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Kidney Stone Basics for the Physician Assistant Exam

Kidney stones (renal calculi) cause pain and hematuria and will be covered on the Physician Assistant Exam. They commonly occur in young and middle-aged men. Here are several big payoff areas dealing with renal calculi.

Signs and symptoms of kidney stones

Many kidney stones pass through the GU tract on their own. For a stone to wreak havoc, it usually has to be bigger than 5 mm. Here are two key points concerning the presentation of an acute kidney stone flare, also known as renal colic:

  • The pain is rapid onset and is usually a unilateral, sharp, stabbing flank pain with radiation to the inguinal region (groin area) on the same side. The pain can be so debilitating that it makes a grown man cry.

  • Hematuria, either macroscopic or microscopic, can be present. The urinalysis dips positive for blood and shows many RBC/HPF (red blood cells per high power field) in the microscopic sediment. The presence of gravelly or sandy urine is indicative of small crystals.

Diagnose kidney stones with imaging studies

When evaluating someone with acute flank pain, you’ll likely need to choose a radiologic modality to further evaluate the cause of the pain. You have these choices:

  • Radiograph: A KUB (kidneys, ureter, bladder) radiograph can detect the presence of many types of stones, except uric acid stones. The radiograph cannot evaluate for a hydronephrosis or a pyonephrosis — you need a more definitive study for that. A radiograph is often used to follow the size of a stone chronically but has limited use acutely.

  • Renal ultrasound: A renal ultrasound can detect stones higher up in the GU tract (that is, in the kidneys and proximal ureters) but can’t fully evaluate the lower GU tract.

  • Spiral CT scan without contrast: A spiral CT scan of the abdomen and pelvis without contrast is the gold standard not only for evaluating for kidney stones but for evaluating the whole GU tract.

An intravenous pyelogram (IVP) is currently much less popular for evaluating kidney stones because it requires the administration of contrast dye. Other techniques, such as ultrasonography, frequently provide similar or more detailed information. An MRI is not indicated.

Kidney stones by type

You should know about four types of kidney stones: calcium oxalate, calcium phosphate, uric acid, and magnesium ammonium phosphate (struvite).

Calcium oxalate

Calcium oxalate stones are the most common type of kidney stone. Patients with calcium oxalate stones often have elevated urinary levels of calcium and oxalate and lower levels of urinary citrate. Medical conditions that can be associated with these types of stones include the following:

  • Primary hyperparathyroidism: Recall the triad of hyperparathyroidism: elevated serum calcium, elevated PTH, and increased 24-hour urine calcium.

  • Absorptive (idiopathic) hypercalcinuria: The cause of absorptive hypercalcinuria is unknown, but experts think that the affected person has increased intestinal absorption of calcium.

Calcium phosphate

Calcium phosphate stones are the second most common type of kidney stone. You can see this type of kidney stone in combination with calcium oxalate or on its own. Here’s the most common clinical presentation:

  • It’s common in young women.

  • A nongapped metabolic acidosis is present.

  • Hypokalemia is predominant.

The treatment is administration of potassium citrate.

Uric acid

Uric acid stones usually occur in acidic urine, with a pH of 6.0 or lower. Here are two big points about uric acid stones:

  • Uric acid stones can be associated with other medical conditions, including gout, diabetes, obesity, and malabsorptive states such as chronic diarrhea and Crohn’s disease.

  • A uric acid stone is radiolucent, meaning it can’t be seen via radiograph. Even Superman with his X-ray vision would likely miss a uric acid stone.

The treatment is hydration (as is true for all stones), alkalinization of the urine (to a pH of 6.5 or greater), and allopurinol (but only if a 24-hour urine demonstrates an overproduction of uric acid).

Struvite

The struvite kidney stone consists of magnesium ammonium phosphate. The usual presentation is a young woman with a history of recurrent urinary tract infections, usually with a urease-positive organism (urease produces an alkaline urine pH). You can also see this type of stone in anyone with altered GU anatomy or who has either a chronic Foley catheter or requires frequent catheterizations.

How to treat kidney stones

Here’s the basic treatment for chronic kidney stones of all types:

  • Increase fluid intake to >= 2 liters (about 8 cups) a day.

  • Where urine citrate concentration is low, increase it with potassium citrate.

  • For calcium-based stones — all the main types except uric acid stones — prescribe a low-sodium, low-oxalate diet and add the diuretic hydrochlorothiazide (HCTZ) to lower calcium excretion.

If a patient has an acute stone attack, you’re likely to be evaluating him or her in the emergency room. IV fluids and analgesics for pain are first-line. The results of a CT scan concerning the size of the stone indicate what to do next:

  • If the stone is <= 5 mm, it should pass on its own.

  • If the stone is > 5 mm, surgical intervention may be needed. The stone can be extracted via a procedure called a ureteroscopy. Extracorporeal shock wave lithotripsy (ESWL) can be used as well to break up the stones so they can pass.

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