Large intestine

THE ABDOMEN 1 65

Barium enema (Fig. 5.20) If the lumen of the appendix is patent, it may f i ll on bariumenema examination. The lumen is often obliterated in patients past mid-adulthood. To f i ll the appendix the patient should be supine because its orifice is on the posterior aspect of the caecum. Some elevation of the head is also helpful.

CT and MRI The normal appendix can usually be identified arising from the caecum inferior to the insertion of the terminal ileum.

Appendiceal abscess Because the appendix is on a mesentery and mobile, pus from an infected appendix may cause abscess formation in a variety of locations. Pus may travel inferiorly to the pelvic peritoneum to the rectovesical (or rectouterine) pouch. Pus may also travel superiorly in the right paracolic gutter to the intrahepatic spaces (see section on the peritoneum).

THE LARGE INTESTINE (Fig. 5.20) (see also Rectum and Anus in the section on the pelvis) The length of the large intestine is very variable, w i th an average length of 1.5 m. It is wider in diameter than the small intestine, w i th a maximum diameter of the caecum at 9 cm and the transverse colon at 5.5 cm.

As far as the rectum the colon is marked by taeniae coli. These are three flattened bands of longitudinal muscle that represent the longitudinal muscle layer of the colon. The taeniae converge on the appendix proximally and the

Fig. 5.20 Barium study of the colon.

1. Rectum 2. Valve of Houston (lateral mucosal fold) 3. Sigmoid colon 4. Descending colon 5. Splenic flexure 6. Transverse colon 7. Hepatic flexure 8. Ascending colon 9. Caecum 10. Caecal pole 11. Base of appendix 12. Tip of appendix

rectum distally, and these structures have a complete longitudinal muscle layer. The taeniae coli are about 30 cm shorter than the colon and cause the formation of sacculations along its length. On radiographs these give rise to the appearance of incomplete septa, called haustra.

Scattered over the free surface of the large intestine, except for the caecum and rectum, are fat-filled peritoneal tags called appendices epiploicae. These are especially numerous in the sigmoid colon. Arteries supplying these perforate the muscle wall. Mucous membrane may herniate through these vascular perforations, giving rise to diverticulosis.

The caecum is a blind pouch of large bowel proximal to the ileocaecal valve. It is approximately 6 cm long and usually has its own mesentery, making it mobile and easily distensible.

The ascending colon runs from the ileocaecal valve to the inferior surface of the liver, where it turns medially into the hepatic flexure.

The transverse colon runs from the hepatic flexure across the midline to the splenic flexure.

The descending colon runs from the splenic flexure inferiorly to the sigmoid colon. Peritoneal attachments of the colon (see Fig. 5.17) The ascending and descending parts of the colon are usually retroperitoneal (covered anteriorly and on both sides by peritoneum). The peritoneal spaces lateral to these are called the paracolic gutters and may act as a route of spread of pus in peritoneal infection. Occasionally the ascending colon has a mesentery. The caecum does not have a mesentery, but is completely invested in peritoneum and is relatively mobile.

The transverse colon, however, always has a mesentery, the mesocolon, on which it hangs in a loop between the hepatic and splenic flexures, which are fixed points. The splenic flexure is attached to the diaphragm by the phrenicocolic ligament. The convexity of the greater curve of the stomach lies in the concavity of the loop of transverse colon. The gastrocolic ligament attaches the stomach and transverse colon. This continues below the transverse colon as the greater omentum (see section on the peritoneum).

The sigmoid colon also has a mesentery. This is attached to the posterior abdominal wall to the left of the midline in an inverted V shape whose limbs diverge from the bifurcation of the common iliac artery over the sacroiliac (SI) joint at the pelvic brim.

The rectum has peritoneum anteriorly and laterally in its upper third and anteriorly only in its middle third. The lower third of the rectum is below the pelvic peritoneum. (The sigmoid colon, rectum and anus are described in Chapter 6).

Relations of the colon

The parts of the colon that are retroperitoneal have as their posterior relations the structures of the posterior abdominal wall, that is, the psoas and iliac muscles, the quadratic lumborum muscles and the kidney. The more mobile transverse and sigmoid colon are related posteriorly to loops of small intestine. The colon is an anterior structure in the abdomen (cf. development) and has the anterior abdominal wall as an anterior relation, particularly when f u l l. The liver, gallbladder and spleen overlap superiorly, and the sigmoid and rectum are related anteriorly to the bladder and retrovesical structures in the male and the uterus in the female. The arterial supply of the colon (Figs 5.21-5.24) (see also rectum in pelvic section and superior and inferior mesenteric arteries.) That part of the colon derived from the midgut (i.e. to the midtransverse colon) is supplied by the superior mesen¬ teric artery as follows: • The ileocolic artery (a continuation of the main trunk of the superior mesenteric artery) supplies the caecum, appendix and the beginning of the ascending colon. • The right colic artery supplies the remainder of the ascending colon. • The middle colic artery supplies the transverse colon to its midpoint. The inferior mesenteric artery supplies the colon as far as the upper rectum as follows: • The left colic artery to the descending colon; • The sigmoid artery to the sigmoid colon; and • The superior rectal (superior haemorrhoidal) artery to the upper rectum. Each of these vessels anastomoses w i th its neighbour forming a marginal artery (of Drummond) close to the colon. The vessels that enter the bowel are, however, end arteries.

Venous drainage of the colon

Veins corresponding w i th the arteries drain to the superior and inferior mesenteric veins.

Lymphatic drainage of the colon

Lymph drains to nodes near the bowel wall, which drain to nodes in the mesentery and retroperitoneum along w i th the mesenteric vessels. • The drainage of the right colon to midtransverse colon is w i th the superior mesenteric vessels to the peripancreatic nodes and superior mesenteric group of para-aortic nodes. • The drainage of the left side of transverse and left colon is along the inferior mesenteric vessels to the inferior mesenteric nodes at the origin of the inferior mesenteric artery at the level of the third lumbar vertebra.

THE ABDOMEN 167

Fig. 5.21 Superior mesenteric angiogram. The inferior pancreaticoduodenal artery (not shown) also arises from the proximal superior mesenteric artery and runs superiorly.

1. Catheter in aorta 2. Superior mesenteric artery 3. Jejunal branches 4. Ileal branches 5. Terminal superior mesenteric artery 6. Ileocolic artery 7. Right colic artery 8. Middle colic artery running superiorly 9. Contrast-filled bladder

Fig. 5.22 Branches of the superior mesenteric artery.

168 ANATOMY FOR DIAGNOSTIC IMAGING

Fig. 5.23 Inferior mesenteric angiogram.

1. Catheter in right common iliac artery 2. Catheter in aorta 3. Inferior mesenteric artery 4. Left colic artery 5. Sigmoid arteries 6. Marginal artery of Drummond 7. Superior rectal artery and branches 8. Middle rectal artery (filling by reflux: a branch of the internal iliac artery) 9. Gas in ascending colon 10. Descending colon 11. Sigmoid colon 12. Rectum

Development of the colon

In the fifth fetal week the midgut herniates into the umbilical cord, w i th the vitellointestinal duct at the apex of the hernia. At the tenth week the loops of gut return to the abdominal cavity, proximal bowel before distal and rotating 270O anticlockwise as it does so. This results in the jejunum being to the left and deeper than the colon, and the caecum being to the right and superficial.

In malrotation the bowel returns without twisting and the small intestine lies on the right and the colon on the left. The small intestinal mesentery is very short and is prone to volvulus.

In exomphalos, midgut herniation at the umbilicus persists at birth.

Radiological features of the colon

Plain films of the abdomen (see Fig. 5.1) Gas w i t h in the colon outlines the colon or parts of it. The sacculation of the colon by the taeniae coli gives rise to septa called haustra. The haustra are fixed anatomical structures in the proximal colon, but in the distal colon require active contraction for their formation. Haustra may be absent distal to the midtransverse colon.

The upper limit of normal diameter of the transverse colon on plain films is taken to be 5.5 cm, and of the caecum 9 cm. Beyond these limits, in the right clinical setting, there is a risk of caecal perforation. Numerous gas-fluid levels may be normal and 18% of normal films have f l u id levels in the caecum.

As it has a mesentery, the transverse colon is mobile and may interpose itself between the liver and the right hemi¬ diaphragm in the normal subject. The resulting intracolic gas seen between the liver and diaphragm on an abdominal radiograph mimics a pneumoperitoneum. This is called Chilaiditis syndrome and is commoner in patients w i th chronic lung disease.

In intestinal obstruction, distinguishing dilated loops of small intestine from dilated loops of colon is based on several anatomical features (see Table 5.2).

Table 5.2 Anatomical features of small and large intestine in intestinal obstruction

Small intestine Colon

Position of loops Central

Peripheral Septa Complete Incomplete (valvulae conniventes) (haustra) Number of loops Several Few

Diameter < 5 cm > 5 cm

Solid faeces Absent May be present Double-contrast barium-enema examination (Fig. 5.20) The entire colon and appendix may be outlined. The technique for filling the colon w i th barium and air requires an understanding of anatomy. Because the transverse colon, for example, hangs anteriorly between the relatively posteriorly positioned splenic and hepatic flexures, this is easiest to f i ll when the patient is prone. Resumption of a supine position allows filling of the hepatic flexure and the ascending colon w i th barium. The junction of the caecum w i th both the appendix and the ileum is posterior, and these therefore f i ll w i th barium in the supine position.

Visualization of the sigmoid colon may require oblique views and caudally angled views to overcome the problem of overlapping loops. Similarly, views of the ileocaecal area are best obtained w i th the patient's left side raised because of the posteromedial position of this junction.

The haustra can be seen well on double-contrast views of the colon. Gaseous distension may obliterate these distally as far as the midtransverse colon.

Small mucous glands in the mucosa of the colon (crypts of Lieberkuhn) may f i ll w i th barium on good doublecontrast views. These are up to 1 mm deep and perpendicular to the colonic wall on tangential views, and on 'en-face' views are seen as thin, transverse, parallel lines w i th short intercommunicating branches called innominate lines.

Lymphoid follicles are visible in 13% of adults on double-contrast barium studies. These are low elevations of 1-3 mm in diameter. They are larger in the rectum and up to 4 mm in diameter are considered normal.

A barium enema is used sometimes in children to diagnose malrotation of the bowel (a barium meal is also used). The caecum may be high in partial defects or on the left side of the abdomen in complete malrotation.

Angiography For f u ll evaluation of the blood supply of the colon both superior and inferior mesenteric arteries must be shown (see Figs 5.21 and 5.23).

CT of the abdomen (see Figs 5.2-5.4; 5.11; 5.55-5.60) The caecum and ascending colon can be seen anterior to the muscles of the posterior abdominal wall on the right side. The right paracolic gutter lies lateral to the ascending colon. The right infracolic space lies medially. The hepatic flexure can be seen lateral to the second part of the duodenum inferior to right lobe of liver.

The transverse colon varies in its position because of its variable length and its mobility. Fat, blood vessels and lymph nodes may be seen in the mesocolon. The splenic flexure is seen behind the greater curvature of the stomach and the anterior splenic tip. The descending colon is seen on the muscles of the posterior abdominal wall on the left side. The left paracolic gutter lies lateral to the descending colon and the left infracolic space is medial. The sigmoid