Urolithiasis

Dr Henry Knipe and Dr Jeremy Jones et al.

Urolithiasis refers to the presence of calculi anywhere along the course of the urinary tracts. For the purpose of the article the terms urolithiasis, nephrolithiasis and renal or kidney stones are used interchangeably, although some authors have slightly varying definitions of each. 

See main articles: ureteric calculi and bladder stones for further discussion of these.

Epidemiology

Most patients tend to present between 30-60 years of age 1

The lifetime incidence of renal stones is high, seen in as many as 5% of women and 12% of males. By far the most common stone is calcium oxalate, however the exact distribution of stones depends on the population and associated metabolic abnormalities (e.g. struvite stones are more frequently encountered in women, as urinary tract infection as more common) 8.

Clinical presentation

Although some renal stones remain asymptomatic, most will result in pain. Small stones that arise in the kidney are more likely to pass into the ureter where they may result in renal colic. Haematuria, although common, may be absent in ~15% of patients 1.

Pathology

The composition of urinary tract stones varies widely depending upon metabolic alterations, geography and presence of infection, and their size varies from gravel to staghorn calculi. The more common composition of stones include (more detail below):

  • calcium oxalate +/- calcium phosphate: ~75%
  • struvite (triple phosphate): 15%
  • pure calcium phosphate: 5-7%
  • uric acid: 5-8%
  • cystine: 1%
Risk factors

Certain risk factors have been identified including 8:

Calcium-containing stones

Most renal calculi contain calcium, usually in the form of calcium oxalate (CaC2O4) and often mixed with calcium phosphate (CaPO4) 1,6. In most instances no specific cause can be identified, although most patients have idiopathic hypercalciuria without hypercalcaemia. 

Brushite is a unique form of calcium phosphate stones that tends to recur quickly if patients are not treated aggressively with stone prevention measures and are resistant to treatment with shock wave lithotripsy. 

Interestingly hyperuricosuria is also associated with increased calcium containing stone formation, and is thought to be related to the the uric acid crystals acting as a nidus on which calcium oxalate and calcium phosphate can precipitate 6

Rarely the underlying cause is primary oxaluria, a liver enzyme deficiency leading to massive cortical and medullary nephrocalcinosis, and renal failure.

Certain medications 14 can predispose to calcium oxalate or calcium phosphate calculi, including:

  • loop diuretics
  • acetazolamide
  • topiramate
  • zonisamide
Struvite stones

Struvite (magnesium ammonium phosphate or "triple phosphate") stones are usually seen in the setting of infection with urease producing bacteria (e.g. Proteus, Klebsiella, Pseudomonas and Enterobacter), resulting in hydrolysis of urea into ammonium and increase in the urinary pH 6,10. They can grow very large and form a cast of the renal pelvis and calices resulting in so-called staghorn calculi. The struvite accounts for ~70% of these calculi, and is usually mixed with calcium phosphate thus rendering them radiopaque. Uric acid and cystine are also found as minor components. 

Uric acid

Hyperuricosuria is not always associated with hyperuricaemia, and is seen in a variety of settings (see above), although in most instances uric acid stones occur in patients with no identifiable underlying aetiology 6. Uric acid crystals form and remain insoluble at acidic urinary pH (below 5).

Cystine stones

Cystine stones are also formed in acidic urine, and are seen in patients with congenital cystinuria.

Others
  • medication stones 14:
    • indinavir stones (in HIV patients) which are typically radiolucent (see case 23)
    • magnesium trisilicate which are typically poorly radiopaque
    • ciprofloxacin which are typically radiolucent
    • sulphonamides which are typically radiolucent
    • triamterene which are typically poorly radiopaque
    • guaifenesin/ephedrine which are typically radiolucent
  • pure/protein matrix stones
    • mostly (~65%) made of organic proteins, carbohydrates, and glucosamines (c.f. with other stones which are crystalline with only a minor organic element) 15

Radiographic features

These depend on stone composition, and vary according to modality. The much greater sensitivity of CT to tissue attenuation means that some stones radiolucent on plain radiography are nonetheless radiopaque on CT.

Abdominal radiography

Calcium containing stones are radiopaque

  • calcium oxalate +/- calcium phosphate
  • struvite (triple phosphate) - usually opaque but variable
  • pure calcium phosphate

Lucent stones include

  • uric acid
  • cystine
  • Indinavir stones
  • pure matrix stones (although may have radiodense rim or centre 15)
Fluoroscopy

Intravenous urography (IVU) is a traditional radiographic study of the renal parenchyma, pelvicalyceal system, ureters and the urinary bladder. It involves administration of intravenous contrast. This exam has been largely replaced by noncontrast CT. 

CT

On CT almost all stones are opaque, but vary considerably in density.  

  • calcium oxalate +/- calcium phosphate: 400-600 HU
  • struvite (triple phosphate): usually opaque but variable
  • pure calcium phosphate: 400-600 HU
  • uric acid: 100-200 HU
  • cystine: opaque

Two radiolucent stones are worth mentioning 11:

  • Indinavir stones: (antiretroviral drug) radiolucent and usually undetectable on CT 5
  • pure matrix stones

Ninety-nine percent of renal tract calculi are visible on a noncontrast CT. Given that one of the commonest sites for a stone to become lodged is the vesicoureteric junction, some centres perform the study in the  prone position to establish if the stone is retained within the intravesical component of the ureter or has already passed into the bladder itself.

Dual energy CT

DECT is a technique allowing determination of calculus composition, by assessing stone attenuation at two different kVp levels. Each CT vendor has its own algorithms for the use of dual energy CT for assessing stone composition.

Ultrasound

Ultrasound is frequently the first investigation of the urinary tract, and although by no means as sensitive as CT, it is often able to identify calculi. Small stones and those close to the corticomedullary junction can be difficult to reliably identify. Ultrasound compared to CT KUB reference showed a sensitivity of only 24% in identifying calculi. Nearly 75% of calculi not visualised were <3 mm 13. Features include 7

Treatment and prognosis

Treatment depends on the location of the stone, composition and size.

Irrespective of stone composition, patients who present with renal colic require assessment. Approximately 90% of stones <4 mm are likely to pass down the ureter and into the bladder, and thus often require no more than analgesia and hydration 1. This is of course provided that the kidney is not obstructed and infected in which case a percutaneous nephrostomy should be performed on an emergency basis to save the organ and prevent sepsis.

Calcium stones

Small asymptomatic stones in the kidney can be safely ignored, and if patients maintain good states of hydration, the risk of recurrent symptoms can be dramatically reduced 10. In all settings a search for a possible underlying cause of hyperoxaluria/hypercalciuria should be sought and if present corrected when possible.

Larger stones may be treated with:

Struvite stones

Struvite stones are usually large (staghorn calculi) and result from infection. These stones need to be treated surgically and the entire stone removed, including small fragments, as otherwise these residual fragments act as a reservoir for infection and recurrent stone formation.

Uric acid stones

Uric acid stones usually are the result of low urinary pH, and hydration and elevation of urinary pH to approximately 6 is usually sufficient (note rendering the urine too alkali (e.g. >pH 6.5) may result in calcium stone formation) 10.

Cystine stones

Cystine stones may be difficult to treat and are difficult to shatter with ESWL. Hydration and alkalinisation are usually first line therapy.

Complications

Recognised complications include:

Differential diagnosis

The differential of renal calculi is essentially that of abdominal calcifications. On CT there is usually little confusion as not only is CT exquisitely sensitive in detecting stones, but their location can also be precisely noted.

Thus the differential diagnosis is predominantly on plain radiograph, and to a lesser degree ultrasound:

  • cholethiasis overlying right kidney
  • pancreatic calcification
  • phleboliths
  • calcified mesenteric lymph nodes
  • renal artery calcification 7 
  • intrarenal gas (only a differential for ultrasound)
    • acoustic shadow is usually 'dirtier'
    • gas typically more mobile than stones
  • pure/protein matrix stones may mimic an upper tract soft tissue mass 15
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Article Information:

rID: 6212
System: Urogenital
Section: Pathology
Synonyms or Alternate Spellings:
  • Renal calculi
  • Renal stones
  • Kidney stones
  • Renal tract calculi
  • Kidney stone
  • Renal stone
  • Nephrolithiasis
  • Renal calculus
  • Urinary tract stones
  • Renal tract calculus
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    Figure 1
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    Bilateral renal s...
    Case 1: bilateral nonobstructing renal stones
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    Case 2: in pelvic kidney
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    Coronal reformat
    Case 3: coral calculi
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    Case 4: in horseshoe kidney
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    Staghorn calculus
    Case 5: staghorn calculus
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    Ureterolithiasis
    Case 6: stone in proximal ureter
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    Case 7
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    Case 8: Randall plaques
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    Urolithiasis on b...
    Case 9: with hydronephrosis
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    Case 10: calculus in ureterocoele - MRI
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    Case 11: pre-vesical stone
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    Left renal caclul...
    Case 12: with twinkle artefact
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     Case 15: urethral calculus
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    Case 16: right VUJ calculus
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    Case 17
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    CT-IVU component ...
    Case 18: on CT-IVU
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    Right ureteral ca...
    Case 19: ureteric calculus on ultrasound
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    Case 20: urethral stone
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    Case 21: urethral stone
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    Case 22: urethral on ultrasound
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    Case 23: radiolucent indinavir stone
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