Kidney stones (nephrolithiasis) and stones in the urinary tract (urolithiasis) are solid concretions of dissolved minerals that may form usually in the kidney or bladder (albeit uncommonly). They are also known as renal calculi.
The most common type of renal calculi is calcium stones, usually composed of calcium oxalate. They may form due to a lack of water intake, which allows calcium that would otherwise dissolve, to precipitate and crystallize. They may also form in response to certain conditions that can raise the blood calcium level, including hyperparathyroidism (too much parathyroid hormone), sarcoidosis (a disease that causes small lumps, or granulomas, due to chronic inflammation), cancer, vitamin D overdose, too much intestinal absorption of oxalate, or too little citrate (which prevents calcium from crystallizing).
Other types of calculi include:
- Uric acid stones – can form with an excessive intake of purines, an organic compound primarily found in meats. Highly acidic urine can also facilitate crystallization of uric acid.
- Struvite stones – can form during urinary tract infections by urea-splitting bacteria such as proteus mirbilis
- Cysteine stones – can form due to high blood levels of cysteine resulting from a rare genetic condition (cystinuria)
- Drug-induced stones – some medications can crystallize to form stones. Other medications and/or their metabolites can lead to metabolic abnormalities that allow stones to form more easily.
About 1-2 percent of renal calculi are drug induced. When there is urinary supersaturation of an agent, it may crystallize and become its primary component. Drugs that can induce calculi through this process include:
- Indinavir – a protease inhibitor used to treat human immunodeficiency virus infection (HIV), Indinavir induces stones in a significant number of patients who take it.
- Magnesium Trisilicate – present in foods such as vegetables, seafood, whole grains, and even drinking water, it is also available without a prescription as an antacid to treat GERD. While dietary silicates are easily excreted in the urine, excessive amounts of magnesium trisilicate can induce silicate stone formation.
- Ciprofloxacin – a fluoroquinolone antibiotic that may induce ciprofloxacin crystalluria at doses greater than 1000 mg and a urinary pH greater than 7.3.
- Triamterene – a diuretic used to treat edema and hypertension. The mechanism by which it promotes the formation of urinary calculi is not fully understood.
- Sulfa Medications – although used in a variety of antibacterial, anticonvulsant, and diuretic applications, among others, urinary calculi-associated problems can be further complicated by the crystalline aggregates in these drugs. Generally, however, the solubility of sulfonamides and their ability to induce calculi is significantly increased by higher urinary pH levels.
- Ephedrine and guaifenesin – preparations containing these substances are available over-the-counter in the U.S. Ephedra is purported to promote increased energy, a sense of euphoria, heightened sexual sensation, increased muscle mass and weight loss. Patients who chronically abuse this medication, however, have been known to develop ephedrine calculi. A nonprescription preparation that combines ephedrine with guaifenesin for its expectorant and bronchodilation properties has induced calculi containing both of these substances in persons who have consumed it in large quantities.
Other drugs may induce physiologic changes that can lead to metabolic abnormalities that facilitate the formation of “metabolic stones”. Drugs that can induce calculi through this process include:
- Loop diuretics – these include such drugs as furosemide (Delone, Furosemide, Lasix) and bumetanide (Bumex, Burinex). The calculi induced by these medications are composed of calcium oxalate. Up to 64 percent of low-birth-weight infants who’ve received furosemide therapy have developed renal calculi.
- Carbonic anhydrase inhibitors – include drugs such as acetazolamide (Diamox), which is used to treat glaucoma, altitude sickness, and epilepsy among other conditions. Used on a long-term basis, the metabolic alterations it induces increases the risk for developing calcium phosphate calculi.
- Topiramate (Topamax) – an anticonvulsant medication used to treat partial or refractory seizures may induce calcium phosphate calculi in about 1.5 percent of patients who are treated with it.
- Zonisamide (Zonegran) – an sulfonamide anticonvulsant used to treat partial-onset seizures. Initial trials of the drug found a 4 percent incidence of primarily calcium phosphate calculi.
- Laxatives – potential laxative abuse should be considered by patients who form ammonium acid urate calculi in the absence of urinary tract infection or bowel disease.
Urinary calculi tend to trigger intense pain. If it causes physical trauma, blood may be seen in the urine (hematuria). If a large irregularly shaped stone fills the space within the kidney, there may be signs of infection, as fever and low blood pressure, rather than pain.
Depending on the medical history and symptoms, any of the following tests may be performed to diagnose kidney stones:
- CT scan of the abdomen and pelvis
- Abdominal x-rays
- Ultrasound of the kidneys
- Urinalysis
- Complete blood count
- Serum chemistries
The first step in treatment is to relieve any urinary obstruction. Depending on the location of the obstruction, urine is drained with either a Foley catheter or nephrostomy tube. For stones smaller than 5mm, medications such as Hytrin or Flomax may be given to help relax smooth muscle in the ureter to aid in its passage. Treatment for larger stones that are unable to pass may include:
- Breaking the stone with extracorporeal shock-wave lithotripsy (ESWL)
- Ureteroscopy – a thin, tube-like instrument with a light and a lens is passed through the urethra and a small tool at the end is used to extract the stone
- Nephrolithotomy – surgically opening the kidney
Analysis of the stone is then undertaken to determine its composition. This aids in addressing the underlying condition that led to its formation.