DescriptionUrine is an ionic solution containing many dissolved salts and other products. Sometimes these salts crystallise and the crystals aggregate to form stones. Most urinary tract calculi (kidney stones) consist of a crystal component and a matrix component (protein and debris). Kidney stones are usually named according to the crystal component of the stone. When a stone is found in the bladder it is called a bladder stone. Stones may be asymptomatic or severely symptomatic. Pain is usually caused by obstruction, especially when a stone blocks the ureter (the tube that drains urine from the kidney into the bladder). Stones can also cause infection and bleeding.
The medical name for the process of stone formation is called urolithiasis (renal lithiasis or nephrolithiasis). Stones may form because:
- the urine becomes too saturated with salts that can form stones or
- the urine lacks the normal inhibitors of stone formation.
- obstruction causes urine stasis and subsequent stone formation
- stones can form on foreign bodies in the urinary tract such as indwelling catheters and stents.
Types of stones and relative incidence
- Calciumoxalate: 75%
- Uric acid stones: 8%
- Struvite (Infection) stones: 15%
- Calcium, ammonium, magnesium phoshate
- Caciumphosphate: rare
- Cystine: rare
Calciumoxalate stones are by far the most common. Stones containing calcium are clearly seen on X-rays. Cystine stones are poorly visible on routine X-rays and uric acid stones are radiolucent.
Struvite stones – a mixture of magnesium, ammonium, and phosphate – are also called infection stones, because they form only in infected urine
Stones vary in size from too small to be seen with the eye alone to 1 inch or more in diameter. A large staghorn calculus (stone) may be shaped by the renal pelvis, and may fill it and the tubes that drain into it.
Cystine stones form due to a hereditary defect in the metabolism of the amino acid cystine. People with cystinuria excrete large amounts of cystine in the urine. Cystine is poorly soluble in urine and crystallises to form stones.
- Stones may not cause any symptoms.
- Stones in the bladder may cause pain in the lower abdomen.
- Stones that obstruct the ureter or renal pelvis or any of its drainage tubes may cause back pain or a severe colicky pain (renal colic). Renal colic is characterised by an excruciating intermittent pain, usually in the flank, that spreads across the abdomen, often to the genital area and inner thigh.
- Other symptoms include nausea and vomiting, abdominal distention, chills, fever, and blood in the urine.
- A person may need to urinate frequently, particularly as a stone passes down the ureter.
- Stones may cause a urinary tract infection. When stones block the flow of urine, bacteria become trapped in urine that pools above the blockage, leading to an infection. When stones block the urinary tract for a long time, urine backs up in the tubes inside the kidney, producing pressure that can distend the kidney (hydronephrosis) and eventually damage it.
- The combination of obstruction and infection is an emergency because the kidney can be permanently damaged in 24 to 36 hours.
Stones that cause no symptoms may be discovered by chance during a routine microscopic analysis of the urine (urinalysis). Stones that cause pain are generally diagnosed on the basis of the symptoms of renal colic. This is characterised by severe colicky intermittent pain in the loin that radiates to the groin or genitals. Despite the severe pain, physical examination is usually inconclusive. Microscopic analysis of the urine may disclose blood or pus in the urine as well as small stone crystals.
When someone presents with renal colic the diagnosis is usually confirmed by intravenous urography. In intravenous urography, a radio- opaque substance, which is visible on x-rays, is injected into a vein and travels to the kidneys where it outlines uric acid stones so they can be seen on x-rays. Intravenous urography will not only confirm the diagnosis, but will also indicate the position of the stone and whether any obstruction is present or not. More recently, spiral CT scanning has emerged as an alternative initial imaging investigation in patients with renal colic. Spiral CT scanners are available in some but not all hospitals in South Africa. Spiral CT scanning is as accurate as intravenous urography to make the diagnosis and does not involve the use of intravenous injection of contrast medium.
Additional tests that help make the diagnosis involve collecting 24-hour urine samples and blood samples, which are analyzed for levels of calcium, cystine, uric acid, and other substances known to produce stones.
Small stones which aren't causing symptoms, obstruction, or an infection usually are not treated.
Drinking plenty of fluids increases urine production and helps wash out some stones. Once a stone is passed during urination, no other immediate treatment is needed.
The pain of renal colic may be relieved with narcotic analgesics
Sometimes a stone in the renal pelvis or uppermost part of the ureter that's 1cm or less in diameter can be broken up by ultrasound waves (extracorporeal shock wave lithotripsy). The pieces of stone are then passed in the urine. Certain stones are removed through a small incision in the skin (percutaneous nephrolithotomy), followed by ultrasound treatment.
Small stones in the lower part of the ureter may be removed by an endoscope (a small, flexible tube) inserted into the urethra and through the bladder.
Uric acid stones are sometimes dissolved gradually by making the urine more alkaline (for example, with potassium citrate), but other types of stones can't be removed this way.
Rarely, larger stones that are causing an obstruction may need to be removed surgically.
Measures to prevent the formation of new stones vary, depending on the composition of the existing stones. Most people with calcium stones have a condition called hypercalciuria, in which excess calcium is excreted in the urine.
- Thiazide diuretics such as trichlormethiazide reduce new stone formation in such people.
- Drinking large amounts of fluids – 8 to 10 glasses a day – is recommenced.
- A normal calcium intake is recommended.
- Paradoxically, a low calcium intake may increase the risk of stone formation due to increased oxalate absorption from the gut.
- Taking sodium cellulose phosphate, a resin, may help.
- Potassium citrate may be given to increase a low urine level of citrate, a substance that inhibits calcium stone formation.
- A high level of oxalate in the urine, which contributes to calcium stone formation, may result from excess consumption of foods high in oxalate, such as rhubarb, spinach, cocoa, nuts, pepper, and tea, or from certain intestinal disorders. A change in diet may help, and the underlying disorder is treated.
- Rarely, calcium stones result from another disorder, such as hyperparathyroidism, sarcoidosis, vitamin D toxicity, renal tubular acidosis, or cancer. In such cases, the underlying disorder is treated.
- For stones that contain uric acid, a diet low in meat, fish, and poultry is recommended, because these foods increase the level of uric acid in the urine.
- Allopurinol may be given to reduce the production of uric acid.
- Potassium citrate may be given to make the urine alkaline, because uric acid stones form when urine acidity increases. Drinking large amounts of fluids also helps.
- If there are struvite stones, which always indicates a urinary tract infection, antibiotics are given.
Reviewed by Dr Pieter J le Roux MBChB, FRCS(Eng), FRCSI, FCS(SA)Urol.