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What is Meckels diverticulum and explain in detail?
A Meckels diverticulum, a true congenital diverticulum, is a slight bulge in the small intestine present at birth and a vestigial remnant of the omphalomesenteric duct (also called the vitelline duct or yolk stalk). It is the most common malformation of the gastrointestinal tract and is present in approximately 2% of the population, with males more frequently experiencing symptoms.
Meckels diverticulum was first explained by Fabricius Hildanus in the sixteenth century and later named after Johann Friedrich Meckel, who described the embryological origin of this type of diverticulum in 1809.

Signs and symptoms
The majority of people with a Meckels diverticulum are asymptomatic. An asymptomatic Meckels diverticulum is called a silent Meckels diverticulum. If symptoms do occur, they typically appear before the age of two years. The most common presenting symptom is painless rectal bleeding such as melaena-like black offensive stools, followed by intestinal obstruction, volvulus and intussusception. Occasionally, Meckels diverticulitis may present with all the features of acute appendicitis. Also, severe pain in the epigastric region is experienced by the person along with bloating in the epigastric and umbilical regions. At times, the symptoms are so painful that they may cause sleepless nights with acute pain felt in the foregut region, specifically in the epigastric and umbilical regions. In some cases, bleeding occurs without warning and may stop spontaneously. The symptoms can be extremely painful, often mistaken as just stomach pain resulting from not eating or constipation. Rarely, a Meckels diverticulum containing ectopic pancreatic tissue can present with abdominal pain and increased serum amylase levels, mimicking acute pancreatitis.

Complications
The lifetime risk for a person with Meckels diverticulum to develop certain complications is about 4–6%. Gastrointestinal bleeding, peritonitis or intestinal obstruction may occur in 15–30% of symptomatic people (Table 1). Only 6.4% of all complications require surgical treatment, and untreated Meckels diverticulum has a mortality rate of 2.5–15%.Table 1 – Complications of Meckels Diverticulum:

Bleeding
Bleeding of the diverticulum is most common in young children, especially in males who are less than 2 years of age. Symptoms may include bright red blood in stools (hematochezia), weakness, abdominal tenderness or pain, and even anaemia in some cases.Bleeding may be caused by:

Ectopic gastric or pancreatic mucosa:Where diverticulum contains embryonic remnants of mucosa of other tissue types.
Secretion of gastric acid or alkaline pancreatic juice from the ectopic mucosa leads to ulceration in the adjacent ileal mucosa i.e. peptic or pancreatic ulcer.
Pain, bleeding or perforation of the bowel at the diverticulum may result.
Mechanical stimulation may also cause erosion and ulceration.Gastrointestinal bleeding may be self-limiting but chronic bleeding may lead to iron deficiency anaemia.The appearance of stools may indicate the nature of the bleeding:

Tarry stools: Alteration of blood produced by slow bowel transit due to minor bleeding in upper gastrointestinal tract
Bright red blood stools: Brisk bleeding
Stools with blood streak: Anal fissure
"Currant jelly" stools: Ischaemia of the intestine leads to copious mucus production and may indicate that one part of the bowel invaginates into another intussusception.

Diverticulitis
Inflammation of the diverticulum can mimic symptoms of appendicitis, i.e., periumbilical tenderness and intermittent crampy abdominal pain. Perforation of the inflamed diverticulum can result in peritonitis. Diverticulitis can also cause adhesions, leading to intestinal obstruction.Diverticulitis may result from:

Association with the mesodiverticular band attaching to the diverticulum tip where torsion has occurred, causing inflammation and ischaemia.
Peptic ulceration resulting from ectopic gastric mucosa of the diverticulum
Following perforation by trauma or ingested foreign material e.g. stalk of vegetable, seeds or fish/chicken bone that become lodged in Meckels diverticulum.
Luminal obstruction due to tumors, enterolith, foreign body, causing stasis or bacterial infection.
Association with acute appendicitis

Intestinal obstruction
Symptoms: Vomiting, abdominal pain and severe or complete constipation.
The vitelline vessels remnant that connects the diverticulum to the umbilicus may form a fibrous or twisting band (volvulus), trapping the small intestine and causing obstruction. Localised periumbilical pain may be experienced in the right lower quadrant (like appendicitis).
"Incarceration": when a Meckels diverticulum is constricted in an inguinal hernia, forming a Littré hernia that obstructs the intestine.
Chronic diverticulitis causing stricture
Strangulation of the diverticulum in the obturator foramen.
Tumors e.g. carcinoma: direct spread of an adenocarcinoma arising in the diverticulum may lead to obstruction
Lithiasis, stones that are formed in Meckels diverticulum can:Extrude into the terminal ileum, leading to obstruction
Induce local inflammation and intussusception.The diverticulum itself or tumour within it may cause intussusception. For example, from the ileum to the colon, causing obstruction. Symptoms of this include "currant jelly" stools and a palpable lump in the lower abdomen. This occurs when the diverticulum inverts into the lumen of the ileum, due to either:An active peristaltic mechanism of the diverticulum that attempts to remove irritating factors
A passive process such as the transit of food

Umbilical anomalies
Anomalies between the diverticulum and umbilicus may include the presence of fibrous cord, cyst, fistula, or sinus, leading to:
Infection or excoriation of periumbilical skin, resulting in a discharging sinus
Recurrent infection and healing of sinus
Abscess formation in the abdominal wall
Fibrous cord increases the risk of volvulus formation and internal herniation

Neoplasm
Tumors in Meckels diverticulum may cause bleeding, acute abdominal pain, gastrointestinal obstruction, perforation or intussusception.
Benign tumors:Leiomyoma
Lipoma
Vascular and neuromuscular hamartomaMalignant tumors:Carcinoids: most common, 44%
Mesenchymal tumors: Leiomyosarcoma, peripheral nerve sheath and gastrointestinal stromal tumors, 35%
Adenocarcinoma, 16%
Desmoplastic small round cell tumor

Other complications
A diverticulum inside a Meckels diverticulum (daughter diverticula)
Stones and phytobezoar (a bezoar of vegetable fibers) in Meckels diverticulum
Vesicodiverticular fistula

Pathophysiology
The omphalomesenteric duct (omphaloenteric duct, vitelline duct, or yolk stalk) normally connects the embryonic midgut to the yolk sac ventrally, providing nutrients to the midgut during embryonic development. The vitelline duct narrows progressively and disappears between the 5th and 8th weeks of gestation.In Meckels diverticulum, the proximal part of vitelline duct fails to regress and involute, which remains as a remnant of variable length and location. The solitary diverticulum lies on the antimesenteric border of the ileum (opposite to the mesenteric attachment) and extends into the umbilical cord of the embryo. The left and right vitelline arteries originate from the primitive dorsal aorta, and travel with the vitelline duct. The right becomes the superior mesenteric artery that supplies a terminal branch to the diverticulum, while the left involutes. Having its own blood supply, Meckels diverticulum is susceptible to obstruction or infection.
Meckels diverticulum is located in the distal ileum, usually within 60–100 cm (2 feet) of the ileocecal valve. This blind segment or small pouch is about 3–6 cm (2 inch) long and may have a greater lumen diameter than that of the ileum. It runs antimesenterically and has its own blood supply. It is a remnant of the connection from the yolk sac to the small intestine present during embryonic development. It is a true diverticulum, consisting of all 3 layers of the bowel wall which are mucosa, submucosa and muscularis propria.As the vitelline duct is made up of pluripotent cell lining, Meckels diverticulum may harbor abnormal tissues, containing embryonic remnants of other tissue types. Jejunal, duodenal mucosa or Brunners tissue were each found in 2% of ectopic cases. Heterotopic rests of gastric mucosa and pancreatic tissue are seen in 60% and 6% of cases respectively. Heterotopic means the displacement of an organ from its normal anatomic location. Inflammation of this Meckels diverticulum may mimic appendicitis. Therefore, during appendectomy, ileum should be checked for the presence of Meckels diverticulum, if it is found to be present it should be removed along with appendix.A memory aid is the rule of 2s:
2% (of the population)
2 feet (proximal to the ileocecal valve)
2 inches (in length)
2 types of common ectopic tissue (gastric and pancreatic)
2 years is the most common age at clinical presentation
2:1 male:female ratioHowever, the exact values for the above criteria range from 0.2–5 (for example, prevalence is probably 0.2–4%).It can also be present as an indirect hernia, typically on the right side, where it is known as a "Hernia of Littré". A case report of strangulated umbilical hernia with Meckels diverticulum has also been published in the literature. Furthermore, it can be attached to the umbilical region by the vitelline ligament, with the possibility of vitelline cysts, or even a patent vitelline canal forming a vitelline fistula when the umbilical cord is cut. Torsions of intestine around the intestinal stalk may also occur, leading to obstruction, ischemia, and necrosis.

Diagnosis
A technetium-99m (99mTc) pertechnetate scan, also called Meckel scan or nuclear scintigraphy scan, is the investigation of choice to diagnose Meckels diverticula in children. This scan detects gastric mucosa; since approximately 50% of symptomatic Meckels diverticula have ectopic gastric or pancreatic cells contained within them, this is displayed as a spot on the scan distant from the stomach itself. In children, this scan is highly accurate and noninvasive, with 95% specificity and 85% sensitivity; however, in adults the test is only 9% specific and 62% sensitive.
Patients with these misplaced gastric cells may experience peptic ulcers as a consequence. Therefore, other tests such as colonoscopy and screenings for bleeding disorders should be performed, and angiography can assist in determining the location and severity of bleeding