What are the causes and treatment options for left flank pain?

Left Flank Pain: Causes and Diagnostic Approach

Primary Causes

Kidney stones (nephrolithiasis) are the leading cause of acute left flank pain, affecting 19% of men and 9% of women by age 70, with particularly high recurrence rates in those with previous stone episodes 1.

Urinary Tract Etiologies

• Ureterolithiasis presents with colicky, wave-like severe flank pain due to ureteral hyperperistalsis, typically accompanied by hematuria and independent of body position 1, 2
• Hydronephrosis from ureteral obstruction represents a serious complication requiring urgent evaluation 1
• Pyelonephritis causes flank pain that characteristically worsens with external pressure on the flank area, distinguishing it from stone disease 3
• Urothelial masses may be detected on excretory phase imaging 1
• Page kidney (subcapsular hematoma causing renal compression) can present with flank pain and hematuria mimicking nephrolithiasis, often associated with secondary hypertension 4

Gastrointestinal Causes

• Acute diverticulitis of the descending or sigmoid colon is the most common cause of left lower quadrant pain extending to the flank, with prevalence increasing 50% between 2000-2007 5
• Colitis and inflammatory bowel disease frequently present with left-sided abdominal and flank pain 5
• Acute pancreatitis (particularly tail involvement) can rarely present solely as isolated left flank pain with fluid extending to the left perirenal space 6
• Epiploic appendagitis and bowel obstruction are additional gastrointestinal causes 5

Gynecologic Causes (in Women)

• Ovarian and fallopian tube pathology including benign adnexal masses 5, 2
• Pelvic congestion syndrome 2

Musculoskeletal Causes

• Thoracolumbar junction syndrome involving deep paravertebral muscles at T10-T12 levels can cause severe flank pain, particularly after prolonged static positioning 2, 7
• Facet joint pathology or referred pain from lumbar spine disease 2

Other Causes

• Hernia (abdominal wall or internal) 5
• Vascular pathology including renal artery stenosis 7

Diagnostic Imaging Approach

Non-contrast CT of the abdomen and pelvis is the gold standard imaging modality for evaluating acute left flank pain, with 98-100% sensitivity and specificity for detecting urinary stones while identifying alternative extraurinary diagnoses in approximately one-third of patients 1, 2, 8.

Standard Imaging Protocol

• Low-dose non-contrast CT should be used preferentially over conventional dosing to minimize radiation exposure while maintaining diagnostic accuracy 5
• CT with IV contrast is recommended when flank pain occurs with negative urinalysis and no stone history, as contrast enhancement better characterizes alternative diagnoses including bowel wall pathology, pericolic abnormalities, vascular pathology, and intraabdominal fluid collections 5, 1
• The "soft tissue rim" sign on contrast-enhanced CT helps differentiate ureteral stones from phleboliths 1
• Unenhanced CT has similar accuracy (64-68%) to contrast-enhanced CT (68-71%) in elderly patients ≥75 years with acute abdominal pain, with substantial interrater agreement 5

Alternative Imaging Modalities

• Ultrasound combined with plain radiography provides 79-90% sensitivity for clinically significant stones, offering an acceptable radiation-sparing alternative 5, 3
• Ultrasound is up to 100% sensitive and 90% specific for hydronephrosis, ureterectasis, and perinephric fluid; absence of hydronephrosis makes larger ureteral stones (>5 mm) less likely 2
• Plain abdominal radiography (KUB) alone is only 72% sensitive for large stones (>5 mm) in the proximal ureter and 29% sensitive overall, making it usually not appropriate as initial imaging 5, 2
• MRI is less accurate for identifying stones and extraluminal air, more time-consuming, and generally not useful for initial evaluation of acute flank pain 5
• Intravenous urography cannot guide physicians toward alternative diagnoses if stones are ruled out and has been largely replaced by CT 5

Special Population Considerations

• Pregnant patients: Ultrasound is first-line imaging (rating 8/9, usually appropriate) due to reasonable sensitivity for stone detection without ionizing radiation to mother or fetus 5, 1, 2
• Low-dose non-contrast CT may be used in second and third trimesters if renal calculus is strongly suspected and ultrasound is non-diagnostic 5
• Recurrent stone formers: Limited imaging protocols, ultrasound for hydronephrosis evaluation, or review of previous imaging should be considered to avoid cumulative radiation exposure 5, 1

Key Distinguishing Clinical Features

Red Flags Requiring Urgent Evaluation

• Fever, chills, or signs of systemic infection suggest pyelonephritis or complicated stone disease requiring immediate assessment 2
• Inability to urinate or decreased urine output indicates possible complete obstruction 2
• Large-volume extraluminal air on CT suggests large-caliber perforation with higher mortality rates 5

Differentiating Pain Characteristics

• Classic renal colic: Colicky, wave-like severe pain independent of body position, caused by internal obstruction rather than external compression 1, 2, 3
• Pyelonephritis/perinephric abscess: Pain significantly worsens with external flank pressure 3
• Musculoskeletal origin: Pain occurring after prolonged static positioning, worsens with movement or palpation of paraspinal muscles 2, 7
• Nonobstructing calyceal stones: Can cause moderate to severe flank pain despite lack of obstruction; surgical removal provides significant pain reduction (86% achieve ≥20% pain reduction at 12 weeks) 9

Common Pitfalls to Avoid

• Do not assume all flank pain is kidney-related—the positional nature of symptoms and response to external pressure are key distinguishing features 2
• Do not anchor on nephrolithiasis when hematuria and flank pain are present; consider Page kidney, particularly in hypertensive patients without trauma history 4
• Do not dismiss isolated left flank pain as purely urologic; rare presentations of acute pancreatitis can manifest solely as flank pain without typical epigastric symptoms 6
• Do not overlook musculoskeletal causes, particularly thoracolumbar junction syndrome, which may persist despite treatment of concurrent urologic findings 7
• Do not use repetitive conventional-dose CT in recurrent stone formers; utilize low-dose protocols, ultrasound, or limited imaging to minimize cumulative radiation exposure 5, 1