What is the most likely diagnosis and urgent management for a 25‑year‑old man with a prior appendectomy who now presents with acute sharp left lower‑quadrant pain that worsens when supine or in the lateral decubitus position and is not relieved by passing flatus?

Diagnosis and Urgent Management of Acute Left Lower Quadrant Pain Worsening with Supine and Lateral Decubitus Positioning

This 25-year-old man with prior appendectomy presenting with sharp left lower quadrant pain that worsens when lying down and is not relieved by passing flatus most likely has acute diverticulitis with peritoneal irritation, and requires immediate CT abdomen/pelvis with IV contrast followed by urgent surgical evaluation if complicated disease or perforation is identified.

Most Likely Diagnosis

Acute sigmoid diverticulitis with peritoneal involvement is the primary diagnosis, given the sharp left lower quadrant pain that worsens with positional changes (supine and lateral decubitus), which indicates peritoneal irritation 1. The fact that pain is not relieved by passing flatus excludes simple bowel distension and points toward an inflammatory or infectious process involving the peritoneum 1.

Key Diagnostic Features Supporting Diverticulitis

• Pain worsening with supine positioning is a classic sign of peritoneal irritation, occurring when inflamed peritoneal surfaces are compressed against the abdominal wall 1
• Lack of relief with flatus passage distinguishes this from functional bowel disorders or simple obstruction 1
• Acute sigmoid diverticulitis is the most common cause of left lower quadrant pain in adults, though this patient's young age (25 years) is atypical 1, 2

Critical Alternative Diagnoses to Exclude

• Perforated colon cancer can mimic diverticulitis clinically and radiographically; CT findings of pericolonic lymphadenopathy >1 cm favor malignancy over diverticulitis 1
• Post-appendectomy complications including abscess or adhesive obstruction must be considered given the surgical history 3
• Epiploic appendagitis presents with sharp, localized left lower quadrant pain but typically lacks systemic symptoms 1, 2

Urgent Diagnostic Workup

Immediate Imaging (Within 1 Hour)

Order CT abdomen and pelvis with IV contrast immediately—this is rated 8/9 (usually appropriate) by the American College of Radiology and provides 98% diagnostic accuracy 1. Clinical examination alone has a 34-68% misdiagnosis rate and cannot reliably distinguish between uncomplicated diverticulitis, perforation, abscess, or alternative pathology 1.

Critical CT Findings to Identify

• Pericolonic fat stranding with bowel wall thickening confirms diverticulitis 1
• Free intraperitoneal air indicates perforation requiring emergency surgery 1
• Abscess size: <4 cm can be managed with antibiotics alone; ≥4 cm requires percutaneous drainage 1
• Pericolonic lymphadenopathy >1 cm suggests colon cancer rather than diverticulitis 1

Laboratory Evaluation

• Complete blood count with differential: leukocytosis with left shift ≥75% neutrophils suggests bacterial infection 4
• C-reactive protein: CRP >170 mg/L predicts severe diverticulitis with 87.5% sensitivity and 91.1% specificity 4
• Lactate level: elevated lactate signals sepsis and metabolic derangement requiring aggressive resuscitation 4
• Blood cultures if fever is present or sepsis is suspected 4

Common Pitfall: Only 25% of diverticulitis patients present with the classic triad of left lower quadrant pain, fever, and leukocytosis—normal vital signs and labs do NOT exclude serious disease 1, 4.

Urgent Management Algorithm Based on CT Findings

If CT Shows Uncomplicated Diverticulitis (No Abscess, Perforation, or Fistula)

• For immunocompetent patients: Conservative management WITHOUT antibiotics is the standard recommendation 1
• For immunocompromised or elderly patients: Antibiotics for maximum 7 days using Ertapenem 1 g q24h or Eravacycline 1 mg/kg q12h 1
• Outpatient management is appropriate if patient can tolerate oral intake and has no signs of peritonitis 1

If CT Shows Complicated Diverticulitis with Small Abscess (<4 cm)

• Antibiotics alone for 7 days without drainage 1
• Use Ertapenem 1 g q24h or Eravacycline 1 mg/kg q12h 1
• Stop antibiotics at 4 days if source control is adequate and patient is immunocompetent and non-critically ill 1

If CT Shows Complicated Diverticulitis with Large Abscess (≥4 cm)

• Percutaneous drainage PLUS antibiotics for 4 days 1
• If drainage is not feasible, consider antibiotics alone for immunocompetent patients or surgical intervention for critically ill patients 1

If CT Shows Perforation with Free Air or Diffuse Peritonitis

• Emergency surgical consultation immediately 1
• Broad-spectrum carbapenem therapy: Meropenem 1 g q6h by extended infusion, Doripenem 500 mg q8h by extended infusion, or Imipenem/cilastatin 500 mg q6h by extended infusion if septic shock is present 1
• NPO status, IV fluid resuscitation, and nasogastric decompression 5
• Surgical options: Primary resection and anastomosis with or without diverting stoma (clinically stable patients) or Hartmann's procedure (critically ill patients) 1

Red Flags Requiring Emergency Surgery

• Free air on CT indicating perforation 1
• Fecal peritonitis 1
• Signs of peritonitis: diffuse abdominal tenderness, guarding, rebound tenderness, or absent bowel sounds 1
• Toxic appearance: fever, rapid breathing, rapid heartbeat, confusion, or signs of septic shock 1
• Hemodynamic instability despite resuscitation 6

Critical Pitfalls to Avoid

• Do not rely on clinical examination alone—misdiagnosis rates are 34-68% without imaging 1
• Do not assume diverticulitis based solely on history of diverticulosis—colon cancer must be excluded 1
• Do not continue antibiotics beyond 7 days in uncomplicated cases, as this increases resistance without improving outcomes 1
• Do not perform routine colonoscopy after CT-confirmed uncomplicated diverticulitis unless concerning CT features (lymphadenopathy, mass, abscess, perforation, or fistula) are present 1

Follow-Up Imaging Indications

• Repeat CT abdomen/pelvis with IV contrast if symptoms persist >2-3 days despite conservative management 1
• Repeat imaging if signs of infection persist beyond 7 days to assess for inadequate source control 1