Kidney Stones: A Practical Update

Educational Objectives

The goal of this program is to improve management of kidney stones. After hearing and assimilating this program, the clinician will be better able to:

1. Choose the appropriate imaging method to diagnose kidney stones.
2. Differentiate the clinical manifestations of kidney stones from other conditions.
3. Manage primary and recurrent kidney stones.

Summary

Epidemiology: kidney stones are common, affecting ≈16% of adult men and 8% of women; the male-to-female ratio has shifted from 3:1 to <2:1; the prevalence of kidney stones doubled from 4% to 8% during the COVID-19 pandemic due to higher intake of vitamins C and D; they are more prevalent among White individuals, with African American individuals being the least affected

Clinical manifestations: stones in the kidney are very common, but most patients are asymptomatic; pain occurs when the stone passes through the ureter and ranges from mild to severe; it comes in waves, lasting 20 to 60 sec as the stone moves, and can radiate to the testicles or labia due to obstruction of the ureteral and nerve irritation in the renal capsule; microscopic hematuria is common, and white blood cells (WBCs) may also be present; however, its absence does not rule out kidney stones; other symptoms include nausea, vomiting, dysuria, and urinary urgency, especially when a stone is in the bladder; on examination, patients often appear restless and describe unilateral back pain

Diagnostic workup: check for an elevated WBC count, acute kidney injury, and electrolyte imbalances, and perform a urinalysis including pH, hematuria, and crystal analysis by microscopy; urine pH indicates the type of stone and is important to assess in patients with recurrence; x-rays are useful for localizing stones in the ureter or kidney that are >2 mm, but they cannot visualize uric acid stones that are radiolucent; x-rays may be helpful for patients with known stones; ultrasonography (USG) is a good option for pregnant patients (no radiation) and can detect stones >1 mm in the kidney or bladder; however, it misses ureteral stones, is less sensitive than computed tomography (CT), and often overestimates stone size; stone protocol CT uses 10% of the radiation needed for regular abdomen-pelvis CT with contrast; CT is the gold standard for kidney stone imaging as it detects most stones and may predict their composition based on Hounsfield units; the radiation in CT is ≈3 times that of a kidney-ureter-bladder x-ray; intravenous pyelography is no longer used because of high radiation, and CT is more useful; magnetic resonance imaging is not useful

Differential diagnosis (DD): patients with renal bleeding from renal carcinoma do not typically have flank pain; active kidney inflammation does not typically cause pain unless there is significant swelling; pyelonephritis usually involves fever, flank pain, and pyuria; intestinal obstruction due to appendicitis or diverticulitis causes pain but not hematuria; biliary colic, cholecystitis, and mesenteric ischemia do not cause hematuria; DD includes ectopic pregnancy or torsion of an ovarian cyst; flank pain that comes in waves, hematuria, and the presence of crystals in the urine are suggestive of kidney stones

Management: administer fluids; opioids can help with acute pain but do not address the underlying pathophysiology; large doses of nonsteroidal anti-inflammatory drugs (NSAIDs) reduce inflammation and pain during stone passage; evaluating microscopic hematuria is costly; CT urography typically yields negative results and leads to cystoscopy; most patients with kidney stones do not develop infection; recurrent kidney obstruction leads to tubular dysfunction in the medulla; the kidney cannot concentrate urine or effectively excrete hydrogen, which increases risk for renal tubular acidosis (RTA) and kidney stones; recurrence rate is 50% to 75%

Types of kidney stones: the majority are calcium (Ca)-containing, ie, Ca oxalate, Ca phosphate, or a combination; uric acid stones constitute 15% to 20%; struvite and cystine stones are uncommon; other stones are associated with medications; struvite stones (managed by urologists) cannot be prevented as they are related to infection; acidic urine (pH <5.5) favors uric acid and cystine stone formation, whereas alkaline urine (pH >6.5) favors Ca phosphate and struvite stones; with the latter, pH is typically ≈9 because urea-splitting organisms lead to increased urine ammonia; to prevent stones, encourage reduced sodium (Na) and increased fluid intake

Acute management: use NSAIDs or opioids for acute pain relief and provide hydration; (if the patient can tolerate oral fluids (eg, salt water, electrolyte supplements [eg, CeraSport, GeriAide, PediaLyte]), observation is fine; expulsive therapy can be used for, eg, a 4-mm stone stuck in the ureter; use tamsulosin or nifedipine to relax the ureter and allow the stone to pass; advise the patient to collect the calculus by straining their urine; guidelines state that stones ≤10 mm can often be managed conservatively, but the speaker believes that a first stone measuring >4 mm is less likely to pass; hospitalization and urgent referral to urology — indicated for patients with obstruction and pyelonephritis, a solitary kidney and obstruction, bilateral obstruction, or acute kidney failure; hospitalization is also indicated for nausea and vomiting with inability to take anything orally and for refractory pain, which may require intravenous (IV) analgesics, including opioids; IV ketorolac (Toradol) is effective but is not used often as it has a higher risk for papillary necrosis than oral NSAIDs

Referral to urology: indicated for stones >10 mm or not passed ≤4 wk; urologists can use shockwave lithotripsy for a single stone, stones <15 mm in the pelvis or proximal ureter, and Ca-based and cystine stones, but it is not effective for uric acid stones; lithotripsy machines have limited availability, but ureteroscopy can visualize from the bladder to the distal kidney and allows laser lithotripsy and basket extractions; percutaneous nephrolithotomy is used for large (>15 mm) and complex stones

Metabolic management: indicated for patients with >2 kidney stones, a complicated first stone requiring surgery or hospitalization, or a family history of Ca or uric acid kidney stones; evaluate stone composition through x-ray, serum Ca and bicarbonate levels; if the patient has excess Ca in the urine, check parathyroid hormone, vitamin D, and dihydroxy vitamin D (hyperparathyroidism [HPT] is seen in ≈1% of the population); perform two 24-hr urine collections without dietary modifications; dietary and drug management — involves increasing fluid intake, reducing salt and oxalate consumption, maintaining normal Ca levels, and limiting protein to ≤1 gm/kg of lean body weight; thiazide diuretics are commonly used because of their affordability; citrate salts (especially potassium citrate) are often costly; allopurinol is useful; follow-up — annual imaging is recommended to assess changes; use x-rays unless recurrent x-rays show excessive stool, in which case USG is preferred (lower radiation and cost than stone protocol CT); if the patient has uric acid stones, USG or stone protocol CT is preferred

Ca Stones

Risk factors (RFs): include low urine output (UO), excess Ca (hypercalciuria), uric acid and oxalate in the urine, and not enough citrate; advise patients that 80 oz of fluid daily is the minimum (even with heart failure)

Hypercalciuria: is responsible for 50% of Ca stones and is often associated with a high-Na diet and Na excretion (salt-sensitive hypercalciuria); advise patients to reduce Na intake to ≤2000 mg/day, which aligns with a typical Mediterranean diet; thiazide diuretics are commonly used; indapamide and chlorthalidone (CTD) provide 24-hr efficacy, but indapamide is more effective because of its carbonic anhydrase activity; hydrochlorothiazide (HCTZ) only works for 8 hr but can still be used; potassium-sparing diuretics are effective; spironolactone has an antihypertensive effect; amiloride blocks the epithelial Na channel without affecting blood pressure; spironolactone can be used with HCTZ or CTD; the long-term action of indapamide and CTD can lead to potassium wasting; hypotension is an adverse effect of diuretics

Idiopathic hypercalciuria: the current thresholds are 250 mg/day in men, 200 mg/day in women, and 4 mg/kg in children; rule out hypercalcemia, sarcoidosis and other granulomatous disease, primary HPT, or RTA; treatment — increase fluid intake, restrict dietary Na (<2300 mg), use thiazides, amiloride, and potassium supplements; do not restrict Ca intake in any patient with Ca stones, as Ca is necessary to bind oxalate and phosphate from the diet; ≥600 mg of Ca intake daily with meals is recommended (studies show that intake <600 mg increases the incidence of new kidney stones)

Hyperuricemia: is concerning at >800 mg of uric acid in the urine; patients usually report passing “gravel” in their urine all the time; patients may be overweight with diabetes, hypertension, and sleep apnea; can result from a high-purine diet or from consuming high-fructose corn syrup; allopurinol and a purine-restricted diet is recommended, regardless of the serum uric acid level

Hyperoxaluria: the threshold for oxalate in urine is >40 mg; this results from low Ca intake, high-oxalate diets, or vitamin C >1000 mg (excess converts into oxalate); people with >80 mg of oxalate in their urine may have primary hyperoxaluria, inflammatory bowel disease, or after gastric bypass surgery and should be referred; do not advise a low-oxalate diet; advise calcium intake of 800 to 1000 mg with meals; for enteric hyperoxaluria, prescribe Ca supplementation, low-fat diet, cholestyramine, and vitamin B₆; liver transplantation may be required for uncontrolled disease

Low citrate: citrate is a natural inhibitor of kidney stone formation, as it competes with oxalate and phosphate for binding; young children and people >70 yr of age tend to have low levels; citrate is reduced in acidic states, eg, patients with diarrhea, RTA, hypokalemia (<3.6 mEq/L), and kidney failure (because citrate is used to manage acidosis); most cases are idiopathic; address any reversible causes and supplement with potassium citrate 20 to 100 mEq/day; the goal is to raise citrate to 400 to 600 mg/day; citrate supplementation is effective for most types of Ca stones except Ca-phosphate and treatment-resistant stones

Primary hyperparathyroidism: patients often have Ca-phosphate stones, Ca deposits in the medulla, hypercalciuria, and alkaline urine; parathyroidectomy is the treatment

Renal tubular acidosis: impairs the kidney’s ability to excrete hydrogen ions (H+) and leads to acidosis; H+ reaches the bones and activates osteoclasts that cause bone loss, resulting in excretion of Ca and phosphorus in the urine (pH >6), eventually resulting in stones; potassium citrate supplementation is recommended to prevent osteomalacia and osteoporosis; patients typically have high urinary Ca, low citrate, and alkaline urine

Uric acid stones: RFs are low fluid intake, acidic urine, and sometimes hyperuricemia; patients with metabolic syndrome have low UO, elevated uric acid levels (6-7 mg/dL), and a urine pH ≈5; uric acid is insoluble in acidic urine; alkalinize the urine (eg, with potassium citrate or Na bicarbonate) to a pH ≈6 to convert uric acid to soluble urate; in rare cases, acetazolamide can be used at night; this is not for long-term use as it affects bone health; treat high serum uric acid with allopurinol or febuxostat; adverse effects are common but tolerable; monitor excretion of uric acid and urate, address other RFs, and increase fluid intake

Stenting: urologists stent the ureter when there is evidence of stone impaction, stones present for >2 wk, or anomalies

Readings

Bhojani N, Bjazevic J, Wallace B, et al. UPDATE - Canadian Urological Association guideline: Evaluation and medical management of kidney stones. Can Urol Assoc J. 2022;16(6):175-188. doi:10.5489/cuaj.7872; KC M, Leslie SW. Uric acid nephrolithiasis. StatPearls Publishing. 2023 Oct 15. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560726/; Leslie SW, Sajjad H, Murphy PB. Renal calculi, nephrolithiasis. StatPearls Publishing. 2024 Apr 20. Available from: https://www.ncbi.nlm.nih.gov/books/NBK442014/; Rey, Rodolfo SF, Niedlich. Indapamide effects on hypercalciuria and bone mineral density. Int J Diabetes Metab Disord, 2022; 7(2), 206-214; Shastri S, Patel J, Sambandam KK, et al. Kidney stone pathophysiology, evaluation and management: Core Curriculum 2023. Am J Kidney Dis. 2023;82(5):617-634. doi:10.1053/j.ajkd.2023.03.017; Vijayakumar M, Ganpule A, Singh A, et al. Review of techniques for ultrasonic determination of kidney stone size. Res Rep Urol. 2018;10:57-61. Published 2018 Aug 10. doi:10.2147/RRU.S128039.

Disclosures

For this program, members of the faculty and planning committee reported nothing relevant to disclose.

Acknowledgements

Dr. Patel was recorded at Updates in Nephrology for the Primary Care Provider, held April 13, 2024, in Ann Arbor, MI, and presented by University of Michigan Medical School. For information about upcoming CME activities from this presenter, please visit https://umich.cloud-cme.com. Audio Digest thanks the speakers and presenters for their cooperation in the production of this program.

CME/CE INFO

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Lecture ID:

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