Kidney stones or renal calculi (from Latin ren, renes , "kidney" and calculi , "pebbles") are solid concretions or calculi (crystal aggregations) formed in the kidneys from dissolved urinary minerals. Nephrolithiasis (from Greek nephros , "kidney" and λιθoς ( lithos , "stone")) refers to the condition of having kidney stones. Urolithiasis refers to the condition of having calculi in the urinary tract (which also includes the kidneys), which may form or pass into the urinary bladder. Ureterolithiasis is the condition of having a calculus in the ureter, the tube connecting the kidneys and the bladder. The term bladder stones usually applies to urolithiasis of the bladder in non-human animals such as dogs and cats.
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 causes dilation or stretching of the upper ureter and renal pelvis (the part of the kidney where the urine collects before entering the ureter) as well as muscle spasm of the ureter, trying to move the stone. This leads to 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. There can be blood in the urine, visible with the naked eye or under the microscope (macroscopic or microscopic hematuria) due to damage to the lining of the urinary tract.
There are several types of kidney stones based on the type of crystals of which they consist. The majority are calcium oxalate stones, followed by calcium phosphate stones. More rarely, struvite stones are produced by urea-splitting bacteria in people with urinary tract infections, and people with certain metabolic abnormalities may produce uric acid stones or cystine stones.
The diagnosis of a kidney stone can be confirmed by radiological studies or ultrasound examination; urine tests and blood tests are also commonly performed. When a stone causes no symptoms, watchful waiting is a valid option. In other cases, pain control is the first measure, using for example non-steroidal anti-inflammatory drugs or opioids. Using soundwaves, some stones can be shattered into smaller fragments (this is called extracorporeal shock wave lithotripsy). Sometimes a procedure is required, which can be through a tube into the urethra, bladder and ureter (ureteroscopy), or a keyhole or open surgical approach from the kidney's side. Sometimes, a tube may be left in the ureter (a ureteric stent) to prevent the recurrence of pain. Preventive measures are often advised such as drinking sufficient amounts of water, although the effect of many dietary interventions has not been rigorously studied.
Causes
Kidney stones can be due to underlying metabolic conditions, such as renal tubular acidosis, Dent's disease, hyperparathyroidism and medullary sponge kidney. Patients with recurrent kidney stones should be screened for these disorders. 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.
A 1998 paper in the Archives of Internal Medicine examined the sources of a widely-held belief in the medical community that vitamin C can cause kidney stones, and found it to be based on several circular references, ultimately attributing the belief to a wider pattern of skepticism regarding efficacy of vitamin supplements. A more recent study suggested a causal relationship may exist, but it was not conclusive.
The American Urological Association has projected that increasing global temperatures will lead to greater future prevalence of kidney stones, notably by expanding the "kidney stone belt" of the southern United States. Astronauts seem to show a higher risk of developing kidney stones during or after long duration space flights.
Calcium oxalate stones
The most common type of kidney stone is composed of calcium oxalate crystals, occurring in about 80% of cases, and the factors that promote the precipitation of crystals in the urine are associated with the development of these stones.
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.
Uric acid (urate)
About 5–10% of all stones are formed from uric acid. Uric acid stones form in association with conditions that cause hyperuricosuria with or without high blood serum uric acid levels (hyperuricemia); and with acid/base metabolism disorders where the urine is excessively acidic (low pH) resulting in uric acid precipitation. A diagnosis of uric acid nephrolithiasis is supported if there is a radiolucent stone, a persistent undue urine acidity, and uric acid crystals in fresh urine samples.
Other types
Other types of kidney stones are composed of struvite (magnesium, ammonium and phosphate); 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. Struvite stones are always associated with urinary tract infections.
The formation of calcium phosphate stones is associated with conditions such as hyperparathyroidism and renal tubular acidosis.
Formation of cystine stones is uniquely associated with people suffering from cystinuria, who accumulate cystine in their urine. Cystinuria can be caused by Fanconi's syndrome.
Urolithiasis has also been noted to occur in the setting of therapeutic drug use, with crystals of drug forming within the renal tract in some patients currently being treated with Indinavir, Sulfadiazine or Triamterene .
Symptoms
Symptoms of kidney stones include:
- Colicky pain: "loin to groin". Often described as "the worst pain ever experienced".
- Hematuria: blood in the urine, due to minor damage to inside wall of kidney, ureter and/or urethra.
- Pyuria: pus in the urine.
- Dysuria: burning on urination when passing stones (rare). More typical of infection.
- Oliguria: reduced urinary volume caused by obstruction of the bladder or urethra by stone, or extremely rarely, simultaneous obstruction of both ureters by a stone.
- Abdominal distension.
- Nausea/vomiting: embryological link with intestine– stimulates the vomiting center.
- Fever and chills.
- Hydronephrosis
- Postrenal azotemia: when kidney stone blocks ureter
- Frequency in micturition: Defined as an increase in number of voids per day (>than 5 times), but not polyuria, an increase of total urine output per day (2500 ml).
- Loss of appetite
- Loss of weight
Diagnosis
Clinical diagnosis is usually made on the basis of the location and severity of the pain, which is typically colicky 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.
X-rays
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
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