What are the common causes of flank pain?

Common Causes of Flank Pain

Kidney stones (nephrolithiasis) are the leading cause of acute flank pain, affecting 19% of men and 9% of women by age 70, followed by acute pyelonephritis as the second most common urinary tract etiology. 1

Primary Urinary Tract Causes

Nephrolithiasis (Most Common)

• Kidney stones present with characteristic colicky flank pain caused by ureteral hyperperistalsis and obstruction, often accompanied by hematuria 1, 2
• The pain occurs in waves of severe intensity followed by periods of less intense discomfort, and is caused by internal pressure from obstruction rather than external compression 2
• Ureteral obstruction can progress to hydronephrosis as a serious complication requiring urgent intervention 1
• Stones are particularly likely in patients with previous stone history, who have high recurrence rates 3, 1

Acute Pyelonephritis (Second Most Common)

• Flank pain is nearly universal in acute pyelonephritis, and its absence should raise suspicion of an alternative diagnosis 4
• Most patients present with fever (though it may be absent early), flank pain, and positive urinalysis with pyuria and/or bacteriuria 3, 4
• On ultrasound, kidneys may be enlarged with hypoechoic parenchyma and loss of normal corticomedullary junction 5
• Pain that significantly worsens with external flank pressure suggests pyelonephritis or perinephric abscess rather than nephrolithiasis 2

Other Urinary Tract Causes

Hydronephrosis from Various Obstructive Causes

• Can result from stones, ureteral strictures, or external compression 1
• Ultrasound has 96% sensitivity for detecting pyeloureteral dilatation when combined with direct stone visualization and absence of ureteral ejaculation 6

Urothelial Masses

• May be detected on excretory phase imaging and can present with flank pain 1

Extraurinary Causes (Critical Pitfall)

Non-contrast CT identifies extraurinary causes of flank pain in approximately one-third of patients presenting with suspected renal colic, making it essential not to anchor solely on nephrolithiasis 1, 6

Important Alternative Diagnoses to Consider:

• Page kidney (subcapsular hematoma causing renal compression) can present identically to nephrolithiasis with flank pain and hematuria but requires different management 7
• Musculoskeletal causes 6
• Gastrointestinal pathology 6
• Vascular abnormalities 7

Diagnostic Approach Algorithm

Initial Imaging Selection:

For non-pregnant adults with acute flank pain:

• Non-contrast CT abdomen/pelvis is the gold standard with 98-100% sensitivity and specificity for stones and identifies alternative diagnoses 1, 6
• Low-dose CT protocols should be used to minimize radiation exposure while maintaining diagnostic accuracy 3

If flank pain with negative urinalysis and no stone history:

• Contrast-enhanced CT better characterizes alternative diagnoses and differentiates ureteral stones from phleboliths via the "soft tissue rim" sign 1

For pregnant patients:

• Ultrasound is first-line imaging with good sensitivity for hydronephrosis without radiation exposure 3, 1
• Low-dose CT may be considered in second and third trimesters if ultrasound is inconclusive (rating 6/9) 3

For recurrent stone formers:

• Limited imaging or ultrasound should be considered to avoid cumulative radiation exposure 1
• Review previous images to track stone location and number 3

Clinical Correlation Requirements:

• Obtain urinalysis in all patients - positive findings confirm UTI-related causes in compatible clinical presentations 4
• Urine culture should be obtained in all patients to guide antibiotic therapy if pyelonephritis is suspected 4
• Assess for fever, which when present with flank pain strongly suggests pyelonephritis over simple nephrolithiasis 4
• Evaluate whether pain worsens with external flank pressure - this suggests infectious/inflammatory causes rather than stone disease 2

Common Pitfalls to Avoid

• Do not anchor on nephrolithiasis diagnosis when hematuria and flank pain are present - consider Page kidney and other rare causes 7
• Plain abdominal radiography alone has narrow capabilities (40-60% direct stone detection) and often leads to repeat imaging, cancelling radiation benefits 3
• Absence of fever does not exclude pyelonephritis 3
• Physiologic hydronephrosis in pregnancy can mimic pathologic obstruction 3