Kidney Stones for USMLE Step 2

Kidney Stones for USMLE Step 2 Exam

Kidney stones (nephrolithiasis) are solid deposits formed from minerals and salts that crystallize in the kidneys. They can vary in size and composition and cause significant pain and obstruction as they pass through the urinary tract.

Calcium Oxalate Stones:
The most common type (~80%). Formed in patients with hypercalciuria, hyperoxaluria, and hypocitraturia. Risk factors include high dietary oxalate (e.g., spinach, nuts), low calcium intake, and dehydration.
Calcium Phosphate Stones:
These form in alkaline urine and are associated with renal tubular acidosis (RTA) and hyperparathyroidism.
Uric Acid Stones:
Associated with acidic urine (pH <5.5), hyperuricemia, and diets rich in purines (e.g., red meat, seafood). Uric acid stones are radiolucent on X-ray but visible on non-contrast CT.
Struvite Stones:
Formed in the presence of urinary tract infections (UTIs) caused by urease-producing bacteria (e.g., Proteus, Klebsiella). These can form large staghorn calculi.
Cystine Stones:
Rare, caused by cystinuria, a genetic disorder. Cystine stones form in acidic urine and are recurrent.

Stone formation occurs when urine becomes supersaturated with minerals or solutes like calcium, oxalate, or uric acid, which crystallize and aggregate. Factors contributing to stone formation include:
Dehydration: Concentrates solutes in the urine, promoting crystallization.
Hypercalciuria: High urinary calcium levels, often from idiopathic causes or hyperparathyroidism.
Diet: High sodium intake increases calcium excretion, and oxalate-rich foods (e.g., chocolate, spinach) promote calcium oxalate stone formation.
Infection: Urease-producing bacteria increase urinary pH, facilitating struvite stone formation.

Renal Colic:
Sudden, severe, sharp pain originating in the flank and radiating to the groin or lower abdomen, caused by obstruction in the urinary tract. The pain is colicky, with patients often restless.

Hematuria:
Blood in the urine (gross or microscopic) due to irritation of the urinary tract.
Nausea and Vomiting:
Commonly associated with renal colic.
Urinary Symptoms:
Dysuria, urgency, and frequency may occur if the stone is near the bladder.
Signs of Infection:
Fever and chills, indicating a UTI or pyelonephritis, may accompany the stone.

Urinalysis:
Hematuria is common, and crystals (e.g., calcium oxalate) may be seen. Pyuria and bacteriuria suggest infection.
Urine pH:
Acidic urine is associated with uric acid and cystine stones, while alkaline urine suggests struvite or calcium phosphate stones.
Imaging:
Non-contrast CT: The gold standard for diagnosing stones, identifying both radiopaque and radiolucent stones.
Ultrasound: Used in pregnant patients or those needing to avoid radiation exposure, though less sensitive than CT for small stones.

Conservative Treatment:
Hydration: Encouraged to produce more than 2 liters of urine daily to promote stone passage.
Pain Control: NSAIDs are the first-line treatment for renal colic, with opioids reserved for severe cases.
Alpha-Blockers (e.g., tamsulosin): Relax the ureter and facilitate passage of stones, especially for stones 5-10 mm in size.
Surgical Treatment:
Extracorporeal Shock Wave Lithotripsy (ESWL): Non-invasive method for breaking stones <2 cm into smaller fragments for passage.
Ureteroscopy: Involves using a scope to visualize and remove or fragment stones, often used for mid or distal ureter stones.
Percutaneous Nephrolithotomy: Used for large or complex stones (>2 cm) that cannot pass on their own.

Hydration: Patients should aim to produce 2-2.5 liters of urine daily.
Dietary Modifications:
Low-sodium diet: Reduces calcium excretion.
Moderate calcium intake: Adequate calcium (800-1,200 mg/day) binds oxalate in the gut and prevents its absorption.
Avoid high-oxalate foods: Such as spinach, nuts, and chocolate.
Limit animal protein: Reduces uric acid production.
Medications:
Thiazide diuretics: Decrease calcium excretion and are used for recurrent calcium stone formers.
Allopurinol: Used for patients with uric acid stones or hyperuricemia.
Potassium citrate: Alkalinizes urine to prevent uric acid and cystine stones.

Key Points

Kidney stones are classified based on composition, with calcium oxalate stones being the most common. Other types include uric acid, struvite, and cystine stones.
Classic symptoms include sudden onset of severe flank pain (renal colic), hematuria, and nausea. Fever suggests infection and requires urgent evaluation.
Non-contrast CT is the gold standard for diagnosis, while urinalysis aids in identifying stone type and ruling out infection.
Small stones may pass with hydration and medical expulsive therapy, while larger stones often require surgical intervention (e.g., ESWL, ureteroscopy).
Prevention includes adequate hydration, dietary modifications, and medications such as thiazides or allopurinol in high-risk patients.