Small Bowel Flashcards
Except for the duodenum which is a foregut structure, the rest of the small intestine is derived from?
midgut
What are the four main cell types in the small intestine?
absorptive enterocytes (make up 95% of all cell types)
Paneth cells
goblet cells
enteroendocrine cells
During fifth week of embryological development, when intestine is rapidly growing, herniation occurs via the umbilicus around what artery?
SMA
this herniation continues until up to 10 weeks
Entire small bowel extends from where to where?
pylorus to cecum
What’s the entire length of small bowel?
approx. 300 cm
duodenum–> 20 cm
jejunum–> 100 cm
ileum–> 150 cm
What divides the duodenum from the jejunum?
ligament of Treitz
Where does the jejunum begin?
ligament of Treitz
How do we separate the jejunum from ileum?
no actual demarcation
jejunum makes up proximal 2/5
ileum makes up remaining 3/5
How do we distinguish jejunum from ileum on gross inspection?
jejunum has;
larger circumference
is thicker
has only 1 or 2 arcades sending long vasa recta thru the mesentery
(ileum has 4 or 5 arcades with shorter vasa recta)
What are the tranverse folds of small bowel, commonly seen in distal duodenum and jejunum called?
plicae circulares
valvulae conniventes
What is the blood supply of the small intestine?
jejunum + Ileum and distal duodenum–> SMA
proximal duodenum–> celiac axis
Anatomic location of SMA in relation to the pancreas and duodenum?
the SMA is anterior to uncicate process of pancrease
the SMA is anterior to 3rd part of duodenum
What is the venous drainage of the small bowel?
SMV–> drains into the splenic vein behind the neck of pancreas–> portal vein
In this part of the small bowel the circumference is smaller and the walls are thinner, there are multiple vascular arcades with short vasa recta;
ileum
This part of intestine is relatively thick, with prominent plicae circulares, and you see only one or two arcades with long vasa recta;
jejunum
What’s the innervation of the small bowel?
PSNS–> Vagus
SNS–> from splanchnic ganglia
Where do we find the lymphatics of the small bowel?
Peyer’s patches of distal small bowel
Lymphatic drainage of small bowel is carried via?
cysterna chyli —> thoracic duct—> neck
What are the four layers of the small bowel?
mucosa
submucosa
muscularis propria
serosa
What is the outermost layer of the small intestine consisting of a single layer of flattened mesoepithelial cells?
serosa
What are the two muscle types found in the muscularis propria layer of the small bowel?
thin outer longitudinal muscle layer
thick inner circular muscle layer
Where do we find the myenteric Auerbach’s plexus in the small bowel wall?
in the muscularis propria layer between the thin longitudinal outer layer and thick inner circular layer
What’s the strongest layer of the small intestinal wall?
submucosa
This is the strongest layer of the small intestinal wall and needs to be incorporated during anastomotic sutures;
submucosa
In what part of the small bowel wall do we find blood vessels and nerves?
submucosa (strongest part of wall)
This part of the small bowel wall contains lymphatics, arterioles, venules, and an extensive network of nerve fibers:
submucosa
Where do we find Meissner’s plexus in the small bowel wall?
submucosa
What are the four main cell types of the small intestine and where do we find them in the small bowel lumen?
Goblet cells–> make mucus
Paneth cells–> lysozyme, TNF, leukocyte defensins
absorptive enterocytes (most abundant type)
enteroendrocrine cells–> make GI hormones
** all found in mucosal layer
Microscopically, the mucosa of the small intestine is designed for what?
maximum absorptive capacity
How is the mucosa of the small intestine designed for maximum absorptive potential?
villi protrude into the luminal cavity
** increase absorptive capacity 30-fold
Main role of small intestine?
absorption of dietary carbs, proteins fats, vitamins, ions, H2O
How much daily carbs consumed by avg human in western diet?
300-350 g/day (50% starch, 30% sucrose)
In what form are carbs absorbed from small intestine?
monosacharides via active transporters; SGLT1, GLUT2, GLUT5
Where does protein digestion begin?
stomach via stomach acid (denatures protein)
continued in small intestine where protein comes in contact with pancreatic proteases
Pancreatic protease trypsinogen is secreted by pancreas in the inactive form, but it’s activated by what brush border enzyme in the duodenum?
enterokinase
** once trypsin activated, it activates other pancreatic enzymes in the protelytic pathway
In what part of the small intestine is digestion and absorption of proteins 80-90% completed?
jejunum
What part of intestine do we see majority of digestion and absorption of proteins (80-90%)?
jejunum
Most adults consume how much fat/day?
60-100 g
Essentially all fat digestion occurs in small intestine, where first step is what?
fat globules broken down into smaller molecules, process call emulsification (increase surface area for further digestion)
How does the gallbladder help emulsify dietary fats?
bile secretions contain bile salts and lecithin
incorporate into fat globules and help aggregate them and increase their surface area for digestion
What is the most important enzyme in the digestion of dietary fats?
pancreatic lipase
How is majority of fat absorbed in small intestine (80-90%)?
chylomicrons pass from epithelial cells to lacteals
pass thru lymphatics into venous system via thoracic duct
How much water enters the small bowel daily and how much leaves it for colon?
8-10 L /day
500 cc makes it to the colon
Where is Fe absorbed in the small intestine?
proximal duodenum
deposited within the cell as ferritin or transferred to plasma bound to transferrin
How are fat soluble ADEK vitamins absorbed?
incorporated into chylomicrons
absorbed via lacteals, thoracic duct, venous circulation
Where is vitamin B12 absorbed?
terminal ileum
How is vitamin B12 absorbed?
b12 derived from cobalamin (free in duodenum by pancreatic proteases)
cobalamin binds to IF secreted by stomach
receptors in terminal ileum pick up cobalamin-IF complex
Small bowel secretes what immunoglobulin?
IgA
Produced by plasma cells in lamina propria, and secreted into intestine, prevents adherence of bacteria to epithelial cells:
IgA
I cells of duodenum make what hormone?
CCK
What does CCK do?
stimulates pancreatic secretions
stimulates GB contraction
relaxes sphincter of oddi
inhibits gastric emptying
Antigens in the gut come in contact with what cells in Peyer’s patches which then process the antigen and present it to the immune system;
M cells
Most common causes of SBO?
- adhesions (60%)
- tumors (20%)
- hernias (10%)
- Crohn’s (5%)
These are responsible for 60% of all cases of SBO:
adhesions from previous abdominal surgeries
Malignant tumors account for what % of SBO?
20%
usually tumors from other abdominal sources, primary SB tumors very rare
Third leading cause of SBO?
hernias (10%)
How does Crohn’s dx cause SBO?
often from acute inflammation + edema
pts with chronic Crohn’s get strictures
Misc. causes of SBO?
2-4 %
intussussception
foreign bodies
gallstones
phytobezoars
Describe the pathophyiology of SBO:
early on–> intestinal contractility increases to overcome the obstruction
increased peristalsis occurs proximal and distal to obstruction—> see diarrhea early on
later on–> intestine gets fatigued, dilates, contractions less severe, less intense
bowel dilates–> accumulates water and electrolytes intraluminally and in bowel wall–> this third spacing causes hypovolemia, dehydration
With a more proximal obstruction with resultant vomiting, what metabolic derangements do we see?
hypochloremia
hypokalemia
metabolic alkalosis
How does SBO compromise ventilation?
get increased intra-abdominal pressures
decreased venous return
elevation of diaphragm
As the intraluminal pressure increases inside the bowel, a decrease in mucosal blood flow can occur, causing;
ischemia, perforation
This type of SB obstruction, commonly caused by a twist in the bowel, can progress to ischemia and perforation rapidly if not corrected quickly;
closed loop obstruction
In the abscence of obstruction, the jejunum and ileum are almost sterile, with obstruction we see change in bacterial flora;
most commonly e.coli, streptococcus, klebsiella
Cardinal symptoms of SBO?
colicky abdominal pain, N/V
abdominal distention
obstipation (not passing flatus/feces)
Failure to pass flatus/feces is known as?
obstipation
Early on in course of SBO, pts may report a hx of diarrhea, how is this possible?
early on we see increased contractions and peristalsis as the body tries to overwhelm the obstruction
As a SBO progresses, and more bacteria accumulate, the vomitus becomes?
more feculent indicating a late established obstruction
Tachycardia and hypotension are signs of what in someone suspected of SBO?
dehydration
Accuracy of diagnosis of SBO on plain abdominal xray?
60%
What do we see on xray of suspected SBO?
dilated loops of SB without colonic distention
multiple air fluid levels
What are some late signs seen on CT scan of irreversible ischemia due to SBO?
pneumatosis intestinalis
portal venous gas
Pre-operatively how can we tell if bowel is ischemia or strangulated in cases of SBO? What parameters or labs can we use?
no definitive test or lab value shown to be useful
What % of pts with partial SBO improve with conservative medical management and tube decompression?
60-85%
How can we ensure that a questionable area of small bowel is viable or not?
place area of questionable bowel in warm saline sponge for 15-20 mins–> reassess
second look laparotomy
Indications for laparoscopic exploration for suspected SBO?
mild abd distention
proximal obstruction
partial obstruction
anticipated single band obstruction
Most effective means of reducing post-op adhesions is?
good surgical technique
In the early post-operative period, SBO presents a challenge, bc it’s a question of is it SBO vs ileus, with >90% of post-op obstructions being partial and;
spontaneously resolve with conservative management
This describes intestinal distention and the slowing or abscence of passage of luminal contents without a mechanical obstruction;
ileus
What are some metabolic causes of ileus?
hypokalemia
hypomagnesemia
hyponatremia
Which age distribution does Crohn’s disease affect?
young adults 20-30s
smaller peak in 6th decade of life
What is the association between Crohn’s dx and smoking?
risk of getting Crohn’s is twice as likely in smokers
Genetic component of Crohn’s?
67% concordance in monozygotic twins
Cause of Crohn’s?
likely combo of immunologic, genetic, environmental
What is the single strongest risk factor for developing Crohn’s dx?
having a first degree relative with Crohn’s
Most common sites of Crohn’s dx are?
colon + small intestine
Crohn’s is discontinuous and segmental, and in pts with colonic involvement, this is characteristic, distinguishing it from UC:
rectal sparing
What are skip areas commonly seen with Crohn’s dx?
areas of diseased bowel separated by areas of grossly normal appearing bowel
What is fat wrapping?
circumferential growth of mesenteric fat around the bowel wall
pathognomonic for Crohn’s
What’s the earliest lesion we see in Crohn’s on gross inspection of lumen?
superficial apthous ulcer noted in mucosa
as dx progresses, ulcer becomes pronounced and transmural inflammation occurs
What gives characteristic cobblestone appearance in Crohn’s dx?
apthous ulcers are linear, they covalesce to form sinuses with islands of normal tissue in between
What are two characteristic histological lesions seen in Crohn’s?
non-caseating granulomas w/Langerhan’s giant cells
appear later in disease course (seen in 60-70% of pt)
Most common sxs of Crohn’s?
lower colicky abdominal pain
diarrhea second most common sxs
Compare BMs of UC vs Crohn’s dx;
Crohn’s dx–> have less BMs than UC
BMs rarely have mucus, pus, blood
Common intestinal complications of Crohns?
obstruction + perforation
These are two autoantibodies associated with IBD:
pANCA
anti-saccharomyces cervisiae
Perianal involvement and racto-vaginal fistulas are more common UC or Crohn’s?
Crohn’s
Most commonly prescribed drug for Crohn’s?
sulfasalazine (an aminosalycilate)
mesalamine is a new sulfasalazine like drug that’s used as first line for Crohn’s
What’s an altrernative medication used in pts with active Crohn’s flare?
budesonide (steroid)
not used for long-term remission maintenance because of side effects
What antibiotic commonly used in Crohn’s?
metronidazole
may be useful in treating perianal disease, fistulas, etc
What are the two immunosuppressive agents used in Crohn’s pts?
azathioprine
6-MP
Side effects of azathioprine and 6-MP?
pancreatitis
hepatitis
fever
rash
Most concerning side effects of azathioprine and 6-MP?
bone marrow suppression
malignancy potential
This other immunosuppressive agent used in Crohn’s for pts with fistula, with MOA of blocking production of IL-2 by helper T-cells:
tacrolimus
This is a monoclonal antibody to TNF-a and has shown promise in tx of moderate to severe Crohn’s dx:
infliximab
MOA of infliximab?
monoclonal antibody to TNF-a
Side effects of infliximab?
TB reactivation
fungal and other opportunisitc infexns
reactivation of laten MS
CHF exacerbation
What extra-intestinal manifestations of Crohn’s dx do not resolve after surgery?
ankylosing spondylitis
hepatic complications
Principle of surgery when operating on pt with Crohn’s?
only resect the segment causing the problem/complication
do not resect anything you don’t have to, even if it looks diseased
frozen sections during OR are unreliable to determine disease involvement
What is acute ileitis?
pts have RLQ abdominal pain, confused with appendicitis
appendix normal–> but should be removed to prevent future confusion
acute ileitis is a self-limited dx–> no resection needed
What is the most common indication for surgical intervention in pts with Crohn’s dx?
intestinal obstruction (often partial)
What is the treatment of choice for pts with intestinal obstruction due to Crohn’s?
for complete obstruction and partial non-responsive to conservative tx–> segmental resection w/ anastomosis
If a segment or multiple segments of bowel are involved with strictures, what can we do beside resect bowel and re-anastomose?
perform a stricturoplasty of involved segment
How is a stricturoplasty performed?
longitudinal incision is made thru narrowed area
followed by closure in a tranverse fashion
What is a Heineke-Mikulicz pyloroplasty?
make a longitudinal incision on stricture bowel segment
pull the open area superior and inferiorly, and run a suture in a tranverse manner (up and down)
For shorter strictured segments, what kind of stricturoplasty do we perform?
Heineke-Mikulicz pyloroplasty
For strictured segments > 10 cm what kind of stricturoplasty do we perform?
Finney pyloroplasty
What is a Finney pyloroplasty?
side-to-side iso-peristaltic stricturoplasty
Advantages of stricturoplasty?
preserve intestinal length
similar outcomes/complications compare to resection and anastomosis pts
Given the high concern for carcinoma developing at stricture sites, what needs to be done when performing stricuturoplasty?
full thickness biopsy of stricture needed
Most common urologic complication assc with Crohn’s dx?
ureteral obstruction
due to ileo-colic dx with retroperitoneal abscess
Why do pts with Crohn’s dx who are found to have cancer have a worse prognosis than pts without Crohns?
usually dx is delayed in Crohn’s pt
Crohns dx of duodenum seen in 2-4% of cases, and primary indication for surgery in these pts is duodenal obstruction, what’s the procedure of choice?
we don’t resect
we bypass obstruction with a gastro-jejunostomy
Leading cause of disease related death in pts with Crohn’s
GI cancer
Small bowel neoplasms are rare, with mean age at onset?
60
Most common benign tumors of small bowel?
leiomyomas
adenomas
What is the most malignant neoplasm of the small bowel?
adenocarcinoma vs carcinoid
Risk factors or conditions that predispose to small bowel neoplasms?
hereditary conditions like; FAP, HNPCC, peutz-jeghers, Crohn’s, celiac sprue,
smoking, heavy etoh use, consuming red meat
What are the most common benign tumors of the small bowel?
GISTs
adenomas
lipomas
These are benign small bowel tumors of smooth muscle origin:
GISTs
** MC symptomatic benign tumors of small bowel
What causes GISTs?
arise from interstitial cells of Cajal (intestinal pacemaker of mesodermal origin)
Most GISTs are benign, (3-4 x more common than malignant GISTs), and most GISTs, >90%, express what tumor marker?
CD117
it’s a c-kit proto-oncogene protein
These small bowel neoplasm expressed CD117 and CD 34;
GISTs
What neoplasm of small bowel expressed CD 117, a c-kit proto-oncogene protein?
GISTs
When are GISTs most commonly diagonsed age wise?
5th decade
equally seen in men/women
What is the most common reason for surgery in pts with benign GISTs?
these tumors sometimes outgrow their blood supply and cause bleeding
This small bowel tumor looks firm, grayish-white, with a whorled appearance, with well differentiated smooth muscle cells:
GIST
These make up 15% of benign small bowel tumors:
adenomas
20%–> duo
30%–>Jej
50–> ileum
How do we manage villous adenomas of small bowel?
depend on location (have malignant potential of 35-55%)
jejunum + ileum–> resect
duodenum –> endoscopic polypectomy, if invasive or recurs–> pancreaticoduodenectomy
What is a Brunner gland adenoma of small bowel?
benign hyperplastic lesions found in Brunner gland of proximal duodenum
produce sxs similar to PUD
Tx–> simple excision, no malignant potential, no need to radical surgery
These benign small bowel lesions commonly seen in ileum, and present as single intramural lesions found in submucosa of lumen;
lipomas
Hamartomas of the small bowel occur as a result of this entity;
Peutz-Jeghers syndrome
This is an AD inherited syndrome of mucocutaneous melanotic pigmentation and gastrointestinal polyps;
Peutz-Jeghers syndrome
These are developmental malformations consisting of submucosal proliferation of blood vessels;
hemagiomas
Hemangiomas commonly seen in what small bowel segment?
jejunum
make up 5% of benign small bowel neoplasms
Hemangiomas of small bowel may occur as part of a syndrome called;
Osler-Weber-Rendu dx
Most common symptom of small bowel hemangiomas is bleeding, what studies do we get?
CTA
red-blood cell tagged study
