Small and Large Bowel Obstruction

Extraluminal (compressive) causes — the majority of SBO

• Postoperative adhesions: fibrous bands from prior laparotomy tether or kink bowel; the single most common cause of SBO in the United States, and the reason "prior abdominal surgery" is the highest-yield stem detail
• Incarcerated hernias: inguinal, femoral, umbilical, and incisional; the leading cause of SBO in a patient with a virgin abdomen. Femoral hernias have narrow necks and strangulate disproportionately
• Volvulus: axial twist around a mesenteric pedicle creating a closed-loop; sigmoid (elderly, institutionalized, chronically constipated, Chagas megacolon) and cecal (younger patients with incomplete right colon fixation)
• Carcinomatosis / extrinsic tumor: peritoneal implants from ovarian, gastric, or colorectal primaries; endometriosis in reproductive-age women

Intramural causes

• Crohn's strictures: transmural inflammation with fibrostenosis, classically terminal ileum
• Malignancy: annular apple-core left-sided colorectal adenocarcinoma dominates LBO; small bowel adenocarcinoma, carcinoid, and lymphoma are rarer
• Diverticular stricture after repeated sigmoid inflammation; radiation enteritis after pelvic radiotherapy; intramural hematoma in anticoagulated or blunt-trauma patients

Intraluminal causes

• Gallstone ileus: a large stone erodes through a cholecystoenteric fistula and impacts at the ileocecal valve
• Bezoar, foreign body, fecal impaction; meconium ileus in cystic fibrosis; intussusception (children: ileocolic, often postviral; adults: always look for a pathologic lead point such as a tumor or Meckel diverticulum)

Small Bowel Obstruction

• Mechanical obstruction from adhesions (post-surgical, most common), hernias, malignancy, Crohn's disease, or volvulus creates a physical barrier; proximal bowel dilates with accumulated fluid and gas while distal bowel collapses
• Intestinal wall changes include increased intraluminal pressure leading to mucosal edema, impaired blood flow, and potential ischemic necrosis if complete obstruction persists >6-12 hours
• Fluid and electrolyte sequestration into the bowel lumen and interstitial space causes third-spacing, hypovolemia, and dehydration; bacterial overgrowth and increased intraluminal pressure promote bacterial translocation across the compromised mucosa
• Strangulation occurs when venous and arterial blood flow are compromised (closed-loop obstruction), leading to transmural necrosis and peritonitis—a surgical emergency distinguishable by fever, severe pain, and elevated lactate

Large Bowel Obstruction

• Functional and mechanical obstruction from malignancy (60%), diverticular strictures, volvulus (sigmoid most common), or pseudo-obstruction (Ogilvie syndrome)
• Increased intraluminal pressure in a closed system with intact ileocecal valve creates extremely high wall tension; the cecum (widest diameter) is most vulnerable to perforation when diameter exceeds 10-12 cm (risk of perforation increases significantly >12 cm)
• Fecal impaction and gas-liquid levels develop more distally, with marked proximal small bowel dilation if obstruction is complete
• Perforation risk is substantially higher than SBO due to thin cecal wall and high pressures, leading to fecal peritonitis with high mortality

Small Bowel Obstruction

• Colicky abdominal pain (crampy, intermittent) is the hallmark, typically periumbilical or supraumbilical; pain occurs in waves as the bowel contracts against the obstruction
• Nausea and vomiting occur early and are projectile if high obstruction; vomiting may become feculent (suggesting distal SBO or prolonged obstruction with bacterial overgrowth)
• Abdominal distension develops over hours and is more pronounced with distal SBO; palpable "ladder-like" bowel loops may be apparent
• Absolute constipation (no flatus or stool) occurs with complete obstruction; partial obstruction may allow flatus and minimal stool passage
• Tachycardia, hypotension, and signs of dehydration indicate significant third-spacing; fever and severe focal tenderness suggest strangulation (surgical emergency requiring urgent intervention)

Large Bowel Obstruction

• Insidious onset of constipation progressing over days to weeks, often with gradual distension and dull abdominal pain (less colicky than SBO)
• Abdominal distension is marked and progressive; patient may report paradoxical diarrhea from fecal impaction with overflow
• Nausea and vomiting occur late, only after prolonged obstruction develops
• Severe abdominal pain, fever, leukocytosis, and peritoneal signs indicate perforation—a surgical emergency with mortality >30%
• Critical pearl: Elderly patients with LBO may present subtly with minimal pain but significant distension; malignancy should be strongly suspected

Clinical History and Examination

• Previous abdominal surgery (highest risk for adhesive SBO), family history of colorectal cancer, or chronic constipation suggests etiology
• Examination for hernias (including incarcerated hernias), peritoneal signs, and rectal examination (impaction, mass, blood) is mandatory
• High-pitched bowel sounds ("tinkling") in SBO or absent sounds in paralytic ileus/late SBO help guide diagnosis

Imaging—First-Line

• Abdominal X-ray (supine and upright/decubitus) shows:
• SBO: Dilated small bowel loops (valvulae conniventes crossing entire width), air-fluid levels on upright films, collapsed distal colon; "string of pearls" sign (small air bubbles in clustered bowel)
• LBO: Dilated colon with haustra (not crossing entire diameter), transition point to collapsed distal colon, possible bird's-beak appearance if volvulus
• Free air suggests perforation (surgical emergency)
• Sensitivity for obstruction is ~70%; normal films do NOT exclude obstruction

Imaging—Advanced

• CT abdomen/pelvis with IV contrast is gold standard (95% sensitivity) for both diagnosis and etiology; shows:
• Transition point (location of obstruction)
• Bowel wall enhancement and thickening (assess viability)
• Strangulation signs: Mesenteric edema, free fluid, pneumatosis intestinalis (ominous finding), portomesenteric venous gas
• Malignancy, diverticular disease, or other specific etiologies
• Obtain CT early if clinical uncertainty exists or if imaging will change management

Laboratory Studies

• CBC: Leukocytosis (especially if strangulation/perforation)
• BMP: Electrolyte derangements from vomiting (hypochloremic, hypokalemic metabolic alkalosis common), elevated BUN/creatinine ratio (dehydration)
• Lactate: Elevated lactate suggests ischemia/strangulation (sensitivity ~50% but high specificity); useful prognostic marker
• Urinalysis: Rule out alternative diagnoses (UTI, kidney stone)

Initial Management (Nasogastric Decompression & Resuscitation)

• Nil per os (NPO) immediately upon diagnosis
• Nasogastric tube (NGT) placed to suction for gastric and proximal bowel decompression; reduces vomiting, improves comfort, and allows assessment of output
• Aggressive IV fluid resuscitation with normal saline or lactated Ringer's to correct hypovolemia and third-spacing; target urine output 0.5 mL/kg/hr; replace ongoing losses from NGT output
• Electrolyte correction: Replete potassium and other lost ions; correct metabolic alkalosis (hypochloremic, hypokalemic)
• Broad-spectrum antibiotics (e.g., ceftriaxone 1-2 g IV q12h + metronidazole 500 mg IV q6-8h) if signs of strangulation, perforation, or sepsis; antibiotics cover gram-negative and anaerobic bacteria

Small Bowel Obstruction—Conservative Management

• Partial/adhesive SBO without strangulation: 70-80% resolve with NPO, NGT, and IV fluids ("bowel rest") within 2-7 days; serial abdominal exams and imaging assess progress
• Criteria for continued conservative trial: Improving pain, tolerating some oral intake, no signs of strangulation, adequate urine output
• Transition to diet: Begin sips of water if tolerated; advance to clear liquids then regular diet as obstruction resolves (bowel sounds return, flatus/stool passage)
• Monitor closely: Serial abdominal exams q4-6h, reassess imaging if deterioration or no improvement after 48-72 hours

Indications for Surgical Intervention

• Complete obstruction (particularly early SBO from hernias or volvulus)
• Strangulation (fever, severe pain, lactate >2 mmol/L, peritoneal signs, imaging findings)
• Failed conservative management after 48-72 hours of adequate trial (deteriorating clinical status, recurrent v

Emergencies of the disease itself

• Strangulation and transmural infarction (surgical emergency): closed-loop obstruction raises intramural pressure above venous then arterial perfusion pressure. Signals are constant rather than colicky pain, fever, tachycardia, peritoneal signs, rising lactate, and CT findings of reduced wall enhancement, mesenteric edema, or pneumatosis intestinalis with portal venous gas
• Perforation with feculent peritonitis (surgical emergency): Laplace's law makes the thin-walled cecum the rupture point in LBO with a competent ileocecal valve; free air under the diaphragm or septic shock signals it
• Hypovolemic shock and AKI: third-spacing plus emesis and NG losses; oliguria and a rising BUN:creatinine ratio are the tell
• Aspiration pneumonitis (emergency): high-volume feculent emesis in an obtunded or supine patient; new hypoxemia and dependent infiltrates
• Bacterial translocation and sepsis across ischemic mucosa, producing bacteremia without frank perforation

Complications of treatment

• NG tube injury: epistaxis, sinusitis, and pharyngeal discomfort; continued high output drives worsening hypochloremic, hypokalemic metabolic alkalosis if losses are not replaced
• Iatrogenic enterotomy and anastomotic leak: adhesiolysis of matted bowel risks unrecognized enterotomy; leak classically declares itself on postoperative day 5–7 with fever, tachycardia, ileus, leukocytosis, or feculent drain output (emergency)
• Prolonged postoperative ileus, surgical site infection, incisional hernia, enterocutaneous fistula, and recurrent adhesive obstruction — surgery for adhesions creates new adhesions
• Short bowel syndrome after extensive resection for infarcted bowel
• Procedure-specific: self-expanding metal colonic stents used as a bridge to surgery in obstructing colon cancer (NCCN) can perforate or migrate; neostigmine for Ogilvie syndrome causes bradycardia and bronchospasm, so ASCRS advises continuous cardiac monitoring with atropine available and exclusion of mechanical obstruction first; endoscopic detorsion of sigmoid volvulus carries a high recurrence rate without definitive resection.

• Adhesions are the default answer for SBO in a patient with a surgical scar; in a virgin abdomen, examine the groin — an incarcerated hernia is the answer, and femoral hernias strangulate most readily.
• Upright film buzzwords: air-fluid levels at differing heights in dilated loops give the step-ladder pattern; string of pearls reflects trapped gas bubbles along the valvulae conniventes. Plain films are insensitive — a normal film never excludes obstruction, and CT is the next step when the diagnosis or etiology is unclear.
• Single best next step in stable adhesive SBO is NPO, NG decompression, and resuscitation, with a water-soluble contrast (Gastrografin) challenge; EAST guidance notes that contrast reaching the colon on follow-up imaging predicts nonoperative resolution and can itself be therapeutic. Contrast is also a decision tool — failure to progress means the operating room.
• Do not wait on strangulation. Constant pain out of proportion, fever, leukocytosis, lactic acidosis, or peritonitis mandates operation regardless of how recently conservative therapy began.
• Volvulus split: sigmoid volvulus shows the coffee-bean / bent inner tube sign pointing to the RUQ and is first managed with endoscopic detorsion, followed by same-admission sigmoidectomy because recurrence is high (ASCRS). Cecal volvulus is not reliably fixed endoscopically and requires surgery — usually ileocecal resection.
• The one association examiners love: new LBO in an adult is colorectal adenocarcinoma until proven otherwise; after the obstruction resolves, complete colonoscopy to evaluate the remaining colon.
• Gallstone ileus = Rigler triad (pneumobilia, small bowel obstruction, ectopic radiopaque stone) in an elderly woman with prior biliary colic.
• Common distractors: ileus has gas distributed diffusely through colon and rectum with no transition point — do not treat it operatively; Ogilvie syndrome is colonic pseudo-obstruction, and neostigmine is only appropriate once mechanical obstruction is excluded. Barium is contraindicated if perforation is suspected.