Nephrolithiasis

Background

Formation of kidney stones occurs when urine becomes supersaturated with stone-forming substances, leading to crystallization. This process can be influenced by various factors, including metabolic disorders, genetic predispositions, dietary habits, and dehydration
Classified into four main types based on their composition:
- Calcium stones (oxalate or phosphate)
  - ~75-80% of stones
  - Driven by increased intestinal absorption of calcium, enhanced bone resorption, or decreased renal tubular reabsorption of calcium with low levels of urinary citrate further increasing risk
- Uric acid stones
  - ~9-10% of stones
  - Primary risk factors are high levels of uric acid in the urine, low urinary volume, and persistently low urinary pH, which makes uric acid less soluble
- Cystine stones
  - ~1% of stones
  - Occur in setting of genetic disorder with defective reabsorption of cystine and other dibasic amino acids (ornithine, lysine, and arginine) in the renal proximal tubules
- Struvite stones
  - ~10% of stones
  - Typically take months to years to form, more common in women with recurrent UTIs from urease producing bacteria

BMP
Uric acid
Parathyroid Hormone level should be measured if primary hyperparathyroidism is suspected, particularly when serum calcium is high or high normal
Urinalysis for pH, erythrocytes, leukocyte esterase, citrate, nitrites, urine culture
- Patients with uric acid stones may have persistent urine pH of 5-5.5 rather than the expected variation in pH of 5 in the morning and 6.5 in the evening
- Patients with RTAs usually have urine pH of 6.5
- Patients with struvite stones usually have urine pH of 8.5
For high-risk or recurrent stone formers, additional metabolic testing is recommended. At VUMC most commonly will see Litholink, which is a send out Labcorp test. This includes one or two 24-hour urine collections analyzed for total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine. This testing helps identify metabolic and environmental risk factors, informing dietary and medical therapy

CT abdomen and pelvis w/o contrast is typically first line
MR urography (MRU) without and with IV contrast or CT abdomen and pelvis with IV contrast may be appropriate as the next imaging study if CT w/o cntrast is inconclusive
Renal and bladder ultrasound may be used in pregnant patients

Stones less than 5 mm often pass spontaneously. Follow-up imaging within 14 days is recommended to monitor stone position and assess for hydronephrosis
Alpha blockers, such as tamsulosin, may be used to facilitate the passage of distal ureteral stones less than 10 mm
NSAIDs are ideal for acute pain control if no contraindications
Thiazide Diuretics are indicated for patients with recurrent calcium stones and hypercalciuria
Potassium Citrate is recommended for patients with hypocitraturia and calcium phosphate stones, as well as for uric acid and cystine stones to raise urinary pH
Allopurinol for patients with recurrent calcium oxalate stones and hyperuricosuria

Extracorporeal Shock Wave Lithotripsy (ESWL): Suitable for renal stones between 10 and 20 mm, especially in favorable anatomical locations
Ureteroscopy: An option for lower pole stones between 1.5 and 2 cm, and for stones resistant to ESWL
Percutaneous Nephrolithotomy (PCNL): Indicated for stones larger than 20 mm, staghorn calculi, and stones in patients with CKD

50% of patients will have recurrent stone at 10 years and 80% will have recurrent stone at 20 years
Increase fluid intake to achieve a urine volume of more than 2.5 L/day and adhere to a low-sodium diet
Weight loss and exercise
- Diets consisting of low sodium, low animal protein (source of uric acid), and low oxalate (beets, berries, chocolate, rhubarb, nuts and leafy greens) with no calcium restriction may be beneficial. During a meal it may help to combine calcium rich food with oxalates that will bind in the intestinal lumen and be excreted in stool
- Adding citrus fruits may increase citrate and reduce stone formation