colorectal carcinoma
DESCRIPTION
decribes about anatomy, physiology, pathology and managementTRANSCRIPT
COLORECTAL CARCINOMA
EMBRYOLOGY
Foregut Midgut
• Small intestine, ascending colon and proximal transverse colon
• Blood supply: Superior mesenteric artery
Hindgut
• Distal transverse colon, descending colon, rectum and proximal anus
• Inferior mesenteric artery
The distal anal canal
• derived from ectoderm • receives its blood supply from the internal
pudendal artery. • The dentate line divides the endodermal
hindgut from the ectodermal distal anal canal.
• The colon consists of five layers: mucosa, submucosa, circular muscle layer, longitudinal muscle layer, and serosa
• Microscopically, the colonic mucosa is a columnar epithelium marked by crypts and goblet cells.
• Unlike the small intestine, the columnar epithelium of the colon and rectum does not have villi.
BLOOD SUPPLY
• the superior mesenteric artery (1) the ileocolic artery supplies blood flow to the terminal ileum and
proximal ascending colon(2) the right colic arterysupplies the ascending colon(3) the middle colic arterywhich supplies the transverse colon.
• The inferior mesenteric artery (1) the left colic artery supplies the descending colon(2) several sigmoidal branches which supply the sigmoid colon (3) the superior rectal artery supplies the proximal rectum. The terminal branches of each artery form anastomoses
with the terminal branches of the adjacent artery and communicate via the marginal artery of Drummond.
• Except for the inferior mesenteric vein, the veins of the colon parallel their corresponding arteries and bear the same terminology.
• The inferior mesenteric vein ascends in the retroperitoneal plane over the psoas muscle and continues posterior to the pancreas to join the splenic vein.
• During a colectomy, this vein often is mobilized independently and ligated at the inferior edge of the pancreas.
LYMPHATIC DRAINAGE
• originates in a network of lymphatics in the muscularis mucosa.
• Lymphatic vessels and lymph nodes follow the regional arteries.
• Lymph nodes are found on the bowel wall (epicolic), along the inner margin of the bowel adjacent to the arterial arcades (paracolic), around the named mesenteric vessels (intermediate), and at the origin of the superior and inferior mesenteric arteries (main).
NERVE SUPPLY
• innervated by both sympathetic (inhibitory) and parasympathetic (stimulatory) nerves, which parallel the course of the arteries.
• Sympathetic nerves arise from T6–T12 and L1–L3. • The parasympathetic innervation to the right and
transverse colon is from the vagus nerve• The parasympathetic nerves to the left colon arise
from sacral nerves S2–S4 to form the nervi erigentes.
• The main function of the colon is absorption of water, Na+, and other minerals.
• By removal of about 90% of the fluid, it converts the 1000-2000 mL of isotonic chyme that enters it each day from the ileum to about 200-250 mL of semisolid feces.
• Sodium is absorbed actively via a Na-K ATPase.• The colon can absorb up to 400 mEq of sodium
per day. • Water accompanies the transported sodium and
is absorbed passively along an osmotic gradient. • Potassium is actively secreted into the colonic
lumen and absorbed by passive diffusion.• Chloride is absorbed actively via a chloride–
bicarbonate exchange.
• The diameter of the colon is greater than that of the small intestine.
• Its length is about 100 cm. • Solitary lymph follicles are present, especially
in the cecum and appendix.
• Characteristics unique to the colon are (a) taeniae coli, (b) haustra, and (c) appendices epiploicae.
• There are three taeniae (anterior, posterior medial, and posterior lateral), which are condensations of the outer longitudinal muscle layer in the colon.
• The taeniae originate at the base of the appendix, course along the length of the colon, and then converge at the rectosigmoid junction.
• The haustra are pockets of colon wall, between the taeniae, that result from the fact that the length of the taeniae is one sixth of the colon length.
• The epiploicae appendices are fat appendages seen on the colonic serosa. • Microscopically, the colonic mucosa is a columnar epithelium marked by
crypts and goblet cells. Unlike the small intestine, the columnar epithelium of the colon and rectum does not have villi.
Defecation
• Distention of the rectum with feces initiates reflex contractions of its musculature and the desire to defecate.
• the sympathetic nerve supply to the internal (involuntary) anal sphincter is excitatory, whereas the parasympathetic supply is inhibitory.
• This sphincter relaxes when the rectum is distended.• The nerve supply to the external anal sphincter, a skeletal
muscle, comes from the pudendal nerve.• The sphincter is maintained in a state of tonic contraction,
and moderate distention of the rectum increases the force of its contraction.
• The urge to defecate first occurs when rectal pressure increases to about 18 mm Hg.
• When this pressure reaches 55 mm Hg, the external as well as the internal sphincter relaxes and the contents of the rectum are expelled.
• Before the pressure that relaxes the external anal sphincter is reached, voluntary defecation can be initiated by voluntarily relaxing the external sphincter and contracting the abdominal muscles (straining), thus aiding the reflex emptying of the distended rectum.
• Defecation is therefore a spinal reflex that can be voluntarily inhibited by keeping the external sphincter contracted or facilitated by relaxing the sphincter and contracting the abdominal muscles
• Colorectal cancer is the first most frequent cancers among males and the third most important cancer for women in 2003.
• The incidence of colorectal cancers has been rising slowly in Malaysia. It is decreasing around 16% cases compare to 2002.
• In year 2003, Malaysia recorded at least 2552 cases of both colon and rectal cancers among male and female.
• Male recorded for 52.3% (1335 cases) while female recorded 47.7% (1217 cases) both for colon and rectal cancers.
• The incidence of this cancer increased exponentially after age of 40 for both sexes.
• According to National Cancer Registry, Chinese have a highest incidence of colon and rectal cancers, there were 59.8% of cases involved Malaysian Chinese followed by Malay with 34.6% cases and Indian with 5.6% of cases.
PATHOPHYSIOLOGY• Colon cancer arises from mucosal colonic polyps. • The two most common histologic types are(a) hyperplastic (b) adenomatous.
Histologically• hyperplastic polyps : contain an increased number of glandular cells with
decreased cytoplasmic mucus, but lack nuclear hyperchromatism, stratification, or atypia .
• Adenomatous nuclei : usually hyperchromatic, enlarged, cigar-shaped, and crowded together in a palisade pattern .
classified : tubular (composed of branched tubules) or villous (digitiform villi arranged in a frond) .
Tubulovillous adenomas contain both elements.
PATHOPHYSIOLOGY
• Most carcinomas are initially exophytic (i.e. protruding into the lumen) and later ulcerate and progressively invade the muscular bowel wall.
• Eventually, the tumour involves the serosa and surrounding structures.
• Stromal fibrosis may cause luminal narrowing, which is responsible for the common acute presentation of large bowel obstruction.
RISK FACTORS
• Age (>50 years)• Hereditary risk factor (20%)• Environmental and dietary factors (diet high in
animal fat, low fibre diet)• Inflammatory bowel disease (long standing
colitis) : carries an independent risk of bowel neoplasia. After 10 years of active disease, the cancer risk rises by 1% each year.
• Others (smoking, pelvic irradiation)
• Age: More than 90 per cent of the colorectal cancer cases in Malaysia occur in people over the age of 40.
• Polyps: Most colorectal cancer begins in polyps. This non-cancerous growth occurs in the lining of the large intestine. Certain types of polyps can develop into cancer.
• Family history: People whose family members have colorectal cancer especially at a young age, have a higher risk of developing this disease.
• Personal history of cancer: Women who have had cancer of the ovary, uterus or breast are more likely to develop colorectal cancer.
• Diet: A diet high in fat and low in fibre is linked to the development of the disease.
• Tobacco: Smoking increases the risk of polyp formation. • Body weight and physical activity: People with sedentary lifestyles and who are
overweight or obese increase their risk of colorectal cancer. • Nonsteroidal anti-inflammatory drugs (NSAIDS): Some studies suggest that
taking aspirin and other NSAIDS may reduce the risk of developing polyps.
POLYPOSIS SYNDROME• familial adenomatous polyposis (FAP) An autosomal dominant defect in the APC gene causes a hundred or more
adenomatous polyps to develop in the large bowel by the mid teen years. Affected patients usually have one parent with the condition. Each affected individual is certain to develop colorectal cancer, at an
average age of 40, unless preventative measures are taken. Ideally, prophylactic surgery to remove the area at risk should be
performed in early adulthood. One option is subtotal colectomy and ileorectal anastomosis which
removes nearly all the large bowel but has the disadvantage that the retained rectum requires careful long-term surveillance.
The alternative is to remove the rectum as well (panproctocolectomy) and then perform an ileostomy or an ileal pouch restorative procedure.
• Peutz-Jeghers syndromewhich causes hamartomatous polyps throughout the
gastrointestinal tract. Patients often have freckles around the mouth and on
the hands, feet and genitalia. Half of these patients are likely to die by the age of 50
because of polyp-related emergencies such as bowel intussusception or cancer.
These patients are prone to develop cancers of small and large bowel, stomach, pancreas, testis and breast.
Hereditary non-polyposis colorectal cancer (HNPCC) (Lynch syndrome)
• as results from defects in mismatch repair genes which mend damaged DNA.
• The condition carries a 70% lifetime risk of colorectal cancer, but also a substantially increased risk of other 'indicator' cancers such as those of endometrium, ovary, urothelium, small bowel and brain, and sometimes several of these.
• Families can be difficult to identify because of the diversity of cancers and incomplete genetic penetrance (i.e. not everyone carrying the genetic defect will develop cancer).
• When patients under 45 develop indicator cancers, they can be tested for markers which suggest the genetic condition.
• If positive, formal genetic tests are then undertaken. Those at risk should be offered colonoscopy every 2 years from the age of 25 if practicable.
Presentation
• insidious onset of chronic symptoms, • acute onset of intestinal obstruction, • and acute perforation.
• A change in bowel habits • Diarrhoea, constipation or feeling that the bowel does
not empty completely • Blood (either bright red or very dark) in the stool • Stools that are narrower than usual • General abdominal discomfort (frequent gas pains,
bloating, fullness, and/or cramps) • Weight loss with no known reason • Constant tiredness • Vomiting
RECTAL BLEEDING
• Carcinomas distal to the splenic flexure often cause visible blood to be passed per rectum.
• The character of the blood and the nature of its mixing with stool depend on how far proximally the lesion is from the anus.
• Large bowel carcinomas metastasise via lymphatics and the bloodstream, and by the time of diagnosis as many as 25% of patients already have widespread metastases.
• Lymphatic spread is sequential, first to mesenteric nodes and then onward to para-aortic nodes.
• Occasionally lymph node involvement is directly responsible for the clinical presentation.
• For example, para-aortic nodes may present as a palpable mass or cause duodenal obstruction. Other enlarged nodes may compress the bile ducts in the porta hepatis causing jaundice.
• Haematogenous spread is predominantly to the liver and usually occurs later than lymphatic spread; therefore a patient with early lymph node involvement at the time of presentation has a better chance of avoiding liver metastases.
• Despite this, hepatic involvement does occur without evidence of lymphatic spread.
• Haematogenous spread to other sites such as lung or bone is uncommon but can occur, as may systemic manifestations.
Pathology
• Carcinoma of the colon or rectum is an adenocarcinoma with a fibrous stroma that may progress in different ways:
• as an exophytic cauliflower-type of growth;• as an ulcerating lesion penetrating through the
bowel wall;• as an annular constricting growth;• as a diffuse infiltrating tumour;• as the rare colloidal mucus-secreting tumour.
• It is likely that all carcinomas start as a benign adenoma, the so called ‘adenoma–carcinoma sequence’.
• Tumours are more common in the left colon and rectum.
Local spread
• The tumour can spread in a longitudinal, transverse or radial direction;
• it spreads round the intestinal wall and usually causes intestinal obstruction before it invades adjacent structures.
• The ulcerative type more commonly invades locally, and an internal fistula may result, for example into the bladder.
• There may also be a local perforation with an abscess or even an external faecal fistula. This type of radial spread to adjacent organs has the largest impact on prognosis.
• The progression of invasion occurs across the submucosa into the muscularis propria and thence out into the serosa and fat, lymphatics and veins in the mesentery alongside the bowel wall.
Lymphatic spread
Lymph nodes draining the colon are grouped as follows:• N1: nodes in the immediate vicinity of the bowel wall;• N2: nodes arranged along the ileocolic, right colic,
midcolic, left colic and sigmoid arteries;• N3: the apical nodes around the superior and inferior
mesenteric vessels where they arise from the abdominal aorta. Involvement of the lymph nodes by the tumour progresses in a gradual manner from those closest to the growth along the course of the lymphatic vessels to those placed centrally.
Bloodstream spread• Metastases are carried to the liver via the portal
system, sometimes at an early stage before clinical or operative evidence is detected (occult hepatic metastases).
Transcoelomic spread• Rarely, colorectal cancer can spread by way of cells
dislodging from the serosa of the bowel or via the subperitoneal lymphatics to other structures within the peritoneal cavity.
Dukes’ classification
• was originally described for rectal tumours (but has been adopted for histopathological reporting of colon cancer as well.
A: confined to the bowel wall;B: through the bowel wall but not involving the free
peritoneal serosal surface;C: lymph nodes involved.• Dukes himself never described a D stage, but this is
often used to describe either advanced local disease or metastases to the liver.
TNM classification
• T Tumour stage;T1 Into submucosa;T2 Into muscularis propria;T3 Into pericolic fat but not breaching
serosa;T4 Breaches serosa or directly involving
another organ;• N Nodal stage;N0 No nodes involved;N1 One or two nodes involved;N2 Three or more nodes involved;• M Metastases; M0 No metastases;M1 Metastases;
• Ly Lymphatic invasion;L0 No lymphatic vessels
involved;L1 Lymphatics involved;• V Venous invasion; V0 No vessel invasion; V1 Vessels invaded;• R Residual tumour;R0 No residual tumour;R1 Margins involved, residual
tumour present.
Clinical features
• usually occurs in patients over 50 years• but it is not rare earlier in adult life. • Twenty per cent of cases present as an emergency with
intestinal obstruction or peritonitis.• In any case of colonic bleeding in patients over the age of
40 years, a complete investigation of the colon is required.
• A careful family history should be taken. Those with first-degree relatives who have developed colorectal cancer at the age of 45 years or below are at high risk and may be part of one of the colorectal cancer family syndromes.
Change of bowel habit and large bowel obstruction
• blood and mucus into the lumen. • This tends to alter the bowel habit towards a looser stool. • Thus a recent history of loose stool is more likely to predict cancer than
increasing constipation, especially since constipation is so common in the elderly population.
• Faeces in the left colon are more solid and the intraluminal pressure is higher, thus cancers here are more likely to obstruct.
• The more distal the tumour, the more likely it is to cause obstruction. • Colonic cancers tend to progressively encircle the bowel wall, encroaching on
the lumen and producing an annular stenosis.• It has been estimated that it takes a year to involve each quarter of the bowel
circumference.• Large bowel obstruction may be partial or complete. • Partial obstruction may present as a change in bowel habit, often noticed as
constipation with intermittent 'overflow' diarrhoea.• Complete obstruction will precipitate emergency hospital admission
Perforation• A cancer invading through the bowel wall may stimulate a
vigorous local inflammatory process resulting in a pericolic abscess which contains the perforation, at least for a while.
• This occurs most often in the recto-sigmoid area and usually presents with left iliac fossa pain and tenderness and a swinging fever.
• The differential diagnosis is acute diverticulitis or a diverticular abscess.
• A carcinoma anywhere in the colon (but rarely in the rectum) may perforate and present as an acute abdomen with peritonitis.
• Occasionally a carcinoma may erode into a nearby organ creating a malignant fistula. Fistulation can occur into stomach, bladder, uterus or vagina, or direct to the skin.
Carcinoma of the left side of the colon
• They are usually of the stenosing variety.• The main symptoms are those of increasing
intestinal obstruction.• This includes lower abdominal pain, which
may be colicky in nature, and abdominal distension.
• The patient may have a change in bowel habit with alternating diarrhoea and constipation
Carcinoma of the sigmoid
• symptoms of intestinal obstruction,• Lesions (carcinomas or polyps) in the lower two-
thirds of the rectum may be perceived as masses of faeces. This stimulates a persistent defaecation response, causing an unpleasant sensation of incomplete evacuation known as tenesmus.
• accompanied by the passage of mucus and blood. • Bladder symptoms are not unusual and, in some
instances, may herald a colovesical fistula.
Carcinoma of the transverse colon
• This may be mistaken for a carcinoma of the stomach because of the position of the tumour together with anaemia and lassitude.
Carcinoma of the caecum and ascending colon
• This may present with the following: anaemia, severe and unyielding to treatment; the presence of a mass in the right iliac fossa; colonoscopy may
be needed to confirm the diagnosis; a carcinoma of the caecum can be the apex of an
intussusception presenting with the symptoms of intermittent obstruction.
cause obstruction unless the ileo-caecal valve is involved. This is because the right colon has a larger diameter than the left
colon and the faecal stream is more fluid. However, occult bleeding from the tumour surface commonly
causes iron deficiency anaemia, and these patients typically present with anaemia and a palpable mass in the right iliac fossa
Metastatic disease
• Patients may present for the first time with liver metastases and an enlarged liver, ascites from carcinomatosis peritonei and, more rarely, metastases to the lung, skin, bone and brain.
COLONOSCOPY
• Offer colonoscopy to patients without major comorbidity, to confirm a diagnosis of colorectal cancer.
• If a lesion suspicious of cancer is detected, perform a biopsy to obtain histological proof of diagnosis, unless it is contraindicated (for example, patients with a blood clotting disorder).
STAGING
• Offer contrast-enhanced computed tomography (CT) of the chest, abdomen and pelvis, to estimate the stage of disease, to all patients diagnosed with colorectal cancer unless it is contraindicated.
Rectal examination
• is mandatory in all suspected cases as a high proportion of carcinomas occur in the lowest 12 cm of the large bowel and can be reached with an examining finger.
• In addition, intraperitoneal tumour spread into the pouch of Douglas may be palpable anteriorly through the rectal wall.
• The degree of fixation of a rectal tumour to surrounding structures can also be evaluated digitally and this gives some indication of potential operative difficulty.
• Finally, the glove should be inspected for blood and mucus as well as stool colour and consistency
Proctoscopy and rigid or flexible sigmoidoscopy
• are usually performed at the initial consultation for all patients complaining of bowel symptoms.
• Lesions can be biopsied through SIGMOIDOSCOPE• A history of rectal bleeding should be fully investigated in patients over
about 45 years and in any patient if the symptoms or signs suggest malignancy.
This applies even if a local cause such as haemorrhoids is found, since these are so common that they will often be found coincidentally.
• Flexible sigmoidoscopy is the investigation of choice for rectal bleeding as the causative lesion has a high probability of being found in the left side of the colon.
If a tumour is found, the rest of the bowel must still be examined for synchronous tumours or further polyps.
• In patients complaining of a change in bowel habit (particularly looser stools) or unexplained anaemia, a bowel lesion could be left or right sided, and thus the entire colon must be examined by colonoscopy or barium enema examination
Colonoscopy
• investigation of choice if colorectal cancer is suspected• provided the patient is fit enough to undergo the bowel
preparation. • picking up a primary cancer but also having the ability to detect
synchronous polyps or even multiple carcinomas, which occur in 5% of cases.
• Ideally, every case should be proven histologically before surgery.
• Full bowel preparation and sedation are necessary. • However, one must be aware of a small risk of perforation and
also the failure to get to the caecum in 10% of cases, even by experienced endoscopists.
Radiology
• Double-contrast barium enema is used when colonoscopy is contraindicated.
• It shows a cancer of the colon as a constant irregular filling defect .
• Ultrasonography is often used as a screening investigation for liver metastases over the size of 1.5 cm, and CT is used in patients with large palpable abdominal masses, to determine local invasion, and is particularly used in the pelvis in the assessment of rectal cancer.
• Spiral CT is particularly useful in elderly patients when contrast enemas or colonoscopy are not diagnostic or are contraindicated.
• Liver and lung metastases are sought, along with any other evidence of spread within the abdomen or to bone.
• CT scanning is the most useful investigation for all of these but liver ultrasound scanning may be more sensitive for detecting small liver metastases.
• If CT is not available, chest X-ray will reveal whether there are lung metastases. • MRI scanning can add important information about the extent of local spread
of rectal cancer to aid treatment planning.• In a patient presenting as an emergency with complete large bowel
obstruction, plain abdominal X-rays often show large bowel dilated by gas down to the level of obstruction and empty of gas beyond it.
• The level is often at the sigmoid colon or recto-sigmoid junction.• CT scan or sigmoidoscopy may confirm the likely diagnosis of carcinoma. • Similarly, an 'instant' Gastrografin enema (i.e. without bowel preparation) can
confirm the diagnosis and at the same time exclude pseudo-obstruction
BLOOD TESTS
• Anaemia often results from a bowel neoplasm,• Abnormal liver function tests suggest substantial liver
metastases.• Raised blood urea can result from rectal lesions
compressing the ureters• Hypokalaemia occasionally results from lesions producing
excess mucin. • Tumour markers are neither sensitive nor specific for a
primary diagnosis of colorectal cancer but carcino-embryonic antigen (CEA) is used to monitor for cancer recurrence.
The test of operability• The abdomen is opened and the tumour assessed for resectability.1 The liver is palpated for secondary deposits, the presence of which is
not necessarily a contraindication to resection because the best palliative treatment for carcinoma of the colon is removal of the tumour.
2 The peritoneum, particularly the pelvic peritoneum, is inspected for signs of small, white, seed-like, neoplastic implantations. Similar changes can occur in the omentum.
3 The various groups of lymph nodes that drain the involved segment are palpated. Their enlargement does not necessarily mean that they are invaded by metastases, because the enlargement may be inflammatory.
4 The neoplasm is examined with a view to mobility and perability. Local fixation, however, does not always imply local invasion because some tumours excite a brisk inflammatory response.
• The operations to be described are designed to remove the primary tumour and its draining locoregional lymph nodes, which may be involved by metastases.
Carcinoma of the caecum
• Is treated when resectable by right hemicolectomy• The abdomen is opened, the peritoneum lateral to the ascending colon is incised
and the incision is carried around the hepatic flexure. • The right colon is elevated, with the leaf of peritoneum containing its vessels
and lymph nodes, from the posterior abdominal wall, taking care not to injure the ureter, spermatic vessels in the male or the duodenum.
• The peritoneum is separated medially near the origin of the ileocolic artery, which is divided together with the right colic artery when this has a separate origin from the superior mesenteric. The mesentery of the last 30 cm of ileum and the leaf of raised peritoneum attached to the caecum, ascending colon and hepatic flexure, after ligation of the mesenteric blood vessels, is divided as far as the proximal third of the transverse colon.
• When it is clear that there is an adequate blood supply at the resection margins, the right colon is resected, and an end-to-end anastomosis is fashioned between the ileum and the transverse colon.
Carcinoma of the hepatic flexure• When the hepatic flexure is involved, the resection must be extended correspondinglyCarcinoma of the transverse colon• When there is no obstruction, excision of the transverse colon and the two flexures
together with the transverse mesocolon and the greater omentum, followed by end-to-end anastomosis, can be used.
• An alternative is an extended right hemicolectomy For lesions of the transverse colon, a transverse colectomy is accomplished by proximal ligation of the middle colic artery (Fig. 68.10). Cancer of the splenic flexure can be treated with a segmental resection in which the middle transverse colon is anastomosed to the middle descending colon. For this procedure, the left colic artery is divided and the middle colic artery is preserved. Mobilization of the splenic flexure requires care to avoid injury to the spleen.
Carcinoma of the splenic flexure or descending colon• The extent of the resection is from right colon to descending colon. • Sometimes, removal of the colon up to the ileum, with an ileorectal anastomosis, is
preferable.
• Carcinoma of the pelvic colon• The left half of the colon is mobilised completely . • So that the operation is radical, the inferior mesenteric artery
below its left colic branch, together with the related paracolic lymph nodes, must be included in the resection.
• This entails carrying the dissection as far as the upper third of the rectum.
• Many surgeons advocate flush ligation of the inferior mesenteric artery on the aorta (high ligation).
• Provided that there is no obstruction, primary anastomosis is the rule. Occasionally, a protecting upstream stoma may be necessary.
• A left hemicolectomy with removal of intestine from the middle transverse to the distal sigmoid colon can be used for tumors of the descending colon (Fig. 68.10). High ligation of the inferior mesenteric artery is necessary in this operation. For cancers of the sigmoid colon, a segmental resection can be P.1111
•performed with ligation of the sigmoid artery near its origin. Rectosigmoid cancers and tumors confined to the upper third of the rectum are removed by an anterior resection. The upper third of the rectum is about 12 to 16 cm from the anal verge and is located above the peritoneal reflection ending proximally where the tenia flare (Fig. 68.11). The pelvic peritoneum is incised circumferentially around the rectum, and the intestine is sharply mobilized from the presacral fascia. Laterally, the middle hemorrhoidal vessels are ligated. Anteriorly, the rectum is mobilized from the seminal vesicles and prostate or the vagina. The mesenteric vessels are divided at the origin of the sigmoid artery or higher, at the origin of the inferior mesenteric artery, if further mobilization of the splenic flexure is required to obtain a tension-free anastomosis.
• Cancers located in the lower third of the rectum, between the anorectal ring and 7 to 8 cm from the anal verge, are reliably treated by abdominoperineal resection.
• The procedure involves wide excision of the rectum to include the lateral attachments and pelvic mesocolon and establishment of a colostomy.
• With the patient in a modified lithotomy position, the abdominal and perineal procedures can be performed simultaneously by two teams or sequentially by one team. Alternatively, the abdominal procedure can be completed with the patient in the supine position, and the perineal portion completed afterward, with the patient turned in the lateral position.
• On opening of the abdomen, evidence of intra-abdominal spread is ascertained. The discovery of extensive disseminated disease may eliminate the need for an abdominoperineal resection because a local excision or fulguration to preserve anal function may be more appropriate for palliation.
• If an abdominoperineal resection is performed, ligation of the superior rectal artery at its origin, along with the superior rectal vein, is required. Occasionally, if extensive nodal disease is present, higher arterial ligation may be necessary. The rectum is mobilized in a fashion similar to that described for an anterior resection, but the dissection is carried down to the pelvic floor muscles, which are excised en bloc with the anus. An end-sigmoid colostomy is brought out through the rectus sheath. Efforts to exclude small intestine from a future radiation field by use of the omentum should be considered. Primary closure of the perineal wound over drains often can be accomplished, but a greater than 50% wound infection rate is to be expected. Use of gracilis or other muscle flaps at the time of perineal wound closure should be considered in patients who have undergone neoadjuvant radiation.
• Cancer of the middle third of the rectum, between 8 and 12 cm from the anal verge (Fig. 68.14), can be managed by various techniques. For these tumors, abdominoperineal resection does not yield results superior to those of other procedures that spare the anal sphincter. Therefore, an effort should be made to maintain intestinal continuity. Low anterior resection is a commonly used technique that involves resection of the middle rectum with primary anastomosis. The introduction of the end-to-end anastomosis stapler has increased the use of this sphincter-saving P.1112
•procedure (Fig. 68.13A). If a transanal reconstruction with a stapler is contemplated, the patient should be placed in the lithotomy position. The initial stages of the operation, with complete mobilization of the rectum to the level of the pelvic floor, are identical to those for an abdominoperineal resection. After removal of the tumor, an end-to-end or end-to-side anastomosis is joined with sutures or staples (Fig. 68.13B and C). After ascertaining an airtight anastomosis, a temporary transverse colostomy or loop ileostomy is recommended for patients in whom preoperative radiation was used, the anastomosis is up to 4 cm from the anal verge, or the integrity of the anastomosis is a concern.
When a growth is found to be inoperable• In the upper part of the left colon, an ileostomy is
performed. • In the pelvic colon, a left iliac fossa colostomy is preferable.• With an inoperable growth in the ascending colon, a bypass
using an ileocolic anastomosis is the best procedure. • A total colectomy needs to be considered for multiple
tumours. • Over 95% of colonic carcinomas can, however, be resected.
PATHOLOGY• 90- 95% are adenocarcinomas• with the remaining histologic types being : squamous cell
carcinomas, adenosquamous carcinomas, lymphomas, sarcomas, and carcinoid tumors.
• Most colonic adenocarcinomas are moderately differentiated or well-differentiated tumors. About 20% of adenocarcinomas are poorly differentiated or
undifferentiated (a poorer prognosis) . • 10-20 % of tumors : mucinous or colloid carcinomas based on
the abundant production of mucin (poorer 5-year survival rate ).• Blood vessel invasion, lymphatic vessel invasion, and the absence
of a lymphocytic response to the tumor (poorer prognosis).
NATURAL HISTORY
• three processes: local invasion, lymphatic spread, and hematogenous spread. • Often local growth of an adenocarcinoma is characterized by
intramural expansion of the tumor into the bowel lumen. Subsequent lateral invasion into the intestinal wall usually progresses in a transverse direction rather than longitudinally and thereby leads to circumferential involvement of the intestine. Although the incidence of lymphatic metastasis increases with extent of local invasion through the intestinal wall, 10% to 20% of patients with cancer limited to the submucosa are found to have positive lymph nodes.
LOCAL GROWTH
• characterized by intramural expansion of the tumor into the bowel lumen.
• Subsequent lateral invasion into the intestinal wall usually progresses in a transverse direction rather than longitudinally and thereby leads to circumferential involvement of the intestine.
• Although the incidence of lymphatic metastasis increases with extent of local invasion through the intestinal wall, 10% to 20% of patients with cancer limited to the submucosa are found to have positive lymph nodes.
• Lymphatic spread is sequential, first to mesenteric nodes and then onward to para-aortic nodes.
• Occasionally lymph node involvement is directly responsible for the clinical presentation.
• For example, para-aortic nodes may present as a palpable mass or cause duodenal obstruction.
• Other enlarged nodes may compress the bile ducts in the porta hepatis causing jaundice.
HEMATOGENOUS SPREAD
• The liver is the most common site • The liver is the first capillary network exposed to tumor
emboli traveling through the portal system and represents the major site of venous drainage of the colon and upper rectum.
• The liver can be the sole site of tumor metastasis, as evidenced by the successful resection of liver metastases for cure in selected patients.
• By contrast, the lower rectum has a dual drainage system, draining into the portal system and the vena cava by way of the middle and inferior hemorrhoidal veins, respectively.
• Isolated lung metastases can develop from lower rectal tumors when tumor emboli travel through the systemic venous drainage system.
• second most common site of metastasis from colorectal tumors.
• Tumor involvement of other sites in the absence of liver and lung metastases is unusual.
• In certain circumstances, isolated bone metastases to the sacrum or vertebral bodies can arise when tumor emboli travel through portal-vertebral venous communications known as the Batson plexus.
• intraluminal or extraluminal exfoliation of tumor cells with subsequent implantation.
• Tumor implantation may occur during surgical resection; spillage of tumor cells can cause recurrences in bowel anastomoses, abdominal incisions, or other intra-abdominal sites.
• When tumors penetrate the intestinal wall, shed tumor cells can be implanted intraperitoneally and cause peritoneal carcinomatosis.
MANAGEMENT
• Surgical resection : the main treatment for colorectal carcinoma.
• For tumours localised to the bowel wall, resection offers an excellent chance of complete cure; for tumours at a more advanced stage, chemotherapy and radiotherapy may be required to increase the chances of cure.
• For rectal cancers, chemoradiotherapy may be given preoperatively (known as neoadjuvant therapy) to shrink the tumour to improve the chances of successful surgical removal.
• For advanced disease, even with very extensive tumours, palliative resection is usually worthwhile to relieve obstruction or to prevent continuing blood loss.
• In frail patients with metastatic disease in whom any surgery is too risky, a stent can often be placed endoscopically to hold open the bowel and relieve obstruction
RECTUM
• The rectum is approximately 12 to 15 cm in length. • Three distinct submucosal folds, the valves of Houston, extend
into the rectal lumen.• Posteriorly, the presacral fascia separates the rectum from the
presacral venous plexus and the pelvic nerves.• At S4, the rectosacral fascia (Waldeyer's fascia) extends forward
and downward and attaches to the fascia propria at the anorectal junction.
• Anteriorly, Denonvilliers' fascia separates the rectum from the prostate and seminal vesicles in men and from the vagina in women.
• The lateral ligaments support the lower rectum.
• The surgical anal canal measures 2 to 4 cm in length.• It begins at the anorectal junction and terminates at the anal
verge. • The dentate or pectinate line marks the transition point between
columnar rectal mucosa and squamous anoderm. • The 1 to 2 cm of mucosa just proximal to the dentate line shares
histologic characteristics of columnar, cuboidal, and squamous epithelium and is referred to as the anal transition zone.
• The dentate line is surrounded by longitudinal mucosal folds, known as the columns of Morgagni, into which the anal crypts empty. These crypts are the source of cryptoglandular abscesses
• In the distal rectum, the inner smooth muscle is thickened and comprises the internal anal sphincter that is surrounded by the subcutaneous, superficial, and deep external sphincter.
• The deep external anal sphincter is an extension of the puborectalis muscle.
• The puborectalis, iliococcygeus, and pubococcygeusmuscles form the levator ani muscle of the pelvic floor
• The rectum begins where the taenia coli of the sigmoid colon join to form a continuous outer longitudinal muscle layer at the level of the sacral promontory.
• The rectum follows the curve of the sacrum, to end at the anorectal junction.
• The puborectalis muscle encircles the posterior and lateral aspects of the junction, creating the anorectal angle (normally 120°).
• The rectum has three lateral curvatures: the upper and lower are convex to the right, and the middle is convex to the left.
• On the luminal aspect, these three curves are marked by semicircular folds (Houston’s valves).
• That part of the rectum that lies below the middle valve has a much wider diameter than the upper third and is known as the ampulla of the rectum.
The adult rectum is approximately 12–18 cm in length and is conveniently divided into three equal parts:
• the upper third, which is mobile and has a peritoneal coat;
• the middle third where the peritoneum covers only the anterior and part of the lateral surfaces;
• the lowest third, which lies deep in the pelvis surrounded by fatty mesorectum and has important relations to fascial layers.
• The lower third of the rectum is separated by a fascial condensation – Denonvilliers’ fascia – from the prostate/vagina in front,
• and behind by another fascial layer – Waldeyer’s fascia – from the coccyx and lower two sacral vertebrae
• These fascial layers are surgically important as they are a barrier to malignant invasion
Blood supplyThe superior rectal artery • the direct continuation of the inferior mesenteric artery• the main arterial supply of the rectum.• The arteries and their accompanying lymphatics lie within the loose fatty tissue
of the mesorectum, surrounded by a sheath of connective tissue (the mesorectal fascia).
The middle rectal artery • arises on each side from the internal iliac artery • and passes to the rectum in the lateral ligaments.• It is usually small and breaks up into several terminal branches.The inferior rectal artery• arises on each side from the internal pudendal artery as it enters Alcock’s canal.• It hugs the inferior surface of the levator ani muscle as it crosses the roof of the
ischiorectal fossa to enter the anal muscles
Venous drainage• The venous drainage of the rectum parallels the arterial supply.• The superior rectal vein drains into the portal system via the inferior
mesenteric vein.• The middle rectal vein drains into the internal iliac vein. • The inferior rectal vein drains into the internal pudendal vein, and
subsequently into the internal iliac vein. • A submucosal plexus deep to the columns of Morgagni forms the
hemorrhoidal plexus and drains into all three veins. • The superior haemorrhoidal veins draining the upper half of the anal
canal above the dentate line pass upwards to become the rectal veins: these unite to form the superior rectal vein, which later becomes the inferior mesenteric vein. This forms part of the portal venous system and ultimately drains into the splenic vein.
LYMPHATIC DRAINAGE
• Lymphatic drainage of the rectum parallels the vascular supply. • Lymphatic channels in the upper and middle rectum drain
superiorly into the inferior mesenteric lymph nodes. • Lymphatic channels in the lower rectum drain both superiorly
into the inferior mesenteric lymph nodes and laterally into the internal iliac lymph nodes.
• Proximal to the dentate line, lymph drains into both the inferior mesenteric lymph nodes and the internal iliac lymph nodes.
• Distal to the dentate line, lymph primarily drains into the inguinal lymph nodes, but also can drain into the inferior mesenteric lymph nodes and internal iliac lymph nodes.
Lymphatic drainage
• The lymphatics of the mucosal lining of the rectum communicate freely with those of the muscular layers.
• The usual drainage flow is upwards, and only to a limited extent laterally and downwards.
• For this reason, surgical ablation of malignant disease concentrates mainly on achieving wide clearance of proximal lymph nodes.
• However, if the usual upwards routes are blocked (by carcinoma), flow can reverse, and it is then possible to find metastatic lymph nodes on the side walls of the pelvis (along the middle rectal vessels) or even in the inguinal region (along the inferior rectal artery).
NERVE SUPPLY
• Both sympathetic and parasympathetic nerves innervate the anorectum.
• Sympathetic nerve fibers are derived from L1–L3 and join the preaortic plexus.
• The preaortic nerve fibers then extend below the aorta to form the hypogastric plexus, which subsequently joins the parasympathetic fibers to form the pelvic plexus.
• Parasympathetic nerve fibers are known as the nervi erigentes and originate from S2–S4. These fibers join the sympathetic fibers to form the pelvic plexus.
• Sympathetic and parasympathetic fibers then supply the anorectum and adjacent urogenital organs.
• The internal anal sphincter is innervated by sympathetic and parasympathetic nerve fibers; both types of fibers inhibit sphincter contraction.
• The external anal sphincter and puborectalis muscles are innervated by the inferior rectal branch of the internal pudendal nerve.
• The levator ani receives innervation from both the internal pudendal nerve and direct branches of S3 to S5.
• Sensory innervation to the anal canal is provided by the inferior rectal branch of the pudendal nerve.
• Although the rectum is relatively insensate, the anal canal below the dentate line is sensate.
SURGERY
• Surgical resection is the main treatment • For tumours localised to the bowel wall, resection
offers an excellent chance of complete cure• for tumours at a more advanced stage, chemotherapy
and radiotherapy may be required to increase the chances of cure.
• For rectal cancers, chemoradiotherapy may be given preoperatively (known as neoadjuvant therapy) to shrink the tumour to improve the chances of successful surgical removal.
• There is a trend for patients with cancers to be referred for multidisciplinary team discussion, where surgeons, oncologists, radiologists, palliative care doctors and colorectal specialist nurses discuss all aspects of the case and formulate a plan of management.
• For advanced disease, even with very extensive tumours, palliative resection is usually worthwhile to relieve obstruction or to prevent continuing blood loss.
• In frail patients with metastatic disease in whom any surgery is too risky, a stent can often be placed endoscopically to hold open the bowel and relieve obstruction
The principles of colorectal tumour resection
• Operative access is achieved by laparotomy, usually via a long midline incision. In specialist units, laparoscopic or laparoscopic-assisted surgery is sometimes employed
• The affected segment of bowel is removed with a margin of normal bowel.• A minimum of 5 cm clear each side of the tumour removes local lymphatics
likely to be involved.• In practice, the precise lines of resection are determined by the distribution of
mesenteric blood vessels . For example, lesions in the ascending colon are treated by removal of the whole right colon (right hemicolectomy), as the right colic artery has to be ligated in order to remove a section of the right colon. There must be a good blood supply to the cut ends of bowel to ensure healing
• A wedge-shaped section of colonic mesentery is removed with the bowel. This contains the primary field of lymph node drainage. If there are other obvious lymph node metastases, these are usually included in the resection specimen
Rectal cancers• are a special case • The preferred operation is a sphincter-saving anterior resection of
rectum; provided the lower edge of the tumour is 1-2 cm above the anal sphincters, the sphincter can be preserved in most patients.
• This operation involves excising the tumour with an appropriate length of bowel plus an intact envelope of fat around it (the mesorectum containing local lymph nodes).
• Ideally this is to a distance of 5 cm below the primary tumour, although a margin of 1 cm is acceptable when the tumour is very low.
• The proximal end of bowel is then anastomosed to the distal stump.
• Alternatively, a pelvic reservoir is created using a J-pouch technique
• This has been shown to reduce the frequency and urgency of defaecation without increasing surgical complications.
• A temporary ileostomy or colostomy is sometimes used to aid healing of a low anastomosis.
• If the sphincter is involved, the entire rectum and anus has to be removed via an abdomino-perineal resection (APR), with the proximal end of bowel brought out as a colostomy
• In most cases, the two cut ends of bowel can be joined (anastomosed) without the need for a temporary or permanent colostomy.
• The method used to rejoin the bowel depends on the site of the anastomosis, the preference of the surgeon and whether there is much disparity in diameter between the ends to be joined.
Follow-up after apparently curative resection
• Offer patients regular surveillance with:a minimum of two CTs of the chest, abdomen,
and pelvis in the first 3 years and• regular serum carcinoembryonic antigen tests
(at least every 6 months in the first 3 years).
Treatment options
• vary and are assessed taking into account the following variables:
• Tumour size• Stage of diagnosis• The location of the tumour in the colon or
rectum• The risk of the cancer returning• The physical health of the patient
MANAGEMENT
• The principles of the operation involve radical excision of the neoplasm, removal of the mesorectum and high proximal ligation of the inferior mesenteric lymphovascular pedicle.
• Once the rectum has been adequately mobilised, it is removed, and the rectal stump is washed out.
• Restoration of continuity by direct end-to-end anastomosis (manually or by stapling) must be carried out by a meticulous technique to reduce the risks of suture line breakdown
notes
anatomy
• In the colon, the outer longitudinal muscle is separated into three teniae coli, which converge proximally at the appendix and distally at the rectum, where the outer longitudinal muscle layer is circumferential.
• In the distal rectum, the inner smooth muscle layer coalesces to form the internal anal sphincter.
• The intraperitoneal colon and proximal one third of the rectum are covered by serosa; the mid and lower rectum lack serosa
• The rectosigmoid junction is found at approximately the level of the sacral promontory
• and is arbitrarily described as the point at which the three teniae coli coalesce to form the outer longitudinal smooth muscle layer of the rectum.
• The cecum is the widest diameter portion of the colon (normally 7.5 to 8.5 cm) and has the thinnest muscular wall.
• As a result, the cecum is most vulnerable to perforation and least vulnerable to obstruction.
• The ascending colon usually is fixed to the retroperitoneum.• The hepatic flexure marks the transition to the transverse colon. • The intraperitoneal transverse colon is relatively mobile, but is tethered by the
gastrocolic ligament and colonic mesentery. • The greater omentum is attached to the anterior/superior edge of the
transverse colon.• These attachments explain the characteristic triangular appearance of the
transverse colon observed during colonoscopy.
• The cecum extends approximately 6 to 8 cm below the ileocecal valve (where the terminal ileum enters the posteromedial aspect of the cecum) .
• From the cecum, the right colon ascends to the hepatic flexure (approximately 15 cm).
• The hepatic flexure is anterior to the inferior pole of the right kidney and overlies the second portion of the duodenum.
• The hepatic flexure is marked by medial, anterior, and downward angulation of the colon.
• When the right colon is mobilized during a colectomy, care must be taken to avoid injury to the underlying duodenum.
• Only the anterior surface of the right colon is invested with peritoneum; laterally, the white line of Toldt marks the extent of the peritoneal covering and serves as an important landmark during surgical mobilization of the colon.
• The transverse colon stretches from the hepatic flexure to the splenic flexure and is the longest segment of colon (between 30 and 60 cm).
• The transverse colon is suspended by the transverse mesocolon and is completely intraperitoneal.
• It is the most mobile portion of the colon.• The greater omentum descends from the greater curve of the stomach in front of the
transverse colon and then ascends to attach to the transverse colon on its anterosuperior edge.
• The splenic flexure is situated high in the left upper quadrant, more cephalad than the hepatic flexure.
• The flexure lies anterior to the mid-left kidney and abuts the lower pole of the spleen. • There are attachments from the colon to the diaphragm and spleen (phrenocolic and
splenocolic ligaments) and these must be carefully divided during mobilization of the splenic flexure to avoid splenic injury.
• The descending colon is approximately 25 cm long and courses from the splenic flexure to its junction with the sigmoid colon at the pelvic brim.
• It lies anterior to the left kidney and, like the right colon, the anterior, lateral, and medial portions of the descending colon are covered by peritoneum.
• The sigmoid colon extends from the pelvic brim to the sacral promontory, where it continues as the rectum and generally measures 30 to 40 cm in length. The rectosigmoid junction is marked by the convergence of the colonic taenia. The sigmoid colon is extremely mobile and has a generous mesentery that extends along the pelvic brim from the iliac fossa across the sacroiliac joint to the second or third sacral segment. Because of its mobile mesentery, the sigmoid colon can twist and cause an obstruction, termed sigmoid volvulus. The left ureter runs in the intersigmoid fossa. When the sigmoid colon is being mobilized, the left ureter should be identified to avoid inadvertent injury. Preoperative placement of urinary stents can be useful for locating the ureter intraoperatively in complex, reoperative pelvic surgery.
• The rectum, which is 12 to 15 cm long, begins at the level of the sacral promontory and extends to the anorectal ring.
• The rectum proceeds posterior and caudal along the curvature of the sacrum and coccyx, passing through the levator ani muscles, at which point it turns abruptly caudal and posterior, becoming the anal canal.
• Anterior to the rectum are the uterine cervix and posterior vaginal wall in women, and the bladder and prostate in men.
• Posteriorly, the rectum occupies the sacral concavity where the median sacral vessels, presacral veins, and sacral nerves run, all of which are invested in the presacral fascia.
• The rectum is marked by three curves that correspond to the valves of Houston.
• The valves are only visible from the lumen and separate the lower third, middle third, and upper third of the rectum, important landmarks when the location of a rectal abnormality is established endoscopically (the lower rectal valve is at 7 to 8 cm from the anal verge, middle rectal valve at 9 to 11 cm, and upper rectal valve at 12 to 13 cm).3
• The anterior and lateral surfaces of the upper third of the rectum are intraperitoneal, whereas only the anterior surface of the middle third of the rectum is intraperitoneal in location. The lower third of the rectum is entirely extraperitoneal. The mesorectum is the term used to describe the areolar tissue surrounding the rectum that contains nerves, lymphatics, and terminal branches of the superior hemorrhoidal branch of the inferior mesenteric artery. Although it invests the rectum circumferentially, the mesorectum is most prominent posterior to the rectum. It is invested by the fascia propria of the rectum, a continuation of the parietal endopelvic fascia (Fig. 63.3). A total mesorectal excision entails removal of the entire rectum without violating the fascia propria of the rectum. This is accomplished by mobilizing the rectum using the plane between the fascia propria of the rectum and the presacral fascia.
Blood supply• The superior mesenteric artery arises from the aorta, runs posterior to the
pancreas, and passes anterior to the third portion of the duodenum.• The inferior mesenteric artery arises from the anterior surface of the aorta, typically
3 to 4 cm above the aortic bifurcation, • The inferior mesenteric artery gives rise to the left colic artery and sigmoidal
branches, then continues in the sigmoid mesentery, and after crossing the left iliac vessels, is renamed the superior hemorrhoidal artery.
• The superior hemorrhoidal artery descends behind the rectum and splits into right and left branches in the mesorectum.
• The middle and inferior hemorrhoidal arteries arise from the hypogastric arteries and supply the distal two thirds of the rectum.
• The presence of the middle rectal artery, in particular, can be variable.• A series of arterial arcades along the mesenteric border of the entire colon, known
as the marginal artery of Drummond, connect the superior mesenteric and inferior mesenteric arterial systems.
• The marginal artery may be attenuated or absent at the distal transverse colon/splenic flexure, the delineation between the midgut and hindgut, and thus ischemic colitis most commonly affects this region.
• The splenic flexure marks the transition from the transverse colon to the descending colon. The attachments between the splenic flexure and the spleen (the lienocolic ligament) can be short and dense, making mobilization of this flexure during colectomy challenging. The descending colon is relatively fixed to the retroperitoneum. The sigmoid colon is the narrowest part of the large intestine and is extremely mobile. Although the sigmoid colon usually is located in the left lower quadrant, redundancy and mobility can result in a portion of the sigmoid colon residing in the right lower quadrant. This mobility explains why volvulus is most common in the sigmoid colon and why diseases affecting the sigmoid colon, such as diverticulitis, may occasionally present as right-sided abdominal pain. The narrow caliber of the sigmoid colon makes this segment of the large intestine the most vulnerable to obstruction.
• The sentinel lymph nodes are the first one to four lymph nodes to drain a specific segment of the colon, and are thought to be the first site of metastasis in colon cancer. The use of sentinel lymph node dissection and analysis in colon cancer remains controversial.
• Sodium is absorbed actively via a Na-K ATPase.• The colon can absorb up to 400 mEq of sodium
per day.• Water accompanies the transported sodium and
is absorbed passively along an osmotic gradient.• Potassium is actively secreted into the colonic
lumen and absorbed by passive diffusion. • Chloride is absorbed actively via a chloride–
bicarbonate exchange.
• Continence• The maintenance of fecal continence is at least as complex as the mechanism of
defecation. Continence requires adequate rectal wall compliance to accommodate the fecal bolus, appropriate neurogenic control of the pelvic floor and sphincter mechanism, and functional internal and external sphincter muscles. At rest, the puborectalis muscle creates a "sling" around the distal rectum, forming a relatively acute angle that distributes intra-abdominal forces onto the pelvic floor. With defecation, this angle straightens, allowing downward force to be applied along the axis of the rectum and anal canal. The internal and external sphincters are tonically active at rest. The internal sphincter is responsible for most of the resting, involuntary sphincter tone (resting pressure). The external sphincter is responsible for most of the voluntary sphincter tone (squeeze pressure). Branches of the pudendal nerve innervate both the internal and external sphincter. Finally, the hemorrhoidal cushions may contribute to continence by mechanically blocking the anal canal. Thus, impaired continence may result from poor rectal compliance, injury to the internal and/or external sphincter or puborectalis, or nerve damage or neuropathy.