STEPTRACKERGASTROINTESTINAL | PATHOLOGY + ANATOMY + EMBRYOLOGY | HIGH-YIELD REVIEW

Small Bowel Disorders

Obstruction, telescoping, twisting, starving of blood — and the embryologic remnant that bleeds.

FOCUS ON
Small bowel obstruction is most often from adhesions (prior surgery). Children: intussusception (currant jelly stool, target sign) and Meckel diverticulum (painless bleeding, rule of 2s). Adults with atrial fibrillation: acute mesenteric ischemia — pain far worse than the exam. Ischemic colitis hits watershed zones.
DISORDER
MECHANISM · CAUSE
FEATURES
CLUE · DIAGNOSIS
Small bowel obstruction
Adhesions (most common), hernias, tumors, Crohn strictures, gallstone ileus
Colicky pain, vomiting, distension, obstipation; high-pitched bowel sounds
Dilated loops with air-fluid levels
Ileus
Loss of peristalsis without blockage: surgery, opioids, hypokalemia, sepsis
Distension, no flatus, absent bowel sounds
Gas through small and large bowel
Intussusception
Proximal segment telescopes into distal; ileocecal most common; lead point: lymphoid hyperplasia (post-viral, rotavirus vaccine), Meckel, polyp, IgA vasculitis; adults → tumor
6 months–3 years: intermittent colicky pain, drawing up legs, currant jelly stool, sausage-shaped mass
Ultrasound target sign · air or contrast enema is diagnostic and therapeutic
Volvulus
Bowel twists around its mesentery → obstruction + ischemia
Sigmoid (elderly, constipation — "coffee bean"); cecal (younger); midgut (infants with malrotation)
Bilious vomiting in an infant = malrotation until proven otherwise
Acute mesenteric ischemia
Embolus to the SMA (atrial fibrillation, MI), thrombosis on atherosclerosis, venous thrombosis, nonocclusive (low flow)
Severe pain out of proportion to exam, then bloody stool, peritonitis
↑ lactate, metabolic acidosis · CT angiography
Chronic mesenteric ischemia
Atherosclerosis of ≥ 2 mesenteric arteries
"Intestinal angina": postprandial pain → food fear → weight loss
Smokers with vascular disease
Ischemic colitis
Low flow in watershed areas: splenic flexure (SMA/IMA), rectosigmoid
Older patient: crampy left-sided pain, then bloody diarrhea
"Thumbprinting" (submucosal edema)
Meckel diverticulum
True diverticulum from a persistent vitelline (omphalomesenteric) duct
Painless lower GI bleeding (ectopic gastric mucosa) in a child; may cause intussusception, volvulus, diverticulitis
Rule of 2s · Tc-99m pertechnetate scan
Vitelline duct remnants
Incomplete obliteration
Patent duct → meconium from umbilicus · fibrous cord → volvulus · vitelline cyst
Meckel is the most common form
Hernias
Indirect inguinal (patent processus vaginalis, lateral to inferior epigastrics) · direct (Hesselbach triangle, medial) · femoral (below inguinal ligament)
Bulge; incarceration → obstruction; strangulation → ischemia
Femoral hernias: women, highest strangulation risk

Mechanism — obstruction to infarction

Blockage or twist
→
Proximal dilationFluid + gas
→
↑ Wall tensionVenous congestion
→
Arterial compromiseStrangulation
→
Transmural infarctPerforation, sepsis

Bowel infarction is hemorrhagic (red) because of the dual blood supply and venous congestion.

Meckel diverticulum — rule of 2s

FEATURE
VALUE
Prevalence
~2% of people
Location
Within ~2 feet of the ileocecal valve
Length
~2 inches
Ectopic tissue
2 types: gastric (bleeds) and pancreatic
Age at presentation
Usually first 2 years of life
Symptomatic
~2% of carriers

How it's tested

COMMON CONFUSIONObstruction has high-pitched bowel sounds and a transition point. Ileus has quiet or absent sounds and gas throughout the bowel.
THE TRAPA soft, barely tender abdomen in a patient screaming in pain is not reassuring. With atrial fibrillation, that is acute mesenteric ischemia.
YOUR APPROACHAge + onset + blood in stool + vascular risk + surgical history → mechanical, vascular or embryologic → best test (US, enema, CT angiography, Meckel scan).

If you see… think…

Prior laparotomy, colicky pain, vomiting, air-fluid levels → adhesive SBO
Postoperative distension, absent bowel sounds, low K⁺ → ileus
Toddler with episodic pain, currant jelly stool, sausage mass → intussusception
Intussusception in a 60-year-old → tumor lead point
Elderly constipated nursing-home patient, coffee bean sign → sigmoid volvulus
Newborn with bilious vomiting → malrotation with midgut volvulus
Atrial fibrillation + severe pain + benign exam + ↑ lactate → SMA embolus
Weight loss from fear of eating, postprandial pain → chronic mesenteric ischemia
Bloody diarrhea after hypotension, splenic flexure injury → ischemic colitis
2-year-old with painless bright red rectal bleeding → Meckel — pertechnetate scan