Kidney Stones for USMLE Step 3

Kidney Stones for the USMLE Step 3 Exam

Kidney stones (nephrolithiasis) are solid deposits that form in the kidneys from minerals and salts in the urine. They can vary in size and may cause obstruction, pain, and hematuria as they pass through the urinary tract.

Calcium Oxalate Stones:
The most common type (~80%). Risk factors include hypercalciuria, hyperoxaluria, hypocitraturia, and diets high in oxalate (e.g., spinach, nuts) or sodium.
Calcium Phosphate Stones:
Form in alkaline urine and are associated with conditions like renal tubular acidosis (RTA) and hyperparathyroidism.
Uric Acid Stones:
Form in acidic urine (pH <5.5) and are linked to hyperuricemia, gout, and high-purine diets (e.g., red meat, seafood). Uric acid stones are radiolucent on X-ray but visible on non-contrast CT.
Struvite Stones:
Composed of magnesium ammonium phosphate, these stones form in the setting of UTIs with urease-producing bacteria (e.g., Proteus, Klebsiella). They can grow rapidly into large staghorn calculi.
Cystine Stones:
Rare, caused by cystinuria, a genetic disorder that leads to high levels of cystine in the urine. These stones form in acidic urine.

Kidney stones form when urine becomes supersaturated with solutes like calcium, oxalate, uric acid, or cystine, which crystallize. Contributing factors include:
Dehydration: Low urine volume increases solute concentration, promoting crystallization.
Hypercalciuria: High urinary calcium excretion, commonly due to idiopathic causes, hyperparathyroidism, or high dietary sodium.
Dietary factors: High oxalate intake (e.g., spinach, nuts), excessive animal protein, and high salt intake can contribute to stone formation.
Infection: Urease-producing bacteria raise urinary pH, facilitating struvite stone formation.

Renal Colic:
Sudden, severe, sharp pain originating in the flank and radiating to the groin. Pain is colicky, and patients are often restless. It is the hallmark of stone obstruction.

Hematuria:
Gross or microscopic blood in the urine due to irritation of the urinary tract.
Nausea and Vomiting:
Frequently accompany renal colic.
Urinary Symptoms:
Dysuria, urgency, and frequency if the stone is near the bladder.
Signs of Infection:
Fever and chills indicate a potential UTI or pyelonephritis, which may require urgent intervention.

Urinalysis:
Reveals hematuria and sometimes crystals (e.g., calcium oxalate). If infection is present, pyuria and bacteriuria may be seen.
Urine pH:
Acidic urine (pH <5.5) suggests uric acid or cystine stones, while alkaline urine (pH >7.0) suggests struvite or calcium phosphate stones.
Imaging:
Non-contrast CT: The gold standard for diagnosing kidney stones, detecting both radiopaque and radiolucent stones.
Ultrasound: Used in pregnant patients or those needing to avoid radiation. It detects hydronephrosis and larger stones but is less sensitive for small stones.

Conservative Treatment:
Hydration: Encouraged to produce more than 2 liters of urine daily.
Pain Control: NSAIDs (e.g., ibuprofen) are first-line for renal colic, while opioids may be needed for severe pain.
Alpha-Blockers (e.g., tamsulosin): Relax the ureter and facilitate passage of stones, particularly for stones 5-10 mm in size.
Surgical Treatment:
Extracorporeal Shock Wave Lithotripsy (ESWL): Non-invasive treatment that uses shock waves to break stones <2 cm into smaller fragments.
Ureteroscopy: Involves direct visualization and removal or fragmentation of stones, useful for mid or distal ureter stones.
Percutaneous Nephrolithotomy: Used for large (>2 cm) or complex stones (e.g., staghorn calculi).

Hydration: Patients should aim to produce 2-2.5 liters of urine daily.
Dietary Modifications:
Low-sodium diet: Reduces urinary calcium excretion.
Moderate calcium intake: Adequate calcium (800-1,200 mg/day) reduces oxalate absorption.
Limit oxalate-rich foods: Reduce intake of spinach, nuts, and chocolate.
Limit animal protein: Reduces uric acid production.
Medications:
Thiazide diuretics: Decrease urinary calcium excretion for recurrent calcium stone formers.
Allopurinol: Reduces uric acid production, useful for patients with uric acid stones or hyperuricemia.
Potassium citrate: Alkalinizes urine, preventing uric acid and cystine stone formation.

Key Points

Kidney stones are classified by composition, with calcium oxalate being the most common type. Other types include uric acid, struvite, and cystine stones.
Symptoms include sudden, severe flank pain (renal colic), hematuria, and nausea. Fever suggests a UTI and requires urgent treatment.
Non-contrast CT is the gold standard for diagnosis. Ultrasound is used in pregnancy or when radiation exposure is a concern.
Conservative management includes hydration and pain control for stones <5 mm, while larger stones may require surgical treatment (e.g., ESWL, ureteroscopy).
Prevention includes adequate hydration, dietary changes, and medications like thiazides or allopurinol for patients prone to recurrent stones.