What are Kidney Stones?
What are Kidney Stones and What Causes them?
A kidney stone, also known as a renal calculus is a crystal aggregation formed in the kidneys from dietary minerals in the urine.
Urinary stones are typically classified by their location in the kidney (nephrolithiasis), ureter (ureterolithiasis), or bladder (cystolithiasis), or by their chemical composition (calcium-containing, struvite, uric acid, or other compounds).
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 (usually at least 3 millimeters (0.12 in)) they can cause obstruction of the ureter. Ureteral obstruction causes postrenal azotemia (abnormally high level of nitrogen waste products in the blood) and hydronephrosis (distension and dilation of the renal pelvis and calyces), as well as spasm of the ureter.
This leads to pain, most commonly felt in the flank (the area between the ribs and hip), lower abdomen, and groin (a condition called renal colic). Renal colic can be associated with nausea, vomiting, fever, blood in the urine, pus in the urine, and painful urination.
Renal colic typically comes in waves lasting 20 to 60 minutes, beginning in the flank or lower back and often radiating to the groin or genitals.
The diagnosis of kidney stones is made on the basis of information obtained from the history, physical examination, urinalysis, and radiographic studies. Ultrasound examination and blood tests may also aid in the diagnosis.
When a stone causes no symptoms, watchful waiting is a valid option. For symptomatic stones, pain control is usually the first measure, using medications such as nonsteroidal anti-inflammatory drugs or opioids. More severe cases may require surgical intervention.
For example, some stones can be shattered into smaller fragments using extracorporeal shock wave lithotripsy. Some cases require more invasive forms of surgery.
Examples of these are cystoscopic procedures such as laser lithotripsy or percutaneous techniques such as percutaneous nephrolithotomy.
Sometimes, a tube (ureteral stent) may be placed in the ureter to bypass the obstruction and alleviate the symptoms, as well as to prevent ureteral stricture after ureteroscopic stone removal.
The hallmark of stones that obstruct the ureter or renal pelvis is excruciating, intermittent pain that radiates from the flank to the groin or to the inner thigh.
This particular type of pain, known as renal colic, is often described as one of the strongest pain sensations known. Renal colic caused by kidney stones is commonly accompanied by urinary urgency, restlessness, hematuria, sweating, nausea, and vomiting. It typically comes in waves lasting 20 to 60 minutes caused by peristaltic contractions of the ureter as it attempts to expel the stone.
Dehydration from low fluid intake is a major factor in stone formation.
Kidney stones can result from an underlying metabolic condition.
Calcium is not the only electrolyte that influences the formation of kidney stones. For example, by increasing urinary calcium excretion, high dietary sodium may increase the risk of stone formation.
Kidney stones are more likely to develop, and to grow larger, if a person has low dietary magnesium. Magnesium inhibits stone formation.
Calcium-containing stones are relatively radiodense, and they can often be detected by a traditional radiograph of the abdomen that includes the kidneys, ureters, and bladder (KUB film). Some 60% of all renal stones are radiopaque.
In general, calcium phosphate stones have the greatest density, followed by calcium oxalate and magnesium ammonium phosphate stones. Cystine calculi are only faintly radiodense, while uric acid stones are usually entirely radiolucent (allowing the passage of x-rays or other radiation; not radiopaque - objects that block radiation rather than allow it to pass through).
Ultrasound imaging of the kidneys can sometimes be useful, as it gives details about the presence of hydronephrosis, suggesting the stone is blocking the outflow of urine. Ultrasound imaging is useful for detecting stones in situations where X-rays or CT scans are discouraged, such as in children or pregnant women.
Laboratory investigations typically carried out include:
- Microscopic examination of the urine, which may show red blood cells, bacteria, leukocytes, urinary casts and crystals
- Urine culture to identify any infecting organisms present in the urinary tract and sensitivity to determine the susceptibility of these organisms to specific antibiotics
- Complete blood count, looking for neutrophilia (increased neutrophil granulocyte count) suggestive of bacterial infection, as seen in the setting of struvite stones
- Renal function tests to look for abnormally high blood calcium blood levels (hypercalcemia)
- 24 hour urine collection to measure total daily urinary volume, magnesium, sodium, uric acid, calcium, citrate, oxalate and phosphate
- Collection of stones (by urinating through a StoneScreen kidney stone collection cup or a simple tea strainer) is useful.
Chemical analysis of collected stones can establish their composition, which in turn can help to guide future preventive and therapeutic management.
Kidney stones are typically classified by their location and chemical composition.
By far, the most common type of kidney stones worldwide contains calcium. For example, calcium-containing stones represent about 80% of all cases in the United States; these typically contain calcium oxalate either alone or in combination with calcium phosphate.
Oxaluria is increased in patients with certain gastrointestinal disorders including inflammatory bowel disease such as Crohn disease or patients who have undergone resection of the small bowel or small bowel bypass procedures. Oxaluria is also increased in patients who consume increased amounts of oxalate (found in vegetables and nuts).
Calcium oxalate stones appear as 'envelopes' microscopically. They may also form 'dumbbells.'
About 10–15% of urinary calculi are composed of struvite (ammonium magnesium phosphate. Struvite stones (also known as "infection stones", urease or triple-phosphate stones), form most often in the presence of infection by urea-splitting bacteria.
Using the enzyme urease, these organisms metabolize urea into ammonia and carbon dioxide. This alkalinizes the urine, resulting in favorable conditions for the formation of struvite stones.
Struvite stones (triple phosphate/magnesium ammonium phosphate) have a 'coffin lid' morphology by microscopy.
About 5–10% of all stones are formed from uric acid. People with certain metabolic abnormalities, including obesity, may produce uric acid stones.
They also may form in association with conditions that cause hyperuricosuria (an excessive amount of uric acid in the urine) with or without hyperuricemia (an excessive amount of uric acid in the serum). They may also form in association with disorders of acid/base metabolism where the urine is excessively acidic (low pH), resulting in precipitation of uric acid crystals.
Uric acid stones appear as pleomorphic crystals, usually diamond-shaped. They may also look like squares or rods which are polarizable.