Small Intestine Diseases

Published on 16/04/2015 by admin

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Last modified 22/04/2025

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15 Small Intestine Diseases

Anatomy of the Small Intestine

image

Greater Omentum and Abdominal Viscera

Duodenum

• See Chapter 10, Gastroduodenal Diseases.

Jejunum

• 40% of small intestine
• Few large vascular arcades (loops)
• Long vasa recta
• Large, tall, and closely packed plicae circulares
• Less fat in mesentery than ileum
• Locus of maximum water (90%) and nutrient absorption, except for B12, bile acids, iron, and folate
• 95% of water absorbed

Ileum

• 60% of small intestine
• Many small vascular arcades (loops)
• Short vasa recta
• Large, low, and sparse plicae circulares, none distal
• More fat in mesentery than jejunum
• Maximum absorption of nonconjugated bile acids, with conjugated bile acids absorbed in terminal ileum
• B12 and folate maximally absorbed in terminal ileum

Microscopic Anatomy

image

Jejunum and Ileum: Mucosa and Musculature

• Mucosa

image Epithelium: enterocytes (absorptive), goblet cells, Paneth cells, enterochromaffin cells
image Lamina propria: contains Peyer’s patches

image Lymphoid aggregations with B cells in germinal centers and T cell in interfollicular zones
image Densest patches are in ileum.

image Muscularis mucosa

• Submucosa: strongest layer, connective tissue, Meissner’s plexus (parasympathetic ganglion cells and neuronal network)
• Muscularis

image Inner circular muscle
image Outer longitudinal layer
image Auerbach’s plexus: myenteric neurons and parasympathetic ganglion cells

• Serosa: peritoneum
• Mucosal surface area specializations: microvilli, villi, plica circulares (valvulae conniventes)
• Total absorptive surface for water and nutrient transfer: 200-550 cm2
• Water and nutrients absorbed across mucosa

Endocrine Gut Functions

• Cholecystokinin (CCK): secreted by cells of proximal intestine
• Secretin: secreted by S cells of proximal intestine
• Motilin: secreted by M cells of proximal intestine
• Somatostatin (SMS): secreted by D cells throughout gut
• Peptide YY (PYY): secreted by L cells of distal intestine
• Glucagon-like peptide 2 (GLP-2): secreted by L cells of distal intestine
image

Nerves of Small Intestine

Innervation

• Parasympathetic: vagus

image Preganglionic fibers: posterior branches of right and left vagus distributed through celiac and superior mesenteric plexus
image Ganglion cells are located in myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses.

• Sympathetic

image Preganglionic fibers from T8-T10 lateral column distributed via splanchnic nerves to celiac and superior mesenteric ganglia
image Postganglionic fibers distributed through celiac and superior mesenteric plexuses along arterial branches

• Sensory fibers, general visceral afferent

image Vagal afferents distributed through celiac and superior mesenteric plexuses
image Segmental afferents travel back (parallel to sympathetics) through celiac and superior mesenteric plexuses and splanchnic nerves to dorsal root ganglia and thoracic spinal cord segments.

Vessels and Lymphatics

image

Arteries of Small Intestine

Arterial Supply

Celiac Artery Branches

• Common hepatic artery

image Gastroduodenal artery

image Superior pancreaticoduodenal artery: to duodenum proximal to bile duct
image Anastomoses with inferior pancreaticoduodenal

Superior Mesenteric Artery Branches

• Inferior pancreaticoduodenal artery (duodenum distal to bile duct); anastomoses with superior pancreaticoduodenal
• Jejunal branches
• Ileal branches
• Ileocolic artery (with appendiceal branch)
• Arcades link adjoining jejunal and ileal branches.
• Vasa rectae connect from arcades to bowel walls.

Marginal Artery

• Connects superior and inferior mesenteric arteries
image

Veins of Small Intestine

Venous Drainage

• Duodenal veins empty into splenic vein, superior mesenteric vein, and portal vein (which lies posterior to the first part).
• Superior mesenteric vein (right portal drainage) receives jejunal, ileal, and ileocolic veins that run alongside of arterial counterparts.
image

Lymph Vessels and Nodes of Small Intestine

Lymphatic Drainage

• Peyer’s patches occur in greater numbers in more distal small bowel (e.g., ileum).
• Peyer’s patches and intraluminal vessels drain into mesenteric nodes clustered around branches of superior mesenteric artery.
• Deeper drainage flows superiorly into nodes along aorta and into cisterna chyli.

Clinical Correlates

Small Bowel Obstruction

• Hernias are most common cause in absence of previous surgery.
• Adhesions are most common cause with previous surgery.
• Other causes include malignancy, inflammatory bowel disease, Meckel’s diverticulum, and volvulus.
• Midgut volvulus can occur in pediatric patients with malrotations.
image

Intussusception

Intussusception

• Portion of bowel (intussusceptum) invaginates into an adjoining segment of bowel (intussuscipiens), causing obstruction.
• Can occur in adults owing to tumors; lead point is often a malignant tumor.
• Ileocolic is the most common kind in infants.

image Typically occurs between 5 and 10 months
image Timing (after 3 months) suggests infectious etiology.
image Terminal mesenteric node enlargement may be an indicator.
image Enlarged parietal lymphoid aggregates may be lead points that induce invagination.
image Idiopathic

image No other specific pathology is typically associated.
image More common in children < 2 years
image Viral infections and rotavirus vaccine have been implicated.

• Ileoileal

image More common in children > 2 years
image Lead point may be a Meckel’s diverticulum, pancreatic rest, enteric duplication cyst, or hemangioma.

• Ileocolic and ileocecal intussusception can occur after trauma or abdominal surgery.

Diverticular Disease

• Most intestinal diverticula are asymptomatic, discovered incidentally during other procedures.
• Acquired jejunoileal diverticula consist of outpocketing of mucosa and submucosa only.

image Occurring in <2% of the population
image Prevalence increases with age.

image

Meckel’s Diverticulum

Meckel’s Ileal Diverticulum

• Most common congenital anomaly of the GI tract: ~2% of population
• Remnant of the omphalomesenteric (vitelline) duct in distal ileum
• True diverticulum: includes all layers of the bowel
• About 2 feet from the ileocecal junction
• Typically manifests with painless lower GI bleeding in first 2 years of life
• Accounts for ~5% of painless lower GI bleeding in children < 2 years
• Pancreatic tissue most common nonbowel tissue found in Meckel’s diverticula
• Can also include gastric tissue: symptomatic with ulcer occurring in opposite gut wall (due to acid secretion)
• Obstruction: most common presentation in adults
• Diverticulectomy: most common treatment for uncomplicated diverticulitis
• Segmental resection indicated for complicated diverticulitis, neck <1/3 ileal diameter, or inflammation of the base
• Resection on incidental discovery controversial

Cancer of the Small Intestine

• Most common benign neoplasms: adenomas, leiomyomas, and lipomas
• Most common malignant neoplasms: adenocarcinomas, carcinoid tumors, lymphomas, and gastrointestinal stroma tumors, all rare
• Only ~2% of GI malignancies occur in small bowel.
• Adenocarcinoma: most common malignant small bowel tumor, most common in duodenum
• Carcinoid

image Slowly growing tumor of enterochromaffin (argentaffin or Kulchitsky) cells, producing serotonin and bradykinin
image More commonly found in appendix
image Small bowel carcinoids are aggressive and typically first seen when metastatic.
image Carcinoid syndrome, characterized by diarrhea, flushing, hypotension, tachycardia, eventual endocardial fibrosis
image Symptoms may be minimal with small bowel carcinoid because healthy liver metabolizes excess hormones.

• Gastrointestinal stromal tumors (GIST): most common GI mesenchymal neoplasm (1% of all), often associated with Kit gene mutation
• Leiomyosarcoma

image Usually found in jejunum and ileum
image Most commonly extraluminal

• Lymphoma

image Usually found in ileum
image Increased incidence in Wegener’s disease, systemic lupus erythematosus, AIDS, Crohn’s disease, celiac sprue
image Usually B-cell type

image

Crohn’s Disease

Crohn’s Disease

• Idiopathic inflammatory bowel disease, usually involving small and large intestine, but lesions can occur in the GI tract from mouth to anus
• Higher rate of occurrence in Ashkenazi Jews
• Terminal ileum most commonly involved segment
• Asymmetrical distribution of lesions
• Discrete (aphthous) and longitudinal ulcers common
• Gross bleeding may be absent (25%-30%).
• Rectum frequently spared (~50%)
• Perianal disease ~75%
• Fistulization
• Granulomas 5%-75%
• Discontinuous mucosa involvement
• Mucosal friability uncommon
• Relatively normal surrounding mucosa
• Cobblestoning in severe cases
• Normal vascular pattern
• Surgery not curative (unlike ulcerative colitis)

Short-Bowel Syndrome

• Because of absorptive and vascular reserve capacity of small intestine, limited resection of bowel is generally associated with minimal morbidity.
• Extensive resection can result in short-bowel syndrome, with insufficient absorptive activity, intractable diarrhea, malnutrition, weight loss, and dehydration.
• About 75% of cases result from a single massive resection.
• Adults: most common etiologies of short-bowel syndrome include mesenteric ischemia, malignancy, and Crohn’s disease
• Infants and children: atresias, volvulus, and necrotizing enterocolitis are most common causes
• Major public health problem in U.S.: 10,000 to 20,000 affected persons are dependent on total parenteral nutrition (TPN)

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