Nephrolithiasis, also known as kidney stones, is a common condition affecting 5% to 15% of the population at some point, with a yearly incidence of 0.5% in North America and Europe, and is caused by a crystal or crystalline aggregate traveling from the kidney through the genitourinary system.    
There are multiple predictors and risk factors for stone formation. The following are the most common:
Approximately 5% to 15% of the population will be affected by a kidney stone, and of those, 50% will have a recurrent stone within five to seven years of initial presentation if preventive measures are not taken. Over 70% of stones occur in people 20 to 50 years old, and they are more common in men than women by a factor of about 2:1. Patients with obesity, hypertension, and/or diabetes are at increased risk for kidney stone formation.
As a stone moves from the renal collecting system, it can significantly affect the genitourinary tract. A stone can cause obstruction and hydronephrosis of the ureter, decreasing the rate of ureteral peristalsis and causing urine to back up into the kidney. This can cause decreases in the glomerular filtration rate of the affected kidney, and increase renal excretion of the unaffected kidney as well as very severe, excruciating pain. Complete obstruction of the ureter can lead to eventual loss of renal function, with damage becoming irreversible in one to two weeks. Additionally, there is a risk of rupture of a renal calyx with the development of a urinoma. Of even more concern is the possibility that an obstructed renal unit might become infected causing an obstructive pyelonephritis or pyonephrosis. This condition can be life-threatening and requires immediate surgical drainage as antibiotics alone are ineffective
Renal calculi can become impacted, most commonly at the ureteropelvic junction (the renal pelvis narrows abruptly to meet the ureter), near the pelvic brim, or at the ureterovesical junction.
Patients with renal colic typically present with sudden onset of flank pain radiating laterally to the abdomen and/or to the groin. Patients often report a dull constant level of pain with colicky episodes of increased pain. The constant pain is often due to stretching of the renal capsule due to obstruction, whereas colicky pain can be caused by peristalsis of the genitourinary tract smooth muscle against the obstruction. Many patients report associated nausea or vomiting, and some may report gross hematuria. As the stone migrates distally and approaches the bladder, the patient may experience dysuria, urinary frequency, urgency or difficulty in urination.
Patients experiencing renal colic may present in very severe pain. Classically, these patients are unable to find a comfortable position and are often writhing on or pacing around the examination table. The exam may reveal flank pain (more commonly than abdominal pain), and the skin may be cool or diaphoretic. There is often a prior personal history of stones or a family history.
Diagnosis is made through a combination of history and physical exam, laboratory and imaging studies. Urinalysis shows some degree of microscopic or gross hematuria in 85% of stone patients, but should also be evaluated for signs of infection (e.g., white blood cells, bacteria). Urinary pH greater than 7.5 may be suggestive of a urease producing bacterial infection, while pH less than 5.5 may indicate the presence of uric acid calculi.
Basic metabolic panel (BMP) should be obtained to assess for renal function, dehydration, acid-base status, calcium and electrolyte balance. Complete blood count (CBC) can be considered to evaluate for white blood cell count (mild elevation is commonly secondary to white blood cell demargination) if there is a concern for infection.
Consider obtaining a parathyroid hormone (PTH) level if primary hyperparathyroidism is suspected as a cause of any hypercalcemia. If possible, urine should be strained to capture stones for analysis to help determine if there is a reversible or preventable cause of stone development. Further metabolic testing (24-hour urine collection for volume, pH, calcium, oxalate, uric acid, citrate, sodium, and potassium concentrations) should be considered in high-risk first-time stone formers, pediatric patients or recurrent stone formers. It is highly recommended in kidney stone patients with solitary kidneys, renal failure, renal transplants, gastrointestinal (GI) bypass, and any patient with high or increased anesthesia risk.
Unenhanced (or helical) CT is the gold standard of initial diagnosis, with a sensitivity of 98%, specificity of 100%, and negative predictive value of 97%. This modality allows rapid identification of a stone, provides information as to the location and size of the stone, and any associated hydroureter, hydronephrosis, or ureteral edema, and can give information regarding potential other etiologies of pain (e.g., abdominal aortic aneurysm, malignancy). In those patients with no previous history of nephrolithiasis, CT should be performed to guide management. CT scans may underestimate stone size in comparison with an intravenous pyelogram or abdominal x-ray.
However, CT scan does expose patients to a radiation burden and it can be costly. In some patients with a history of renal colic that present with pain similar to previous episodes of renal colic, it may be sufficient to perform ultrasonography (US). However, US is less sensitive (60% to 76%) than CT for detecting calculi less than 5 mm, but can reliably detect hydronephrosis and evidence of obstruction (increased resistive index in the affected kidney). It is also the modality of choice for evaluating a pregnant patient with concern for renal colic. Studies have shown that using ultrasonography as a primary imaging modality does not lead to an increase in complications in comparison to CT. Ultrasound is also a good way to follow a patient known to have uric acid stones.
An abdominal x-ray (KUB) can identify many stones, but 10% to 20% of renal stones are radiolucent and provide little information regarding hydronephrosis, obstruction or the kidneys. Additionally, bowel gas, the bony pelvis and abdominal organs may obstruct visualization of a stone. The KUB is recommended in kidney stone cases when the CT can is positive and the exact location of the stone is known. This helps in clearly identifying those stones that can be tracted by follow-up KUB and those that might be amenable to lithotripsy.
Using both a KUB and a renal US is a reasonable alternative to a CT scan and far cheaper with less radiation exposure. Symptomatic stones are likely to produce hydronephrosis or obstruction (visible on ultrasound) or will be seen directly on the KUB or the ultrasound.
Treatment includes the following:
Calculus size, location, and patient discomfort predict the likelihood of spontaneous stone passage. Approximately 90% of stones less than 5 mm pass within four weeks. Up to 95% of stones larger than 8 mm can become impacted, requiring intervention to pass.
Indications for admission include a renal stone in a solitary kidney, severe kidney injury, an infected renal stone, intractable pain or nausea, urinary extravasation, or hypercalcemic crisis.
Patients with infected stones (e.g., nephrolithiasis plus evidence of urinary tract infection) require special and more urgent treatment. The infected stone acts as a nidus for infection and leads to stasis, decreasing the ability to manage infections. Frequently, these stones need to be removed in their entirety operatively to prevent a repeat infection and new stone formations.
24-hour urine tests are the cornerstone of long-term preventive therapy, but they require very high levels of patient dedication and compliance to be successful. Nevertheless, they should be offered to patients with recurrent stones and a high risk of new stone formation.
The management of renal stones is by a multidisciplinary team that consists of a nephrologist, emergency department physician, radiologist, urologist, and an internist. The majority of renal stones pass within four weeks but stones larger than 8 mm may require some type of intervention before they can pass. Healthcare workers including nurse practitioners who see patients with kidney stones should contact the urologist when large stones fail to pass. In some cases of infected stones, surgery may be required. Because recurrence of kidney stone is common, the patient should be educated on fluid intake and avoidance of certain foods. The prognosis for most patients with kidney stones is good.
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