Acute Abdomen — Surgical Emergencies

• Visceral perforation and peritonitis: When a hollow viscus ruptures (perforated peptic ulcer, appendix, diverticulum, or bowel), intraluminal contents including bacteria, gastric acid, and fecal material spill into the peritoneal cavity. This triggers immediate bacterial translocation and activation of the innate immune system through pathogen-associated molecular patterns (PAMPs) binding to pattern recognition receptors (PRRs) on peritoneal macrophages and mesothelial cells. The peritoneum, with its large surface area (~1.7 m²) and rich vascularization, rapidly absorbs bacterial toxins including lipopolysaccharide (LPS) from gram-negative organisms. This initiates a cascade of pro-inflammatory cytokine release (TNF-α, IL-1β, IL-6, IL-8) leading to local peritoneal inflammation and, if untreated, systemic inflammatory response syndrome (SIRS) progressing to sepsis and septic shock. The magnitude of inflammation correlates with bacterial burden and acidic stomach contents, explaining why early intervention dramatically improves outcomes. Prostaglandin and bradykinin release locally cause visceral pain transmission via visceral afferent fibers in the splanchnic nerves, perceived as diffuse, poorly localized periumbilical pain initially.

• Acute vascular compromise and ischemia: Acute mesenteric ischemia (arterial or venous occlusion) or strangulated bowel obstruction creates a time-critical surgical emergency because intestinal tissue lacks collateral circulation in certain segments. In acute arterial mesenteric ischemia, thromboembolism or low-flow states reduce oxygen delivery below the critical threshold required for aerobic respiration. Ischemic enterocytes rapidly switch to anaerobic metabolism, accumulating lactate and hydrogen ions (local acidosis) while depleting ATP-dependent ion pumps. Cellular necrosis occurs within 2-4 hours of complete arterial occlusion, progressing from mucosa to transmural involvement. Ischemic tissue triggers release of intracellular contents including phosphate, potassium, and myoglobin into portal circulation. Reperfusion injury—occurring either spontaneously or during surgical intervention—paradoxically worsens tissue damage through reactive oxygen species (ROS) generation, particularly superoxide radicals via NADPH oxidase and xanthine oxidase pathways. Bacterial translocation across the compromised intestinal barrier introduces endotoxin into the mesenteric venous system, seeding the liver and systemic circulation. The pain in mesenteric ischemia is characteristically severe and out of proportion to physical findings—a cardinal clinical pearl reflecting the transition from reversible ischemia to irreversible necrosis.

• Mechanical bowel obstruction with transmural pressure necrosis: When intestinal luminal contents accumulate proximal to a mechanical obstruction (adhesions, herniation, malignancy, volvulus), intraluminal pressures increase progressively. This increased pressure compromises mucosal blood flow through compression of capillary beds, initially causing reversible mucosal edema. If obstruction persists beyond 6-12 hours, transmural pressure necrosis develops as venous outflow becomes compromised while arterial inflow continues, creating a compartment syndrome-like state within the bowel wall. Strangulation, defined as obstruction with vascular compromise, represents a surgical emergency because viable bowel is lost. The closed-loop obstruction (segment blocked at two points) carries highest risk for strangulation compared to simple obstruction because both entry and exit points are compromised. Ischemic mucosa becomes permeable to gram-negative bacteria and endotoxin; bacterial translocation progresses to peritonitis within hours. Dehydration occurs from sequestration of fluid into the obstructed bowel segment (third-spacing) and vomiting, reducing circulating blood volume and triggering hypovolemic shock if untreated.

• Inflammatory propagation in contained perforation: When a perforation is walled off by adjacent omentum and viscera before generalizing to the entire peritoneum, a localized abscess forms. Granulomatous inflammation develops as the body attempts to contain bacterial invasion; macrophages, lymphocytes, and fibroblasts organize around the necrotic core. Multiple cytokine gradients (TNF-α, IL-6) are established, drawing neutrophils toward the focus but creating a hostile milieu that impairs antibiotic penetration. Abscesses represent a walling-off mechanism that paradoxically may prevent immediate sepsis while creating a source for persistent or recurrent infection if not adequately drained. The inflammatory mass may erode into adjacent structures, creating fistulas (e.g., diverticular abscess → colovesical fistula with pneumaturia).

• Acute inflammation and histamine-mediated response: In acute cholecystitis (cystic duct obstruction by gallstone with secondary inflammation), mast cells and resident macrophages in the gallbladder wall degranulate, releasing histamine, bradykinin, and other inflammatory mediators. Even without bacterial superinfection, the chemical irritation from concentrated bile and phospholipid accumulation triggers edema and neutrophilic infiltration. In acute pancreatitis, premature activation of pancreatic proenzymes (trypsinogen → trypsin) within the gland triggers autodigestion; released pancreatic enzymes (lipase, amylase, elastase, phospholipase A₂) directly damage acinar cells and activate the coagulation and complement cascades. Phospholipase A₂ is particularly destructive, generating lysophospholipids that cause fat necrosis and further tissue damage. Systemic absorption of these enzymes and their inflammatory byproducts can trigger SIRS and multi-organ dysfunction even in the absence of infection.

Surgical emergencies requiring operative intervention

• Perforated viscus (peptic ulcer disease, appendicitis, diverticulitis, inflammatory bowel disease, malignancy, trauma): Peptic ulcer perforation accounts for 5-10% of acute abdomen presentations and results from acid-peptic erosion through the full thickness of the gastric or duodenal wall, typically on the anterior wall where there is less protection. Risk factors include NSAID use (responsible for 60% of PUD in developed countries), H. pylori infection (more common in developing nations), stress ulceration in ICU patients, and rare conditions like Zollinger-Ellison syndrome. Perforated appendicitis represents a failure to diagnose acute appendicitis before transmural inflammation and necrosis occur; perforation risk increases with duration of symptoms (perforation rate 16% at 24 hours, 32% at 48 hours). Perforated diverticulitis typically occurs in the sigmoid colon in Western populations and represents erosion through an inflamed diverticulum; patients with immunosuppression have higher perforation risk. Perforated colorectal cancer should be suspected in elderly patients with acute abdomen and prior history of weight loss or anemia. Inflammatory bowel disease (Crohn's disease more than ulcerative colitis) can present with perforation, particularly during severe flares or toxic megacolon.

• Acute mesenteric ischemia (arterial thrombosis/embolism, venous thrombosis, nonocclusive): Represents 1-2% of acute abdomen cases but carries mortality approaching 60-80% when diagnosis is delayed. Acute arterial mesenteric ischemia from cardiac embolism (atrial fibrillation accounts for 80-90% of emboli, followed by left ventricular thrombus post-MI) or atherosclerotic thrombosis predominantly affects the superior mesenteric artery (SMA) which supplies the small bowel and proximal colon. Risk factors include advanced age, atherosclerotic disease, hypercoagulable states, cardioembolic sources, and inherited thrombophilias (Factor V Leiden, prothrombin G20210A mutation). Acute mesenteric venous thrombosis (MVT) is less common but particularly affects patients with hypercoagulable states (malignancy, inherited thrombophilia, oral contraceptives), portal hypertension, recent abdominal surgery, or inflammatory conditions (pancreatitis, diverticulitis). Nonocclusive mesenteric ischemia (NOMI) occurs in the setting of systemic hypoperfusion (cardiogenic shock, sepsis, major hemorrhage) without angiographic evidence of vascular occlusion; splanchnic vasoconstriction in response to hypotension reduces mesenteric blood flow below critical threshold.

• Strangulated bowel obstruction (incarcerated hernia, volvulus, internal herniation, adhesions): Mechanical small bowel obstruction accounts for 15-20% of acute abdominal surgical emergencies; 75% are due to adhesions from prior surgery, 8-10% from incarcerated hernia, and 5% from malignancy. Femoral hernias carry the highest strangulation risk among external hernias (40%) followed by inguinal (10%) because of the rigid anatomic boundaries of the femoral ring. Internal herniation through peritoneal defects or within mesenteric attachments can lead to rapid strangulation with minimal warning. Sigmoid volvulus occurs predominantly in elderly patients and those with chronic constipation or megacolon; carries high mortality if not recognized and decompressed promptly. Closed-loop obstructions from adhesions or herniation (blockage at two points) progress to strangulation faster than simple obstructions because of rapid pressure increase and rapid vascular compromise.

• Acute appendicitis with perforation: While non-perforated appendicitis is technically a surgical condition, perforation represents a surgical emergency with increased morbidity. Appendicitis results from obstruction of the appendiceal lumen (by lymphoid hyperplasia, fecalith, malignancy, or stricture) leading to increased intraluminal pressure, bacterial overgrowth, and transmural inflammation. Peak incidence is 10-19 years but can occur at any age. Perforation occurs in 20-30% of cases if surgery is delayed >48 hours and creates localized or generalized peritonitis. In young children (<5 years), elderly patients (>65 years), and pregnant women, diagnostic delay is more common, explaining higher perforation rates in these populations.

• Acute cholecystitis and biliary emergencies: Acute cholecystitis (usually from cystic duct obstruction by gallstone in 90% of cases, or acalculous in 10% associated with sepsis, ischemia, or critical illness) can progress to empyema, perforation, or bile peritonitis if untreated. Gallstone pancreatitis results when a stone transiently obstructs the ampulla of Vater; represents the most common cause of acute pancreatitis in regions with high gallstone prevalence. Acute ascending cholangitis (fever, jaundice, right upper quadrant pain—Charcot's triad) from biliary obstruction (stone, stricture, malignancy) with bacterial superinfection represents a septic emergency requiring urgent biliary decompression.

• Ruptured abdominal aortic aneurysm (AAA): Represents a vascular surgical emergency with extremely high mortality (80-90% overall, 40-50% in those reaching the operating room). Risk factors include male gender (male:female ratio 5:1), age >60, hypertension, smoking, atherosclerotic disease, and family history of AAA. Expansion and rupture risk increases exponentially with aneurysm diameter; aneurysms >5.5 cm have yearly rupture risk of 10-15%.

• Acute pancreatitis (severe): While most cases resolve with supportive care, severe acute pancreatitis (APACHE II score ≥8, presence of organ failure) complicated by infected pancreatic necrosis or acute fluid collections requiring intervention may require surgical debridement. Gallstones cause 40-50% of acute pancreatitis in developed countries; alcohol causes 30-40%, with remaining cases attributable to hypertriglyceridemia, medications, autoimmune disease, or are idiopathic.

Risk factors for surgical emergencies

• Advanced age (>65 years): impaired immune response, higher perforation rates, comorbidities increasing operative risk
• Prior abdominal surgery: adhesions (cause 75% of small bowel obstructions)
• Anticoagulation or thrombophilia: increased VTE risk and mesenteric venous thrombosis
• Immunosuppression: increased perforation risk in IBD and diverticulitis
• Chronic corticosteroid use: increased perforation and poor wound healing
• Male gender: higher AAA incidence and appendicitis perforation rates

Cardinal symptoms

• Acute abdominal pain: The hallmark of surgical abdomen, with character and location providing crucial diagnostic clues. Visceral pain (from splanchnic innervation of abdominal organs) is typically described as dull, aching, or cramping and is poorly localized to the periumbilical region (foregut structures), epigastrium (stomach, proximal duodenum), or lower abdomen (hindgut). In contrast, parietal pain (from peritoneal irritation activating somatic nerve fibers in the parietal peritoneum) is sharp, well-localized, and exacerbated by movement or coughing due to direct stimulation of the parietal peritoneum. Classic examples include the initial periumbilical visceral pain of acute appendicitis followed by sharp parietal pain in the right lower quadrant as inflammation extends to the parietal peritoneum. Pain that is "out of proportion" to physical findings is pathognomonic for acute mesenteric ischemia and should trigger immediate vascular imaging. The onset of pain is critical: sudden onset suggests perforation or acute vascular occlusion, while gradual onset suggests obstruction or inflammatory process. Colicky pain (crescendo-decrescendo pattern) indicates mechanical obstruction with peristalsis against a fixed obstruction; persistent pain suggests perforation or ischemia. Post-prandial pain in a patient with chronic symptoms followed by acute exacerbation with obstruction suggests malignancy. Severe pain followed by relief and then return of pain suggests strangulated bowel (pain from initial ischemia, relief as nerves die, return of pain from peritonitis).

• Nausea and vomiting: Reflects visceral irritation, obstruction, or systemic effects of peritonitis. Bilious vomiting indicates obstruction distal to the pylorus; non-bilious vomiting may occur with gastric outlet obstruction or gastritis. Persistent vomiting causes electrolyte abnormalities (hypokalemia, hypochloremia, alkalosis from loss of gastric acid) and severe dehydration. Feculent vomiting (late vomiting with fecal appearance and odor) indicates advanced small bowel obstruction with bacterial overgrowth.

• Abdominal distension: Reflects accumulation of swallowed air, bowel gas, and fluid in the obstructed bowel. Occurs prominently in complete small bowel obstruction and paralytic ileus; minimal distension may occur with high small bowel obstruction near the pylorus.

• Constipation and obstipation: Complete mechanical obstruction presents with both absolute constipation (no flatus or stool) and obstipation (inability to pass flatus). Patients with partial obstruction may pass stool or flatus initially.

• Diarrhea or loose stools: May paradoxically occur early in complete obstruction as fluid proximal to the obstruction passes around the obstruction. Bloody diarrhea suggests vascular compromise or infectious colitis.

Physical examination findings

• Peritoneal signs: These indicate parietal peritoneal irritation and carry high specificity for surgical pathology. Rebound tenderness (pain with sudden release of abdominal pressure after palpation) reflects peritoneal irritation from underlying inflammation or perforation. Guarding (involuntary muscle contraction during palpation) is mediated by spinal reflex and indicates somatic nerve irritation. Rigidity (boardlike abdominal wall, rigid even at rest) indicates severe peritoneal inflammation and

Immediate bedside steps (before imaging)

• Urine or serum β-hCG: mandatory in every reproductive-age woman with abdominal pain — ruptured ectopic pregnancy is the diagnosis that kills fastest and is missed most often.
• Point-of-care ultrasound (E-FAST): in the hemodynamically unstable patient, free intraperitoneal fluid or a large aortic aneurysm answers the question at the bedside; an unstable patient should not be sent to CT.
• Labs: CBC, CMP, lipase, lactate, type and screen, coagulation studies, urinalysis, ECG (inferior MI mimics epigastric pain). A markedly elevated lactate with metabolic acidosis in a patient with pain out of proportion to exam suggests bowel infarction; a normal lactate does not exclude early ischemia. Lipase ≥3× the upper limit of normal establishes acute pancreatitis (American College of Gastroenterology criteria: two of three — characteristic pain, enzyme elevation, imaging).

Imaging, in sequence

• Upright chest radiograph / left lateral decubitus film: fast, detects free air under the diaphragm (pneumoperitoneum) confirming perforated viscus. Absence of free air does not exclude perforation.
• CT abdomen/pelvis with IV contrast: the workhorse confirmatory study for undifferentiated acute abdomen per ACR Appropriateness Criteria. CT angiography is the test of choice when mesenteric ischemia is suspected — do not delay it for creatinine.
• Right upper quadrant ultrasound: first-line for suspected biliary disease (ACR) — stones, wall thickening >3 mm, pericholecystic fluid, sonographic Murphy sign.
• Ultrasound first, then MRI without gadolinium in pregnancy (ACR/ACOG) to avoid ionizing radiation.

Named criteria and scores

• Alvarado / AIR score for appendicitis risk stratification (MANTRELS).
• Tokyo Guidelines (TG18) define and grade acute cholecystitis and cholangitis; Charcot triad and Reynolds pentad remain the exam buzzwords.
• Revised Atlanta classification grades pancreatitis by organ failure and local complications.
• Water-soluble (not barium) contrast if perforation is possible.

Immediate stabilisation (parallel to diagnosis)

• Resuscitation: two large-bore IVs, balanced crystalloid (lactated Ringer's) for hypovolemia from third-spacing and vomiting; the Surviving Sepsis Campaign supports early fluid resuscitation and vasopressors (norepinephrine first-line) for shock refractory to fluids.
• NPO, nasogastric decompression for obstruction or intractable vomiting; correct hypokalemic hypochloremic alkalosis.
• Analgesia: opioids (e.g., morphine, fentanyl) should be given early — the old teaching that analgesia "masks" the exam is a distractor and does not delay diagnosis.
• Empiric antibiotics covering enteric gram-negatives and anaerobes — piperacillin-tazobactam, or ceftriaxone plus metronidazole — per IDSA/Surgical Infection Society intra-abdominal infection guidance; give within one hour in septic shock (Surviving Sepsis Campaign).

Definitive source control — the decision that matters

• Perforated viscus with peritonitis: emergent laparotomy/laparoscopy for repair (Graham omental patch for perforated duodenal ulcer), lavage, and resection as needed.
• Appendicitis: laparoscopic appendectomy; antibiotics-first management is an option in selected uncomplicated cases (CODA trial), but not with an appendicolith, perforation, or peritonitis.
• Acute cholecystitis: early laparoscopic cholecystectomy per Tokyo Guidelines; percutaneous cholecystostomy in the prohibitive-risk patient. Cholangitis requires urgent biliary decompression by ERCP.
• Acute mesenteric ischemia: systemic heparin anticoagulation plus urgent revascularization (embolectomy, endovascular thrombectomy/stenting) and resection of frankly necrotic bowel, often with planned second-look laparotomy. NOMI is treated by reversing the low-flow state and withdrawing vasoconstrictors.
• Strangulated obstruction, closed-loop obstruction, or hernia incarceration: operative, not conservative.
• Ruptured AAA: permissive hypotension, immediate transfer to the OR/hybrid suite; the Society for Vascular Surgery favors endovascular repair when anatomy permits.

Contraindicated

• Delaying source control for imaging or "trial of antibiotics" once peritonitis is present.
• Barium contrast with suspected perforation; colonoscopy in toxic megacolon.
• Nonoperative management of femoral hernia, gangrenous bowel, or free perforation.

Complications of the disease

• Septic shock (EMERGENCY): peritoneal absorption of endotoxin drives vasodilation and capillary leak; signalled by hypotension unresponsive to fluids, rising lactate, and altered mentation. Requires antibiotics, vasopressors, and source control simultaneously.
• Bowel infarction and perforation (EMERGENCY): transmural necrosis from strangulation or arterial occlusion; heralded by pain that abates then returns, peritonitis, and pneumatosis intestinalis or portal venous gas on CT — a late, ominous finding.
• Intra-abdominal abscess: contained perforation walled off by omentum; presents days later with persistent fever, leukocytosis, and ileus. Treated by image-guided percutaneous drainage plus antibiotics.
• Enterocutaneous or enterovesical fistula: chronic inflammation erodes into an adjacent structure; pneumaturia and fecaluria in a diverticular colovesical fistula.
• Abdominal compartment syndrome (EMERGENCY): massive bowel edema and resuscitation fluid raise intra-abdominal pressure, compressing the IVC and diaphragm — oliguria, rising peak airway pressures, and a tense abdomen. Confirmed by bladder pressure; treated by decompressive laparotomy with temporary abdominal closure.
• Hypovolemic/hemorrhagic shock: third-spacing in obstruction or exsanguination from ruptured AAA or ectopic pregnancy.

Complications of treatment

• Anastomotic leak (EMERGENCY): ischemia or tension at the suture line; postoperative days 5–7 with fever, tachycardia, new peritonitis, or feculent drain output. Return to the OR.
• Reperfusion injury after revascularization: oxygen free radicals plus washout of potassium, hydrogen ion, and myoglobin → hyperkalemia, acidosis, arrhythmia.
• Short bowel syndrome: after extensive resection; malabsorptive diarrhea and TPN dependence.
• Postoperative ileus vs. early mechanical obstruction: ileus shows diffuse gas including colon; mechanical obstruction shows transition point.
• **Surgical site infection, fascial dehiscence with evisceration, adhesions causing later obstruction, and Clostridioides difficile colitis** from broad-spectrum antibiotics (IDSA/SHEA: test by stool PCR/toxin, treat with oral vancomycin or fidaxomicin).

• Pain out of proportion to exam = acute mesenteric ischemia until proven otherwise. The association examiners test is atrial fibrillation (or recent MI with LV thrombus) embolizing to the superior mesenteric artery. Best next step: CT angiography, not plain film, not observation.
• Free air under the diaphragm on an upright chest radiograph = perforated viscus. The single best next step is operative source control, not more imaging; give fluids and broad-spectrum antibiotics on the way.
• The unstable patient does not go to CT. Hypotensive elderly smoker with back/flank pain and a pulsatile abdominal mass goes to the OR for ruptured AAA; bedside ultrasound only. Ordering CT here is the classic distractor.
• β-hCG before anything else in a reproductive-age woman. Ruptured ectopic is the surgical abdomen most often missed on exams and in practice.
• Analgesia does not mask the acute abdomen. Withholding opioids pending surgical evaluation is an outdated answer choice.
• Blunted exam in the elderly, diabetic, immunosuppressed, chronic steroid user, and late-pregnancy patient: peritoneal signs may be absent despite perforation; a "soft abdomen" does not exclude catastrophe, and these groups have the highest perforation and mortality rates.
• Buzzword-to-diagnosis shortcuts: coffee-bean sign → sigmoid volvulus; Charcot triad (fever, jaundice, RUQ pain) → ascending cholangitis, and Reynolds pentad (plus hypotension and confusion) → urgent ERCP decompression; pneumatosis intestinalis with portal venous gas → infarcted bowel; sonographic Murphy sign → acute cholecystitis.
• Femoral hernia carries the highest strangulation risk of the external hernias and should be repaired rather than observed — a favorite distractor is "elective repair after weight loss."