This document discusses the mechanisms of abdominal pain. It begins by explaining the embryological development of pain pathways in the embryo that determine how we perceive pain as adults. Different patterns of abdominal pain can indicate different underlying pathologies. Large bowel problems cause pain in the periphery while small bowel issues are usually more central. Basic causes of abdominal pain include inflammation, infection, trauma, vascular issues, tumors, obstruction, distension, and perforation. The nerve supply of the bowel is also described, explaining how visceral pain is vague while somatic pain from irritated peritoneum is sharp and localized. Location, timing, character, and worsening/relieving factors of pain can provide clues to its source.

1. Mechanisms of Abdominal Pain
Embryology of pain: formation of the embryo and arrangements of pain fibres and
pathways that determine how we perceive pain as an adult.
Different patterns of pain can give a close indication of the pathology.
***ANATOMY ATLAS***
Large bowel problems largely in the periphery. Small bowel problems are usually
more central.
Some structures in the chest and pelvis are included in the abdomen.
Basic pathology
 Inflammation (influx of inflammatory cells result of stimulus)
 Infection (a stimulus)
 Trauma (bowel can be split in a car accident)
 Vascular (ischemia, infarction (tissue death)-thrombosis and embolism or
atherosclerosis; usually arterial)
 Tumour (usually doesn’t cause pain in itself-usually when is obstructive or
presses on a nerve doe sit cause pain-indirect means)
 Obstruction (wall tries to contract to move obstruction): outside wall, inside
wall, inside lumen.
 Distension (the result of obstruction to accommodate material, can give rise to
ischemia an then perforation (in extreme form))
 Perforation (tumour, trauma or ischaemia)
Narrowest part of the small intestine ¼-1/2 a metre from the ileocolic junction back
up the ileum. Common places for obstruction.
Pith (fruits), fibrous material (corn, nuts) and hair are not broken down by peristalsis
of the bowel or enzymes and cause obstruction.
Obstruction due to tumour and faecal boluses (faecaliths)
Clinical Pain
Visceral (vague)
Parietal (sharp)
Bowel Nervous system
Intrinsic Plexuses (sub mucosal and intermuscular plexuses)
Extrinsic inputs (brain and spinal cord)
Peristalsis: intrinsic automatic bowel contractions from proximal to distal. (one
direction only is a property o the arrangement of the nervous system and intrinsic
pacemaker function of the small bowel)
-helps move material along and keep it clean
-Physiological (housekeeping function to prevent stasis)
-Pathological (attempts to overcome artificial obstruction)

2. Bowel Nerve Supply
1. Distension and contraction produced visceral pain which is vague ad poorly
localised which is referred to the abdominal wall: not at the site of origin in
the internal organ where it is arising.
2. When inflamed bowel irritates the peritoneal lining which is supplied by
somatic nerves, sharp ‘somatic pain can be directly localised to the site of
origin where it is occurring
Embryology of Pain
Foregut= lower oesophagus, stomach,
Duodenum=episgastric region
Midgut=periumbilical region
Hindgut=suprapubic region
Forgeut=lower oespgaus, stomach, duodenum, gallbladder, pancreas
Midgut=small bowel caemcum, appendix, asceding and proximal transverse colon,
Hind gut=distal transevrse and descending colon, sigmoid, bladder.
Sonial cord runs doen the embryo
Braches supply each segement directly
Spinal ord retracts back, rst gets longer
Spinal cord ends up at L2-L4
All the nerves are now going down the body
They now no longer directly supply the segments next to it.
Different parts of the gastrointestinal tract develop along side as a tube
The tube starts to elongate and then form the separate parts that are twisted and
moved around that results in the adult anatomy.
What supplies body wall and the gut is concurrent.
As the development progresses, the organs are no longer in the same position.
The basics of how the gut is supplied persist into the adult form.
The same nerves that supply the epigastric region for the skin supplies the foregut.
Same for he peri umbilical and midgut, and the hypogastric and hindgut.
Embryology of Pain
Vague visceral (jnternal organ) pain is ‘referred’ to the area of the abdominal wall
wher ehte same nerves that supply the gut, supple the abdominal wall. The
corresponding same spinal segemnsts supply both the organ ans the abdominal skin.
Epigastirum = T7,T8
Peri-umbilical region= T10 (91011)
Suprrapubic=T12, L1
Some overlap between dermatomes

3. Clinical Signs
VISECRAL –vague; diffuse; poorly lcoalised
 Colic –contraction/distension
 Types of Colic
o Colic– Biliary, renal, long waves, hours
o Colicky Pain – shirt waves, seconds., more of a spasm like pain.
SOMATIC
 Sharp;direct;localised.
Typical features
 Location
 Timing /duration
 Character
 Worsening /relieving factors
 Tenderness, guarding, rebound.
LOCATION
 The four quadrants: RUQ,LUQ,RLQ,LLQ
o Radiation – shoulder tip, thigh, back, chest, arm, pelvis.
o Can lead to the confusion of pain – because of embryological origin
and one level supplies many dermatomes.
 The nine quadrants: R and L hyperchondrium, epigastrium, R and L lumbar,
umbilical, R and L iliac fossa, Hypogastrium.
TIMING
 Intermittent/constant; draction (time)
 Small bowel: sharp crmapy, spasm central (lasts seonds)
 Biliary – contant sevre-RUQ to back – lasts hours
 Renal – constant and sever – loin tog roin (lateralising)
 Ischaemic –dull, sevre – symtks worse than signs
 Labour – intermittent dull, inceraing in severity-minutes, and contraction that
is everely and then reliqusihes. 9uterus e.g.)
 Oesphageal – sharpconatsnt and sevrer. – lasts minytes
 Bladder/Large Bowel/Uterine(Pelvic) – deull, severe –lsts hrs, suprapubic.
Isolated episode or recurrent
Character
Sharp/dull, sudden/gradual
Perforation-sudden, sever eonset oain or worsening. Affects peritoneum.
Inflammation –gradual osnet, worsening of dull pain.
Ischaemia –vague, dull worsening relentless, severe.
Tube Obstruction – ‘Colic”, cramp type (crampy, colicky;Bilairy;Renalcolic;
distension)
Shift of pain –due to the embryological origin of the pain fibres (the visceral pain),
then moves to somewhere else supplied by somatic fibres
Small gut – sharper contractions
Large gut- longer contractions
-can tell due to the location of the pain.