Kidney Stones for ABIM

Kidney Stones for the American Board of Internal Medicine

Kidney stones (nephrolithiasis) are solid concretions or crystals formed in the kidneys from dissolved urinary minerals. They can lodge anywhere along the urinary tract, leading to pain, obstruction, and hematuria. Stones are classified based on their composition, with calcium oxalate stones being the most common.

Calcium Stones:
Calcium oxalate stones: Most common type (about 80%). Form in the presence of high oxalate and calcium levels. Risk factors include hypercalciuria, hyperoxaluria, and hypocitraturia.
Calcium phosphate stones: Associated with conditions causing alkaline urine, such as renal tubular acidosis (RTA) and hyperparathyroidism.
Uric Acid Stones:
Form in acidic urine and are linked to hyperuricemia, gout, and high purine intake (e.g., from red meat and shellfish). They are radiolucent on X-ray but visible on CT.
Struvite Stones:
Composed of magnesium ammonium phosphate. These stones are associated with recurrent urinary tract infections (UTIs) caused by urease-producing organisms like Proteus mirabilis and Klebsiella. They tend to grow rapidly, forming large "staghorn" calculi.
Cystine Stones:
Rare, caused by cystinuria, an autosomal recessive disorder. These stones form in acidic urine and are difficult to manage due to their tendency to recur.

Stone formation results from supersaturation of the urine with solutes (e.g., calcium, oxalate, uric acid) that crystallize and aggregate. Contributing factors include:
Dehydration: Low urine volume increases the concentration of solutes, promoting crystallization.
Diet: Excessive dietary sodium, oxalate (found in spinach, nuts, and chocolate), and animal protein intake can increase stone risk.
Hypercalciuria: Increased urinary calcium excretion, often idiopathic or secondary to hyperparathyroidism or malignancy, contributes to stone formation.
Metabolic disorders: Hyperuricemia, gout, and hyperoxaluria (due to gastrointestinal conditions like Crohn's disease) increase the likelihood of stone formation.
UTIs: Urease-producing bacteria raise urine pH, facilitating struvite stone formation.

Renal Colic:
Sudden, severe, sharp, or cramping flank pain radiating to the groin, caused by stone obstruction. The pain typically comes in waves and is often associated with restlessness as the patient tries to find relief.

Hematuria:
Microscopic or gross blood in the urine is common due to mucosal irritation from the stone.
Nausea and Vomiting:
Often accompany renal colic due to the severe pain.
Urinary Symptoms:
Dysuria, urgency, and frequency may occur if the stone lodges near the bladder.
Signs of Infection:
Fever, chills, and purulent urine suggest a concurrent UTI or pyelonephritis, which is a urologic emergency.

Urinalysis:
Typically reveals hematuria, and if infection is present, pyuria and bacteriuria. Crystals may be seen in the urine, correlating with the stone type (e.g., calcium oxalate crystals in calcium stones).
Urine pH:
Useful for identifying stone types. Acidic urine (<5.5) is associated with uric acid and cystine stones, while alkaline urine (>7.0) suggests struvite stones.
Imaging:
Non-contrast CT scan of the abdomen and pelvis: The gold standard for diagnosing kidney stones, detecting even small or radiolucent stones (e.g., uric acid stones).
Ultrasound: Preferred in pregnant patients or those who need to avoid radiation. It detects hydronephrosis and larger stones but is less sensitive for small stones.
KUB X-ray (Kidneys, Ureters, Bladder): Can detect radiopaque stones (calcium-containing stones) but misses radiolucent stones (uric acid).
Stone Analysis:
Retrieval and analysis of passed or surgically removed stones help identify the composition and guide preventive measures.

Conservative Management:
Stones <5 mm often pass spontaneously. Initial management includes:
Hydration: High fluid intake to maintain urine output >2 L/day.
Pain Control: NSAIDs (e.g., ibuprofen) are first-line for renal colic. Opioids may be necessary for severe pain.
Alpha-Blockers (e.g., tamsulosin): Promote stone passage by relaxing the smooth muscles of the ureter.
Antiemetics: For nausea associated with renal colic.
Medical Expulsive Therapy:
For stones between 5-10 mm, a combination of alpha-blockers and hydration can assist with stone passage. Stones larger than 10 mm typically require intervention.
Surgical Management:
Extracorporeal Shock Wave Lithotripsy (ESWL): Uses sound waves to break stones into smaller fragments that can be passed. Best for stones <2 cm.
Ureteroscopy: A scope is passed into the ureter to directly remove or fragment stones. Useful for mid-ureteral stones and when ESWL fails.
Percutaneous Nephrolithotomy: Indicated for large (>2 cm) or complex stones (e.g., staghorn calculi) and involves direct removal via a small incision in the back.
Management of Infection:
Infected stones or concurrent UTIs require urgent decompression of the urinary tract with a stent or nephrostomy tube. Antibiotics (e.g., ceftriaxone or piperacillin-tazobactam) are initiated, followed by definitive stone treatment once infection is controlled.

Hydration:
Patients should aim to drink enough fluids to produce at least 2-2.5 liters of urine daily.
Dietary Modifications:
Low-sodium diet: Reduces calcium excretion and stone formation.
Limit oxalate: Patients with calcium oxalate stones should reduce oxalate intake from foods like spinach, nuts, and chocolate.
Moderate calcium intake: Paradoxically, a moderate calcium intake (800-1,200 mg/day) is protective, as calcium binds oxalate in the gut and reduces absorption.
Limit animal protein: Reducing meat consumption can decrease uric acid levels.
Medications:
Thiazide diuretics: Reduce calcium excretion and are useful for patients with recurrent calcium stones.
Allopurinol: Used in patients with hyperuricemia or uric acid stones to reduce uric acid production.
Potassium citrate: Alkalinizes the urine, preventing the formation of uric acid and cystine stones.

Key Points

Kidney stones are categorized by their composition, with calcium oxalate being the most common. Other types include uric acid, struvite, and cystine stones.
Clinical features include sudden onset of severe flank pain (renal colic), hematuria, and nausea. Fever and chills suggest a concurrent infection, which is an emergency.
Non-contrast CT is the gold standard for diagnosis, while urinalysis and stone analysis provide valuable information about stone composition.
Stones <5 mm often pass spontaneously with conservative management (hydration, pain control), while larger stones may require medical expulsive therapy or surgical intervention.
Preventive strategies include adequate hydration, dietary modifications (low sodium, moderate calcium, low oxalate), and medications like thiazides and allopurinol for high-risk patients.