Nephrolithiasis for the OB/GYN
/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.