Urology for the urogynecologist

Introduction

One out of 13 women will become symptomatic with a kidney stone in her lifetime; that incidence is rising in the United States resulting from the increasing prevalence of obesity and diabetes. Often, the symptoms are acute and painful, and require a visit to the emergency room. Unfortunately, half of patients who pass a kidney stone will have a recurrence within 5 years. This section will provide a brief review of urinary tract stone disease and recommendations for prevention in women with known stone disease, as well as when to refer a patient with an asymptomatic stone found on an evaluation for microscopic or gross hematuria.

Epidemiology/gender differences

The incidence of asymptomatic stones is 8% to 10% of the population. In a study of over 5000 patients undergoing a screening computed tomography colonography, asymptomatic stones were found in 9.7% of men and 6.3% of women. The average number of stones found was 2.1, with a mean stone size of 3 mm ( ). Survey studies are used to assess the prevalence of symptomatic stones. The National Health and Nutrition Examination Survey (NHANES) conducted in 2007 to 2010 ( n = 12,100) included questions about a history of kidney stones. The prevalence was found to be 8.8% overall, at 10.6% in men and 7.1% in women ( ). This was a marked increase from the NHANES III cohort (1976–1994), in which the prevalence was 6.3% among men and 4.1% among women ( ). found that kidney stones were more prevalent among obese individuals compared with normal-weight individuals (11.2% vs. 6.1%). Black, non-Hispanic and Hispanic individuals were less likely to report a history of stone disease than White, non-Hispanic individuals. Obesity and diabetes were strongly associated with a history of kidney stones in multivariate analysis.

Pathophysiology

The pathophysiology of stone formation is complex; however, there are a few concepts that are important in understanding renal stone formation. Urinary supersaturation is the driving force for the phase change of minerals from a dissolved salt to a solid. Although supersaturation is essential, by itself it does not result in the formation of a stone. The solid precipitates out of the urine and forms crystals in the urine that are often identified by a clinical laboratory with urine microscopy. The process of stone formation requires nucleation, crystal growth, crystal aggregation, and crystal retention. Once formed, the crystals may flow in the urine or become retained in the urinary tract at sites that promote growth, leading to stone formation.

The majority of stones seen in the United States contain calcium, most commonly calcium oxalate (60%), followed by hydroxyapatite (20%) and brushite (2%). Of the non–calcium-containing stones, uric acid (7%) and struvite stones (7%) are the most common. Along with low urine volume and low urine pH, urinary calcium and oxalate are important and equal contributors to calcium oxalate stone formation. Inhibitors of stone formation include high urinary volume, higher urinary pH, citrate, and magnesium.

Radiologic imaging for the presence of stones

Noncontrast computed tomography (CT stone protocol) is the gold standard for imaging urinary tract stones because of its lower radiation exposure (85% decrease) as compared with three-phase CT. It is also beneficial because it can assess renal anatomy and other associated findings such as hydronephrosis and perinephric stranding. An alternative imaging modality includes the use of renal ultrasound in combination with a plain film of the kidneys, ureters, and bladder (KUB). KUB imaging is typically required with ultrasound imaging because ultrasound measurements of renal stone size are not as accurate and are less able to visualize stones in the ureter.

Treatment

There is not much evidence on how or when to treat asymptomatic stones. Stones that are asymptomatic in the kidney can become symptomatic while passing through the ureter. Ureteral stones usually become impacted at three distinct ureteral sites: at the ureteropelvic junction, crossing the iliac vessels, and at the ureterovesical junction. Stone passage rates are dependent on the size of the stone and will be discussed in detail later. Stones that are 2 mm or less have excellent passage rates and require intervention less than 5% of the time. That said, the average time to passage could be a week. Renal stones 2 mm or greater should be referred to a urologist, who can clearly state the risk and benefits of a stone removal from the kidney. Kidney stones larger than 1 cm are less likely to traverse the ureteropelvic junction and result in acute pain. Although asymptomatic, they should be addressed, because the stone will continue to enlarge. Stones less than 2 cm may be amenable to more conservative surgical treatments, such as extracorporeal shock wave lithotripsy and ureteroscopic interventions. Stones greater than 2 cm require percutaneous nephrolithotomy or open/laparoscopic stone removal, which are more morbid procedures.

Upper urinary tract stones can be the cause of pain, infection, obstruction, active stone growth, or hematuria. In a study of 180 women presenting to the emergency room with a symptomatic upper tract stone, one-third had kidney stones, and two-thirds had ureteral stones ( ). Of those with a kidney stone on presentation, the average stone size was 7.9 mm. The majority (45%) required no intervention other than medications and a urology referral, whereas 33% required ureteroscopy, 10% required a stent, and 12% required percutaneous nephrolithotomy. Ureteral stones had an average size of 4.3 mm. The majority (86%) of the stones were located in the distal ureter, 11% were in the proximal ureter, and 3% were located in the midureter. The majority of the women (77%) were managed without surgical intervention. Of the individuals for whom follow-up information was obtained (41%), the average size of the stones that passed spontaneously was 3.3 mm.

Spontaneous passage rates of stones in the ureter are dependent on the stone size and location. In a study of 850 male and female patients with acute flank pain, 172 (13.5%) were found to have ureteral stones ( ). The majority of stones (67%) passed spontaneously, and the rest required intervention. The spontaneous passage rate for stones 1 mm in diameter was 87%; for stones 2 to 4 mm, 76%; for stones 5 to 7 mm, 60%; for stones 7 to 9 mm, 48%; and for stones larger than 9 mm, 25%. Spontaneous passage rates as a function of the stone location were 48% in the proximal ureter, 60% for midureteral stones, 75% for distal stones, and 79% for ureterovesical junctional stones. Limited studies have demonstrated that 84% of pregnant women with ureteral colic spontaneously pass renal calculi when treated conservatively with hydration, analgesics and, if infected, antibiotics. If stents are required, they can be done cystoscopically using ultrasound or minimal radiographic imaging ( ).

Time to passage of stones is also dependent on stone size. In a study of 75 men and women who were prospectively followed for stone passage, stones smaller than 2 mm passed in 8.2 days, with 4.8% requiring intervention, whereas stones between 2 and 4 mm took an average of 12 days and up to 40 days to pass, with 17% requiring intervention ( ). Stones that were larger than 4 mm took 22 days to pass, and 11% needed an intervention, whereas intervention was required for 50% of patients with ureteral calculi greater than 5 mm. Factors that predicted a spontaneous stone passage were right-sided stones, distal location in the ureter, and smaller size.

Recurrence

The recurrence rate for patients who have passed their first stone is approximately 50%. Risk factors for stone recurrence are anatomic upper tract abnormalities (pelvic kidney), a family history of renal stones, intestinal diseases that result in chronic diarrhea, osteoporosis, urinary tract infections, and gout. There is some debate if a complete metabolic work-up for stone disease should be undertaken after passage of the first stone. Many experts advocate fluid and dietary recommendations until patients suffer a recurrence. A metabolic work-up may be required if a woman has the previously listed risk factors, is premenopausal, has a solitary or transplanted kidney, or has stones that were composed of cystine, uric acid, or struvite. Blood tests such as a basic metabolic panel, serum calcium, serum uric, and intact parathyroid hormone are relatively inexpensive and can be performed in first-time stone formers as a simple screen for underlying metabolic issues.

Dietary recommendations for individuals with stones

The following dietary recommendations should be reviewed with women who have a history of kidney stones:

• Increase fluid intake to achieve a urine output of 2 L or more a day. Carbonated water is preferred to still water because it has been found to increase urinary citrate levels, which inhibit stone formation.

• Increase the dietary intake of citrate to increase urinary citrate levels, which inhibit stone formation. Natural juices that are highest in citrate are grapefruit, lemon, and orange juice. Of the commercially available citrus-based beverages, Crystal Light (Kraft Foods) has the highest concentration of citrate.

• Abstain from high protein diets, because protein increases urinary calcium, oxalate, and uric acid excretion and can increase the probability of stone formation even in normal subjects.

• Reduce dietary sodium, because dietary sodium and not dietary calcium restriction has been found to prevent recurrent nephrolithiasis.

• Reduce dietary oxalate by limiting foods that are rich in oxalate, such as spinach, beets, chocolate, nuts, and tea. There is debate about the conversion of ascorbic acid to urinary oxalate; however, most experts agree that daily vitamin C intake should be limited to less than 2 grams.