The vermiform appendix, a true diverticulum at the base of the cecum, is located near the ileocecal valve where the taenia coli converge. The term vermiform is Latin for worm-like and ascribes to its long, tubular architecture. In contrast to an acquired diverticulum, it is a true diverticulum of the colon and contains all of the colonic layers: mucosa, submucosa, longitudinal and circular muscularis, and serosa. While the location of the appendicular orifice at the base of the cecum is a consistent anatomical feature, the position of its tail is not. Variations in position include retrocecal, subcecal, pre-ileal and post-ileal, and pelvic. In addition, factors such as posture, respiration, and distention of adjacent bowels can influence the position of the appendix. This can cause clinical confusion in diagnosing appendicitis as variations in position can produce different symptoms. The histological distinction between the colon and the appendix depends upon the presence of B and T lymphoid cells in the appendicular mucosa and submucosa.
The function of the appendix has traditionally been a topic of debate. There is no clear evidence to its function in humans. The presence of gut-associated lymphoid tissue in the lamina propria has led to the belief that it serves a function in immunity, although the specific nature of this has never been identified. As a result, the organ has mostly maintained its reputation as a vestigial organ. However, as the recent understanding of gut immunity has improved, a theory that the appendix is a “safe house” for symbiotic gut microbes has emerged. This is especially important in developing countries where diarrhea due to infectious gastroenteritis is prevalent. Extreme bouts of diarrhea that may clear the gut of commensal bacteria can be replaced by that contained in the appendix. This suggests an evolutionary advantage for the maintenance of the vermiform appendix and weakens the theory that the organ is vestigial.
The appendix arises from the midgut, which is the portion of the alimentary tract from the duodenum to the proximal two-thirds of the transverse colon. The midgut receives its blood supply from the superior mesenteric artery. The cecal diverticulum appears at week 6 and is the precursor of the cecum and vermiform appendix. With developmental elongation of the colon, the cecum and appendix descend into the lower right abdomen where the tail of the appendix can then take on variable positions. During weeks 14 and 15, the mucosa develops lymphoid tissue, lending to its proposed function in immunity.
The appendicular artery, a terminal branch of the ileocecal artery, supplies blood to the appendix. This artery is a branch of the superior mesenteric artery, coinciding with its origin as a midgut derivative. Lymph from both the appendix and cecum drain into the ileocolic lymph nodes. However, while drainage from the cecum is via several intermediate mesenteric lymph nodes, the appendix drains through a single intermediate node. From the ileocolic lymph nodes, drainage proceeds to the superior mesenteric nodes.
The autonomic innervation of the appendix arises from the superior mesenteric plexus. Afferent sensory fibers from the appendix are carried on the sympathetic nerve fibers to enter the spinal cord at T10 which corresponds to the umbilical dermatome.
Appendectomy due to acute appendicitis is one of the most frequent indications for emergent abdominal surgery. An important anatomical landmark for surgeons performing appendectomy is the convergence of the taeniae coli which marks the area of the appendix. By following them inferiorly, the appendix can be located and resected.
Acute appendicitis follows a pathogenesis similar to that of other hollow viscous organs and is thought to be most often caused by obstruction. A fecalith, or sometimes a gallstone, tumor, or worms obstruct the appendiceal orifice, causing increased intraluminal pressure and compromised venous outflow. In the young, obstruction is more often caused by lymphoid hyperplasia. Ischemic injury results, encouraging bacterial overgrowth and triggering an inflammatory response. This becomes a surgical emergency because perforation of the inflamed appendix can leak bacterial contents into the abdominal cavity.
As the appendiceal wall becomes inflamed, visceral afferent fibers are stimulated. These fibers enter the spinal cord at T8-T10, producing the classic diffuse periumbilical pain seen at the onset of appendicitis. As it progresses, it begins to irritate the nearby peritoneum, stimulating somatic nerve fibers and producing more localized pain. The localization depends on the position of the tail. For example, a retrocecal appendix can produce right flank pain. Extending the patient’s right leg can elicit this pain; pain produced by stretching the iliopsoas muscle with the patient in the left lateral decubitus position is known as “psoas sign.” Another classic finding in acute appendicitis is McBurney’s sign, where pain two-thirds the distance from the umbilicus to the anterior superior iliac spine, known as McBurney’s point, is elicited upon palpation. Unfortunately, these signs and symptoms are not always present, making clinical diagnosis difficult. The clinical picture often includes nausea, vomiting, low-grade fever, and a slightly elevated white count.
In addition to acute appendicitis, cancer of the appendix is another clinical concern. The most common of these is a carcinoid tumor. This neoplasm often forms a 2 cm to 3 cm mass at the distal tip. Carcinoid tumors are of neuroendocrine origin and can secrete serotonin or other vasoactive substances. Carcinoid syndrome can arise as a result and produce symptoms such as flushing, diarrhea, wheezing, or right-sided valvular heart disease. Symptoms may be general or related to the organ system in which the carcinoid arises. Because most of these arise in the distal one-third, they rarely cause obstruction and can remain asymptomatic. However, 10% of neuroendocrine tumors arise at the base of the appendix and can cause appendicitis.
Appendiceal adenocarcinoma is another possible neoplasm of the appendix; its clinical presentation is often indistinguishable from that of acute appendicitis as it also causes obstructive pathology. Treatment involves a surgical approach involving either a right colectomy or appendectomy.