Abdominal Pain

Embryology – embryonic gut tube is derived from endoderm and mesoderm

it is divided into three parts based upon segmental artery supply and autonomic innervation

these divisions remain, even though many parts of the gut rotate or become secondarily retroperitoneal

(1) foregut – esophagus Þ proximal duodenum – celiac artery, greater splanchnic nerve (enter T5-9)

(2) midgut – distal duodenum to transverse colon – superior mesenteric artery, lesser splancnic nerve (enter T8-11)

(3) hindgut – descending colon to anus - inferior mesenteric artery, least splancnic nerve (enter T12-L3)

At 6-8 weeks, midgut herniates out and makes a 270° counterclockwise rotation, and returns by 10^th week

therefore appendix, a midgut structure ends up in lower RH corner

Three Different Types of Abdominal Pain – from different nerves

(1) Visceral pain – arising from visceral peritoneum (splanchnic mesoderm)

Quality: pain is generally vague, not well localized, dull, crampy, sickening, hard to accurately describe

Stimuli: three major pain stimuli:

(a) mechanical – receptors respond to perforation and stretching (distention) due to obstruction
(b) chemical receptors – receptors respond to inflammation (bradykinin) due to infection, bile problems, etc.
(c) ischemia – vasoactive agents such as K⁺, lactate, H⁺
also some extraabdominal (pneumonia, PE); cutting/crushing of gut does not produce pain

Transmission: sympathetic autonomic nerves (visceral afferent) – NOT parasympathetic (vagotomy has no effect)

secondary autonomic effects – sweating, restlessness, nausea, emesis, perspiration, pallor, tachycardia

transmitted by C fibers – slower, large receptive field, accounts for 50% of pain neurons from abdomen

cell bodies in retroperitoneal ganglia – sphlancnic nerves Þ bilateral entry to spinal cord

Localization: since embryonic gut tube is midline, visceral pain is midline; vertical location depends on precursor location

foregut seems epigastric (T5-9), midgut seems periumbilical (T8-11), hindgut seems hypogastric (T12-L3)

(2) Parietal pain – arising from parietal peritoneum (somatic mesoderm)

Quality: well localized, sudden onset, sharp

Stimuli: anything causing irritation of the parietal paritoneum (perforation, compression, etc.)

Transmission: somatic neurons – to dorsal root ganglion on one side (unilateral)

transmitted by Ad (A delta) neurons – 25% of pain neurons to abdomen, fast, small receptive field, sharp

Localization: localized to dermatome segment closest to pathological process

(3) Referred pain – pain arising from intraabdominal problems that is experienced somewhere else

convergence projection hypothesis – visceral afferents and somatic neurons comingle at dorsal root ganglion
example: shoulder pain if diaphragm is irritated

Example of abdominal pain: Appendicitis

The appendix is a small tubular structure at the start of the colon (derived from midgut)

if lumen obstructed (fecalith, tumor, lymphoid tissue, foreign body) Þ accumulate mucous Þ distention Þ visceral pain

appendix is from the midgut, so midline periumbilical pain

if obstruction continues Þ ischemia Þ decreased mucosal blood flow, no normal mucosal protection Þ bacteria

eventually can get perforation Þ parietal peritoneum – then parietal pain (right lower quadrant)

Eventually, the arterial supply will be cut off, leading to ischemic necrosis, gangrene, perforation, and diffuse peritonitis.

Diagnosis of Abdominal Pain

Signs of parietal peritonitis – severe tenderness, guarding (voluntary or involuntary spasm of abdominal wall musculature), rigidity, rebound tenderness, absent bowel sounds (ileus – reflex)

Rate of onset:

immediate: perforation, ishemia, rupture
rapid (minutes): biliary colic, renal colic, small bowel obstruction, pancreatitis
gradual (hrs): gastritis, appendicitis, cholecystitis

Modifiers of Abdominal Pain – age, pregnancy, sensory loss, ascities (can affect location), steroids (interfere with inflammatory response), anxiety/pain threshold

Acute abdomen – sudden onset of acute pain – usually accompanied by nausea, vomiting, sweating, distention – 4 types:

(1) steady with peritonitis – appendicitis, diverticulitis, salpingitis, perforation (peptic ulcer, malignant ulcer, trauma), abscess rupture, hemorrhage (of speen, due to ectopic pregnancy, or ruptured aneurism, or traumatic injury

(2) steady without peritonitis – cholecystitis, tumor infiltration, abscess, hematoma, bowel ischemia without rupture, tortion (of ovary/omentum/appendices epiploica), pancreatis, absess, hepatosplenic enlargement, ischemia

(3) intermittent with distention – bowel obstruction (adjesions, hernia, volvulus, intussusception, tumor,gallstone ileus)

(4) intermittent without distention – gastroenteritis, inflammatory bowel disease, early bowel obstruction or appendicitis, biliary colic, renal/ureteral colic, labor, diabetic ketoacidosis, porphyria, poisoning lead/mercury/arsenic