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Urolithiasis


Anatomy, embryology, pathophysiology

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    Renal calculi result from crystallization of supersaturated inorganic and organic compounds and proteins within renal tubular fluid, renal interstitial fluid, or in the urinary collecting system along the papillary surfaces. Supersaturation and crystallization of stone constituents may result from a combination of increased constituent excretion, reduced urinary fluid volume, abnormal urinary pH, urinary stasis from anatomic abnormality or obstruction, and chronic infection.

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    Calcium compounds including calcium oxalate and calcium phosphate make up the majority (70%–80%) of upper urinary tract stones. Next to calcium, struvite or magnesium ammonium phosphate and uric acid are the next most common components. Uric acid stones are unique in that they can be dissolved by alkalinization of urine, often obviating the need for urological intervention. Less common stone constituents include cystine and precipitation of medications, such as indinavir, triamterene, guaifenesin, and sulfa drugs.

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    Genitourinary tract anomalies that predispose to urolithiasis include horseshoe kidney, ectopic pelvic kidney, and the ectopic moiety in a cross-fused renal ectopia.

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    Most symptomatic urolithiasis presents as renal colic or flank pain. If there is acute collecting system obstruction, stone passage is often associated with acute symptom relief. Aside from obstruction and pain, calculi can lead to hematuria from urothelial irritation and can serve as a nidus for infection. Common locations for obstructing urolithiasis include areas of inherent ureteral narrowing, specifically the ureterovesical junction, the ureteropelvic junction, and where the ureter crosses anterior to the iliac vessels/pelvic brim.

Techniques

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    Unenhanced computed tomography (CT) is the preferred method for diagnosis, treatment planning, and posttreatment follow-up of urolithiasis. When a calcification cannot be confidently diagnosed as a ureteral stone, excretory or urographic phase images after intravenous (IV) iodinated contrast administration are helpful to delineate ureteral anatomy and stone location ( Fig. 22.1 ).

    Fig. 22.1, Delayed phase of a contrast-enhanced computed tomography scan (coronal reformatted image) demonstrates a dilated right ureter and a distal obstructing stone.

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    Ultrasonography (US) is useful as a screening modality for obstruction because it readily demonstrates hydronephrosis. The presence of urinary jets from the ureterovesical orifices rules out significant obstruction.

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    Radiographs lack the sensitivity and additional anatomic information of CT but can be useful to follow patients with known radiographically visible calculi.

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    Magnetic resonance imaging (MRI) has little role in the evaluation of urolithiasis but can demonstrate urinary tract findings associated with urolithiasis such as obstruction, ureteral strictures, and genitourinary anomalies.

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    IV pyelogram (IVP) has been replaced by CT in most settings.

Computed tomography

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    Unenhanced CT is the modality of choice for the diagnosis of renal stones, because of widespread availability, efficiency, and high diagnostic accuracy with reported sensitivity of 95% to 98% and specificity of 96% to 100%.

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    In addition to being more sensitive than radiography, IVP, and US in the detection of urolithiasis, CT better delineates nonurinary tract causes of flank pain, which have been reported to account for up to 15% of identifiable causes by CT, such as appendicitis, diverticulitis, pancreatitis, and ovarian torsion. CT is particularly more sensitive than abdominal radiography for detection of ureteral calculi, small renal calculi, and radiographically lucent calculi, such as uric acid stones ( Figs. 22.2 , 22.3 and 22.4 ).

    Fig. 22.2, 34-year-old woman presented with right-sided flank pain. A, Axial unenhanced computed tomography scan shows a nonobstructing right renal stone. B, Plain film radiograph also demonstrates the right renal stone and could be used for follow-up imaging.

    Fig. 22.3, 51-year-old woman presented with abdominal pain. A, Sagittal ultrasound image shows two nonobstructing stones in the mid and lower left kidney as echogenic foci ( white arrows ) with posterior acoustic shadowing ( yellow arrows ). B, Coronal computed tomography shows the stones ( red arrows ).

    Fig. 22.4, A, Scout image shows a uric acid bladder stone as minimally opaque. Unenhanced axial (B) and reformatted coronal (C) Computed tomography images show the 17- by 12-mm stone, measuring 400 Hounsfield units, in a left posterolateral bladder diverticulum.

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    A perinephric fluid collection in the setting of obstructing urolithiasis could represent a perinephric abscess or a urinoma from a ruptured calyx.

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    CT also has utility in differentiating ureteral calculi from pelvic phleboliths, a common finding. Ureteral calculi may demonstrate a “soft tissue rim” sign of the edematous ureteral walls surrounding the calculus, distinguishing it from pelvic phleboliths. Phleboliths may demonstrate the “comet tail” sign, which represents a tapering noncalcified pelvic vein adjacent to the phlebolith, which is not in the path of the ureter. Lastly, although calculi typically have more uniform density, phleboliths often have a radiolucent center.

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    Large intrarenal stones occupying most of the renal pelvis and some of the calyces are known as staghorn calculi.

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    CT may show renal parenchymal or medullary calcifications in cases of cortical and medullary nephrocalcinosis, respectively.

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    CT urography is the definitive imaging study in the evaluation of hematuria. Unlike IVP, CT urography allows the evaluation of both the renal parenchyma and the urothelium in a single examination. If there is uncertainty about distinguishing a ureteral calculus from another source of calcification, urographic phase images can aid in delineating the exact path of the collecting systems and will show a contrast column upstream from an obstructing calculus. Urographic phase images can also distinguish hydronephrosis in the setting of obstruction from parapelvic or peripelvic cysts, which do not fill with excreted contrast and exist within normal sinus fat rather than obliterate it.

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    CT also provides management-guiding and prognostic information, including stone burden, composition, and fragility. Identification of urolithiasis by CT has been reported to alter management in 55% of patients suspected of having renal colic based on clinical and laboratory findings alone. Most common renal stone compositions are readily identified by routine CT, except for crystallized protease inhibitors, such as indinavir used for human immunodeficiency virus therapy, which may be CT occult on unenhanced images because of similar attenuation (15–30 HU) to adjacent soft tissues ( Fig. 22.5 ).

    Fig. 22.5, Coronal reformatted computed tomography image shows hydronephrosis and hydroureter on the right side with a distal ureteral stone. The stone is only slightly hyperattenuating relative to the urine-filled ureter. The patient was undergoing therapy with indinavir for human immunodeficiency virus infection.

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    Attenuation of calculi at 120 kV typically fall within predictable ranges: uric acid, 200 to 450 HU; struvite, 600 to 900 HU; cystine, 600 to 1100 HU; calcium phosphate, 1200 to 1600 HU; and calcium oxalate, 1700 to 2800 HU. The mixed composition of many stones and partial-volume effects make attenuation measurement of small stones with single energy CT less accurate. Nevertheless, differentiation of pure uric acid from calcium containing stones using attenuation measurements is feasible and reliable.

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    Dual-energy CT (DECT) uses two different tube potentials to differentiate materials based on their unique energy-dependent photoelectric absorption of the x-rays. DECT may be used to differentiate stone chemical composition more accurately than by attenuation at a single kV; allowing differentiation of uric acid from nonuric acid renal stones. Because uric acid stones are typically treated with urinary alkalinization rather than urological intervention, this differentiation provides management changing information for urologists and other referring physicians ( Fig. 22.6 ). Another benefit of DECT is the ability to create virtual-unenhanced images from CT urographic protocols, allowing increased visualization of calculi and potentially reducing radiation dose by eliminating the need for noncontrast scans.

    Fig. 22.6, Axial dual-energy computed tomography images show a nonobstructing left mid-renal calculus ( arrows ), which is hyperattenuating on both standard ( white arrow ) images (A) and on material decomposition uric acid ( yellow arrow ) images (B).

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