7.1 Acute Abdomen

Section 7.1: Acute Abdomen

An acute abdomen refers to a rapid-onset, severe abdominal clinical presentation that frequently requires urgent surgical evaluation and intervention. Clinicians must differentiate benign, self-limiting causes of abdominal pain from life-threatening intra-abdominal pathology. A structured approach focusing on the chronicity, location, and character of pain, coupled with targeted physical examination maneuvers and appropriate diagnostic imaging, is essential.

Pathophysiology of Abdominal Pain

Understanding the neuroanatomy of abdominal pain helps in localizing the pathology:

1. Visceral Pain: Mediated by autonomic sensory fibers in the visceral peritoneum and walls of hollow viscera. It is stimulated by distension, stretch, or contraction, and is typically dull, aching, poorly localized, and felt in the midline (foregut structures to epigastrium, midgut to periumbilical area, hindgut to suprapubic region).
2. Somatic (Parietal) Pain: Mediated by somatic sensory fibers in the parietal peritoneum. It is stimulated by chemical irritation (e.g., blood, pus, bile, gastric juice) or mechanical inflammation. It is sharp, intense, well-localized to the site of irritation, and exacerbated by movement or coughing.
3. Referred Pain: Pain felt at a site distant from the affected organ, sharing common dermatomal pathways (e.g., right shoulder pain in cholecystitis via the phrenic nerve).

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Acute Appendicitis

Acute appendicitis is one of the most common causes of the acute abdomen worldwide. It typically results from obstruction of the appendiceal lumen, most commonly by a fecalith in adults, or lymphoid hyperplasia in children.

Clinical Presentation

The classic presentation begins with vague, dull periumbilical visceral pain (stimulated by appendiceal luminal distension). As the inflammatory process progresses to involve the overlying parietal peritoneum, the pain migrates to the right lower quadrant (RLQ), becoming sharp and localized. This migration is accompanied by anorexia (highly sensitive), nausea, vomiting, and a low-grade fever.

Physical Examination Maneuvers

• McBurney's Point Tenderness: Maximal tenderness located two-thirds of the distance from the umbilicus to the right anterior superior iliac spine (ASIS).
• Rovsing's Sign: Palpation of the left lower quadrant causes pain in the right lower quadrant, indicating referred peritoneal irritation.
• Psoas Sign: Pain on passive extension of the right hip, suggesting a retrocecal appendix irritating the psoas major muscle.
• Obturator Sign: Pain on passive internal rotation of the flexed right hip, suggesting a pelvic appendix irritating the obturator internus muscle.

Diagnostic Workup and Imaging

• Laboratory Studies: Mild leukocytosis (white blood cell count 10,000–18,000/µL) with a left shift.
• Imaging Choices:
  • Computed Tomography (CT) scan of the abdomen and pelvis with intravenous contrast is the gold standard for non-pregnant adults. Key CT findings include an appendiceal diameter >6 mm, wall thickening (>2 mm), periappendiceal fat stranding, and presence of an appendicolith.
  • Ultrasonography is the preferred initial imaging modality in pediatric patients and pregnant women to avoid ionizing radiation. A non-compressible, blind-ended loop measuring >6 mm in diameter is diagnostic.
  • Magnetic Resonance Imaging (MRI) is utilized in pregnant patients when ultrasound is non-diagnostic.

Management

Management involves maintaining NPO status, administering intravenous crystalloids for hydration, initiating broad-spectrum intravenous antibiotics (e.g., cefoxitin, or ceftriaxone plus metronidazole), and performing an urgent laparoscopic appendectomy.

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Acute Cholecystitis

Acute cholecystitis is inflammation of the gallbladder, almost always caused by cystic duct obstruction by a gallstone (calculous cholecystitis).

Clinical Presentation

Patients present with severe, constant right upper quadrant (RUQ) or epigastric pain that often radiates to the right scapula or shoulder. The pain is frequently precipitated by ingestion of fatty meals. Associated symptoms include fever, nausea, vomiting, and mild leukocytosis.

Physical Examination

• Murphy's Sign: Exquisite tenderness and arrest of inspiration when the examiner's hand is held under the right costal margin and the patient is asked to inhale deeply.

Diagnostic Workup and Imaging

• Right Upper Quadrant Ultrasound: The initial imaging test of choice. Findings diagnostic of cholecystitis include gallstones, gallbladder wall thickening (>3 mm), pericholecystic fluid, and a sonographic Murphy's sign (localized pain when the ultrasound transducer is pressed over the gallbladder).
• Cholescintigraphy (HIDA Scan): Performed if the ultrasound is equivocal. A normal scan demonstrates filling of the gallbladder within 1 hour. Non-visualization of the gallbladder after 4 hours indicates cystic duct obstruction and confirms acute cholecystitis.

Management

Initial management includes NPO status, IV fluid resuscitation, analgesics (NSAIDs like ketorolac, or opioids), and IV antibiotics (targeting Gram-negative rods and anaerobes). Definitive management is laparoscopic cholecystectomy, typically performed within 24 to 72 hours of admission.

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Bowel Obstruction

Bowel obstructions are classified into small bowel obstruction (SBO) and large bowel obstruction (LBO).

Small Bowel Obstruction (SBO)

• Etiology: The most common cause is postoperative adhesions (accounting for ~60%), followed by hernias, malignancy, and Crohn's disease.
• Clinical Presentation: Characterized by colicky, cramping mid-abdominal pain, bilious vomiting (more prominent in proximal obstructions), abdominal distension, and obstipation (inability to pass flatus or stool).
• Imaging:
  • Upright Abdominal X-ray: Shows dilated loops of small bowel (>3 cm) with air-fluid levels arranged in a step-ladder pattern, and an absence of gas in the colon.
  • CT scan with oral/IV contrast: Highly sensitive and specific; it identifies the transition point (where dilated bowel meets decompressed bowel) and evaluates for complications like ischemia or perforation (indicated by pneumatosis intestinalis or free air).
• Management: Non-operative management is appropriate for uncomplicated SBO (partial or early complete): NPO status, aggressive IV fluid resuscitation (due to third-spacing), and nasogastric (NG) tube decompression to relieve distension and vomiting. Urgent surgical exploration is indicated for signs of bowel strangulation, ischemia, or peritonitis (e.g., fever, tachycardia, localized tenderness, metabolic acidosis).

Large Bowel Obstruction (LBO)

• Etiology: The most common cause is colorectal malignancy, followed by volvulus (sigmoid or cecal) and diverticulitis.
• Clinical Presentation: Gradual onset of abdominal pain, progressive abdominal distension, and obstipation. Vomiting is a late finding.
• Imaging: Abdominal X-ray reveals peripheral colonic distension (>6 cm, or >9 cm for the cecum) with haustral markings. Sigmoid volvulus shows a classic "coffee bean" sign.
• Management: Typically requires surgical intervention or endoscopic decompression (e.g., detorsion of sigmoid volvulus, or stenting of a malignant obstruction) because the risk of cecal perforation is high, especially if the cecal diameter exceeds 10–12 cm.

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Summary of Key Clinical Signs & Imaging in Acute Abdomen