Sigmoid colon

The sigmoid colon is the S-shaped terminal segment of the large intestine situated between the descending colon and the rectum. It continues the absorption of water and electrolytes from intestinal contents while secreting mucus to facilitate passage of increasingly dehydrated faeces into the rectum. Owing to its characteristic curved course and variable length, it is one of the most mobile segments of the large intestine.

Location, Extent and Parts

Traditionally, the sigmoid colon may be divided into two parts: the iliac part and the pelvic part.

1. iliac part of sigmoid colon

The iliac part (iliac colon) lies within the left iliac fossa and measures approximately 12-15 cm in length. It begins at the level of the iliac crest as a continuation of the descending colon and descends infero-medially in front of the iliacus and psoas major muscles to reach the superior aperture of the lesser pelvis (pelvic inlet), where it becomes continuous with the pelvic part of the sigmoid colon. Unlike the pelvic part, the iliac segment is usually covered by peritoneum only on its anterior and lateral surfaces and is therefore relatively less mobile.

2. Pelvic part of sigmoid colon

The pelvic part forms the characteristic sigmoid loop and averages approximately 40 cm in length, although considerable anatomical variation may occur. It begins at the pelvic inlet and initially passes transversely across the anterior surface of the sacrum toward the right side of the pelvis. It then curves upon itself, turns toward the left, and reaches the midline at the level of the third sacral vertebra (S3), where it continues as the rectum. Although normally located within the pelvis, its long mesentery allows considerable mobility, and it may extend into the lower abdominal cavity.

Peritoneal relations and sigmoid mesocolon

The pelvic portion of the sigmoid colon is completely invested by visceral peritoneum and is suspended from the posterior pelvic wall by the sigmoid mesocolon. This mesentery provides the colon with a significant degree of mobility while simultaneously transmitting its blood vessels, lymphatics and nerves.

The sigmoid mesocolon is longest at the centre of the sigmoid loop and gradually shortens toward both ends, becoming continuous with the posterior abdominal wall at the junctions with the descending colon proximally and the rectum distally. Consequently, the central portion of the sigmoid colon is highly mobile, whereas its proximal and distal ends are relatively fixed.

The root of the sigmoid mesocolon has an inverted V-shaped attachment extending from the region of the left common iliac vessels to the midline at the level of S3.

Anatomical relations

1. Posterior relations

Posteriorly, the sigmoid colon is related to the external iliac vessels, left ureter, left piriformis muscle, and the left sacral plexus. These structures lie behind the mesocolon and may be endangered during pelvic or colorectal surgery.

2. Anterior relations

Anteriorly, the sigmoid colon is separated from the urinary bladder in males, and the uterus in females, by loops of small intestine.

External Features

Like other portions of the colon, the sigmoid colon possesses the characteristic features of the large intestine, including:

• Three taeniae coli

• Haustra (sacculations)

• Appendices epiploicae

At the rectosigmoid junction, the three taeniae coli broaden and merge to form the continuous longitudinal muscle layer of the rectum.

Blood supply

• The arterial supply of the sigmoid colon is derived primarily from the sigmoid arteries, which arise from the inferior mesenteric artery (IMA).

• Venous drainage follows the arterial pattern. Blood from the sigmoid colon drains into the sigmoid veins, which empty into the inferior mesenteric vein. The inferior mesenteric vein subsequently joins the splenic vein and contributes to the portal venous system, carrying blood to the liver.

Innervation

• Sympathetic fibres originate from spinal cord segments L1-L2, travel through the lumbar splanchnic nerves, and reach the bowel via the inferior mesenteric and hypogastric plexuses. Sympathetic stimulation generally inhibits intestinal motility, decreases secretory activity, and promotes vasoconstriction.

• Parasympathetic innervation is supplied by the pelvic splanchnic nerves (S2-S4). These fibres stimulate peristalsis, increase glandular secretion, and facilitate defecation.

• Visceral afferent fibres accompany autonomic pathways and convey sensations of distension, spasm, and inflammation to the central nervous system.

Clinical correlates

• Sigmoid volvulus: Twisting of the sigmoid colon around its mesentery, causing bowel obstruction and possible ischaemia.

• Diverticular disease: Commonly affects the sigmoid colon and may lead to diverticulitis, presenting with left lower quadrant pain.

• Surgical relevance: The left ureter lies posterior to the sigmoid mesocolon and is at risk during colorectal surgery.