Kidney Stones

Detailed Overview

Stones form when the urine becomes supersaturated with stone-forming salts (calcium, oxalate, uric acid) and lacks adequate inhibitors (citrate, fluid volume). The vast majority are calcium oxalate stones. They represent a major source of acute, severe emergency department visits and can lead to recurrent infections or chronic kidney disease if obstructive.

Epidemiology & Demographics

Lifetime prevalence is around 10% in the United States. They are more common in men (approx 12%) than women (approx 7%), with peak incidence between 30 and 50 years of age. Recurrence rates are up to 50% within 10 years if untreated.

Etiological Mechanism

Multifactorial, involving low fluid intake, dietary indiscretion (high sodium, high animal protein), metabolic derangements (hypercalciuria, hypocitraturia), and anatomical anomalies.

Primary Causes

Dehydration (low urine volume)

Hypercalciuria (idiopathic or secondary to hyperparathyroidism)

Hyperuricosuria (gout, high purine diet)

Infection (Proteus or Klebsiella splitting urea to form struvite stones)

• Low fluid intake: The most significant and modifiable risk factor; concentrates urine.
• High sodium diet: Sodium forces calcium out into the urine, promoting calcium stones.
• Obesity and Metabolic Syndrome: Causes acidic urine, highly predisposing to uric acid stones.
• Bariatric Surgery (Roux-en-Y): Leads to severe fat malabsorption, which binds calcium in the gut, leaving free oxalate to be absorbed and excreted in urine (enteric hyperoxaluria).

Stones form through nucleation, growth, and aggregation of crystals in supersaturated urine. Calcium oxalate (80%) forms over a nidus of calcium phosphate (Randall's plaque) in the renal papillae. Uric acid stones (10%) form in highly acidic urine (pH < 5.5). Struvite stones (magnesium ammonium phosphate) form rapidly in alkaline urine caused by urease-producing bacterial infections, often growing to form massive 'staghorn' calculi filling the renal pelvis.

Characteristic Clinical Presentation

• Renal Colic: Excruciating, unilateral, spasmodic flank pain that radiates to the groin or testicle/labia. Patients constantly writhe and cannot find a comfortable position.
• Nausea and Vomiting: Severe pain triggers vagal reflex causing profound nausea.
• Hematuria: Grossly visible or microscopic blood in the urine from stone mucosal trauma.
• Urgency and frequency: Occurs when the stone reaches the ureterovesical junction (UVJ), irritating the bladder.

Physical Examination Signs

• Costovertebral angle (CVA) tenderness to percussion
• Patient writhing in pain (unlike peritonitis patients, who lie perfectly still)
• Tachycardia and hypertension (secondary to extreme pain)

• Obstructive Pyelonephritis: Infected urine trapped behind a stone. A true urologic emergency requiring immediate decompression.
• Acute Kidney Injury (AKI): Particularly dangerous in patients with a solitary kidney or bilateral obstructing stones.
• Stricture formation: Scarring of the ureter from chronic stone impaction.

Diagnostic Criteria & Guidelines

Classic clinical presentation combined with imaging demonstrating a radiopaque or radiolucent stone in the urinary tract, often with associated proximal hydronephrosis.

Differential Diagnosis

• Abdominal Aortic Aneurysm (AAA) rupture
• Acute Appendicitis
• Ectopic Pregnancy
• Pyelonephritis

Laboratory Tests & Biomarkers

• Urinalysis: Typically shows microscopic hematuria. Must check leukocyte esterase and nitrites to rule out concurrent infection.
• Basic Metabolic Panel (BMP): Check creatinine to evaluate for AKI.
• 24-Hour Urine Collection: (Done outpatient) Measures volume, calcium, oxalate, citrate, uric acid, and pH to determine metabolic etiology.

Imaging Modalities & Findings

• Non-contrast CT Abdomen and Pelvis: The gold standard. Detects nearly all stones (radiopaque and radiolucent), identifies exact size/location, and shows hydroureter/hydronephrosis.
• Renal Ultrasound: Preferred in pregnant patients and children. Shows hydronephrosis and stones in the kidney/proximal ureter, but often misses mid-ureteral stones.

• Non-obstructing
  Stones residing in renal calyces. Often completely asymptomatic.
• Obstructing
  Stone lodged in the ureter (UPJ, pelvic brim, or UVJ) causing hydronephrosis and colic.
• Complicated
  Obstructing stone associated with fever and infection (septic physiology).

Second-Line & Adjunctive Therapy

For Uric Acid Stones: Urinary alkalinization with Potassium Citrate (aiming for urine pH 6.5-7.0) can actually dissolve uric acid stones. For recurrent calcium stones: Thiazide diuretics (Chlorthalidone) to reduce urine calcium.

Surgical & Procedural Management

For stones > 10mm, failing to pass, or causing severe pain/AKI: 1. Ureteroscopy (URS) with laser lithotripsy and stent placement. 2. Extracorporeal Shock Wave Lithotripsy (ESWL) for smaller upper tract stones. 3. Percutaneous Nephrolithotomy (PCNL) for large (>2cm) or staghorn stones.

Recommended Lifestyle Changes

• Increase fluid intake to produce > 2.5 liters of urine daily (the single most effective prevention strategy).
• Consume a normal calcium diet (1000-1200 mg/day). Paradoxically, low calcium diets INCREASE stone risk because free oxalate is absorbed in the gut.
• Limit sodium (<2g/day) and animal protein intake.
• Increase dietary citrate (lemon juice).

Patient Counseling & Advice

Use a urine strainer at home to catch the stone for laboratory analysis, which dictates the specific dietary and medical prevention strategy.

Follow-Up & Monitoring Schedule

Outpatient urology follow-up with KUB X-ray or ultrasound in 2-4 weeks to confirm stone passage if asymptomatic. Metabolic evaluation (24-hour urine) for recurrent stone formers.

Preventive Strategies

Thiazide diuretics for hypercalciuria. Allopurinol for hyperuricosuria. Potassium citrate for hypocitraturia. High fluid intake for all.

Most small stones pass within 2-4 weeks. Without metabolic management and lifestyle changes, 50% of patients will have another stone within 5-10 years.