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Urolithiasis: A Complete Guide from Scratch to Pro Level

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Part 1 - Foundations: What Is Urolithiasis?

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Part 2 - Epidemiology

• Up to 20% of men and 10% of women will develop at least one stone during their lifetime (NHANES 2007-2016)
• Prevalence is rising globally, driven by Westernization of diet, increasing BMI, and metabolic syndrome
• Prevalence is ~50% lower in Black individuals compared to white individuals
• Peak incidence in white men: ~3.5 cases/1000/year at age 40
• Peak incidence in white women: ~2.5 cases/1000/year in their thirties
• Recurrence is the rule, not the exception - without intervention, 50% recur within 5 years
• Stone disease has a major economic impact because those affected are often of working age

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Part 3 - Stone Types and Their Distribution

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Part 4 - Pathophysiology: How Stones Form

The Core Concept: Supersaturation

Supersaturation → Nucleation → Crystal Growth/Aggregation → Stone

Randall's Plaques - The Modern Understanding

Key Inhibitors of Stone Formation

1. Urinary citrate - most clinically important; chelates calcium, prevents nucleation
2. Magnesium - competes with oxalate binding
3. Pyrophosphate - inhibits crystal growth
4. Nephrocalcin (glycoprotein) - inhibits crystal aggregation
5. Tamm-Horsfall protein (uromodulin) - inhibits crystal aggregation
6. Osteopontin

Risk Factors for Stone Formation

• Low urine volume (< 1.5 L/day) - most correctable factor
• Hypercalciuria (Ca > 250 mg/day women, >300 mg/day men)
• Hyperoxaluria (Ox > 40 mg/day)
• Hyperuricosuria (UA > 750 mg/day women, >800 mg/day men)
• Hypocitraturia (Cit < 320 mg/day)
• Alkaline urine (promotes calcium phosphate, struvite)
• Acidic urine (promotes uric acid, calcium oxalate)

• Primary hyperparathyroidism (elevated Ca)
• Renal tubular acidosis type 1 (distal) - alkaline urine, hypocitraturia
• Gastrointestinal disease (Crohn's, gastric bypass) - enteric hyperoxaluria
• Gout / hyperuricemia
• Obesity and metabolic syndrome - acid urine, hyperuricosuria
• Type 2 diabetes mellitus
• Medullary sponge kidney

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Part 5 - Stone-by-Stone Deep Dive

5A. Calcium Oxalate Stones

1. Absorptive hypercalciuria - excess intestinal calcium absorption; suppresses PTH; normal/low serum Ca; normal fasting urine Ca
2. Resorptive hypercalciuria - primary hyperparathyroidism (elevated PTH, serum Ca, urine Ca)
3. Renal leak hypercalciuria - renal tubular defect leaking Ca into urine; high PTH secondary response; normal serum Ca

• Primary hyperoxaluria - autosomal recessive enzyme defects (AGXT, GRHPR, HOGA1); severe, early-onset, can cause oxalosis
• Enteric hyperoxaluria - fat malabsorption (Crohn's, bariatric surgery) → increased colonic oxalate absorption (fat binds Ca, leaving oxalate free to be absorbed)
• Dietary hyperoxaluria - excessive intake of spinach, nuts, chocolate, tea, rhubarb (see Box 60.4 from CCN)

5B. Calcium Phosphate Stones

• Distal renal tubular acidosis (type 1) - inability to acidify urine below pH 5.5; classic triad: nephrocalcinosis + alkaline urine + hypocitraturia
• Primary hyperparathyroidism
• Pregnancy (most common stone type in pregnancy)
• Topiramate use (carbonic anhydrase inhibitor → urinary alkalinization)

5C. Uric Acid Stones

1. Hyperuricosuria - gout, high purine diet, tumor lysis syndrome, uricosuric drugs
2. Persistently acidic urine (pH < 5.5) - most important factor; seen in obesity, metabolic syndrome, diabetes type 2 (insulin resistance impairs ammoniagenesis → low NH3 to buffer urine)
3. Low urine volume

5D. Struvite Stones (Infection/Triple Phosphate Stones)

• Proteus mirabilis (most common)
• Klebsiella
• Pseudomonas
• Staphylococcus epidermidis
• Haemophilus, Yersinia
• Ureaplasma urealyticum (doesn't grow on routine media - request specifically!)
• NOT E. coli (does not produce urease)

5E. Cystine Stones

• Presents in 2nd-3rd decade of life
• Stones are mildly radiopaque (sulfur content) - described as "ground glass"
• Hexagonal crystals on urinalysis - pathognomonic
• Often recurrent, bilateral, staghorn-forming

1. High fluid intake (target urine volume > 3 L/day)
2. Alkalinize urine to pH > 7.5 (potassium citrate, acetazolamide)
3. Reduce dietary sodium and protein (reduces cystine excretion)
4. Thiol drugs for refractory cases: D-penicillamine or tiopronin - these form soluble disulfide complexes with cystine

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Part 6 - Urinary Crystals Under the Microscope

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Part 7 - Clinical Presentation

The Classic Presentation

• Colicky - comes in waves as the ureter peristaltically contracts against the stone
• Unilateral
• Associated with nausea and vomiting
• Associated with gross or microscopic hematuria (90% of cases)
• Patient is typically restless and cannot find a comfortable position (vs. peritonitis where patients lie still)

Location-Specific Features

Key Clinical Points

• "Pus under pressure" = infected obstructed kidney (UTI + obstructing stone) = urologic emergency requiring urgent drainage (ureteral stent or nephrostomy)
• Stones in the UVJ are the most painful because the UVJ is the narrowest point of the ureter
• Most stones (up to 80% of stones ≤ 4 mm) pass spontaneously
• A fever in a patient with an obstructing stone demands urgent intervention, not just antibiotics

Three Natural Narrowings of the Ureter

1. Ureteropelvic junction (UPJ) - pelvis narrows to upper ureter
2. Pelvic brim - ureter crosses the iliac vessels
3. Ureterovesical junction (UVJ) - most narrow point; most stones impact here

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Part 8 - Diagnosis

Initial Workup

• BMP (creatinine - assess renal function, electrolytes)
• Calcium, uric acid, phosphate
• PTH if hypercalcemia present

Imaging Hierarchy

• Sensitivity 96-100%, specificity 92-100%
• Identifies ALL stone types including radiolucent uric acid stones (except indinavir stones)
• Shows size, location, degree of obstruction, hydronephrosis, perinephric stranding
• Detects alternate diagnoses (appendicitis, aortic aneurysm)
• Low-dose CT protocols reduce radiation by 50-75% with ~96-97% sensitivity/specificity and are now preferred for initial workup
• Look for "soft-tissue rim sign" around UVJ stones = edema of ureteral wall confirms it's a true stone vs. phleboliths

• No radiation; first-line in pregnancy and children
• Good at detecting hydronephrosis
• Sensitivity 29-69% for ureteral stones (lower than CT)
• Absence of ureteral jets on contralateral decubitus position suggests obstruction

• Only shows radiopaque stones
• Used to monitor known calcium stones
• Cannot see uric acid stones

• Historical gold standard; largely replaced by CT
• Still useful in pregnancy (limited IVU: ~93% accuracy)

• Pregnancy option if ultrasound non-diagnostic
• Findings: signal void at stone, perinephric/periureteral edema, abrupt ureteral termination

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Part 9 - Metabolic Evaluation (24-Hour Urine)

24-Hour Urine Collection Measures:

• Volume (target >2 L)
• Calcium (>250 mg/day women, >300 mg/day men = hypercalciuria)
• Oxalate (>40 mg/day = hyperoxaluria)
• Uric acid (>750/800 mg/day = hyperuricosuria)
• Citrate (<320 mg/day = hypocitraturia)
• Sodium (marker of dietary salt intake - drives calciuria)
• pH (average and range throughout the day)
• Creatinine (validates completeness of collection: ~20-25 mg/kg/day)
• Supersaturation indices (calculated for CaOx, CaP, uric acid)

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Part 10 - Acute Management of Renal Colic

Step-by-Step Approach

• NSAIDs (ketorolac, diclofenac) - first-line; most effective for renal colic
  • Mechanism: reduce ureteral prostaglandin-mediated spasm and renal pelvis pressure
  • As effective as opioids with fewer side effects
• Opioids (morphine, hydromorphone) - if NSAIDs contraindicated or insufficient
• Antispasmodics (e.g., hyoscine) - adjunctive

• Alpha-1 blockers (tamsulosin 0.4 mg daily) - relax ureteral smooth muscle, increase stone passage rates, especially for distal ureteral stones 5-10 mm
• Calcium channel blockers (nifedipine) - alternative; used in pregnancy for tocolysis at doses similar to MET doses
• Most beneficial for stones 5-10 mm; less evidence for smaller or larger stones

• Strain urine to capture the stone for analysis
• Re-imaging at 2-4 weeks if stone has not passed
• If no passage in 4-6 weeks → consider urologic intervention

Indications for Urgent/Emergency Intervention

• Infected obstructed kidney (fever + obstruction = emergency)
• Bilateral obstruction
• Obstruction of solitary kidney
• Uncontrolled pain
• Renal function deterioration
• Complete obstruction

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Part 11 - Surgical Management

Extracorporeal Shock Wave Lithotripsy (ESWL)

• High-energy shock waves focused on stone under fluoroscopy/ultrasound guidance
• Works best for: stones < 2 cm, upper ureter, kidney (non-lower pole)
• Done under general/IV sedation
• Contraindications: pregnancy, coagulopathy, aortic aneurysm, distal obstruction, pacemaker
• Shock wave-resistant stones: calcium oxalate monohydrate, brushite, cystine (avoid ESWL, prefer URS/PCNL)
• Lower-pole stones have poor ESWL outcomes if: steep infundibular-pelvic angle, long lower-pole calyx (>10 mm), narrow infundibulum (<5 mm), skin-to-stone distance >10 cm
• Fragments ("steinstrasse") can obstruct - risk with large stones

Ureteroscopy (URS)

• Flexible or semi-rigid scope passed up the ureter
• Allows direct visualization + fragmentation (holmium laser) + stone basket retrieval
• Stone-free rates ~91-93% for stones up to 2 cm
• First-line for distal ureteral stones
• Preferred over ESWL for stones > 10 mm (higher stone-free rate)
• Can reach anywhere in the ureter and kidney (flexible URS)
• Risks: ureteral injury, stricture, UTI, ureteral avulsion (rare)

Percutaneous Nephrolithotomy (PCNL)

• Dilated tract percutaneously through the flank directly into the renal collecting system
• Nephroscope passed through tract; stone fragmented and removed
• Indications:
  • Stones > 2 cm (in kidney)
  • Staghorn / struvite stones
  • Lower-pole stones > 10 mm
  • ESWL-refractory stones < 2 cm
  • Multiple stones
  • Cystine stones
• Higher morbidity (bleeding, sepsis, adjacent organ injury) but highest stone-free rates
• Mini-PCNL / micro-PCNL - smaller tracts for pediatric patients or smaller stones

Surgical Decision Summary (AUA/EAU Guidelines)

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Part 12 - Long-Term Prevention (Medical Management)

Universal Measures (All Stone Types)

1. Increased fluid intake - target urine output > 2 L/day (ideally 2.5 L)
   • This single intervention reduces recurrence risk by ~50%
   • Optimal fluids: water, citrus juice (lemon/orange juice), coffee, tea, beer, wine
   • Avoid: sugar-sweetened sodas (high fructose → vasopressin-dependent urine concentration, lower pH, uricosuria)
2. Dietary sodium restriction - < 2 g/day sodium (reduces calciuria)
3. Dietary protein moderation - animal protein increases calciuria, uricosuria, and lowers citrate
4. Normal calcium intake - Do NOT restrict dietary calcium; this is a critical point
   • Low dietary calcium increases intestinal oxalate absorption → more oxalate in urine
   • Target 1000-1200 mg/day from diet (not supplements)

Stone-Specific Medical Prevention

High-Oxalate Foods to Avoid:

High-Purine Foods to Avoid (for Uric Acid/Hyperuricosuric Stones):

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Part 13 - Associated Conditions and Systemic Implications

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Part 14 - Special Populations

Pregnancy

• Incidence similar to non-pregnant women (~1:200-4600)
• Calcium phosphate stones most common (unlike non-pregnant: CaOx predominates)
• Pregnancy is a state of relative urinary stasis + increased Ca/uric acid excretion - BUT also increased inhibitors (magnesium, citrate, nephrocalcin), which balance stone risk
• Diagnosis: Ultrasound first-line (radiation-free); sensitivity 29-69%; add transvaginal ultrasound for distal ureteral stones
• If US non-diagnostic: MR urography or limited IVU (93% accuracy)
• CT should be avoided but fetal radiation from abdominal CT is below malformation threshold
• Management: 64-84% pass spontaneously with conservative care
• Conservative: hydration, analgesia, antiemetics, antibiotics if infected; lie with symptomatic side up
• Nifedipine for MET (safe; comparable doses used for tocolysis)
• Surgical: ureteral stent or nephrostomy for persistent obstruction; ureteroscopy if needed

Children (Pediatrics)

• Prevalence: 1-5% in Asia, 5-10% in Europe, 15% in North America
• Metabolic evaluation is mandatory in all pediatric stone patients
• Common metabolic risk factors: hypercalciuria, hyperoxaluria, hypocitraturia
• ~25% are infection stones (urease-producing bacteria)
• Always evaluate for anatomic anomalies (UPJ obstruction, polycystic kidney, neurogenic bladder)
• Imaging: US preferred (radiation avoidance)
• PCNL available for stones > 2 cm; URS generally avoided < 5 years
• ESWL effective and safe under general anesthesia

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Part 15 - Pro-Level Pearls and Exam Traps

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Quick Summary Table

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• Harrison's Principles of Internal Medicine 22e, Chapter on Nephrolithiasis
• Comprehensive Clinical Nephrology 7e, Chapter 60 (Kidney Stones) and Chapter 61 (Obstructive Uropathy)
• Campbell-Walsh-Wein Urology, Urolithiasis/Upper Tract Endoscopy chapters
• Bailey and Love's Short Practice of Surgery 28e, Chapter 20
• Creasy & Resnik's Maternal-Fetal Medicine, Chapter on Urolithiasis in Pregnancy
• National Kidney Foundation Primer on Kidney Diseases 8e, Chapter on Nephrolithiasis
• Bargagli M et al. Nat Rev Nephrol. 2025 Nov [PMID 40790363]
• Tamborino F et al. Int J Mol Sci. 2024 Mar [PMID 38474319]
• Veronese N et al. Minerva Urol Nephrol. 2025 Aug [PMID 40891477]