Lower Left Abdominal Pain Followed by Back Pain: Differential Diagnosis and Workup
The most likely diagnosis is acute sigmoid or descending colonic diverticulitis, which is the most common cause of left lower quadrant pain in adults, and you should obtain a CT abdomen and pelvis with IV contrast as the initial imaging study. 1
Primary Differential Diagnosis
Most Common Causes
- Acute diverticulitis is the leading cause of left lower quadrant pain in adults, with 10-25% of patients with diverticulosis ultimately developing diverticulitis 1
- Renal colic/urolithiasis can present with left lower quadrant pain radiating to the back, representing one of the most common alternative diagnoses 1, 2
- Urinary tract infection should be considered, particularly with back pain suggesting possible pyelonephritis 3
- Perforated colon cancer can mimic diverticulitis clinically and radiographically 1
Less Common but Important Causes
- Epiploic appendicitis presents with acute left lower quadrant pain 3
- Other inflammatory colonic diseases including inflammatory bowel disease 4
- Gynecologic disorders in women of reproductive age (ovarian torsion, ectopic pregnancy, pelvic inflammatory disease) 1, 5
- Abdominal wall pathology 5
- Referred visceral pain from gastrointestinal sources (though duodenal ulcers typically cause upper abdominal symptoms, visceral pain can refer to the back) 6
Immediate Imaging Strategy
First-Line Imaging
CT abdomen and pelvis with IV contrast is the most appropriate initial test with the following characteristics: 1
- Diagnostic accuracy of 98-100% for diverticulitis 7, 8
- Sensitive for detecting small quantities of extraluminal air indicating perforation 1
- Can identify abscess formation, bowel wall inflammation, and pericolonic changes 7
- Guides appropriate management regardless of patient sex or body habitus 1
- Changes leading diagnosis in 51% of patients and admission decisions in 25% of cases 1
Alternative Imaging Considerations
- Unenhanced CT is appropriate only for suspected urolithiasis (sensitivity and specificity near 100% for urinary calculi) but has severely limited sensitivity for diverticulitis 1, 7
- Ultrasound may be used to reduce CT utilization in stable patients and can identify diverticulitis, but is less commonly used in the United States for nongynecologic left lower quadrant pain 1, 3
- MRI abdomen/pelvis with gadolinium is the preferred alternative if IV contrast is contraindicated, with 86-94% sensitivity and 88-92% specificity for inflammatory conditions 7, 9
- Plain radiography is not useful as initial imaging because CT is more accurate, though it may identify large-volume free air or bowel obstruction 1
Critical Red Flag Symptoms Requiring Urgent Evaluation
The following symptoms mandate immediate emergency department assessment: 7, 9
- Fever (suggests complicated diverticulitis or abscess)
- Inability to pass gas or stool (suggests obstruction)
- Severe abdominal tenderness with guarding or rebound (suggests peritonitis)
- Vomiting with progressive worsening pain
- Bloody stools
- Signs of shock or hemodynamic instability
- Toxic appearance
Essential Laboratory Studies
Obtain the following tests to assess for occult inflammation and guide management: 7, 5
- Complete blood count with differential
- C-reactive protein
- Urinalysis (to evaluate for urolithiasis or urinary tract infection)
- Pregnancy test in women of reproductive age
- Consider lipase, hepatobiliary markers, electrolytes, creatinine, and glucose based on clinical presentation
Key Diagnostic Pitfalls to Avoid
Imaging Errors
- Do not rely on non-contrast CT for suspected diverticulitis, as it can miss abscess formation, subtle bowel wall inflammation, and pericolonic changes that are only visible with contrast enhancement 7
- Do not attribute pain to incidental diverticulosis seen on CT, as diverticulosis without inflammation does not cause pain 7
Clinical Assessment Limitations
- Do not rely on clinical examination alone, as misdiagnosis rates based on clinical assessment alone range from 34-68% 7, 8
- Clinical diagnosis is incorrect in 34-68% of new patients presenting with abdominal pain 8
Malignancy Considerations
- Perforated colon cancer can mimic diverticulitis both clinically and radiographically 1
- CT findings suggesting cancer rather than diverticulitis include pericolonic lymphadenopathy >1 cm in short axis with or without pericolonic edema 1, 9
- The likelihood of colon cancer is higher when abscess, local perforation, or fistula is identified 1
Management Based on CT Results
If CT Confirms Uncomplicated Diverticulitis
- Outpatient management is appropriate if pain is mild and stable, patient can tolerate oral intake, and no signs of peritonitis are present 7, 4
- Patients with typical symptoms and previous history of diverticulitis may be treated medically without imaging 1, 8
- Close follow-up within 24-48 hours is necessary to reassess for evolving symptoms 7
If CT Shows Complicated Diverticulitis
- Patients should be admitted for complications including abscesses, fistulas, obstruction, or perforation 1
- Surgery or interventional radiology procedures may be required 1
Colonoscopy Timing
- Do not order routine colonoscopy after CT diagnosis of acute left-sided diverticulitis, except for age-appropriate colon cancer screening or if abnormal pericolic lymph nodes are present on CT 1, 7
- After complicated diverticulitis, colonoscopy should be delayed 6-8 weeks after the acute episode or until complete resolution of symptoms 8
Special Population Considerations
Women of Reproductive Age
- If gynecologic condition is primarily suspected, obtain pelvic/transvaginal ultrasound 1, 9
- Consider ectopic pregnancy, pelvic inflammatory disease, and adnexal torsion 5
- Cyclical pain suggests gynecologic etiology requiring pelvic ultrasound 9
Elderly Patients
- Diverticulitis is especially common in this population 3
- Laboratory tests may be nonspecific and normal despite serious infection 1
- Imaging is particularly helpful as clinical signs may be atypical 1