  Most right-sided colon CAs can be treated with primary anastomosis without ostomy

•  Rectal pain with rectal CA – patient needs APR

•  Generally need 2-cm margins

  Intraoperative ultrasound (U/S) – best method of picking up intrahepatic metastases

•  Conventional U/S resolution: 10 mm

•  Abdominal CT: 5–10 mm

•  Abdominal MRI: 5–10 mm (better resolution than CT)

•  Intraoperative U/S: 3–5 mm

  Abdominoperineal resection (APR)

•  Permanent colostomy; anal canal is excised along with the rectum

•  Can have impotence and bladder dysfunction (injured pudendal nerves)

•  Indicated for malignant lesions only (not benign tumors) that are not amenable to LAR

•  Need at least a 2-cm margin (2 cm from levator ani muscles) for LAR, otherwise will need APR

•  Risk of local recurrence higher with rectal CA than with colon CA in general

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