Patient is a 37-year-old female with cystic fibrosis, recurrent nephrolithiasis who presents to clinic with asymptomatic bilateral kidney stones (8mm on right; 2mm on left) on imaging.
What type of stone and why cystic fibrosis (CF) is associated with kidney stones?
Patients with CF are at increased risk of calcium oxalate stones. This is thought to be primarily due to enteric hyperoxaluria and hypocitraturia.
Gut malabsorption can lead to hyperoxaluria. In malabsorption, there is an increase in free fatty acids (FFA), which bind to calcium and decrease the amount of calcium in the gut available to bind to oxalate. This leads to an increase in soluble oxalate which is absorbed in the gut (See Nezzal et al. NDT 2016 for an excellent discussion of enteric hyperoxaluria). Furthermore, chronic diarrhea from malabsorption leads to a chronic metabolic acidosis and hypocitraturia. In the urine, citrate is an inhibitor of the calcium oxalate complex. Higher urinary oxalate and less urinary citrate are associated with higher risk of stone formation.
What is the diagnostic work-up recommended in the above patient?
Initial evaluation of all patients with kidney stones, should include two 24-hour urine collections. Evaluating the urine composition provides crucial data for characterizing the patient’s risk of kidney stone formation (both formation of new stones and growth of existing stones). For this patient, she also had a noncontrast CT that again demonstrated a 8mm stone in the lower pole of the right kidney. She also had 2 small (2-3mm) stones on the left (See Curhan, et al. “Diagnosis and acute management of
suspected nephrolithiasis in adults” Up to Date, for more in depth discussion).
How to best manage this patient’s stones?
For her existing calcium oxalate stones we need the help of our urology colleagues. Kidney stones less then 5mm are likely to pass on their own, without procedural intervention. For larger stones, discussion with the patient and urology is important. Options include medical expulsion therapy (MET), extracorporeal shock-wave lithotripsy (ESWL), ureteroscopy (URS), and percutaneous nephrolithotomy (PNL) (See also: AUA/EAU J Urol Vol 178, 2418-2434 2007 and prior blog). Our patient underwent urological
evaluation and ureteroscopy to remove her 8mm stone.
How can we prevent further stones formation?
Our patient’s 24-hour urine revealed total volume of below 2 Liters, oxalate 44, and citrate 87. For her, we discussed increasing
fluid intake to reach a goal of at least 2 L/day. To lower her urinary oxalate we discussed
multiple strategies, including reduction of dietary oxalate intake, increasing
dietary calcium intake to the recommended daily allowance, and starting calcium
supplementation with meals (to bind to oxalate in the gut and reduce oxalate
absorption) [see also
urinary citrate (check this article for the
updated AUA guidelines for the medical management of kidney stones).
Megan Prochaska MD
Second year fellow BWH
Image from oxalate stone (masterfile.com)