Nephrolithiasis (Kidney Stones / Urolithiasis)

Epidemiology

Types of Stones and Pathogenesis

Calcium Stones (Most Common)

• ~5% have hypercalcemia (hyperparathyroidism, sarcoidosis, malignancy)
• ~55% have hypercalciuria without hypercalcemia: absorptive hypercalciuria, renal tubular hypercalciuria, or idiopathic
• ~20% are associated with hyperuricosuria - uric acid crystals nucleate calcium oxalate deposition in the collecting ducts
• ~5% are driven by hyperoxaluria (Crohn disease, small bowel resection, bariatric surgery, radiation enteritis, or rare primary/hereditary oxaluria)
• Hypocitraturia (citrate normally inhibits stone formation) can be idiopathic or due to metabolic acidosis/chronic diarrhea
• Robbins Pathologic Basis of Disease; Rosen's Emergency Medicine

Struvite Stones

• Formed by urea-splitting organisms: Proteus, Providencia, Klebsiella, Pseudomonas, Staphylococcus
• Produce ammonia, raising urinary pH and precipitating MgNH₄PO₄
• Often grow very large - staghorn calculi (75% of staghorn calculi are struvite) that fill much of the renal pelvis
• Common in patients with anatomic abnormalities predisposing to recurrent UTI
• Robbins; Rosen's Emergency Medicine

Uric Acid Stones

• Radiolucent (do not show on plain X-ray) - a classic distinction
• Require persistently acidic urine (pH <5.5) for precipitation
• Associated with gout (~15% of gout patients develop stones), leukemia/rapid cell turnover, uricosuric drugs
• More than 50% of uric acid stone formers have neither hyperuricemia nor hyperuricosuria - urine pH is the primary driver
• Robbins; Rosen's Emergency Medicine

Cystine Stones

• Autosomal recessive defect in renal (and intestinal) reabsorption of dibasic amino acids: Cystine, Ornithine, Lysine, Arginine (mnemonic: COLA)
• Cystine is the least soluble; hexagonal crystals on urine microscopy
• Quick Compendium of Clinical Pathology, 5th Ed.

Urine Crystal Morphology

Sites of Impaction

1. Calyx / renal pelvis
2. Ureteropelvic junction (UPJ) - abrupt narrowing from ~1 cm pelvis to 2-3 mm ureter
3. Pelvic brim - where ureter crosses the iliac vessels
4. Ureterovesical junction (UVJ) - the most constricted point; most common site of impaction
5. Vesical orifice

Variations in ureteral caliber - the UVJ (1-5 mm) is the narrowest point. (Rosen's Emergency Medicine)

Clinical Features

• Renal colic: abrupt onset, crescendo of extreme flank pain radiating laterally around the abdomen and into the groin/labia/testicle
• Hematuria (microscopic or gross) - present in most cases but not universal
• Nausea and vomiting - common
• Pain is typically colicky, not positional (unlike peritonitis)
• Urinary urgency and dysuria with distal ureteral stones
• Fever/chills if there is concomitant infection - this is a urologic emergency
• Rosen's Emergency Medicine

Complications

• Obstruction - causes rapid redistribution of renal blood flow and decreased GFR. Complete obstruction for 1-2 weeks can cause irreversible renal damage
• Infection - obstructed + infected stone = urosepsis risk. Pyelonephritis, perinephric abscess, gram-negative sepsis
• Partial obstruction still risks permanent damage
• Rosen's Emergency Medicine

Diagnosis

Urinalysis

• Hematuria (micro or macro)
• Crystals (see crystal morphology table above)
• Pyuria + bacteriuria suggest concurrent infection

Imaging

• Sensitivity and specificity >90%; detects stones as small as 1 mm
• Identifies hydronephrosis, hydroureter, ureteral edema, and alternative diagnoses (AAA, abscess, malignancy)
• Radiolucent uric acid stones are visible on CT (unlike plain film)
• Low-dose protocols are appropriate for BMI <30 kg/m²
• Patients with known stone history and classic renal colic without fever, infection, solitary kidney, or concern for alternative diagnosis may not need CT

• First-line in pregnancy and pediatrics
• Sensitivity ~54%, specificity ~91% for stones; up to 100% sensitive for ureteral obstruction
• Detects hydronephrosis reliably
• Less accurate for stone size and location

• Detects radiopaque stones (calcium-containing)
• Misses uric acid, cystine, and many small stones

Management

Acute Pain Control

• NSAIDs are first-line (ketorolac 30 mg IV or diclofenac 75 mg IM)
  • Reduce ureteral spasm and renal capsular pressure by lowering GFR in the obstructed kidney
  • Use with caution in renal insufficiency or peptic ulcer disease
• IV opioids (fentanyl 1-2 mcg/kg) are also effective; combination with NSAIDs may reduce ED length of stay
• Antiemetics (ondansetron 4 mg IV) + IV fluids for vomiting
• High-volume IV hydration has NOT been shown to improve stone passage

Medical Expulsive Therapy (MET)

• Alpha-blockers (tamsulosin) facilitate passage of distal ureteral stones by relaxing ureteral smooth muscle

Spontaneous Passage Rates (by stone size)

Urologic Emergency Indications

• Obstructing stone + infection/sepsis
• Bilateral obstructing stones
• Obstruction of a solitary or transplanted kidney
• Significant renal impairment

Surgical Options

• Extracorporeal shock wave lithotripsy (ESWL)
• Ureteroscopy with laser lithotripsy
• Percutaneous nephrolithotomy (PCNL) - for large/staghorn calculi
• Surgery can generally be outpatient if oral intake is tolerated and pain is controlled

Indications for Admission

• Inability to tolerate oral intake / uncontrolled pain
• High-grade obstruction, large stone unlikely to pass
• Concomitant infection
• Solitary/transplanted kidney obstruction
• Significant renal impairment

Prevention (Recurrence Reduction)

Special Considerations: Pregnancy

• Incidence is similar to non-pregnant women (1 in 188 to 1 in 4600)
• Calcium phosphate stones are most common in pregnancy (vs. calcium oxalate outside pregnancy); this is because pregnancy causes physiologic hypercalciuria balanced by increased citrate and nephrocalcin
• ~64-84% of stones pass spontaneously in pregnancy
• Imaging: ultrasound is first-line; MR urography or limited IVU if needed; CT is reserved for cases where benefits outweigh radiation risk
• Risks: increased rates of preterm labor, pyelonephritis, hypertensive disorders
• Comprehensive Clinical Nephrology, 7th Ed.