The transverse colon is the longest and most mobile part of the large bowel. It extends from the right upper quadrant to the LUQ usually dipping down in its mid portion, sometimes as far as the pelvis and then ascending to its highest position of the colon in the abdomen in the LUQ at the level of the splenic flexure.
The transverse colon ranges from 30-60cms.in length and has a diameter that should measure less than 5cms.
It is the most anterior portion of the large bowel, which is an important fact to remember and apply in the execution of a double contrast barium enema.
The first phase of the enema is to get the contrast into the cecum. The radiologist does not want to overfill the bowel with barium since for the double contrast study the barium is just needed to coat the wall and not to fill the lumen. It is the object of the air to fill the lumen and for the barium to outline the mucosa. Less barium is better in the performance and optimization of the barium enema examination. Therefore once the barium column reaches the splenic flexure the radiologist really wants to just roll the barium down the transverse colon up to the hepatic flexure, and then down the ascending colon. – a slow roller coaster ride. As the barium column reaches the splenic flexure, the patient is placed in the prone position so that the barium will be aided by gravity as it moves anteriorly in the transverse colon. In this position the barium should traverse most of the transverse colon after which a right decubitus should take the barium to the hepatic flexure. Getting it over the hepatic flexure hump is sometimes difficult but once accomplished it is downhill for the barium. The patient is placed supine as the technologists uncoils the enema tubes from the patients legs and the table tipped up in the grand hope of bringing the barium home safely into the cecum. Not always so easy. Once it is reached – huge relief for the radiologist. Double contrast enemas are one of the most challenging examinations for the radiologist and knowledge of the position of the parts of the colon is essential to enlist gravity as an aide to move the barium and air.
The transverse colon is suspended by two ligaments and acts as a support for a third. The two supporting ligaments are the gastrocolic ligament and the transverse mesocolon. The transverse mesocolon ™ runs between the pancreas and the posterior and superior aspect of the colon. The gastrocolic ligament (gcl) also known as the lesser omentum runs between the inferior aspect of the stomach and the antero-superior aspect of the transverse colon. The greater omentum is a thin apron of fat that hangs off the inferior aspect of the transverse colon.
The transverse colon receives blood mainly from the middle colic artery, but has crossover supply from both the right and left colic arteries. Venous drainage is via the corresponding veins and into the SMV and portal vein
Disease can spread along these ligaments so that in the case of diseases of the stomach, they can spread along the gastrocolic ligament to the superior aspect of the colon, while diseases such as pancreatitis can spread to the posterior and sometimes superior aspect of the colon. The greater omentum is notorious for collecting ovarian cancer deposits and when identified the complex of the fat of the omentum and the soft tissue elements of the cancer has been described as “omental cake” It is therefore very important for the radiologist to examine the superior, posterior and inferior borders of the transverse colon which may hold clues to important diseases in the abdomen.
Since the transverse colon is anterior and the greater omentum forms an apron of protection just below the anterior abdominal wall, they are both subject to traumatic injury. The following case represents the abdominal CTscan of a football player.
As stated above the greater omentum is a thin double layer of mesentery consisting of fat and connective tissue, that hangs off the transverse colon and seems to collect metastases particularly from the ovary, but also from gastric, pancreatic and colonic carcinoma as well. The nodules on the omentum are described as omental cake.
The transverse colon due to its anterior position as well as its redundancy is sometimes trapped in abdominal wall defects resulting in herniation.. When the defect is large the colon can be reduced either spontaneously or manually. However if the orifice is small strangulation and incarceration of the colon can occur, resulting in a surgical emergency.
When the hernial orifice, the hole through which the bowel protrudes is small, and the structural components in the hernia become more bulky for one of many reasons, the bowel in the hernial sac gets stuck, and cannot be reduced. This entity may be complicated by venous and then arterial compromise with resulting ischemia and infarction – an entity known as an incarcerated hernia.
Another more uncommon hernia (3% of abdominal hernias) is the Morgagni hernia, where the transverse colon or stomach protrudes through an anterior, retrosternal defect of the diaphragm. Although this hernia is a result of a congenital defect, it may not become apparent until adulthood. Sometimes it presents as a mass in the chest on a CXR and hence it is one of the causes of a pseudotumor of the lung. Obstruction of the colon can occur with this hernia.
Toxic megacolon (eMedicine) is a life threatening condition where an underlying colonic disease suddenly accelerates and the patient presents with a severe clinical syndrome. The colon is not always dilated in this syndrome and hence a better term is toxic colitis. The dilatation, when it occurs is caused by an ileus thought to originate as a result of damage to the nerves in the submucosa by the disease. In the absence of normal function of these nerves, motility of the bowel is limited and hence an ileus results.
When toxic colitis presents with megacolon it is usually the transverse colon that is affected and the theory for its involvement relates to the position of the transverse colon. The ill patient usually being confined to bed is lies predominantly in the supine position. Air in any part of the colon will rise to the most anterior part of the colon and therefore accumulates in the transverse colon. The transverse colon is therefore stretched and since the air cannot be transported due to the ileus, there is progressive dilatation of the transverse colon and hence progressive ischemia and worsening of the clinical situation.
There are multiple causes of the toxic colitis or toxic megacolon including ulcerative colitis, ischemic colitis, and pseudomembranous colitis.