1. How common are kidney stones?
In industrialized countries, approximately 12% of men and 7% of women will form at least one kidney stone in their life, and the prevalence of nephrolithiasis is increasing. Historically men were much more likely to form kidney stones than women, but recent data show the prevalence in women is approaching that of men. After presenting with their initial stone, about 50% of patients will have at least one more stone within 8 years.
2. What are kidney stones made of?
Approximately 80% of kidney stones are predominantly calcium salts. Calcium oxalate is the major component in 85% to 90% of calcium stones, the rest being calcium phosphate, in the form of apatite or brushite. It is common for small amounts of apatite to be found in calcium oxalate stones. About 10% of stones are uric acid and 5% to 10% are struvite (magnesium ammonium phosphate). Cystine accounts for about 1% of all stones. Stones may be a single pure substance or may be a mix of crystal types, such as calcium oxalate and uric acid.
3. What is the best radiologic test to diagnose kidney stones?
Noncontrast computed tomography (CT) scan or ultrasound are the tests of choice for imaging patients with suspected kidney stones. Advantages of CT include much greater sensitivity for small stones, no intravenous contrast, ability to detect uric acid stones (which are radiolucent on standard x-rays), and the potential to diagnose other causes of flank/abdominal pain if no stones are found. The main drawback with CT scans is the radiation exposure. If a patient requires serial radiologic evaluation, a KUB (kidneys, ureters, and bladder) x-ray is preferred, but only if the stone of interest can be adequately visualized.
Ultrasound can identify hydronephrosis and document stones, but it is not as sensitive or specific as CT. Ultrasound is clearly the preferred imaging method in pregnant women, and many clinicians prefer to use it in children to avoid excessive radiation.
4. Do all patients with renal colic require urologic intervention to remove the stone?
Most patients with renal colic will pass the stone without surgical intervention. Stones <5 mm will pass without intervention 80% to 90% of the time; stones >6 mm pass only 25% of the time. Pharmaceuticals, such as the alpha blocker tamsulosin or calcium channel blockers, which act as ureteral muscle relaxants, can be used to promote stone passage. Corticosteroids may be used to reduce swelling and inflammation. Indications for emergent removal of an obstructing stone include fever, intractable pain, persistent nausea and vomiting, obstruction of a unilateral kidney, or a stone deemed unlikely to pass because of its size.
5. What is extracorporeal shock-wave lithotripsy (ESWL)?
ESWL is a noninvasive method to remove urolithiasis in the renal pelvis or in the ureter. Shock waves are conducted through water to the patient’s flank. The shock waves are focused on the stone, pulverizing it into multiple small pieces. ESWL is an outpatient procedure that requires only mild anesthesia. Disadvantages include the requirement for the patient to pass the fragments, which may cause renal colic, and the potential for some stone fragments to remain in the kidney, particularly stones in the lower pole of the kidney. Some stones, such as cystine and brushite, are difficult to fragment by ESWL.
6. Is open surgery still used for kidney stones?
Open surgery has been replaced by percutaneous nephrolithotomy (PCNL) and ureteroscopy. PCNL is performed by placing a nephroscope through the patient’s flank into the renal pelvis. Stones can be directly visualized and fragmented, and fragments removed through the scope. PCNL most often is used for large stones (>2 cm), stones in a lower pole of the kidney, or stones that have been resistant to ESWL. Ureteroscopy is used for stones in the ureter and some stones in the renal pelvis. The ureteroscope is introduced through the urethra, into the bladder, and then the ureter. Stones may be removed using a basket or destroyed using a laser. The choice of surgical approach depends on the type of stone, location of the stone, and expertise of the urologist.
7. Is it worthwhile having kidney stone composition analyzed?
Whenever stone material is available, crystallographic analysis should be performed. Although 70% of stones will be composed of calcium oxalate, it is the finding of noncalcium stones that provides the most benefit. Struvite and cystine stones are uncommon but cause serious stone diseases; their diagnosis via stone analysis leads to specific and intensive therapy. Uric acid stones are always treated with alkalinization. Crystallization of medications such as indinavir, guaifenesin, and triamterene in the urinary tract can lead to stones, which can only be diagnosed by the analysis of stone material.
8. If an initial stone has been analyzed, do subsequent stones need to be analyzed?
Yes. Analysis of stones as they recur may reveal that stone composition has changed, which could require revision of the stone prevention protocol. In fact, a change in stone type may be a result of preventive therapy, such as calcium phosphate stones forming because of excessive alkalinization in a patient with cystinuria.
9. How much of a workup should a patient with a single kidney stone have?
When adults present with their initial stone episode, they should have a limited workup to identify the more serious issues related to nephrolithiasis. Serum chemistries will reveal hypokalemia and/or acidosis that would indicate distal renal tubular acidosis (RTA), hypercalcemia to identify patients who should be evaluated for hyperparathyroidism, and serum creatinine to estimate kidney function. Stone analysis should always be performed. Urinalysis and/or urine culture should be done to identify infection and microscopy for stone-related crystals. Radiographic evaluation should be performed to quantify stone burden. If multiple stones are seen on CT scan or x-ray, the patient should be considered a recurrent stone former.
10. Who should have 24-hour urine chemistries measured as part of their evaluation?
Adults with recurrent stone disease should have 24-hour urine chemistries measured. Some adults with a single stone event will require full evaluation as dictated by their career, such as airline pilots. Recent AUA guidelines suggest performing 24-hour urine studies in first-time stone formers if they are highly motivated to prevent future stones. In addition, all children with kidney stones should have urine chemistries performed, because they have a higher likelihood of having a severe inherited form of nephrolithiasis such as cystinuria or primary hyperoxaluria.
11. What chemistries should be ordered on a 24-hour urine collection?
Urine chemistries should include:
Urine volume: All patients should try to maintain a urine volume of at least 2.5 liters/day.
Citrate forms a complex with calcium in the urine; thus low urine citrate is a commonly found risk factor for calcium stone formation.
pH should be measured in a 24-hour urine sample; low pH is a risk factor for uric acid stones, and high pH is a risk factor for calcium phosphate stones.
Creatinine should be measured in all timed urine specimens to ensure that the urine specimen was collected properly.
12. Is a random urine ph an adequate replacement for a 24-hour urine pH?
A random urine pH should not be used to assess stone risk, because pH is too variable for a single measurement to provide meaningful information. Urine pH in a normal subject varies from 5.0 to 7.0, depending on diet and time of day. A 24-hour sample gives the time-averaged urine pH; normal range is 5.7 to 6.3 for a 24-hour collection.
13. Is a single 24-hour urine collection adequate when evaluating a patient with nephrolithiasis?
Urine chemistries vary much more than serum chemistries, because urine excretions are dependent on lifestyle, diet, and each patient’s particular physiology. As such, it is wise to perform at least two 24-hour urine collections during the initial evaluation. Optimally, one collection would be on a weekend and the other on a weekday to assess both the home and work environment, because diet, fluid intake, and levels of exertion can vary tremendously.
14. Are crystals seen during urine microscopy helpful in the diagnosis of stone disease?
Cystine crystals and struvite crystals are always abnormal and diagnostic of an underlying disorder ( Fig. 17.1 ). Cystine crystals are only present in people with cystinuria, an uncommon genetic disorder in which cystine is not reabsorbed normally by the kidney. Struvite crystals only form in humans when there is urinary infection with bacteria that possess urease activity. Uric acid and calcium crystals may be more frequent in the urine of stone formers, but they are not diagnostic of disease because they also may be found in healthy people. One situation in which the finding of calcium or uric acid crystals may be helpful is in patients with renal colic without a documented stone, where transient bursts of crystalluria may be causing symptoms.