| Kidney stones, or Renal calculi, are solid concretions (crystal aggregations) of dissolved minerals in urine; calculi typically form inside the kidneys or ureters. 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 grapefruit. 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 distention with urine 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 embyrological association of the kidneys and the intestinal tract. Hematuria is commonly present due to damage to the wall of the urethra as well as dysuria (when passing stones). Recurrence rates are estimated at about 10% per year.
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Causes |
Kidney stones can be due to underlying metabolic conditions, such as renal tubular acidosis, Dent's disease and medullary sponge kidney. Many centers will screen for such disorders in patients with recurrent kidney stones.
The most common type of kidney stone is composed of calcium oxalate crystals, and factors that promote the precipitation of crystals in the urine are associated with the development of these stones.
Conventional wisdom and common sense has long held that consumption of too much calcium can promote the development of 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.
Other types of kidney stones are composed of struvite (magnesium, ammonium and phosphate); uric acid; calcium phosphate; and cystine.
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.
The formation of uric acid stones is associated 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.
The formation of calcium phosphate stones is associated with conditions such as hyperparathyroidism and renal tubular acidosis.
The formation of cystine stones is uniquely associated with people suffering from cystinuria, who accumulate cystine in their urine.
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Treatment |
90% of stones 4 mm or less in size usually will pass spontaneously, however the majority of stones greater than 6 mm will require some form of intervention. In most cases, a smaller stone that is not symptomatic is often given up to 30 days 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.
One modern medical technique uses a ureter stent (a small tube between the bladder and the inside of the kidney) to provide some relief of a blocked kidney. This is especially useful in saving a failing kidney due to swelling and infection from the stone. This tubing allows urine to drain from kidney and in some cases medicine to be injected directly. Ureter stents vary in shape and size, but most are designed to allow urine to drain and be retained for some length of time as infections reside and as stones are dissolved or sonar blasted. Most stents can be removed during a final office visit. This can range from little associated pain to extreme pain.
Management of pain from kidney stones varies from country to country and even from physician to physician, but may require intravenous medication (eg, narcotic or nonsteroidal anti-inflammatories) in acute situations. Similar classes of drugs may be effective orally in an outpatient setting for less severe discomfort. Intravenous ketorolac has been found to be quite effective in many cases of acute renal colic to control the pain without the need for narcotic medications. Ketorolac is a non-steroidal anti-inflammatory drug that is related to aspirin and ibuprofen. Most acute kidney stone pain will last less than 24 hours and not require hospitalization. Patients are encouraged to strain their urine so they can collect the stone when it eventually passes and send it for chemical composition analysis.
In many cases non-invasive Extracorporeal Shock Wave Lithotripsy or (ESWL) may 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 stones. Percutaneous nephrolithotomy or 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. |
