Kidney stones, also called renal calculi, are solid concretions (crystal aggregations) of dissolved minerals in urine; calculi typically form inside the kidneys or bladder. The terms nephrolithiasis and urolithiasis refer to the presence of calculi in the kidneys and urinary tract, respectively.
Renal calculi can vary in size from as small as grains of sand to as large as a golf ball. Kidney stones typically leave the body by passage in the urine stream, and many stones are formed and passed without causing symptoms. If stones grow to sufficient size before passage—on the order of at least 2-3 millimeters—they can cause obstruction of the ureter. The resulting obstruction with dilation or stretching of the upper ureter and renal pelvis as well as spasm of muscle, trying to move the stone, can cause severe episodic pain, most commonly felt in the flank, lower abdomen and groin (a condition called renal colic). Renal colic can be associated with nausea and vomiting due to the embryological association of the kidneys with the intestinal tract. Hematuria (bloody urine) is commonly present due to damage to the lining of the urinary tract.
Within the United States, about 10–15% of adults will be diagnosed with a kidney stone, and the total cost for treating this condition was US$2 billion in 2003. The incidence rate increases to 20–25% in the Middle East, because of increased risk of dehydration in hot climates. (The typical Arabian diet is also 50% lower in calcium and 250% higher in oxalates compared to Western diets, increasing the net risk.) Recurrence rates are estimated at about 10% per year, totalling 50% over a 5–10 year period and 75% over 20 years. Men are affected approximately 4 times more often than women.
New techniques in lithotomy began to emerge starting in 1520, but the operation remained risky. It was only after Henry J. Bigelow popularized the technique of litholopaxy in 1878 that the mortality rate dropped from about 24% down to 2.4%. However, other treatment techniques were developed that continued to produce a high level of mortality, especially among inexperienced urologists. In 1980, Dornier MedTech introduced extracorporeal shock wave lithotripsy for breaking up stones via acoustical pulses, and this technique has come into widespread use.
Among the famous leaders who were kidney stone formers are Emperor Napoleon Bonaparte, Emperor Napoleon III, Peter the Great, Louis XIV, George IV, Oliver Cromwell, and former U.S. President Lyndon B. Johnson. Other notable individuals who endured stones include Benjamin Franklin, the philosophers Michel de Montaigne and Sir Francis Bacon, the scientist Sir Isaac Newton, the civil servant and diarist Samuel Pepys, the physicians William Harvey and Herman Boerhaave, and the anatomist Antonio Scarpa. Astronauts seem to show a higher risk of developing kidney stones during or after long duration space flights.
Kidney stones can be due to underlying metabolic conditions, such as renal tubular acidosis, Dent's disease and medullary sponge kidney. Many health facilities will screen for such disorders in patients with recurrent kidney stones. This is typically done with a 24 hour urine collection that is chemically analyzed for deficiencies and excesses that promote stone formation. Kidney stones are also more common in patients with Crohn's disease.
There has been some evidence that water fluoridation may increase the risk of kidney stone formation. In one study, patients with symptoms of skeletal fluorosis were 4.6 times as likely to develop kidney stones. However, fluoride may also be an inhibitor of urinary stone formation.
There is a longstanding belief among the mainstream medical community that vitamin C causes kidney stones, which may be based on little science. Although some individual recent studies have found a relationship there is no clear relationship between excess ascorbic acid intake and kidney stone formation.
Common sense has long held that consumption of too much calcium could promote the development of calcium kidney stones. However, current evidence suggests that the consumption of low-calcium diets is actually associated with a higher overall risk for the development of kidney stones. This is perhaps related to the role of calcium in binding ingested oxalate in the gastrointestinal tract. As the amount of calcium intake decreases, the amount of oxalate available for absorption into the bloodstream increases; this oxalate is then excreted in greater amounts into the urine by the kidneys. In the urine, oxalate is a very strong promoter of calcium oxalate precipitation, about 15 times stronger than calcium.
The formation of struvite stones is associated with the presence of urea-splitting bacteria, most commonly Proteus mirabilis (but also Klebsiella, Serratia, Providencia species). These organisms are capable of splitting urea into ammonia, decreasing the acidity of the urine and resulting in favorable conditions for the formation of struvite stones. Struvite stones are always associated with urinary tract infections.
Uric acid stones form in association with conditions that cause high blood uric acid levels, such as gout, leukemias/lymphomas treated by chemotherapy (secondary gout from the death of leukemic cells), and acid/base metabolism disorders where the urine is excessively acidic resulting in uric acid precipitation. The formation of calcium phosphate stones is associated with conditions such as hyperparathyroidism and renal tubular acidosis. Whereas formation of cystine stones is uniquely associated with people suffering from cystinuria, who accumulate cystine in their urine.
Clinical diagnosis is usually made on the basis of the location and severity of the pain, which is typically colic in nature (comes and goes in spasmodic waves). Pain in the back occurs when calculi produce an obstruction in the kidney.
Imaging is used to confirm the diagnosis and a number of other tests can be undertaken to help establish both the possible cause and consequences of the stone. Ultrasound imaging is also useful as it will give details about the presence of hydronephrosis (swelling of the kidney—suggesting the stone is blocking the outflow of urine). It can also be used to show the kidneys during pregnancy when standard x-rays are discouraged. About 10% of stones do not have enough calcium to be seen on standard x-rays (radiolucent stones) and may show up on ultrasound although they typically are seen on CT scans.
The relatively dense calcium renders these stones radio-opaque and they can be detected by a traditional X-ray of the abdomen that includes the Kidneys, Ureters and Bladder—KUB. This may be followed by an IVP (Intravenous Pyelogram; (IntraVenous Urogram (IVU) is the same test by another name)) which requires about 50 ml of a special dye to be injected into the bloodstream that is excreted by the kidneys and by its density helps outline any stone on a repeated X-ray. These can also be detected by a Retrograde pyelogram where similar "dye" is injected directly into the ureteral opening in the bladder by a surgeon, usually a urologist.
Computed tomography (CT or CAT scan), a specialized X-ray, is considered the gold-standard diagnostic test for the detection of kidney stones, and in this setting does not require the use of intravenous contrast, which carries some risk in certain people (eg, allergy, kidney damage). All stones are detectable by CT except very rare stones composed of certain drug residues in the urine. The non-contrast "renal colic study" CT scan has become the standard test for the immediate diagnosis of flank pain typical of a kidney stone. If positive for stones, a single standard x-ray of the abdomen (KUB) is recommended. This additional x-ray provides the physicians with a clearer idea of the exact size and shape of the stone as well as its surgical orientation. Further, it makes it simple to follow the progress of the stone without the need for the much more expensive CT scan just by doing another single x-ray at some point in the future.
Other investigations typically carried out include:
In most cases, a smaller stone that is not symptomatic is often given up to four weeks to move or pass before consideration is given to any surgical intervention as it has been found that waiting longer tends to lead to additional complications. Immediate surgery may be required in certain situations such as in people with only one working kidney, intractable pain or in the presence of an infected kidney blocked by a stone which can rapidly cause severe sepsis and toxic shock.
Patients who are to be treated non-surgically, may also be started on an alpha adrenergic blocking agent (such as Flomax, Uroxatral, terazosin or doxazosin), which acts to reduce the muscle tone of the ureter and facilitate stone passage. For smaller stones near the bladder, this type of medical treatment can increase the spontaneous stone passage rate by about 30%.
After treatment, the pain may return if the stone moves but re-obstructs in another location. Patients are encouraged to strain their urine so they can collect the stone when it eventually passes and send it for chemical composition analysis which will be used along with a 24 hour urine chemical analysis test to establish preventative options.
Most kidney stones do not require surgery and will pass on their own. Surgery is necessary when the pain is persistent and severe, in renal failure and when there is a kidney infection. It may also be advisable if the stone fails to pass or move after 30 days. Finding a significant stone before it passes into the ureter allows physicians to fragment it surgically before it causes any severe problems. In most of these cases, non-invasive Extracorporeal Shock Wave Lithotripsy (ESWL) will be used. Otherwise some form of invasive procedure is required; with approaches including ureteroscopic fragmentation (or simple basket extraction if feasible) using laser, ultrasonic or mechanical (pneumatic, shock-wave) forms of energy to fragment the larger stones. Percutaneous nephrolithotomy or rarely open surgery may ultimately be necessary for large or complicated stones or stones which fail other less invasive attempts at treatment.
A single retrospective study in the USA, at the Mayo Clinic, has suggested that lithotripsy may increase subsequent incidence of diabetes and hypertension, but it has not been felt warranted to change clinical practice at the clinic. The study reflects early experience with the original lithotripsy machine which had a very large blast path, much larger than what is used on modern machines. Further study is believed necessary to determine how much risk this treatment actually has using modern machines and treatment regimens.
More common complications related to ESWL are bleeding, pain related to passage of stone fragments, failure to fragment the stone, and the possible requirement for additional or alternative interventions.
One modern medical technique uses a ureteral stent (a small tube between the bladder and the inside of the kidney) to provide immediate relief of a blocked kidney. This is especially useful in saving a failing kidney due to swelling and infection from the stone. Ureteral stents vary in length and width but most have the same shape usually called a "double-J" or "double pigtail", because of the curl at both ends. They are designed to allow urine to drain around any stone or obstruction. They can be retained for some length of time as infections recede and as stones are dissolved or fragmented with ESWL or other treatment. The stents will gently dilate or stretch the ureters which can facilitate instrumentation and they will also provide a clear landmark to help surgeons see the stones on x-ray. Most stents can be removed easily during a final office visit. Discomfort levels from stents typically range from minimal associated pain to moderate discomfort.
For those patients interested in optimizing their kidney stone prevention options, it's essential to have a 24 hour urine test performed. This should be done with the patient on his or her regular diet and activities. The results can then be analyzed for abnormalities and appropriate treatment given.
|Common high-Oxalate foods|
|Food Item|| Serving|
|Beet greens, cooked||1/2 cup||916|
|Purslane, leaves, cooked||1/2 cup||910|
|Rhubarb, stewed, no sugar||1/2 cup||860|
|Spinach, cooked||1/2 cup||750|
|Beets, cooked||1/2 cup||675|
|Chard, Swiss, leaves cooked||1/2 cup||660|
|Rhubarb, canned||1/2 cup||600|
|Spinach, frozen||1/2 cup||600|
|Beets, pickled||1/2 cup||500|
|Poke Greens, cooked||1/2 cup||476|
|Endive, raw||20 long leaves||273|
|Cocoa, dry||1/3 cup||254|
|Dandelion Greens, cooked||1/2 cup||246|
|Okra, cooked||8 - 9 pods||146|
|Potatoes, sweet, cooked||1/2 cup||141|
|Kale, cooked||1/2 cup||125|
|Peanuts, raw||1/3 cup (1-3/4 oz)||113|
|Turnip Greens, cooked||1/2 cup||110|
|Chocolate, unsweetened||1 oz.||91|
|Parsnips, diced, cooked||1/2 cup||81|
|Collard Greens, cooked||1/2 cup||74|
|Pecans, halves, raw||1/3 cup (1-1/4 oz)||74|
|Tea, leaves (4 min. infusion)||1 level tsp in 7 oz water||72|
|Wheat Germ, toasted||1/4 cup||67|
|Potato, Idaho white, baked||1 medium||64|
|Carrots, cooked||1/2 cup||45|
|Apple, raw with skin||1 medium||41|
|Brussel sprouts, cooked||6 - 8 medium||37|
|Strawberries, raw||1/2 cup||35|
|Celery, raw||2 stalks||34|
|Milk chocolate bar||1 bar (1.02 oz)||34|
|Raspberries, black, raw||1/2 cup||33|
|Orange, edible portion||1 medium||24|
|Green beans, cooked||1/2 cup||23|
|Chives, raw, chopped||1 tablespoon||19|
|Leeks, raw||1/2 medium||15|
|Blackberries, raw||1/2 cup||13|
|Concord grapes||1/2 cup||13|
|Blueberries, raw||1/2 cup||11|
|Currants, red||1/2 cup||11|
|Apricots, raw||2 medium||10|
|Raspberries, red, raw||1/2 cup||10|
|Broccoli, cooked||1 large stalk||6|
|Cranberry juice||1/2 cup (4 oz)||6|
One of the recognized medical therapies for prevention of stones is thiazides, a class of drugs usually thought of as diuretics. These drugs prevent stones through an effect independent of their diuretic properties: they reduce urinary calcium excretion. Nonetheless, their diuretic property does not preclude their efficacy as stone preventive. Sodium restriction is necessary for clinical effect of thiazides, as sodium excess promotes calcium excretion. Though some have said that the effect probably fades after two years or so of therapy (tachyphylaxis), in fact it is only randomized controlled trials lasting 2 years or more that show the effect; there is really no good evidence from studies of calcium metabolism that the thiazide effect does not last indefinitely. Thiazides are the medical therapy of choice for most cases of hypercalciuria (excessive urinary calcium) but may not be suitable for all calcium stone formers; just those with high urinary calcium levels.
Allopurinol is reserved for patients in whom alkalinization is difficult. For patients with increased uric acid levels and calcium stones, allopurinol is one of the few treatments that has been shown in double-blinded placebo controlled studies to actually reduce kidney stone recurrences. Dosage is adjusted to maintain a reduced urinary excretion of uric acid. Serum uric acid level at or below 6 mg/dL is often the goal of the drug's use in patients with gout or hyperuricemia.
In addition to increased fluid intake, one of the simplest fixes is to moderate animal protein consumption. However, despite epidemiologic data showing that greater protein intake is associated with more stones, randomized controlled trials of protein restriction have not shown reduced stone prevalence. In this regard, it is not just dietary calcium per se that may cause stone formation, but rather the leaching of bone calcium. Some diseases (e.g., distal renal tubular acidosis) which cause a chronically acidic state also decrease urinary citrate levels; since citrates are normally present as potent inhibitors of stone formation, these patients are prone to frequent stone formation.
Though caffeine does acutely increase urinary calcium excretion, several independent epidemiologic studies have shown that coffee intake overall is protective against the formation of stones.
Measurements of food oxalate content have been difficult and issues remain about the proportion of oxalate that is bio-available, versus a proportion that is not absorbed by the intestine. Oxalate-rich foods are usually restricted to some degree, particularly in patients with high urinary oxalate levels, but no randomized controlled trial of oxalate restriction has been performed to test that hypothesis.