Kidney Stones for USMLE Step 1

Kidney Stones for the USMLE Step 1 Exam

Kidney stones (nephrolithiasis) are hard, solid deposits made of minerals and salts that form in the kidneys. They can vary in size and composition and may pass through the urinary tract, causing significant pain and possible obstruction.

Calcium Oxalate Stones:
Most common type (~80%). They form in patients with hypercalciuria, hyperoxaluria, or hypocitraturia. Risk factors include high dietary oxalate (e.g., spinach, nuts), high sodium intake, and low urine volume.
Calcium Phosphate Stones:
Occur in patients with alkaline urine, such as those with renal tubular acidosis or hyperparathyroidism.
Uric Acid Stones:
Form in acidic urine (pH <5.5) and are associated with hyperuricemia, gout, and diets rich in purines (e.g., red meat, seafood). Uric acid stones are radiolucent on X-ray but visible on CT scan.
Struvite Stones:
Composed of magnesium ammonium phosphate, these stones form in the setting of urinary tract infections (UTIs) caused by urease-producing organisms like Proteus and Klebsiella. These stones often form large staghorn calculi.
Cystine Stones:
Rare stones caused by cystinuria, a genetic disorder. Cystine stones form in acidic urine and tend to be recurrent and difficult to manage.

Kidney stones form when urine becomes supersaturated with minerals or solutes that crystallize. Contributing factors include:
Dehydration: Low fluid intake concentrates the urine, increasing the risk of crystallization.
Hypercalciuria: High urinary calcium excretion, either idiopathic or due to conditions like hyperparathyroidism, increases the risk of calcium-based stones.
Diet: High sodium, oxalate-rich foods, and excessive animal protein intake promote stone formation.
Infection: Urease-producing bacteria (e.g., Proteus) alkalinize the urine, facilitating struvite stone formation.

Renal Colic:
Severe, sharp, or crampy flank pain radiating to the groin, caused by obstruction of the urinary tract. Pain comes in waves and may be accompanied by restlessness.

Hematuria:
Blood in the urine, either gross (visible) or microscopic, is common due to mucosal irritation from the stone.
Nausea and Vomiting:
These symptoms often accompany renal colic.
Urinary Symptoms:
Dysuria, urgency, and frequency may occur if the stone is located near the bladder.
Signs of Infection:
Fever and chills suggest a UTI or pyelonephritis, which may complicate the case and requires immediate treatment.

Urinalysis:
Detects hematuria, and crystals corresponding to the stone type may be seen (e.g., calcium oxalate crystals). Pyuria and bacteriuria indicate infection.
Urine pH:
Acidic urine (<5.5) is associated with uric acid and cystine stones, while alkaline urine (>7.0) suggests struvite or calcium phosphate stones.
Imaging:
Non-contrast CT scan: The gold standard for detecting stones of all types and sizes, including radiolucent uric acid stones.
Ultrasound: Preferred in pregnant patients or those needing to avoid radiation; detects hydronephrosis and larger stones.
KUB X-ray: Identifies radiopaque stones (e.g., calcium-based stones) but misses radiolucent stones (e.g., uric acid).

Conservative Management:
Stones <5 mm often pass spontaneously. Treatment includes:
Hydration: Patients are advised to drink fluids to produce more than 2 liters of urine daily.
Pain Control: NSAIDs are first-line for renal colic, though opioids may be required for severe pain.
Alpha-Blockers (e.g., tamsulosin): Relax ureteral muscles and facilitate stone passage.
Surgical Management:
Extracorporeal Shock Wave Lithotripsy (ESWL): Uses sound waves to fragment stones for easier passage, effective for stones <2 cm.
Ureteroscopy: Direct visualization and removal or fragmentation of stones using a scope, especially for mid and distal ureter stones.
Percutaneous Nephrolithotomy: Used for large or complex stones (>2 cm), such as staghorn calculi.

Hydration: Aim to produce 2-2.5 liters of urine daily.
Dietary Changes:
Low-sodium diet: Reduces urinary calcium excretion.
Moderate calcium intake: Adequate calcium intake (800-1,200 mg/day) binds dietary oxalate in the gut, preventing its absorption.
Avoid oxalate-rich foods: Reduce intake of spinach, nuts, and chocolate.
Limit animal protein: Reduces uric acid production.
Medications:
Thiazide diuretics: Decrease urinary calcium excretion and are used for patients with recurrent calcium stones.
Allopurinol: Lowers uric acid production, helpful 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 stones being the most common. Other types include uric acid, struvite, and cystine stones.
Renal colic, hematuria, and nausea are hallmark features. Stones causing infection require urgent treatment.
Diagnosis is confirmed via non-contrast CT, the gold standard imaging modality, and urinalysis.
Conservative management includes hydration, NSAIDs for pain, and alpha-blockers to aid stone passage. Larger stones may require surgical intervention (e.g., ESWL, ureteroscopy).
Prevention involves adequate hydration, dietary modifications (low sodium, moderate calcium), and medications like thiazides or allopurinol in high-risk patients.