Overview of Kidney Stones
Stone formation or their presence in the kidney is called “nephrolithiasis.” It is a result of crystallization of specific minerals which come out of solution (the urine) and clump together (aggregation). Nephrolithiasis causes mechanical blockage at its site in the urinary tract, but also acts as a foreign body that, with the stasis of urine collecting proximal to it, can create an ideal environment for infection.
Types of Kidney Stones
- Calcium stones: Calcium oxalate or calcium phosphate. Calcium stones are due to high levels of calcium in the blood (hypercalcemia, hyperparathyroidism), excess filtration of calcium into the urine such as in inflammatory bowel disease, a persistent alkaline pH of the urine which enhances crystallization, or a low urine output which concentrates all stone-making elements.
- Uric acid stones: From persistent hyperacidic urine (pH <5) or from uric acid overproduction and excretion.
- Cystine stones: From a genetic recessive disorder (cystinuria).
- Struvite stones: Magnesium ammonium phosphate stones due to infections with certain bacteria (Proteus or Klebsiella).
Signs and Symptoms
Signs (objective findings) include hematuria on urinalysis and stones identified on X-ray or ultrasound.
Symptoms (subjective complaints) include severe, migrating unilateral flank pain due to stone migration down the ureter, or back pain with stone(s) lodging in the kidney causing backup pressure from accumulating urine that cannot exit to the ureters.
The pain is intermittent in its severity, as the ureters have peristalsis which moves the urine toward the bladder or, in the case of a stone, moves a stone down the same path, distending the walls of the ureter (which are very sensitive to distention) in waves. The pain can also be all along the blocked portion of the ureter, up the flank to the back on one side, due to urine distending the ureter all the way up to the kidney (“hydronephrosis”).
The severity of pain is excruciating and debilitating, with patients often reporting pain scores of 9-10/10.
Diagnosis for Kidney Stones
Urinalysis and imaging are the primary methods of diagnosing kidney stones (calculi, plural; calculus, singular). Initial urinalysis is done in conjunction with a routine chemistry profile (metabolic profile) to identify elevations in serum calcium.
Urinalysis: both biochemical and microscopic evaluation of the urine is helpful in initiating a diagnosis of a stone before imaging confirms it. Microscopic findings include the presence of red blood cells and white blood cells in the urine. Biochemical evaluation, especially useful in those with recurrent stones, can identify calcium It will also indicate the acidity or alkalinity of the urine.
- PH >7 suggests calcium phosphate or struvite calculi (based on the finding of phosphate crystals), or cystine calculi (based on the finding of the characteristic hexagonal crystals seen in cystinuria).
- PH < 5 (acidic) can decrease the solubility of uric acid, leading to crystallization out of solution (urine) of uric acid (as uric acid stones).
The radiopacity (ability to stand out on X-ray) of stones makes them readily identifiable. Occult stones can be diagnoses as a cause of mysteriously recurrent urinary tract infections in spite of appropriate antibiotic management.
Intravenous pyelogram, using dye to visualize the urinary system, from kidneys to bladder, are helpful in identifying where non-migrating stones are.
Patients should be instructed to strain their urine so that any stones or stone fragments can be submitted for examination.
How Can I Manage Kidney Stones?
The symptoms from kidney stones (pain and obstruction) are addressed via:
- Fluids: volume expansion to encourage their passage and elimination
- Medication: to enhance passage
- Pulverization: via ultrasound
- Invasive removal: via cystoscopy or surgery
Increased oral intake of fluids or alternately, intravenous hydration, increase the volume of urine and the chance of passage of the stone to the bladder. The success of passage depends on its size and location. Small stones (<4mm) pass easily. After 4 mm, passage becomes increasingly problematic until unlikely at >10 mm.
If a patient with a newly diagnosed stone is not pain-driven for immediate resolution, a period of observation is acceptable during which surveillance for infection and kidney function is important. Designed to avoid the more invasive or aggressive remedies, passage can be enhanced by the use of antispasmodic agents, calcium channel blockers, and/or alpha-blocker medications, with or without corticosteroids.
Shock wave lithotripsy (ultrasound impacts focused on the stones from an external source) is used for stones higher in the urinary tract (kidneys) and has a high success rate. This makes it prudent to push stones lower in the urinary tract higher (within range) so that lithotripsy can be used (the “push-bang” technique). Stones are thus fragmented for easier passage.
- Stones: Especially those in the lower urinary tract, can be mechanically retrieved with the use of a flexible ureteroscope (small, lighted camera on a thin flexible tube that can steer along the urinary tract after entering the bladder by way of the urethra. Besides direct retrieval, flexible ureteroscopy can also be used to fragment stones in the ureter or bladder for easier passage.
- Laser: Can be used in conjunction with ureteroscopy as a type of “intracorporeal” lithotripsy.
- Percutaneous nephroscopy: Surgical approach to a renal stone through the tissues over the kidney to directly remove it. This is usually reserved for large, impacted kidney or high ureteral stones.
- Retroperitoneal laparascopy: The kidneys are retroperitoneal, i.e., lie under the floor of the abdomen, between the abdomen and the external tissues of the mid-back. Visualizing and working in this area do not, therefore, enter the abdomen, which reduces recovery time.
Prevention of Kidney Stones
While the prevention of complications from kidney stones (pain, obstruction, scarring, infection, and kidney failure) is based on elimination, prevention of stone formation itself relies on adequate hydration and dietary alterations that make less likely overabundance of stone forming minerals in the blood and urine.
A foundation of prevention, first, relies on adequate hydration at all times. Not only will it encourage passage of small fragments as they form, but it will also dilute the crystallizing minerals that form the stones.
Alteration of diet is used primarily to discourage calcium oxalate stones. Since 70-80% of all stones contain calcium oxalate, however, the following dietary recommendations impact all of the mixed-component stones:
- Increase in dietary intake of fluid, as above, and calcium, potassium, and phytate, while
- Decreasing the dietary sources of oxalate, animal protein, sucrose, fructose, sodium, excessive vitamin C or calcium supplements (non-dietary). Also,
- Including coffee and tea, which lower the risk of stones (as does alcohol);
- Increased intake of fruits and vegetables;
- Reduce oxalate in the diet (seen in spinach, rhubarb, potatoes, and some nuts); and
- Low sodium diet.
Recurrence of stones demonstrates the process is continuously active but can benefit from thiazide diuretics to reduce urine calcium, allopurinol to reduce urine uric acid, and potassium citrate or bicarbonate to lower urine citrate.