Nephrolithiasis for the OB/GYN

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Background

  • Kidney stones are super common - affecting up to 10% of US adults.

    • There is a described “kidney stone belt” in the US where they are even more prevalent - mostly the South and Southeast of the US.

  • Presentation is frequently classic, but to review:

    • Unilateral flank pain, colicky in nature, waxing-waning with episodes every 20-60 minutes

    • Can radiate to groin

    • Urinary urgency, hematuria, or dysuria also present

    • Nausea, vomiting, and generalized abdominal pain may also be present 

      • Pain also will relieve itself once the stone passes - many patients familiar with the symptoms will recognize this.

  • Risk factors for stones include:

    • Hypertension

    • Gout

    • Obesity

    • Diabetes

    • Certain dietary characteristics:

      • Excessive protein, carbohydrate, and sodium intake

      • Poor fiber intake

      • High oxalate or consumption of carbonated drinks with phosphoric acid (mostly present in higher quantities in colas)

    • Recurrent UTIs with urease-producing organisms 

      • Most classically, Proteus mirabilis 

    • Certain medications and supplements: 

      • Topiramate, Furosemide, Acetazolamide

      • Vitamin C, Vitamin D

  • Kidney stones are made of different stone materials:

    • The most common is calcium oxalate, followed by calcium phosphate (>80%).

    • Other types are uric acid, struvite, and cystine stones.

Diagnosis and Evaluation of Nephrolithiasis

  • Labs

    • Patients with suspected stone should receive a BMP to assess kidney function

    • Should also have a UA to assess for hematuria and potential infection

      • Concomitant UTI may complicate stone management

  • Imaging

    • Preferred: CT abdomen/pelvis without contrast

      • This allows for good imaging of the kidneys and bladder

      • CT characteristics of stones can also help predict stone composition and guide therapy.

    • Pregnancy: ultrasound of kidneys and bladder

      • This avoids radiation while still providing good imaging to evaluate for presence of stones, and if severe obstruction is present.

      • Bladder follow through is important to evaluate for presence of “ureteral jets,” or visible efflux on ultrasound of urine entering the bladder from the ureters.

    • US is not a great modality - sensitivity of ultrasound for stone detection is only about 50-75%, whereas for CT it is 90%+. 

      • However, complication rates from missed diagnoses are similar between CT and ultrasound (less than 1%), and thus it’s prudent in the pregnant patient to use ultrasound if stones are the primary suspicion.

Basics of Management

  • Most patients can fortunately be managed expectantly with pain medication and hydration until the stone passes through. 

    • Hospitalization may be required if patients can’t take PO, or have uncontrolled pain or fever.

    • Patients should strain their urine for several days and bring any collected stones/gravel to clinic.

      • This will allow for stone analysis and to direct preventive therapy.

  • NSAIDs are preferred for pain management over opioids.

    • In pregnant patients, it can be considered to give a single dose of ketorolac if not near delivery timing to provide some short-term relief; however, they are one class of patient where opioids may be used instead.

  • Stone size is the best predictor of passage:

    • < 5 mm stones almost always pass spontaneously. 

    • Stones > 10mm are unlikely to pass spontaneously, as are stones that are in the proximal ureter.

  • Medical expulsive therapy can be considered with alpha blockers for stones between 5-10mm:

    • Tamsulosin 0.4mg daily for up to four weeks can help facilitate stone passage.

      • These generally have minimal side effects.

      • Most effective for more distal stones.

      • There is not much safety data for pregnancy, so typically are not used in pregnant patients.

    • Nifedipine and other calcium channel blockers can also be considered, but with lower success rates compared to alpha blockers.

Urology Referral / Complex Management

  • Surgical indications include persistent pain, infection, and urinary tract obstruction.

    • Urgent decompression is required in patients with:

      • Suspected/confirmed UTI

      • Bilateral obstruction and AKI

      • Unilateral obstruction and AKI in patients with solitary kidney

    • Elective decompression can be considered in patients with:

      • Stones > 10mm

      • Stones under 10mm that have not passed after 4-6 weeks of observation

      • Pregnant patients with stones after failing observation period

      • Persistent kidney obstruction

      • Recurrent UTI

  • Surgical approaches can be with shockwave lithotripsy, ureteroscopy with stenting, or percutaneous nephrolithotomy.

    • Stenting and shockwave therapy are generally preferred, and urologists will choose based on stone location and characteristics.

    • Shockwave therapy cannot be performed with an indication for urgent decompression, nor in the pregnant patient.

      • Ureteroscopy with stenting is the preferred method in pregnancy.

      • Stent exchange or nephrostomy tube change has to be performed much more frequently in pregnancy (every 4-6 weeks) due to higher GFR and thus higher risk of stent/tube obstruction and/or infection. 

Prevention of Recurrent Stones

  • Fluid intake is very important:

    • Drink enough to produce at least 2L of urine a day. Spread fluid throughout the day. Studies have shown even small amounts of urine volume increase reduces recurrence!

    • Water is the best choice, but avoid sweetened soda at the very least due to phosphoric acid content.

  • Limit sodium intake to under 2300mg / day

    • This is due to calcium reabsorption becoming more favorable in the proximal tubule down the same sodium concentration gradient.

    • By reducing dietary sodium, you enhance reabsorption of sodium (and consequently calcium).

  • Fruits and vegetables rich in potassium help excrete citrate better, limiting stone formation.

  • Weight loss in obese patients may also help prevent stone recurrence, though data is limited.

  • In patients with the most common type of stone (calcium oxalate):

    • A calcium-rich diet may be helpful, while trying to obtain as much from dietary sources as possible.

      • Restricting calcium intake is not advised unless it is excessive at baseline.

    • Reduce animal protein intake

      • High sulfur content in animal proteins generates more acid, which then through a variety of complicated renal mechanisms may increase calcium excretion and stone formation.

    • Limit intake of oxalate, fructose, and sucrose.

      • These all increase calcium excretion and/or oxalate excretion.