MEDI7302 Study Guide - Final Guide: Magnetic Resonance Imaging, Parietal Cell, Decarboxylation
Small Bowel
Learning
objectives
Outline the embryology of the midgut, particularly in relation to blood supply of
abdominal midgut and hindgut structures
Explain the relation between midgut and hindgut structures and periumbilical and
hypogastric pain respectively
Explain why the pain of appendicitis begins in the epigastrium and shifts to the right
iliac fossa
Formulate a differential diagnosis for acute right iliac fossa pain in a young woman
and explain the role of investigations
Generate a differential diagnosis for small bowel obstruction
Explain how adhesions develop and how they cause small bowel obstruction
What is meant by the term ‘closed loop small bowel obstruction’, why is it difficult to
diagnose and what is the role of surgery
Understand the natural history of small bowel and appendiceal carcinoid tumours
and explain the pathology of carcinoid syndrome
Understand the pathogenesis of a Meckel’s diverticulum in relation to embryologic
development and complications
Describe the pathogenesis of gastrointestinal stromal tumour (GIST) and the
relationship between c-KIT and tyrosine kinase inhibitor therapy
Anatomy Midgut embryological development
Midgut extends from distal duodenal -> last 1/3 transverse colon (proximal
duodenum is derived from foregut)
Midgut is suspended from the dorsal abdominal wall by a short mesentery &
communicates with the yolk sac via the vitelline duct/ yolk stalk (eventually obliterated)
Midgut development in 5th week foetal development is characterized by
rapid elongation and protrusion of itself, its mesentery and SMA -> divides the stalk into the
primary intestinal loop
Midgut herniates through umbilicus
oCranial limb develops into distal duodenum (after CBD entry),
jejunum and proximal ileum
oCaudal limbs develops into distal ileum, caecum, ascending colon
and proximal 2/3 transverse colon
oAll supplied by SMA artery
Junction of cranial and caudal limbs (apex of loop) is where vitelline duct
joins yolk sac -> obliterated before birth OR persists as Meckel's diverticulum (2cm from
terminal ileum) in 2% population
Midgut herniation persists until 10wks foetal gestation, then intestine returns
back into abdominal cavity
oProximal jejunum enters abdomen after 270* rotation around axis of
SMA
oCaecum enters abdomen last, migrating from RUQ -> RLQ
In 6th week of foetal development - alimentary canal is supported by dorsal
mesentery (midline anterior to aorta) containing coeliac artery, SMA and IMA
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Gut blood supply
Fore gut - coeliac artery
oLeft gastric artery -> oesophageal branch, stomach branch
oCommon hepatic artery -> proper hepatic artery, right gastric artery,
gastroduodenal artery (giving off branch superior pancreaticoduodenal artery)
oSplenic artery -> dorsal pancreatic artery, short gastric arteries, left
gastro-omental artery, greater pancreatic artery
Mid gut - SMA
oInferior pancreaticoduodenal artery -> head of pancreas, D3 and D4
duodenum
oIntestinal arteries -> ileal branches, jejunal branches
oIleocolic artery -> terminal ileum, caecum, appendix
oRight colic -> ascending colon
oMiddle colic -> transverse colon
Hind gut - IMA
oLeft colic artery -> descending colon
oSigmoidal arteries -> sigmoid colon
oSuperior rectal artery -> superior rectum
Marginal artery of Drummond - collateral supply via terminal branches of
SMA + IMA
Gut nerve innervation
Abdomen is innervated by a complex dual visceral and parietal sensory
network
Visceral Afferent C fibres in walls of hollow viscera & capsules
of solid organs
Pain elicited via distension, inflammation or ischaemia
stimulating receptor neurons OR direct involvement (ie malignant
infiltration) of sensory nerves
Pain characteristics - slow onset, dull, poorly localised,
protracted
Different visceral structures are associated with
different sensory levels
oForegut pain -> epigastric region
oMidgut pain -> periumbilical region
oHindgut pain -> suprapubic/ hypogastric
region
oPain is usually felt in midline due to bilateral
sensory supply to the spinal cord
Parietal Afferent C and A-delta nerve fibres
Pain elicited via direct irritation (pus, bile, faeces) of
somatically innervated parietal peritoneum causing more precisely
localised pain
Cutaneous innervation corresponds to T6-L1 areas
(only 1 side of nervous system, hence better localisation and not midline
pain)
Pain characteristics - acute onset, sharp, better
localised pain
Referred A sensation perceived at a site distant from strong
primary stimulus due to confluence of afferent nerve fibres from widely
disparate areas within posterior horn of spinal cord
Example - subdiaphragmatic irritation (air/blood/pus)
is referred to shoulder via C4-mediated phrenic nerve
Small bowel structures
Duodenum (see upper GIT page)
Jejunum and ileum - both intraperitoneal structures, distal parts of small
intestine
Jejunum Ileum
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LUQ
Thick intestinal wall, large
vasa recta (straight arteries)
Less arcades (arterial loops)
Red colour
RLQ
Thin
intestinal wall, short vasa recta
More
arcades
Pink colour
Small bowel layers
Mucosal layer - epithelial lining, lamina propria, muscularis mucosa (smooth
muscle)
Submucosal layer - connective tissue with proteins (collagen, elastin), glands,
additional blood vessels, Meissner plexus (enteric nervous system)
Muscular propria - two layers of smooth muscle sandwiching Myenteric
plexus (enteric nervous system)
Serosal layer - outermost layer facing addominal cavity
RIF RIF pain GIT - appendicitis, cholecystitis, mesenteric adenitis, Meckel's
diverticulum, Crohn's disease, colon cancer, constipation, IBS
Reproductive (female) - ectopic pregnancy, ovarian cyst rupture/
hemorrhage/ torsion, Mittelschmerz (ovalation pain mid-cycle), PID, endometriosis
Reproductive (male) - seminal vesiculitis, undescended testis
Urinary - renal colic, UTI
Hip pathology, psoas abscess
RLL pneumonia
RIF mass Appendix abscess or mass
Caecal carcinoma
Crohn's disease
Distended gallbladde
Ovarian or tubal mass
Aneurysm of common, internal or external iliac artery
Retroperitoneal tumour
Psoas abscess
Appendicitis Inflammation of the appendix
Most common surgical emergency of abdomen affecting 10% population
Infants aren't usually affected (wide appendix, good drainage) nor are elderly
(almost obliterated)
Pathogenesis
Most common cause is obstruction via faecolith, undigested seeds, pinworm
infections, fibrous stricture from previous inflammation, neoplasia or lymphoid hyperplasia
(eg enlargement of lymphoid follicles with adolescence/ viral infection (adenovirus, measles,
immunisations))
Intestinal lumen continues to secrete mucus and fluid for pathogenic
prevention & maintain moisture of lining, causing rise in intra-appendiceal pressure
Appendix distension pushes on visceral nerve fibres causing abdominal pain
Trapped bacteria flora (E coli, bacteroides fragilis) can accumulate along with
immune system mediators creating pus OR empty obstructed appendix can develop
mucocele owing to mucus secretion from goblet cells
Sustained increase in intra-appendiceal pressure may cause compression of
supplying blood vessels (ileo-caecal artery -> appendicular branch -> end-artery branches),
causing wall ischaemia and death
Deterioration of wall mucosa that supplied bacteria-preventing mucus allows
bacteria to proliferate, invade appendix wall and weaken/ thin it out (gangrenous
Select number of patients may have super thinned wall that eventually
ruptures/ perforates (<12hrs or 3-4 days later), causing spillage of bacteria into peritoneum
(peritonism) OR localised collection (abscess)
Symptoms
Vague epigastric pain ('visceral pain' due to distension pushing on visceral
nerves that bilaterally supply spinal cord) that migrates to localised RLQ ('parietal pain' via
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Document Summary
Outline the embryology of the midgut, particularly in relation to blood supply of abdominal midgut and hindgut structures. Explain the relation between midgut and hindgut structures and periumbilical and hypogastric pain respectively. Explain why the pain of appendicitis begins in the epigastrium and shifts to the right iliac fossa. Formulate a differential diagnosis for acute right iliac fossa pain in a young woman and explain the role of investigations. Generate a differential diagnosis for small bowel obstruction. Explain how adhesions develop and how they cause small bowel obstruction. What is meant by the term closed loop small bowel obstruction", why is it difficult to diagnose and what is the role of surgery. Understand the natural history of small bowel and appendiceal carcinoid tumours and explain the pathology of carcinoid syndrome. Understand the pathogenesis of a meckel"s diverticulum in relation to embryologic development and complications.
