Acute Abdomen, Pancreatitis & Biliary Emergencies

Biliary colic results from transient impaction of a gallstone in the cystic duct during postprandial cholecystokinin (CCK)-mediated gallbladder contraction, causing waxing-and-waning right upper quadrant (RUQ) pain that resolves within 4 to 6 hours as the stone falls back into the gallbladder lumen; fever, leukocytosis, and Murphy sign are absent. When cystic duct obstruction persists beyond 6 hours, trapped bile salts (lysolecithin) and prostaglandins trigger transmural gallbladder wall ischemia and inflammation—Acute Calculous Cholecystitis—presenting with unremitting RUQ pain radiating to the right scapula/shoulder (phrenic C3–C5 and T5–T9 celiac referral), fever, leukocytosis, normal bilirubin/alkaline phosphatase, and a positive sonographic Murphy sign. If RUQ ultrasound (1st-line test, showing gallstones, wall thickening >3 mm, and pericholecystic fluid) is equivocal, a Cholescintigraphy (HIDA) scan is the gold-standard confirmatory test (non-visualization of the gallbladder at 60 minutes confirms cystic duct obstruction).

Migration of a gallstone into the common bile duct (choledocholithiasis) produces obstructive jaundice with elevated conjugated (direct) bilirubin, alkaline phosphatase, and GGT, mild-to-moderate AST/ALT elevation, and a dilated common bile duct (>6 mm) on RUQ ultrasound; definitive management is ERCP sphincterotomy and stone extraction followed by laparoscopic cholecystectomy during the same hospitalization. When stasis behind an obstructed CBD allows enteric bacteria (E. coli, Klebsiella, Enterococcus) to ascend from the duodenum, Acute Ascending Cholangitis develops—presenting with Charcot triad (fever/rigors, RUQ pain, and jaundice) and progressing to Reynolds pentad (adding septic shock/hypotension and altered mental status). Ascending cholangitis requires immediate broad-spectrum IV antibiotics (piperacillin-tazobactam or ceftriaxone + metronidazole) and urgent biliary decompression via ERCP within 24 hours.

Gallstones (40%, typically small stones <5 mm migrating through the ampulla of Vater) and chronic alcohol use (30%, which increases zymogen granule secretion, thickens pancreatic ductal plugs, and sensitizes acinar cells to CCK) account for 70% of acute pancreatitis cases. Among the remaining etiologies, hypertriglyceridemia triggers acute pancreatitis only when serum triglycerides exceed 1,000 mg/dL (pancreatic lipase hydrolyzes chylomicrons in pancreatic capillaries into toxic free fatty acids that injure vascular endothelium and acinar membranes; treated acutely with an intravenous regular insulin infusion, which activates lipoprotein lipase [LPL] to clear triglycerides, followed by fenofibrate for outpatient maintenance). Other classic board triggers include post-ERCP pancreatitis (prevented with rectal indomethacin), hypercalcemia (Ca2+ activates calcineurin and trypsinogen), scorpion stings (Tityus trinitatis), and medications (azathioprine/6-MP, valproate, didanosine, thiazides, furosemide, GLP-1 agonists).

During the first week of severe acute pancreatitis, circulating phospholipase A2 degrades pulmonary alveolar surfactant and injures the alveolar-capillary membrane, making Acute Respiratory Distress Syndrome (ARDS) and pleural effusions the leading cause of early mortality. Locally, fluid collections are classified by the Revised Atlanta Criteria based on duration (<4 weeks vs. >=4 weeks) and necrosis: an acute peripancreatic fluid collection (<4 weeks, non-necrotic) matures after >=4 weeks into an encapsulated Pancreatic Pseudocyst (lined by fibrous granulation tissue without an epithelial lining; asymptomatic pseudocysts Regardless of size are observed, while symptomatic/infected pseudocysts undergo endoscopic transgastric cystogastrostomy drainage). Conversely, acute necrotic collections mature after >=4 weeks into Walled-Off Necrosis (WON). Extension of inflammation to the splenic vein (running immediately posterior to the pancreatic tail) causes splenic vein thrombosis and isolated gastric fundal varices.

Peptic ulcer perforation presents with sudden, catastrophic epigastric pain that rapidly generalizes into a rigid, board-like surgical abdomen with absent bowel sounds and rebound tenderness. Anatomical location dictates the complication: Anterior duodenal bulb ulcers perforate freely into the peritoneal cavity, producing pneumoperitoneum (free air under the diaphragm on upright chest X-ray or left lateral decubitus film) and referred shoulder pain via the phrenic nerve (C3–C5). When a patient presents with a rigid abdomen and free air on upright CXR, proceed directly to emergent exploratory laparotomy (Graham omental patch repair) after IV fluids, broad-spectrum antibiotics, IV proton pump inhibitor, and nasogastric decompression. Conversely, Posterior duodenal ulcers penetrate into the retroperitoneum, causing either acute pancreatitis or massive upper GI hemorrhage from erosion into the Gastroduodenal Artery (a branch of the common hepatic artery).

Acute appendicitis is caused by luminal obstruction of the vermiform appendix—by lymphoid follicular hyperplasia in children/adolescents (often post-viral) and by a fecalith (appendicolith) in adults—leading to venous congestion, bacterial overgrowth (E. coli, Bacteroides fragilis), transmural ischemia, and perforation within 24–48 hours. Clinical presentation follows a classic chronological sequence: anorexia and periumbilical pain FIRST, followed by nausea/vomiting SECOND, and migration to McBurney point (RLQ) with low-grade fever THIRD (if vomiting precedes pain, suspect gastroenteritis or bowel obstruction instead). Diagnostic imaging modality is strictly age- and pregnancy-dependent: (1) Non-pregnant Adults: CT Abdomen and Pelvis with IV contrast (highest sensitivity > 98%; shows dilated non-compressible appendix > 6 mm, wall enhancement, periappendiceal fat stranding); (2) Children and Pregnant Women: Graded-compression Ultrasound first-line, followed by MRI without gadolinium if ultrasound is inconclusive. Treat with laparoscopic appendectomy + preoperative cefoxitin or ceftriaxone/metronidazole (note: if presenting late > 5 days with a contained periappendiceal phlegmon or abscess, treat with IV antibiotics + CT-guided percutaneous drainage followed by interval appendectomy 6–8 weeks later).

Sigmoid diverticulitis results from micro- or macro-perforation of false colonic diverticula (herniations of mucosa and submucosa through the muscularis propria where vasa recta penetrate) in the high-pressure sigmoid colon, presenting as Left Lower Quadrant (LLQ) pain ('left-sided appendicitis'), fever, and leukocytosis. The diagnostic test of choice is CT Abdomen and Pelvis with oral and IV contrast. Colonoscopy and barium enema are strictly contraindicated during acute diverticulitis due to risk of converting a microperforation into frank peritonitis; however, a screening colonoscopy must be performed 6–8 weeks after resolution to exclude underlying colorectal adenocarcinoma. Uncomplicated diverticulitis responds to bowel rest and oral antibiotics (ciprofloxacin + metronidazole or amoxicillin-clavulanate). Complicated diverticulitis with a diverticular abscess < 3–4 cm is treated with IV antibiotics alone; an abscess ≥ 4 cm requires CT-guided percutaneous drainage; and generalized purulent or feculent peritonitis (Hinchey Stage III/IV) or free perforation requires emergent Hartmann procedure (sigmoid resection with end colostomy).

Two systemic complications of acute pancreatitis carry major prognostic weight on board exams: (1) Hypocalcemia occurs because activated pancreatic lipase leaks into the retroperitoneal and mesenteric adipose tissue, enzymatically cleaving triglycerides into glycerol and free fatty acids. Circulating ionized calcium (Ca²⁺) avidly binds to these negatively charged free fatty acids, precipitating insoluble calcium-soap salts ( Enzymatic Fat Necrosis / Saponification—histologically visible as chalky-white areas with basophilic calcium deposits and shadowy 'ghost' adipocytes). Severe hypocalcemia (< 8.0 mg/dL at 48 hours is a classic Ranson criterion) manifests with neuromuscular irritability, Chvostek sign, Trousseau sign, and QTc prolongation. (2) Acute Respiratory Distress Syndrome (ARDS) and pleural effusions occur because circulating phospholipase A₂ degrades pulmonary alveolar lecithin (dipalmitoylphosphatidylcholine surfactant) and disrupts the alveolar-capillary membrane.

Under the Revised Atlanta Classification, acute pancreatitis is stratified into Mild (no organ failure or local complications), Moderately Severe (transient organ failure resolving within < 48 hours and/or local complications), and Severe (persistent organ failure lasting > 48 hours involving cardiovascular shock, respiratory PaO₂/FiO₂ ≤ 300, or renal creatinine ≥ 1.9 mg/dL). At the bedside within the first 24 hours, the BISAP score predicts mortality using 5 simple variables: BUN > 25 mg/dL, Impaired mental status, SIRS (≥ 2 criteria), Age > 60 years, and Pleural effusion on CXR (score ≥ 3 indicates high mortality). An admission hemoconcentration (Hematocrit > 44%) and a rising BUN (> 20–25 mg/dL) over the first 24 hours are the two single most sensitive laboratory markers of severe third-space intravascular volume depletion and impending pancreatic necrosis. Note that absolute serum lipase/amylase levels DO NOT correlate with disease severity!

When serum triglycerides exceed 1,000 mg/dL, chylomicrons sludge within pancreatic capillaries and release cytotoxic free fatty acids. Beyond standard NPO status and IV Lactated Ringer's resuscitation, acute management requires rapidly lowering serum triglycerides below 500 mg/dL. The first-line pharmacologic treatment—even in non-diabetic patients—is a continuous IV Regular Insulin infusion (0.1–0.3 units/kg/hr with concurrent IV 5% Dextrose [D₅W] to maintain euglycemia). Insulin directly upregulates endothelial Lipoprotein Lipase (LPL) activity, which accelerates chylomicron and VLDL triglyceride clearance from the bloodstream while inhibiting hormone-sensitive lipase in adipocytes. Once the patient tolerates oral intake, initiate a fibrate (Fenofibrate, a PPAR-α agonist that upregulates LPL) for long-term prevention.

Historically, patients with acute pancreatitis were kept strictly NPO for weeks to 'rest the pancreas.' Modern randomized trials prove this dogma is harmful. Prolonged bowel rest and Total Parenteral Nutrition (TPN) cause intestinal mucosal villous atrophy, breakdown of enterocyte tight junctions, and translocation of enteric Gram-negative bacteria across the gut wall into the mesenteric lymphatics—the exact mechanism that seeds sterile pancreatic necrosis into infected pancreatic necrosis. In mild-to-moderate pancreatitis, initiate a low-fat solid oral diet within 24–48 hours as soon as nausea subsides and pain is decreasing (bowel sounds or normalization of lipase are NOT required). In severe pancreatitis unable to tolerate oral intake by 72 hours, initiate enteral tube feeding via a nasogastric (NG) or nasojejunal (NJ) tube, which preserves mucosal barrier integrity and significantly reduces multi-organ failure and mortality compared to TPN.

In gallstone pancreatitis, most small stones (< 5 mm) pass spontaneously through the ampulla into the duodenum. Therefore, routine urgent ERCP is NOT indicated in uncomplicated gallstone pancreatitis. Emergent ERCP with endoscopic sphincterotomy and stone extraction (within 24 hours) is indicated strictly in two scenarios: (1) Concomitant Acute Ascending Cholangitis (Charcot triad: fever, jaundice, RUQ pain), or (2) Persistent Common Bile Duct obstruction (visible CBD stone on imaging or progressively rising direct bilirubin > 4–5 mg/dL). However, every surgical candidate with mild gallstone pancreatitis MUST undergo laparoscopic cholecystectomy during the SAME hospital admission prior to discharge; delaying cholecystectomy to the outpatient setting carries a 25–30% risk of recurrent biliary pancreatitis or cholangitis within 6 weeks.

Roughly 15–20% of patients develop necrotizing pancreatitis, characterized on contrast-enhanced CT (performed at 48–72 hours if the patient fails to improve—never earlier, as necrosis takes 72 hours to demarcate) by areas of pancreatic parenchyma that fail to enhance with IV contrast. When enteric bacteria translocate into necrotic tissue (typically in week 2 or 3), the patient develops spiking fevers, worsening leukocytosis, and retroperitoneal gas bubbles within the necrotic collection on CT. Immediately initiate IV Carbapenems (Meropenem or Imipenem-Cilastatin) or a fluoroquinolone/cefepime plus metronidazole, as these agents achieve superior penetration into necrotic pancreatic tissue. Rather than immediate open laparotomy (which carries > 50% mortality), employ the 'Step-Up' approach: start with CT-guided percutaneous catheter drainage or endoscopic transluminal drainage, delaying minimally invasive video-assisted retroperitoneal debridement (necrosectomy) until 4 weeks when the necrosis has walled off.

In Acute Mesenteric Ischemia (AMI), the classic patient is an elderly individual with atrial fibrillation (often subtherapeutic on warfarin or recently cardioverted) or recent myocardial infarction who develops sudden, severe periumbilical pain with a soft, non-tender abdomen on initial palpation. Laboratory findings reflect bowel wall ischemia and bacterial translocation: marked leukocytosis (> 15,000–20,000/μL with left shift), hemoconcentration, elevated serum amylase/phosphate/LDH, and High Anion Gap Metabolic Acidosis from elevated serum L-lactate. Note that lactic acidosis and peritoneal signs (rebound/guarding) are LATE findings that signify progression to irreversible transmural bowel necrosis! In a hemodynamically stable patient without frank peritonitis, the diagnostic gold standard is biphasic CT Angiography (CTA) of the abdomen and pelvis (without oral contrast), which reveals an abrupt SMA filling defect, bowel wall thickening, pneumatosis intestinalis (gas within the bowel wall), and portal venous gas. If frank peritonitis is present, proceed straight to exploratory laparotomy.

Unlike the small intestine (supplied richly by SMA arcades), the colon possesses two vulnerable watershed border zones where terminal arterial branches anastomose poorly, making them exquisitely sensitive to systemic hypotension or low-flow states (Ischemic Colitis): (1) Griffiths Point at the Splenic Flexure is the watershed junction between the Middle Colic Artery (branch of the SMA supplying the midgut) and the Left Colic Artery (branch of the Inferior Mesenteric Artery [IMA] supplying the hindgut), linked by the tenuous Marginal Artery of Drummond; and (2) Sudeck Point at the Rectosigmoid Junction is the watershed junction between the Sigmoid/Superior Rectal Artery (branch of the IMA) and the Middle/Inferior Rectal Arteries (branches of the Internal Iliac Artery). Notably, because the lower rectum receives robust collateral flow from the internal iliac arteries, the distal rectum is classically spared in ischemic colitis—and ischemic colitis frequently occurs after open Abdominal Aortic Aneurysm (AAA) repair when the IMA is sacrificed.

Whereas acute SMA ischemia of the small bowel is a catastrophic surgical emergency with > 60% mortality, Ischemic Colitis involves superficial mucosal and submucosal sloughing in > 80% of cases and resolves with conservative medical therapy. Patients present with sudden left-sided cramping abdominal pain followed within 24 hours by passage of bright red or maroon bloody stools. Abdominal CT or plain radiographs demonstrate colonic wall thickening and 'thumbprinting'—smooth, rounded submucosal nodules projecting into the colonic lumen caused by submucosal edema and hemorrhage—localized segmentally to the splenic flexure or descending/sigmoid colon. Cautious flexible sigmoidoscopy or colonoscopy (with minimal air insufflation to avoid perforation) reveals edematous, friable mucosa with longitudinal ulcerations ('single-stripe sign') and cyanotic nodules with rectal sparing. Over 80% recover with bowel rest (NPO), IV crystalloid hydration, hemodynamic optimization, and empiric IV antibiotics; surgery (colectomy) is reserved for the 20% who develop transmural gangrene, peritonitis, or stricture.