Renal calculi are a common cause of blood in the urine (hematuria) and pain in the abdomen, flank, or groin. They occur in one in 11 people at some time in their lifetimes with men affected 2 to 1 over women. Development of the stones is related to decreased urine volume or increased excretion of stone-forming components such as calcium, oxalate, uric acid, cystine, xanthine, and phosphate. Calculi may also be caused by low urinary citrate levels or excessive urinary acidity.
Urolithiasis occurs when solutes crystallize out of urine to form stones. Urolithiasis may occur due to anatomic features leading to urinary stasis, low urine volume, dietary factors (e.g., high oxalate or high sodium), urinary tract infections, systemic acidosis, medications, or uncommonly genetic factors such as cystinuria. The most common cause of the stone disease is inadequate hydration and subsequent low urine volume. The other four most common factors contributing to urinary stone formation are hypercalciuria, hyperoxaluria, hyperuricosuria, and hypocitraturia.
Overall urinary stone prevalence in the United States has increased from 3.8% in 1970 to 8.8% in 2010. For patients who have a history of a previous urinary stone, recurrence rates approach 50% at ten years. There is traditionally a high incidence of urinary stones in the Southeastern and South Central United States, termed the “Stone Belt,” which probably reflects the hot weather climate and relative dehydration that occurs in these areas. Before the development of modern urologic techniques for treatment, mortality from untreated staghorn (infection) calculi was 27%. Currently, mortality from stone disease is rare, although there is still a significant rate (28%) of renal deterioration with certain stone types, particularly staghorn (struvite or infection) stones.
Most urinary stones start as a Randall's plaque at the junction of the nephron's collecting tubule and the renal pelvis in the papilla. These plaques start suburothelial and then gradually grow until they break through into the renal pelvis. Once in continuous contact with urine, layers of calcium oxalate typically start to form on the calcium phosphate nidus (all Randall's plaques are composed of calcium phosphate). Calcium oxalate stones tend to form when the urinary pH is under 7.2 while calcium phosphate will form in the more alkaline urine. Hyperparathyroidism and similar metabolic disturbances like renal tubular acidosis typically form stones that are primarily or significantly composed of calcium phosphate. Overly acidic urine is the primary cause of uric acid stones (not hyperuricosuria).
Patients with a stone disease will most commonly present with acute, severe flank pain that will often radiate to the abdomen and especially to the groin, testicle, and labia. It is often sharp and severe in nature. It may also be colicky. The pain is often associated with nausea and vomiting which is due to the embryological origins of the urogenital tract.
If infected, patients may also present with fever, chills, or other systemic signs of infection. This condition, called pyonephrosis or obstructive pyelonephritis, is potentially severe and life-threatening, requiring emergency decompression surgery.
Patients often present with hematuria as 85% of patients demonstrate at least microscopic hematuria on urinalysis.
A urinalysis should be obtained on every patient with a suspected kidney stone. Hematuria is usually present, but up to 15% of kidney stone patients will not demonstrate even microscopic hematuria. The presence of urinary crystals may suggest urolithiasis. Positive nitrites, leukocytes, and bacteria suggest infection which should be cultured and treated aggressively.
A KUB can be obtained to screen for the presence of significant nephrolithiasis, but may often miss stones that are small, hidden by bowel or uncalcified. Ultrasound may be very useful for assessing obstruction and resultant hydronephrosis, especially in pregnancy where x-ray studies are discouraged. It can also be used to measure the resistive index which can suggest ureteral obstruction.
Resistive Index = (peak systolic velocity - end diastolic velocity)/peak systolic velocity
Values of 0.70 or less are considered normal while higher values suggest obstructive uropathy. Bilateral high resistive indices suggest medical renal disease while a unilateral high resistive index (0.75 or higher) suggests an obstruction such as from a stone.
Ultrasound can also identify uric acid and other non-calcific stones if they are large enough (usually greater than 4 mm), but it can also miss the presence of stones that are less than 5 mm.
The most sensitive and reliable test to diagnose urolithiasis is a non-contrast abdominal and pelvic CT scan, which will also provide information regarding obstruction with resultant hydronephrosis or concerns for infection. Other labs to obtain would include a WBC with differential, and a urine culture if the patient is febrile or has a urinalysis suggestive of a possible infection. The initial use of IV contrast for CT scans in patients with abdominal pain is not recommended. In many cases, an atypical abdominal pain will ultimately turn out to be a kidney stone that has moved or the presence of a urological anatomical variant such as a horseshoe kidney. Even without IV contrast, in most cases the correct diagnosis can be made. If contrast is absolutely necessary, doing the non-contrast study first eliminates urinary stones from consideration. Certainly, if the urinalysis is abnormal for blood or possible infection, a non-contrast abdominal and pelvic CT should be performed prior to using contrast which will make identification of any urinary stones far more difficult. If this recommendation is not followed, sooner or later contrast will be given to a patient who will ultimately be diagnosed with urinary stones. Obscuring urinary stones with IV contrast can make it much more difficult to determine optimum treatment and possible surgery.
If the CT is positive for stones, a simultaneous KUB should be done. This will provide information useful in tracking or following the progress of the stone, it's degree of calcification, and its shape which cannot always be identified from the CT scan alone.
Many stones may be watched conservatively as an outpatient, with intervention planned as an outpatient. Smaller stones (less than 5 mm) have a greater chance (90%) of passing on their own with medical expulsion therapy (usually tamsulosin, nifedipine or alfuzosin). Any hint of a urinary tract infection should be treated aggressively with antibiotics.
There are several cases where urgent intervention is required.
In the case of a urinary tract infection or urosepsis with an obstructing stone, the obstruction should first be relieved with either a ureteral double J stent or nephrostomy tube placement. The decision of which treatment modality is most appropriate should be made by urology. In general, the more severely ill the patient, the greater the benefit from a nephrostomy tube. Definitive stone management can then occur once the infection is no longer active. Morbidly obese patients and those who cannot be safely taken off of their blood thinners may require double J stent, regardless.
Electively, stones can be surgically managed in several ways. Extracorporeal shockwave lithotripsy (ESWL) can be used to break up stones anywhere in the urinary tract but is primarily used in the kidney and upper ureter. Ureteroscopy with laser lithotripsy can be used to manage stones endoscopically and is preferred for ureteral stones in the lower ureter. For large (greater than 2 cm) stones in the renal pelvis, percutaneous nephrolithotomy can be performed.
Once the patient has had his or her acute stone episode treated, it is recommended to evaluate the patient for the underlying cause for their stone episode, particularly if he or she has had stones in the past. This would involve obtaining a basic metabolic panel as well as a 24-hour urine collection for stone prevention analysis. Patients need to understand that this represents a commitment from them to follow a long-term course of therapy for stone prevention and that no treatment plan is foolproof so an occasional stone may still be produced but is much less likely on therapy than off. Physicians evaluating 24-hour kidney stone results should not only look at the normal ranges but also at what may be optimal. For example, in general, optimal 24-hour urinary calcium should be no more than 250 mg, oxalate less than 25 mg, citrate more than 600 mg, urinary volume more than 2,000 cc and urinary uric acid at 600 mg or less. While these levels may not be realistically obtainable in every patient, they are used as goals for treatment where the intention is to get as many chemistry levels optimal as possible even if they are all technically normal.
Analysis of 24-hour urine tests can be complicated. A companion piece "24 Hour Urine Testing for Nephrolithiasis: Guide to Interpretation" by Leslie S and Bashir K is recommended for more details on 24-hour urine interpretation and preventive therapy.
Ultimately, the success of any kidney stone preventive treatment program will depend on the patient's willingness to follow a long-term course of treatment that will involve some level of dietary modifications, medications, sacrifice and change with no obvious immediate or noticeable benefits. Patients on treatment may still make stones, albeit fewer than otherwise. Patients may cheat on their therapies from time to time, and since they do not seem to pay any immediate price or penalty, many will revert to their previous diets and behaviors. Patients may also develop an over-reliance on drug therapy so they can minimize the dietary changes requested.
Most patients with kidney stones, even those with multiple recurrences, are unaware of the availability of 24 hour urine testing and the potential benefits of preventive measures based on this testing. Successful kidney stone preventive programs require high levels of patient compliance, motivation, and discipline for their efficacy. Patients should be educated about preventive therapy, but only those who are strongly motivated are likely to have long-term success. The health care team involved in the care of nephrolithiasis patients has an absolute obligation to inform patients of the existence of such programs, particularly in cases of multiple stone recurrences, solitary kidneys, high surgical risk factors or those in the pediatric age group.