Left hemicolectomy (left colectomy) is the surgical removal of the left side of the large bowel (see the first figure below); sigmoid colectomy is the surgical removal of the sigmoid colon (see the second figure below). These operations are mostly performed for cancers of the left colon and sigmoid colon, though they have several other indications, as described later in this article.
In most instances, surgical treatment of tumors of the left colon requires a left hemicolectomy, which involves taking the inferior mesenteric blood supply, along with its branches (left colic artery and sigmoid arteries), which supply the splenic flexure to the proximal sigmoid colon. Tumors of the sigmoid colon can be removed by means of a sigmoid resection, which encompasses the distal descending colon and the sigmoid colon, sacrificing the sigmoid and superior rectal arteries. [1, 2, 3]
Indications for left hemicolectomy are as follows:
Left-side colon cancer without metastasis
Left-side colon cancer with metastasis with complications such as bleeding or obstruction
Left colonic malignant polyp (T1 lesion) that, after polypectomy, reveals high-risk pathologic features suggesting a high possibility of lymph node metastasis
High-risk premalignant polyps of the left colon that are not amenable to endoscopic polypectomy
High-risk benign polyps that are not amenable to endoscopic polypectomy (eg, large tubulovillous adenoma)
Carcinoid of the left colon
Noniatrogenic trauma with perforation (eg, gunshot injury to the abdomen causing colonic perforation)
Iatrogenic trauma (eg, perforation during colonoscopy)
Lower gastrointestinal bleeding, localized to the left colon
Chronic diverticular disease, most commonly of the sigmoid colon
Failure of a laparoscopic approach - In a study by Simorov et al, the laparoscopic-to-open conversion rate was 20.7% for left colectomy and 14.3% for sigmoid colectomy 
One contraindication for left colectomy is metastatic colon cancer without complications. In such a case, surgical removal of the colon will not benefit the patient; instead, it will put the patient at risk for surgical complications and cause an unnecessary delay in systemic chemotherapy.
A second contraindication is surgically curable colon cancer in patients with severe medical comorbidities and/or those who are unstable and critically ill. Such patients cannot tolerate a major surgical procedure requiring general anesthesia.
The left colon begins at the midtransverse colon and includes the splenic flexure, the left (descending) colon, and sigmoid colon. The midcolon and the distal transverse colon are covered by peritoneum, and they are relatively mobile, except for the splenic flexure, secondary to the presence of the ileocolic ligament.
The descending colon is covered by peritoneum on the anterior and lateral surfaces and attaches to the retroperitoneum on the posterior side. The sigmoid colon is completely covered by peritoneum and is attached to the abdominal wall by a lateral peritoneal attachment termed the white line of Toldt, which extends upward to include attachment of the left colon as well.
Structures beneath the descending colon include the left kidney, the proximal ureter, and the inferior mesenteric vein (IMV). The intersigmoid fossa is a recess at the base of the mesosigmoid that provides an anatomic landmark for locating the left ureter, which courses beneath the fossa and runs parallel and just medial to the gonadal vein. Mobilization of the flexure requires division of the ileocolic ligament, a maneuver that must be performed carefully to prevent splenic capsular tearing.
Embryologically, the blood supply of the left colon is from the inferior mesenteric artery (IMA). The marginal artery of Drummond provides a vascular anastomosis between the superior mesenteric artery (SMA) and the IMA. In general, the distal transverse colon, including the splenic flexure and the descending colon, is supplied by the left branch of the middle colic artery, which is a branch of the superior mesenteric artery. The rest of the left colon is supplied by branches of the IMA.
The vascular supply of the sigmoid colon comes from the IMA and its sigmoidal and superior rectal artery branches. Collateral flow is provided by the marginal arteries and the arc of Riolan, a meandering artery from the middle colic artery to the IMA. Venous drainage is via the IMV, which joins the splenic vein and superior mesenteric vein to form the portal vein.
The lymphatic drainage follows the arterial supply. Since the main route of spread of carcinoma of the colon is via the lymphatics, an oncologic resection includes resection of the draining lymph nodes along with the lesion.
The superior hypogastric plexus is situated at the bifurcation of the aorta in close proximity to the IMA pedicle, and it provides sympathetic innervation for erectile function. Retrograde ejaculation in male patients can occur if care is not taken to avoid division of the nerve fibers during high IMA ligation and division. [1, 2, 3]