Monday, April 9, 2012

Kidney stones

Authors: Drs Marshall Stoller (university of California SF) and Aaron Berger (Chicago) 2009-03-06


Kidney stones or calculi are solid collections of urinary components that have formed crystals, precipitated out of the urine and formed into stones.  The medical term for the presence of kidney stones is nephrolithiasis.  Most often, stones are calcium based but there are many other types of stones that may form.  The five most common stone types are calcium oxalate, calcium phosphate, uric acid, struvite, and cystine.  Pain from stones can develop when the stones obstruct any part of the urinary system.  This article will discuss the different types of stones, risk factors for forming stones, and the treatment and prevention of calculi. 

Relevant anatomy:

The urinary tract has several main components: the kidneys, which form the urine, the ureters which transport the urine to the bladder, which then stores the urine until it passes out of the body through the urethra. 
The kidneys are paired, bean-shaped organs, located in the back, just below the ribs.  They serve multiple functions in the body, but primarily the kidneys filter the blood to clear toxins and extra water from the body by producing urine.  Blood is filtered by a specialized structure called the glomerulus.  Filtered blood from the glomeruli passes into a complex and highly specialized tubular system that runs through the kidney called the nephron, which adjusts the concentration of the urine, and is responsible for the excretion or absorption of the various electrolytes, such as sodium and potassium.  There are two main parts of the kidney. 
The cortex is the outer portion of the kidney where all the specialized filters (glomeruli) and tubular structures (nephrons) that filter the blood and concentrate the urine are located.   The collecting system is where the urine empties before passing down the ureters, which are the long thin tubes (usually 25-30cm) that channel the urine into the bladder.  The collecting system is comprised of a branch network of structures called calyces which all empty into the renal pelvis which then funnels the urine into the ureters.  Stones may be located in any of these structures.  Pain may result from obstruction in any location in the collecting system of the kidney or the ureters.   However, the most common cause of pain from kidney stones arises when the stones pass down the ureter toward the bladder.  There are three narrow areas in the ureter which typically cause stones to get stuck and cause obstruction.  The first is called the ureteropelvic junction, which is where the ureter and renal pelvis join.  The second narrowed area is where the ureter passes over the large blood vessels in the pelvis called the iliac artery and vein. The final, and narrowest portion of the ureter, is at the ureterovesical junction where the ureter empties into the bladder. 

How common are kidney stones and what are the risk factors for forming them?

Kidney stones are believed to affect roughly 5-10% of the population.  They more commonly develop in men by about a three to one margin and are more frequent in Caucasians than African-Americans.  Once a person forms one kidney stone, the likelihood of experiencing a recurrence is about 10-15% in one year and nearly 50% by 5 years.  The incidence varies in different parts of the world with the United States, much of Europe, Scandinavia, and China having a relatively high incidence of stones, and much of Central and South America and Africa thought to have lower incidence of stones, but this may be secondary to lower detection rates.  Risk factors for the formation of kidney stones include inadequate fluid intake, a diet high in salt and/or animal protein, urinary tract infection, family history of kidney stones, and a variety of other disease states such as primary hyperparathyroidism.  When the usual components of urine become either too high or too low in concentration, stone formation is more likely.  Elevated levels of calcium (hypercalciuria), oxalate (hyperoxaluria), and uric acid (hyperuricosuria) can all lead to stone formation.  Abnormally low levels of citrate (hypocitraturia) and total urine volume can also lead to the development of stones.

How do kidney stones form?

The kidneys are essential to sustain life and their principal function is to filter the blood to excrete bodily waste products that accumulate from our diet and normal metabolic processes.  When the concentration of various components of the urine such as calcium and oxalate becomes too high, crystals can form in the urine and over time, these crystals can group together with a small amount of an organic material called matrix, to form stones.  There are multiple substances in urine that can form stones if the concentration becomes too high, including calcium, oxalate, and uric acid.  To combat this process, however, normal urine contains several inhibitors of stone formation including citrate, magnesium, and several proteins.  In patients who form stones, the balance between the promoters and inhibitors of stone formation shifts towards stone formation.  Once a solid stone forms, it may remain stable in size or it may continue to grow.  The exact mechanism for how a stone initially forms, however, is still unclear.

Signs, Symptoms, and Diagnosis

In many instances, stones can be present in the kidney or ureter and be asymptomatic if they are not obstructing the flow of urine down towards the bladder.  When the stone obstructs the flow of urine, either in the kidney or the ureter, the collecting system behind the stone stretches, resulting in pain.  Pain from a stone is often referred to as colic as it is usually intermittent in nature.  Patients with stone-related pain often move around trying to find a comfortable position to alleviate the pain.  This is in contrast to other causes of abdominal pain, such as appendicitis, where patients try to remain still to prevent the pain from getting worse.  Pain from a stone can be quite severe and may be associated with nausea and vomiting.  Pain from an obstructing stone in the kidney or proximal ureter is often located in the back or flank or upper abdomen on the affected side.  As the stone progresses down the ureter, the location of pain can shift as well.  As the ureters become narrow over the iliac vessels in the pelvis, stones often can get stuck and pain from stones in this location can be in the lower abdomen, groin, testes, or labia.   As the stone moves into the distal ureter and approaches the bladder, it often gets held up at the ureterovesical junction, which can cause bothersome urinary frequency, urgency, and sometimes pain at the tip of the penis.  If there is an infection related to the stone, fever and chills also may also develop. 
Patients presenting with symptoms potentially caused by a kidney stone should be evaluated with several tests.  A urinalysis often will demonstrate some blood in the urine, or hematuria, although the absence of blood does not exclude a stone.  The urinary pH may also provide a clue as to what is causing the stone.  The urinalysis can also help to determine if there is an infection associated with the stone.  Several blood tests may be obtained, including a serum creatinine which is a measure of overall kidney function.  An elevated creatinine can be indicative of a significant obstruction and/or dehydration if patients have been vomiting from stone-related pain.  A white blood cell count also can be useful as an elevated value can be indicative of infection.
 There are several diagnostic imaging modalities which can be used to evaluate for the presence of stones.  A plain x-ray of the abdomen can identify many stones, however some stones, such as those composed of uric acid, are typically not visible on plain x-ray.  An example of a large, staghorn type stone is shown in Figure below.

Furthermore, small stones may be obscured by overlying structures and gas in the digestive tract.  Ultrasound can be a useful modality and does not cause any radiation exposure to the patient.  Ultrasound is useful for determining the presence of swelling of the kidney from obstruction (hydronephrosis), and can visualize most stones in the kidney.  Ultrasound, however, misses many stones in the ureters.  Computed tomography (CT) scanning has become much more widely utilized in the evaluation of stones. A non-contrast CT scan can be performed safely in most patients and can detect every stone type except rare stones caused by protease inhibitors, medications used to treat HIV-positive patients. It also provides useful information about the anatomy of the kidney and size and location of stones to help plan a possible surgical procedure.  A CT scan showing a large left kidney stone is shown in Figure below. 

A CT scan also may be useful in diagnosing other intra-abdominal pathology which could be the source of pain.  Magnetic resonance imaging (MRI) is sometimes used in the evaluation of abdominal pain but MRI and does not cause any ionizing radiation exposure to the patient but in general, MRI, especially without intravenous contrast, is not a reliable technique for visualizing stones.1,2

Developing one kidney stone suggests  a high likelihood of developing a new stone.  To aid in stone prevention and to determine the likely underlying risk factors, a metabolic work-up can be obtained.  This includes several blood tests, including calcium, uric acid, and parathyroid hormone.  The metabolic evaluation also includes a 24-hour urine collection which is useful to determine what the underlying risk factors are, such as low urine volume, high urine levels of calcium, oxalate, uric acid, or low levels of urinary citrate.  Based on this data, the urologist may recommend dietary changes and/or pharmacologic therapy to help reduce the risk of stone recurrence.3

What are the major stone types, what are the causes, and how are they medically treated?

There are five major stone types:  calcium oxalate, calcium phosphate, struvite (magnesium ammonium phosphate), uric acid, and cystine.  Each type of stone has a distinct crystal shape, as depicted in Figure 5. 

Calcium oxalate crystals are square shape with a characteristic “X” mark, struvite stones have a “coffin lid” appearance, uric acid crystals are often needle shaped and can form into rosettes, and cystine crystals are hexagonal in shape.  Calcium-based stones (calcium oxalate and calcium phosphate) are the most common, comprising about 85% of stones, and calcium oxalate stones are the most prevalent.

1) Calcium Stones:
There are several metabolic abnormalities that lead to the development of calcium stones. The most common of these is hypercalciuria or elevated calcium levels in the urine.  Hypercalciuria can be classified into three different types which are absorptive, resorptive, and renal.  In absorptive hypercalciuria, there is an excess of calcium absorbed from the digestive system, mainly the small intestine.  The majority of cases of absorptive hypercalciuria are not dependent on calcium intake. Thus, limiting calcium intake will not fix the problem, contrary to what many believe.4  There are medications available that can correct the elevated urinary calcium of absorptive hypercalciuria.  Cellulose phosphate is an agent that when taken with meals, binds to calcium in the intestine so it is excreted in the stool instead of going into the urine.  Another option is a thiazide diuretic such as hydrochlorothiazide, which decreases the excretion of calcium by the kidney into the urine but does not affect the digestive system.  There are a minority of cases of hypercalciuria that are calcium dependent and a metabolic evaluation (see diagnosis) is necessary to determine if a calcium restricted diet may be beneficial.   Resorptive hypercalciuria occurs when there are elevated levels of parathyroid hormone in the circulation, often from an abnormal growth in the parathyroid gland, which leads to calcium being reabsorbed from the bones, resulting in elevated calcium levels in the blood and eventually in the urine.  The primary treatment of this disorder is by surgical removal of the abnormal parathyroid gland.  Renal hypercalciuria is an intrinsic problem in the kidney which leads to inadequate reabsorption of calcium from the urine.  Thiazide diuretics are the mainstay of treatment for this abnormality. 
In addition to hypercalciuria, there are several other causes of calcium stone formation.  Elevated levels of uric acid in the urine, or hyperuricosuria, can be a contributing factor and can be caused by an excess of purines in the diet or an abnormality of uric acid metabolism.  Foods that are high in purines include very rich foods like sweetbreads, liver, kidney, sardines, and anchovies.  Most meat, oily fish, and shellfish have moderately high amounts of purines so eating these foods in moderation is important in patients with hyperuricosuria.  Several vegetables and grains also contain moderate amounts of protein but it appears, at least in gout which is also uric acid dependent, that purines from vegetable sources are not as problematic as from animal sources.Calcium stones that form in the presence of high uric acid are different than true uric acid stones as these stones are usually a combination of uric acid and calcium and patients do not have the characteristic low urine pH as is seen with pure uric acid stones.
Elevated urinary oxalate, or hyperoxaluria, is caused by abnormal absorption of oxalate from the digestive system.  Oxalate is found naturally in many foods we eat.  Normally, calcium in the diet binds with the oxalate which then is excreted in the stool.  In patients with various types of digestive problems such as chronic diarrhea, inflammatory bowel disease, or patients who have had gastric bypass surgery, there is an increase in the amount of fat passing through the intestines and this binds with calcium, thus making the calcium unavailable to bind with oxalate.  This leads to increased amounts of free oxalate in the intestines which are absorbed and excreted into the urine, which can result in a higher likelihood of stone formation.  There are two primary methods to help decrease urinary oxalate in patients with hyperoxaluria.  One method is to have patients take extra calcium with their large meals by having a serving of dairy or by a calcium supplement which will then bind the excess oxalate.  The other method of reducing oxalate is to restrict the amount of oxalate in the diet.  Many foods contain oxalate but the most common foods that should be restricted include dark green vegetables such as spinach, chocolate, and nuts.  Caution must be taken when attempting to make dietary changes to reduce oxalate to ensure that the new foods do not have just as high or even higher oxalate content as the previous diet. 
Abnormally low levels of urinary citrate, known as hypocitraturia, also can lead to calcium stone formation.  Citrate is a natural inhibitor of kidney stones as it binds calcium, making it unavailable to form stones.  Hypocitraturia can be caused by any condition that causes an acidosis, when the pH of the blood is abnormally low.  There are many causes of acidosis but a few examples include chronic diarrhea, type I renal tubular acidosis, and prolonged fasting.  Urinary citrate levels can be increased by citrate supplementation, usually in the form of potassium citrate which can be taken as tablets or crystals mixed into liquids.  Alternatively, lemonade has been shown to increase levels of urinary citrate as well and can be an option in patients who do not want to take medications.6

2) Uric acid stones
            Uric acid stone formation is dependent on the pH of the urine.  The average urinary pH in the general population is around 5.85 and patients that form uric acid stones consistently have urinary pH values of less than 5.5.  At this pH, uric acid crystals can precipitate out of the urine and form stones.  Therefore, the treatment of uric acid stones is aimed at increasing the urinary pH, also known as alkalinization.  Potassium citrate is the most commonly used medication and the goal of therapy should be to get the pH between 6 and 6.5.  If the pH gets too high, calcium phosphate stones may occur.  If treatment is successful, uric acid stones can be dissolved without surgical intervention, which is not true of calcium-based stones.  With proper alkalinization of the urine, about 1cm of stone can dissolve per month and with continued alkalinization, uric acid stones can be eliminated completely.  Several disease states, including myeloproliferative disorders and malignancies, can lead to elevated uric acid levels in the blood and can lead to uric acid stones.  Elevated uric acid levels in the blood can be corrected with allopurinol, which is the same medication used in the prevention of gout.

3)  Struvite stones
            Struvite stones, also known as magnesium-ammonium-phosphate or infection stones, are stones that form as a result of urinary tract infection.  The bacteria that can lead to struvite stones are known as urea-splitting organisms, the most common of which are Proteus, Pseudomonas, and Providencia.  The pH of the urine in patients with struvite stones is usually elevated, higher than 7.2, which leads to the precipitation of the magnesium-ammonium-phosphate stones.  The only effective means to eradicate the infection in patients with struvite stones is to completely remove the stones.  Struvite stones, unlike calcium-based stones, rarely present with renal colic and often are asymptomatic until they become very large and infections develop.  Therefore, treatment of the stones often requires invasive surgery to clear the stone burden. 

4)  Cystine stones
            Cystine stones are the result of a defect in one of the amino acid transporters in the kidney and intestine causing abnormally high levels of cystine to be excreted in the urine.  This is thought to be an autosomal recessive disorder, meaning that both parents have to be carriers of the defective gene and a child has a 25% chance of inheriting the disease.  There are no known inhibitors of cystine stone formation so prevention is based primarily on increasing fluid intake (enough to have 3-4 liters of urine per day), alkalinizing the urine to above 7.5, and several medications that are thought to bind to cystine to prevent stone precipitation.  However, many patients have difficulties complying with these difficult preventative measures.  Even with good compliance, there is a high likelihood of frequent stone recurrences.

Unusual types of stones
There are several other types of stone which are much less common than the five listed above.  Two of these, 2,8-dihydroxyadenine and xanthine stones, are often the result of a genetic defect.  A variety of medications can also form stones if taken in large enough amounts.  Guaifenesin, a common drug found in cough and cold remedies, can form into stones as can ephedrine, another medication used for congestion and colds.  Sulfa from various antibiotics can form stones as can topiramate (Topamax), a medication used for seizures.  The protease inhibitors indinavir and nelfinavir, used in the treatment of HIV, can also form stones and indinavir stones are unique in that they are the only stones that are not visible on non-contrast CT scan.

How Can I Change My Diet to Prevent Stones?
There are three general guidelines that are useful for all patients hoping to prevent the formation or recurrence of kidney stones.  The first is adequate fluid intake so that the patient urinates between 1.5 and 2 liters of urine per day.  The second is a diet low in sodium.  A diet high in sodium leads to higher levels of calcium excreted in the urine and a higher likelihood of stone formation.  Finally, spacing out animal protein intake over the course of several meals is important as a single meal high in animal protein will cause the blood to become acidic which causes calcium to be reabsorbed from the bones, leading to transiently high urine calcium levels.  For more patient-specific recommendations, the previously discussed metabolic evaluation is necessary. 

Medical Expulsion Therapy:
As mentioned previously, stones are symptomatic when they cause obstruction of the urinary tract.  The most common location for a stone to get stuck and cause obstruction is in the ureter.  Patients presenting with small ureteral stones often will be able to pass the stone without any surgical intervention so management of the symptoms is the mainstay of treatment.  Narcotic pain medications such as Percocet or Vicodin are often utilized for pain control and patients should be aware that these medications must be used with caution and should not be used while driving.  In the setting of acute pain, many patients often present to the emergency department or are admitted to the hospital and in this setting, ketorolac (Toradol), given intravenously or intramuscularly, is a very useful medication as it works very well as both a pain reliever and anti-inflammatory.  In addition to pain medication and anti-nausea medication, several other medications can be useful to aid in stone passage.  One class of medications which has been shown to aid in stone passage are the alpha-blockers which are often used in the management of benign prostatic hyperplasia.7  Examples of alpha-blockers include tamsulosin (Flomax) and alfuzosin (Uroxatral), which help to relax the smooth muscle in the ureter and can be taken daily.  The second class of drugs utilized is steroids, which work by decreasing inflammation and swelling.  Steroids can have multiple side effects if used long term so typically a short course (typically 3 days) is given, often in combination with an alpha-blocker.8  Finally, non-steroidal anti-inflammatory medications such as ibuprofen or naproxen not only help with pain but also limit inflammation.  The use of some or all of these medications in combination can increase the chances of spontaneous stone passage from the ureter into the bladder.  The likelihood of a stone passing depends primarily on the location and the size of the calculus, with stones less than about 5mm having a much higher chance to pass than larger stones.  Once in the bladder, stones pass easily through the urethra and out of the body as the caliber of the urethra is much larger than the ureter.  Patients trying to pass a stone are often encouraged to strain the urine so if the stone passes, it will be available for stone analysis to determine its composition. 

Emergent Indications for Intervention:
Many patients who are passing a kidney stone can be managed with medications and observation.  However, there are three main indications for more urgent intervention.  One is if a patient develops a fever or other signs of infection.  Severe kidney infections can occur if there is a stone completely obstructing an infected kidney.  The second is if the pain is causing such severe nausea and/or vomiting that a patient is unable to tolerate any food or liquids.  The third is if the pain is uncontrollable with oral pain medications.  While this may not always be an indication for emergent surgical intervention, it does mean that patients need to be admitted to the hospital for intravenous pain control. The most common surgical modalities for the relief of obstruction from a stone is the placement of a ureteral stent or a nephrostomy tube which will be discussed in detail in the next section. 
Surgical Management of Stones:
There are multiple options for the treatment of symptomatic kidney stones that are not able to pass out of the urinary system on their own.  In the developed world, most current stone treatments are minimally invasive with very small incisions, or none at all. 
For patients with severe pain who have other medical issues which prevents them from undergoing definitive surgery, or if the necessary equipment is unavailable, temporary relief of the obstructed system can be accomplished by placement of either a ureteral stent or a nephrostomy tube.  A stent is a small plastic tube with a curl on either end that is placed into the ureter to allow the urine to bypass the obstructing stone (Figure 6).

This relieves the obstruction but some patients have bothersome symptoms related to the presence of a stent in their bladder.  When a stent is placed, the urologist has the option to leave a small string attached to the stent which allows for removal of the stent at a later date without the need for an additional procedure.  The string comes out the urethra and when the stent is ready to come out, the patient or doctor simply pulls the string to remove the stent.  If no string is left in place, a cystoscopy, where a small scope is placed into the bladder, must be performed to remove the stent with small graspers.  Another option in the more urgent setting is the placement of a nephrostomy tube.  This is a small tube which is placed directly into the kidney from the back and is an effective means to relieve an obstructed kidney.  A nephrostomy tube, however, has the disadvantage of being an external tube connected to a drainage bag so it can impact a patient’s quality of life.
Shock wave lithotripsy (SWL) is a non-invasive treatment that utilizes shock waves to break up stones into small pieces so that they may pass on their own.  There are a variety of different machines in use today but the basic concept is similar for all of them.  Similar to a thunderstorm where an electric current (lightning) causes a shock wave (thunder), SWL machines all are able to produce shock waves that are targeted at the stone.  The effect of multiple shocks is additive and stones are often administered up to 3000 shocks during one procedure.  These procedures can be performed either with a general anesthetic or with intravenous sedation.  The procedure consists of placing the patient onto a special machine called the lithotriptor.  The stone is then targeted with the fluoroscopy (x-rays), ultrasound, or a combination of the two.  Once the stone is identified, the shock waves are administered to the stone.  After the stone is fragmented, all the tiny fragments will pass down the ureter and into the bladder and eventually will be passed out of the body with urination in an uneventful fashion.  Most of the stone fragments pass within the first two weeks after SWL.  Occasionally, an obstruction can occur from a large number of stones getting stuck in the ureter while attempting to pass into the bladder.  This condition is known as steinstrasse (stone street in German) and may require the placement of a nephrostomy tube.  SWL is effective for stones up to about 2 cm in size that are located in the upper pole of the kidney or the renal pelvis, and has decreased success with larger stones or stones located in the lower pole of the kidney which makes it difficult for the fragments to pass.9  Also, some types of stones are harder and more resistant to shock waves but it is often not possible to tell what the stone is made of pre-operatively.  SWL is a safe and effective procedure and the main complication is bleeding from the kidney, which only rarely can be significant.  Currently, there is no convincing data that shows any long term kidney damage after SWL with modern lithotripters. 
            Ureteroscopy is another method for treating stones in the kidney or ureter.  Ureteroscopy is generally performed under general anesthesia but can be performed with a regional anesthetic.  There are no skin incisions during the procedure as it is all performed through the urethra.  The initial step involves placement of a scope called a cystoscope into the bladder.  Once in the bladder, the opening in the bladder where the ureter enters, called the ureteral orifice, is identified.  Typically, a small guidewire is then placed into the ureter to allow for placement of the smaller caliber ureteroscope.  For stones in the lower portion of the ureter, the urologist may use a semi-rigid ureteroscope to visualize the stone and then either remove it or fragment it into smaller pieces.  Figure 7 shows the appearance of a stone in the ureter as seen through a ureteroscope.

For stones in the more proximal portion of the ureter or the kidney, the flexible ureteroscope is utilized.  This is a very thin scope that flexes at the tip to allow access into all parts of the kidney.  Once stones are visualized through the ureteroscope, there are multiple options for removing them.  There are a wide variety of graspers and basket devices which can be used to grab the stones and pull them out of the kidney or ureter.  If the stones are too large to be removed, they must be broken into smaller fragments.  This can be accomplished with an electrohydraulic lithotriptor (EHL) which uses shock waves to fragment the stone, a pneumatic lithotriptor which is like a small jackhammer on the stone, or most commonly with a laser.  The type of laser used most frequently is called the Holmium laser, and it has been proven to be safe and effective in the treatment of stones.  Once the procedure is complete, a ureteral stent is often left in place as the ureteral orifice may develop swelling from the passage of the scope and lead to kidney obstruction.  In many cases, a stent is left for several days after surgery with a string attached so that it may be removed without an additional procedure.  However, the string itself may be irritating so the urologist may elect to remove it.  Furthermore, if there is any concern about a narrowing or stricture of the ureter, or if the ureter was dilated to allow access for the scope, the stent may be left in place for a longer period of time and in these cases, the string is normally removed prior to placement.  The main risks of ureteroscopy include bleeding, infection, and injury to the ureter, but fortunately these are rare complications. 
Percutaneous nephrolithotomy or PNL is a procedure that is often utilized for larger or more complex stones. In this procedure, an access needle is placed directly into the kidney using radiologic guidance.  This is typically performed through a small incision in the back and once the needle is introduced, a guidewire is placed through the needle into the kidney and used to dilate the tract to allow for the placement of a rigid scope, called a nephroscope, directly into the kidney (Figure 8). 


Once the nephroscope is in the kidney and the stone(s) is identified, it can be removed with a grasper or fragmented with a variety of devices.  The advantage of a percutaneous approach in the treatment of stones, especially large stones, is that after the stone is fragmented, the pieces can be removed or evacuated using a suction device to truly get the patient stone-free prior to leaving the hospital.  

A short video of a percutaneous stone procedure using an ultrasonic lithotriptor.      

After the procedure, a small tube is typically left in place for a period of time, and is subsequently removed.  Recently, some urologists have begun performing a “tubeless” technique where there is no tube left in the back after surgery but a stent is placed into the ureter instead.10  This has the advantage of the patient not having any external tubes protruding out the back, but has the disadvantage that a second procedure is often required to remove the stent.  If stones are very large or if infection is encountered during the access into the kidney, a nephrostomy tube can be left in place and the patient can be brought back for an additional procedure at a later date.  Some complex stones may require multiple procedures and/or multiple tracts in the kidney to remove the extensive stone burden.  The main risks of PNL are bleeding, infection, and injury to adjacent organs which can include the bowels, spleen, liver, or the pleura of the lungs.  Again, the incidence of these injuries is very low and the vast majority of cases are uncomplicated.
Laparoscopic approaches can also be used in special stone cases.  This involves making several small incisions in the abdomen or flank, inflating the abdomen to allow for adequate working space, and using a camera and long, thin instruments to perform the procedure.  For large stones in the renal pelvis, the area where the collecting system funnels into the ureter, a laparoscopic pyelolithotomy can be performed, in which an incision is made into the renal pelvis and the stone is removed.  Laparoscopy also can be utilized for kidneys stones in a caliceal diverticulum which is a small outpouching from a calyx where the urine collects.  These diverticuli often have narrow openings and the stones can be very peripheral in the kidney. In this circumstance it may be easier to remove the stone from outside the kidney via laparoscopy, rather than from inside the kidney. 
In rare cases in the developed world, but more commonly in developing countries, kidney or ureteral stones can be removed via an open approach, using relatively large incisions to gain access to the stone.  These incisions can be in the side, back, or lower abdomen, depending on the stone’s location.  These approaches can be performed with basic, reusable surgical instruments that are used in other surgical procedures.  These procedures, while effective, often result in more post-operative pain and longer recovery times than the minimally invasive alternatives.

Stones in Pregnancy
Kidney stones in pregnant women can present a difficult management problem.  There are several physiologic changes that occur in the kidneys during pregnancy that may lead to the development of kidney stones.  However, the overall incidence of stones is not increased in pregnant women compared with non-pregnant women of the same age.  Ultrasound is the primary diagnostic modality, as all attempts should be made to limit radiation exposure to the fetus.  Most stones occur during the 2nd and 3rd trimesters and the majority can pass on their own without intervention.  However, for severe symptoms, pregnant patients can have ureteral stents of nephrostomy tubes placed to decompress the obstructed system.  Ureteroscopy and laser lithotripsy is also safe in pregnancy, but shock wave lithotripsy is contraindicated.
Kidney stones are a common problem, affecting nearly 10% of the population, and once a patient develops a stone, the risk of a recurrence is high.  The majority of stones are calcium based and may be asymptomatic but if they cause pain or obstruction, they must either pass out of the urinary system on their own or be removed surgically.  A metabolic evaluation can determine a patient’s individual risk factors but in general, a low salt diet and adequate fluid intake can help prevent stones from recurring.

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