Sudden Flank Pain: Causes and Diagnostic Approach
Urolithiasis (kidney stones) is the most common cause of sudden flank pain, accounting for approximately 50-55% of cases in patients presenting with acute flank pain without signs of infection. 1
Primary Causes of Sudden Flank Pain
Urological Causes (Most Common)
Urolithiasis (kidney stones): The leading cause, presenting with sudden severe flank pain due to ureteral hyperperistalsis and obstruction. Men are affected more than women, with 19% of men and 9% of women diagnosed by age 70. 1 Stone size and location determine spontaneous passage rates, with larger and more proximal stones less likely to pass. 1
Acute pyelonephritis: Presents with flank pain or tenderness, often accompanied by fever, chills, and bladder symptoms (urgency, dysuria, frequency), though up to 20% lack bladder symptoms. 1 Pyuria and/or bacteriuria on urinalysis with positive urine cultures (>10,000 colony-forming units/mL) confirm diagnosis. 1
Ureteral obstruction with hydronephrosis: A serious complication of stones causing backup of urine and potential renal damage. 1
Alternative Non-Stone Causes (5-10% of Cases)
When imaging is performed for suspected stone disease, alternative diagnoses are found in approximately 9% of cases overall. 2 In patients with classic flank or back pain without pyuria, acutely important alternative causes occur in only 2.8% of scans. 2
Acutely important alternative causes (requiring immediate intervention) include: 2, 3
- Abdominal aortic aneurysm (life-threatening emergency that must be ruled out) 4
- Acute appendicitis
- Ovarian cyst torsion or complicated ovarian cysts
- Diverticulitis
- Papillary necrosis
Causes requiring follow-up include: 2
- Simple renal cysts
- Uncomplicated ovarian cysts
- Incidental findings
Vascular Causes
- Nutcracker syndrome: Compression of the left renal vein between the superior mesenteric artery and aorta (anterior) or between aorta and spine (posterior), causing atypical left flank pain that worsens with standing and increased lumbar lordosis. 5 May present with hematuria (often microscopic), orthostatic proteinuria, varicocele, or pelvic varicose veins. 5
Diagnostic Approach
Initial Imaging for Suspected Stone Disease
Noncontrast CT abdomen/pelvis is the reference standard for evaluating acute flank pain with suspected urolithiasis, with sensitivity up to 97%. 1 It rapidly detects stones of any size, location, or composition, identifies complications (hydronephrosis, perinephric inflammation), and diagnoses alternative causes. 1, 6
Low-dose CT protocols should be used when available to minimize radiation exposure while maintaining diagnostic accuracy. 1
CT with IV contrast alone is not appropriate as first-line imaging because enhancing renal parenchyma obscures collecting system stones, though it detects stones ≥6 mm with 98% sensitivity. 1
Alternative Imaging Modalities
Ultrasound: Sensitivity of 96% for detecting hydronephrosis when combined with direct stone visualization and absence of ureteral ejaculation. 6 The addition of color Doppler with twinkling artifact increases sensitivity to 99% for small stones (<5 mm), though false-positive rates reach 60%. 1 Ultrasound combined with radiography achieves 90% sensitivity but misses stones <5 mm. 1
Plain radiography (KUB): Detects radiopaque stones in 40-60% of cases directly, with indirect signs increasing detection to 80-90%. 6 Less sensitive than CT but useful when combined with ultrasound. 1
When to Suspect Alternative Diagnoses
Consider non-stone causes when: 2, 3
- Absence of hematuria (though hematuria is frequently but not universally present with stones) 4
- Atypical pain pattern (e.g., pain worsening with standing suggests nutcracker syndrome) 5
- Fever or signs of infection (suggests pyelonephritis or complicated stone disease) 1, 4
- Age >60 years with cardiovascular risk factors (consider vascular catastrophe) 4
Both noncontrast CT and ultrasound have comparable accuracy (74-78% sensitivity, 98-99% specificity) for diagnosing alternative causes of flank pain. 3
Critical Pitfalls to Avoid
Never dismiss sudden severe flank pain without imaging in older patients or those with vascular risk factors—abdominal aortic aneurysm must be excluded as it is life-threatening. 4
Do not assume all flank pain with hematuria is stone disease—pyelonephritis, nutcracker syndrome, and papillary necrosis also cause hematuria. 1, 5
Recognize that absence of hydronephrosis on ultrasound makes large ureteral stones (>5 mm) less likely but does not exclude stone disease entirely. 1
In patients with known stone disease and recurrent symptoms, repeat noncontrast CT remains the test of choice while optimizing radiation dose. 1