What is the differential diagnosis (Ddx) for left-sided abdominal pain?

Differential Diagnosis for Left-Sided Abdominal Pain

Acute sigmoid diverticulitis is the most common cause of left-sided abdominal pain in adults and should be your primary consideration. 1

Primary Differential Diagnoses by Likelihood

Most Common Causes

• Acute sigmoid diverticulitis: The leading cause of left lower quadrant pain in adults, affecting 5-25% of patients with diverticulosis, with approximately 70% of elderly patients having underlying diverticulosis 1, 2
• Colorectal malignancy: Perforated adenocarcinoma can perfectly mimic diverticulitis both clinically and radiographically—the key distinguishing feature is pericolonic lymphadenopathy >1 cm in short axis, which strongly suggests malignancy 2
• Epiploic appendagitis: An often-missed diagnosis that presents identically to diverticulitis but requires no antibiotics or surgery 3
• Inflammatory bowel disease (Crohn's disease or ulcerative colitis): Can present with left-sided pain, particularly in patients without prior diverticulitis history 1

Urologic Causes

• Urolithiasis (kidney stones): Particularly when pain radiates to the flank or groin 4
• Pyelonephritis: Left-sided renal infection with flank tenderness 4

Gynecologic Causes (in patients with female reproductive organs)

• Ectopic pregnancy: Must be excluded in reproductive-age patients 4
• Ovarian torsion: Acute onset with severe pain 4
• Pelvic inflammatory disease: Associated with vaginal discharge and cervical motion tenderness 4

Vascular Emergencies

• Leaking abdominal aortic aneurysm: Life-threatening cause requiring immediate recognition 5
• Mesenteric ischemia: Consider in elderly patients with cardiovascular disease 4

Other Gastrointestinal Causes

• Bowel obstruction: Associated with inability to pass gas or stool 2, 4
• Gastroenteritis: Most common overall cause of acute abdominal pain but typically diffuse 4
• Segmental colitis associated with diverticular disease (SCAD): Inflammatory changes confined to areas of diverticulosis 6

Rare but Important Causes

• Situs inversus with appendicitis: Appendicitis presenting on the left side in patients with reversed anatomy 5
• Psoas abscess: Deep-seated infection with pain on hip flexion 5
• Incarcerated hernia: Palpable mass with tenderness 5

Critical Red Flags Requiring Immediate Action

Emergent surgical consultation is mandatory if any of the following are present: 2

• Fever with inability to pass gas or stool
• Severe abdominal tenderness with guarding or rebound peritonitis
• Signs of shock (hypotension, tachycardia, altered mental status)
• Free air on imaging suggesting perforation

Diagnostic Approach

Initial Clinical Assessment

Focus on these specific historical and examination features: 1, 4

• Pain duration before presentation: >5 days predicts progression to complicated diverticulitis 1
• Pain character: Localized tenderness in left lower quadrant vs. diffuse pain 7
• Associated symptoms: Fever, nausea, vomiting, change in bowel habits, urinary symptoms 4
• Prior history: Previous episodes of diverticulitis, known diverticulosis, inflammatory bowel disease 1

Critical caveat: Clinical diagnosis alone has a 34-68% misdiagnosis rate—do not rely on examination alone 2. In one study, 24% of appendicitis patients had no right lower quadrant findings, and 7% of diverticulitis patients had no left lower quadrant findings 7.

Laboratory Testing

Order these initial tests: 4

• Complete blood count (elevated WBC suggests infection/inflammation)
• C-reactive protein (elevated in diverticulitis)
• Urinalysis and urine pregnancy test (in reproductive-age patients)
• Lipase (if upper abdominal component)
• Hepatobiliary markers, electrolytes, creatinine, glucose

Imaging Strategy

CT abdomen/pelvis with IV contrast is the single most important diagnostic test when there is diagnostic uncertainty, with 98% diagnostic accuracy. 1, 2, 8

Specific CT findings to identify: 2, 8

• Pericolonic lymphadenopathy >1 cm: Strongly suggests malignancy over diverticulitis—mandates colonoscopy
• Extraluminal air: Indicates perforation
• Abscess formation: Determines need for percutaneous drainage
• Bowel wall thickening pattern: Helps differentiate diverticulitis from other colitis

Alternative imaging when CT is contraindicated or unavailable: 1, 8

• Ultrasound (transabdominal): Can reduce unnecessary CT examinations, particularly when directed at maximal pain location
• Ultrasound (transvaginal): First-line in premenopausal women when gynecologic pathology suspected
• MRI: Preferred over CT in pregnant patients when ultrasound is inconclusive 1, 4

When imaging may not be needed: 1

• Patients with known recurrent uncomplicated diverticulitis presenting with typical symptoms may be managed clinically in select cases
• However, err on the side of imaging given the high misdiagnosis rate

Management Implications Based on Diagnosis

Uncomplicated Diverticulitis

Outpatient management with oral antibiotics for maximum 7 days, or conservative management without antibiotics in immunocompetent patients without sepsis. 1, 2

Complicated Diverticulitis with Abscess

• Small abscesses (<3-4 cm): Antibiotics alone 1
• Large abscesses: Percutaneous drainage plus IV antibiotics 1

Follow-up Colonoscopy

Colonoscopy is mandatory to exclude malignancy if: 2, 8

• Pericolonic lymphadenopathy >1 cm present on CT
• Atypical features on imaging
• No recent colonoscopy in patients >40 years with first episode

Common Pitfalls to Avoid

Do not assume diverticulitis without imaging—clinical diagnosis has unacceptably high error rates 2. Do not miss colon cancer, which presents identically to diverticulitis; lymphadenopathy is the key differentiator 2. Do not order routine CT for nonspecific back pain without red flags, even with leukocyturia, as this leads to incidental findings and unnecessary interventions 9. Consider alternative diagnoses like epiploic appendagitis in patients with recurrent "diverticulitis," as this changes management entirely 3.