Gastrointestinal Tract Anatomy and Anomalies

General Information on the Gastrointestinal Tract (GIT)

• The GIT extends as a hollow tube from the mouth to the anus.
• It is a muscular tube that contracts via peristalsis to move contents unidirectionally, which is the normal physiological function.
• Disrupted unidirectional flow indicates a pathological condition.
• Each segment of the GIT has specific, complementary, and integrated functions.
• Damage to one segment’s function can affect others.
• The GIT regulates ingestion, processing, nutrient absorption, and waste elimination.
• Alterations in one segment can directly or indirectly affect another.

Congenital Anomalies

• Because GIT segments originate from the endoderm during organogenesis in the first 8 weeks of embryonic life, an alteration affecting one part likely affects other parts.
• Therefore, more than one anatomical segment can be affected.

Atresia, Fistulas, and Duplications

• Affecting the esophagus are usually evident soon after birth.
• Commonly located near the tracheal bifurcation (often with fistulas).
• Cause regurgitation and even bronchoaspiration in newborns when feeding.
• Require immediate surgical correction.
• Atresia is on the proximal segment of the esophagus which obstructs it where the fistula connects to the trachea which leads to severe complications when pure.
• Diagnosed early, require surgical intervention within 1-2 days of birth.

Stenosis

• Narrowing without complete obstruction usually occurs in the esophagus and small intestine.
• Can be congenital or acquired.
• Usually involves a more pronounced narrowing rather than complete closure.

Esophageal Stricture

• Can be congenital; causes regurgitation because food cannot reach the stomach.
• Dilation with a balloon can be attempted depending on the stricture.
• Surgical correction may be needed if congenital.
• Congenital stenosis requires further specification beyond just "stenosis."

Imperforate Anus

• Occurs when the rectum fails to properly connect to the external opening.
• The condition is independent of the structures of the genitourinary tract.
• In females, imperforate anus can occur with a rectovaginal fistula, where the rectum connects to the vagina instead of the anus.
• Another variation in females is where the rectal tract connects to the vulva instead of passing through the anal sphincter ring.
• It can also have a defect outside anal muscular rings, and can be trained with exercises.
• In males, a rectovesical fistula can occur, where the rectal contents empty into the bladder
• Waste may empty into the urethra.
• A fistula may occur and open into the perineum.

Diaphragmatic Hernia

• Results from incomplete formation of the diaphragmatic domes leading to abdominal viscera displacement into the thoracic cavity.
• Occurs mostly on the left side and may be associated with fatal pulmonary hypoplasia.
• Because the pressure inside the abdominal cavity is much greater than inside the thorax, abdominal contents can herniate through diaphragmatic cupola deficiencies.
• In the embryo, herniation prevents the left lung from developing properly.
• After birth, herniation prevents the lung expansion due to its pressure insufficient to displace the viscera.
• Lowering the viscera can cause intestinal ischemia; severe cases lead to marked reduction of left lung function or lung non-function.
• The most feared result is pulmonary hypoplasia.

Omphalocele

• Associated with a failure of the anterior abdominal wall to close because of a deficiency in the muscles around the umbilicus.
• Herniation contents are covered by peritoneum
• Causes displacement of intra-abdominal content in the umbilical cord structure.

Gastroschisis

• Similar to omphalocele, but the defect in the abdominal wall is larger.
• Characterized by intestinal loops or detached abdominal contents herniating through the abdominal wall.
• Both gastroschisis and omphalocele are corrected with surgery.

Ectopia

• "Patches" of GIT tissue appear in incorrect anatomical locations.
• Esophagus: ectopic pancreatic tissue in stomach spots, gastric heterotopy in small or large intestine, or mucosa in Meckel's diverticula.

Meckel's Diverticulum

• True diverticulum incorporating all three intestinal wall layers rather than pseudodiverticulum.
• Situated on the antimesenteric side of the small intestine caused by the relaxation of the muscle paralell.
• During embryogenesis, a vitelline duct connects the placenta to the intestine; failure of the conduct to close is why it appears.
• Meckel's can have ectopic gastric mucosa; its formation is due to the incomplete involution of vitelline duct.
• Histologically, it is intestinal mucosa that can be gastric or pancreatic.

Congenital Pyloric Stenosis

• More common in males.
• Some chromosomal disorders and antibiotic use (erythromycin) during early pregnancy are risk factors.
• Presents between 3rd and 6th week of life as regurgitation.
• Palpation reveals a mass in the epigastrium caused by hypertrophy of the pyloric ring, resulting in a thickened and contracted pylorus.
• Acquired is secondary as an ulcer.
• Diagnosis is confirmed with ultrasound showing a thickened pylorus.
• Surgical myotomy cures it with the goal to allow food to pass.

Hirschsprung's Disease (Congenital Aganglionic Megacolon)

• Secondary to a mutation that causes functional obstruction of the colon characterized by absence of ganglion cells from the submucosal Meissner and myenteric Auerbach plexuses.
• Individuals lack proper innervation in one colon segment, sometimes extending to the rectum.
• Contractions and intestinal transit are impaired where neurons are absent leading to a buildup of waste, inflammation, and colonic mucosa infection.
• Associated with Down syndrome.
• The ret gene mutation causes failure of neuronal cell migration.
• Diagnosed via histomorphological analysis (H&E or IHC for acetyltransferase).
• Requires surgical resection because the segment is permanently denervated
• Surgeons rely on pathologists to determine the denervated parts extent.
• Can affect other segments of the colon.

Esophageal Disorders

• Can occur as a dysfunction of the lower esophageal sphincter as a primary diffuse esophageal spasm.
• Occurs via a barium bolus to visualize the fluid flowing via fluoroscopy.

Esophageal Diverticula

• Can occur with increased pressure when swallowing or straining.
• Usually causes no symptoms.
• But can accumulate food, causing ulceration, inflammation, and bad breath.

Mucosal Membranes of the Esophagus

• Cause some type of esophageal obstruction.
• Plummer-Vinson syndrome is rare.
• It is associated with iron deficiency anemia and occurs in the upper third of the esophagus.
• It is associated with glottis, cheilosis of the tongue, and esophageal cancer incidence.
• Plummer-Vinson involves anemia, glottis, cheilosis of the tongue, and inflammation of the mucosa.

Schatzki Rings

• Esophageal rings cause some level of collusion.
• Circular rings covered with mucosa occur in parts and may undergo thickening.

Achalasia

• Incomplete relaxation of lower esophageal sphincter.
• Increased tone and aperistalsis due to neuronal degeneration.
• Associated with constant contraction of upper esophageal sphincter and lack of peristalsis of the esophagus.
• Secondary to Chagas disease that attacks GIT innervation.
• Diagnosed with nanometry

Mallory-Weiss Tear

• Occurs from forceful vomiting that causes esophageal tears.
• Most common in people who vomit excessively.
• Mallory-Weiss is esophageal mucosal tearing and bleeding.
• Mallory-Weiss more severe form is esophageal perforation with chest pain from a confunded IAM.

Esophagitis (Chemical or Infectious)

• Caused by ingesting irritants (alcohol, acids, bases, hot drinks, cigarettes) or is infectious (herpes simplex, CMV, fungi).

Reflux Esophagitis

• Most common form of esophagitis caused by relaxation of lower esophageal sphincter due to acid regurgitation into the esophagus.
• Associated with hiatal hernia "sliding," alcohol consumption, tobacco use, obesity, pregnancy, increased intra-abdominal pressure, etc.

Hiatal Hernia

• Causing esophagitis forms in the hiatus of the diaphragm.
• Paraesophageal hiatus hernia may occur lateral which fundic is herniated, and are associated with gastroesophageal reflux.

Esophagitis (Eosinophilic)

• Associated with atopic conditions, allergies, bronchial asthma, or hyperreactivity to environmental antigens.

Esophageal Varices

• Related to alcoholism's severe congestion of venous the the bottom esophagus due to hepatic cirrhosis.
• Liver cirrhosis causes venous drainage through circulation causing congestion of esophageal veins and hematemesis.

Barrett's Esophagus

• Esophageal metaplasia risk factor in adenocarcinoma due to cardia moving into esophagus.
• Histologically, Barrett is columnar with goblet cells and brush borders in the esophagus that normally is squamous.

Esophageal Neoplasms

• Associated with Barrett’s esophagus.
• Can be caused by tobacco use and ionizing radiation.
• Usually in the lower esophagus part.

Squamous Cell Carcinoma

• Associated with alcohol and tobacco consumption, and achalasia or Plummer-Vinson syndrome.
• Occurs in the middle section.

Gastric Disorders

• Divided into gastropathy and gastritis which includes inflammation.
• Physiological characteristics is its ability to withstand incompatible pH levels due to protecting mechanisms.

Gastropathies

• Caused by acid; regulated by cyclooxygenases; NSAIDs or AINES affect cyclooxygenases, so prostaglandins cannot protect the gastric mucosa.
• Can occur due to stress (ischemia), such as with severe burns or with SNC related injuries.

Chronic Gastritis

• Defined by the activity of the presence of inflammatory cells (acute and chronic).
• The presence of abundant neutrophils inside glands constitutes, gastritis while fewer neutrophils and lymphocytes indicates “non-active" gastritis.
• Caused by H. pylori and NASID use in those colonized by the bacteria.
• H. pylori is more likely with tight housing conditions and bad hygiene practices.

Autoimmune Atrophic Gastritis

• Occurs in an atrophic stomach where the cells of the body.
• The lining stops producing intrinsic factor, which enables the body to absorb vitamin B12 in the small intestine.

Peptic Ulcer Disease

• Either duodenal or chronic stomach conditions; has related factors to H. Pylori infection, or use of NSAIDs, or tobacco products.
• Tissue damage or ulceration occurs via hydrochloric acid to stomach lining.

Hypertrophic Gastropathies

• Increase in cells, or Menetrier's Disease.
• TGF- causes a domino effect on the growth process, and in cases can be caused by a viral infection that grows pleats of fluid in the linings.

Gastric Polyps

• In the stomach are inflammatory, or have a neoplastic response to stimuli.
• They are associated with proton alterations and the gastric polyps connect to familial adenomatous polyposis.

Gastric Adenomas

• Atrophic and cause metaplasia infection.
• Occur in the antrum.
• Gastric Metaplasia is either complete or incomplete depending on the location.

Gastric Adenocarcinomas

• Common in Japan, Chile, Costa Rica, or Europe.
• Related to H. Pylori infection or smoking.
• Can be classified histologically of the gland.
• Intestinal includes the gland's lumen while diffuse has a cell structure.

Gastric Lymphomas

• Are mucosal, with gastritis, or associated with M.A.L.T..

Gastrointestinal Stromal Tumors (GISTs)

• Develop from interstitial cells of Kahal below the surface of the wall.

Disorders of the Small Intestine or Small Bowel

• Intestinal Obstruction can occur if there is a predisposition or narrowing found.
• Hernia is a main cause of obstruction, caused by muscle weakness.
• Post Surgery adhesions of the areas that were once connected.
• Volvulus will be twisted if the intestines are locked in and do not free.

Intestinal Ischemia

• Small Bowel obstruction due a blockage in the blood supply.
• The intestinal area lacks oxygen and is irreversable and will require medical assistance.

Malabsorption and Diarrhea

• Related disorders that causes bodily disfunction.
• Cystic Fibrosis can cause malabsorption with the metabolism and also transportability.

Celiac Disease

• Causes damage and inflammation that alter absorption.
• The small bowel develops an incapacitation and cannot process some carbohydrates like, wheat or barley.

Intestinal Angiodysplasia

• Malformation or an irregularity of capillaries found in parts of the body like in the brain.

Intestinal Enterocolitis

• Has multiple causes like bacterial, parasitic, or also viruses.
• The small and large intestines suffer inflammation when this affects the body.

Microscopic Whipple Disease

• Bacteria accumulate and the small bowel is affected by actinomycete.

Disorders of the Large Intestine or Large Bowel

• Inflammatory Bowel: includes both the small and large intestines, and diagnosed by endoscopic means.
• The main primary inflammatory cases are Crohn's and then Colitis.

Crohn's Disease

• Affects the small bowel with inflammation.
• Tobacco increases associated risks, even those who quit.
• Anemia may affect this disease, and have extra intestinal manifestations.

Ulcerative Colitis

• Restricted to only the rectum and the colon.
• Diarrhea occurs hemorrhagically, and with fibroid substances can be painful.

Sigmoid Diverticular Disease

• Has characteristics of colon walls that become herniated, or have structural weaknesses.

Colon Polyposis

• Made with different causes; one being true neoplastic (or having exaggerated cell production).
• Others are hamartomatous, inflammatory, or adenomas.

Colon Adenocarcinoma

• This neoplasm happens most frequently in the digestive tract.
• 10% of deaths in the word is due to cancer, depending on cancer's location.
• Usually the right and left parts of the colon will be affected.

Hernia, Hemorrhoids, & Appendix

• Can stem from sitting too long, or even lifting weights.
• Internal or external sources may be the cause, and cause levels of pain.