What is the differential diagnosis of flank pain?

Differential Diagnosis of Flank Pain

Flank pain has a broad differential diagnosis, with urolithiasis being the most common cause, but approximately one-third of patients presenting with acute flank pain have extraurinary etiologies that must be systematically considered. 1, 2

Urinary Tract Causes

Urolithiasis (Most Common)

• Kidney stones are the leading cause of acute flank pain, affecting 19% of men and 9% of women by age 70, with high recurrence rates in those with previous episodes 1
• Presents with colicky flank pain due to ureteral hyperperistalsis, often accompanied by hematuria 1
• Ureteral obstruction can lead to hydronephrosis as a serious complication 1

Acute Pyelonephritis

• Kidneys may be enlarged with hypoechoic parenchyma and loss of normal corticomedullary junction on imaging 3
• Clinical and laboratory correlation essential for diagnosis 3
• Note: The ACR guidelines specifically exclude infection scenarios, which are covered separately 1

Other Urinary Causes

• Papillary necrosis - can mimic stone disease 4
• Urothelial masses - may be detected on excretory phase imaging 1
• Hydronephrosis from various obstructive causes 1

Vascular Causes

Page Kidney

• Compression of renal parenchyma by subcapsular hematoma or mass causing secondary hypertension through renin-angiotensin-aldosterone system activation 5
• Can present with typical flank pain and hematuria, mimicking nephrolithiasis 5
• May result from trauma, renal cyst rupture, tumor, hemorrhage, or arteriovenous malformation 5

Nutcracker Syndrome

• Compression of left renal vein between abdominal aorta and superior mesenteric artery 6
• Presents with hematuria and abdominal or flank pain 6
• Often undiagnosed condition, particularly important in pediatric populations 6

Gastrointestinal Causes

Acute Appendicitis

• Can present as flank pain and be detected on imaging studies 4
• Identified as alternative diagnosis in patients presenting with suspected renal colic 4

Diverticulitis

• May cause flank pain mimicking urologic pathology 4
• Detected on both CT and ultrasound imaging 4

Gynecologic Causes (in Women)

Ovarian Pathology

• Ovarian cyst torsion - acute presentation requiring urgent intervention 4
• Complicated ovarian cysts - can cause flank pain 4

Other Causes

Musculoskeletal/Pleural

• Pleuritis - can present as flank pain 4
• Musculoskeletal causes should be considered based on clinical presentation 3

Genitourinary

• Epididymitis - may present with referred flank pain 4

Key Clinical Pitfalls

The most critical error is anchoring on nephrolithiasis when hematuria and flank pain are present together, potentially missing serious alternative diagnoses like Page kidney or vascular emergencies. 5

• Contrast-enhanced CT can obscure stones within the renal collecting system 7
• Secondary signs of obstruction may develop late, leading to false-negative ultrasound findings 7
• Absence of hydronephrosis on ultrasound makes larger ureteral stones less likely but does not exclude other pathology 7

Diagnostic Approach Based on Clinical Context

Initial Presentation Without Known Stone History

• Non-contrast CT abdomen/pelvis is the gold standard with 98-100% sensitivity and specificity for stones AND identifies extraurinary causes in approximately one-third of patients 1, 7, 2

When Alternative Diagnosis Suspected

• If flank pain with negative urinalysis and no stone history, contrast-enhanced CT better characterizes alternative diagnoses 7
• Contrast helps differentiate ureteral stones from phleboliths via "soft tissue rim" sign and evaluates other etiologies 1

Special Populations

• Pregnant patients: Ultrasound first-line (sensitivity for hydronephrosis without radiation exposure) 1
• Recurrent stone formers: Consider limited imaging or ultrasound to avoid cumulative radiation 1
• Pediatric patients: Consider nutcracker syndrome in differential, especially with hematuria 6