25.3: Urinary Tract Calculi III: Medical Management

The diagnosis of renal calculi involves several imaging techniques, including non-contrast CT scans and ultrasound. These methods help visualize kidney stones, assess their size and location, and detect possible obstructions. Additionally, Measuring urine pH is useful for diagnosing specific stone types, such as struvite (alkaline pH) and uric acid stones (acidic pH). Cystine stones are primarily linked to cystinuria, a genetic condition. A urinalysis helps detect blood in the urine (hematuria) and the presence of crystals, which may indicate stone formation.

Blood tests measure levels of uric acid and calcium, which can indicate underlying conditions like hyperuricemia (high uric acid) or hypercalcemia (high calcium), both of which can lead to stone formation. A comprehensive serum analysis evaluates calcium, phosphorus, sodium, potassium, bicarbonate, uric acid, blood urea nitrogen (BUN), and creatinine to assess kidney function and identify metabolic factors contributing to stone formation.

For patients with recurrent kidney stones, a 24-hour urine collection is recommended to measure the excretion of substances such as calcium, phosphorus, magnesium, sodium, oxalate, citrate, cystine, sulfate, potassium, and uric acid. This test also assesses total urine volume, providing critical insights into the underlying causes of stone formation.

Care of Patients with Kidney Stones

The management of kidney stones involves two concurrent approaches. The first focuses on treating the acute attack by addressing pain, infection, or obstruction. Opioids or NSAIDs are commonly used to relieve the pain of renal colic. However, NSAIDs should be avoided in patients with impaired renal function.

Most stones smaller than 5 mm typically pass spontaneously. To facilitate the passage of stones, α-adrenergic blockers such as tamsulosin (Flomax) or terazosin may be prescribed to relax the smooth muscle in the ureter and aid in the passage of the stone.

The second approach focuses on preventing further stone formation by determining the underlying cause. Patients are assessed for potential risk factors, such as prolonged immobilization. Treatment for active stone formers emphasizes adequate hydration, sodium restrictions, dietary changes, and medications. The specific drugs prescribed depend on the underlying cause of stone formation. These medications work by altering urine pH, reducing excessive excretion of substances, or addressing a primary condition, such as hyperparathyroidism.

Stone Types and Management

Calcium Oxalate Stones:

• Administer thiazide diuretics to reduce calcium excretion in the urine.
• Prescribe potassium citrate to maintain alkaline urine, which reduces the likelihood of stone formation.
• Avoid vitamin C and calcium supplements. High doses of vitamin C can increase oxalate levels, contributing to stone formation.
• Encourage adequate hydration.

Calcium Phosphate Stones:

• Encourage hydration and reduction of sodium and animal protein intake.
• Increase dietary calcium, which binds oxalate in the intestines, reducing oxalate absorption.
• Treat any underlying conditions, such as hyperparathyroidism, that may contribute to stone formation.

Cystine Stones:

• Administer tiopronin or captopril to prevent cystine crystallization and reduce cystine stone formation.
• Prescribe potassium citrate to maintain alkaline urine, which lowers the solubility of cystine.
• Encourage adequate hydration.

Uric Acid Stones:

• Increase fluid intake to dilute the urine and reduce uric acid concentration.
• Alkalinize the urine with potassium citrate to help dissolve existing uric acid stones.
• Consider prescribing allopurinol to reduce uric acid production.
• Advise patients to limit purine-rich foods, such as red meat and seafood.

Struvite Stones:

• Struvite stones, often associated with infections caused by urea-splitting bacteria, typically require surgical intervention, such as percutaneous nephrolithotomy.
• Medical therapy alone is usually insufficient, but antibiotics may be used to manage infections if surgery is not an option or following stone removal to prevent recurrent infections.
• For patients with retained or recurrent struvite stones, acetohydroxamic acid can be prescribed. This medication inhibits urease, an enzyme produced by bacteria that contributes to stone formation.