Reflux Nephropathy Imaging 

In reflux nephropathy, Hodson initially noted the association of vesicoureteral reflux (VUR) with renal parenchymal scarring. Such scarring is demonstrated in the images below.

Preferred examination

Renal ultrasonography (RUS) and voiding cystourethrography (VCUG) are the preferred radiologic examination methods.^[1] The 2 procedures provide complementary information. RUS helps in evaluating the upper urinary tract (kidneys and proximal portion of the collecting system) for anomalies, masses, calcification, hydronephrosis, and size and can identify renal scarring if moderate to severe in degree. VCUG helps in evaluating for VUR and the anatomy of the lower urinary tract (bladder and urethra).

Nuclear medicine renal scanning with a cortical imaging agent (eg, technetium-99m [^99m Tc] dimercaptosuccinic acid) is a better means of detecting and identifying renal scarring than is ultrasonography.

Limitations of techniques

Imaging with renal scanning may require sedation in younger children. Sedation is increasingly requested, although rarely necessary, for children undergoing VCUG.

Plain images offer no diagnostic information regarding vesicoureteral reflux (VUR) and reflux nephropathy, although they can be used to evaluate spinal anomalies and potential urinary tract calculi, which rarely occur in this population. VCUG is essential in evaluating lower urinary tract anatomy and in identifying or excluding VUR.

With voiding cystourethrography, contrast material is instilled in the urinary bladder via a transurethral catheter and demonstrates increased opacity in the ureters and pelvicaliceal systems when VUR is present. VUR grading (grades 0-5) is based on the distribution and degree of associated upper urinary tract dilatation demonstrated on VCUG.

Intravenous urography (IVU), used to produce the image below, can demonstrate changes reflecting reflux nephropathy and renal scarring, including the following:

• Dilatation and deformity of one or more renal calyces
• Loss of parenchymal thickness of a region of the kidney (especially upper and lower poles)
• Focal contour indentation typically adjacent to a calyx, representing renal scarringA 15-minute postinjection anteroposterior view of the kidneys in a 9-year-old boy with a history of multiple urinary tract infections who presented with hypertension demonstrates bilateral diffuse calyceal clubbing (arrows) and deformity accompanied by thinning of the adjacent renal parenchyma (arrowheads). These findings reflect chronic pyelonephritis and demonstrate the characteristic appearance of reflux nephropathy on intravenous urography.

Degree of confidence

With VCUG, the degree of confidence is high for detecting VUR and low for detecting chronic pyelonephritic scarring. With IVU, the degree of confidence is moderate for detecting chronic pyelonephritic scarring.^{[2, 3]}

False positives/negatives

False findings occur infrequently; however, the soft tissue of the bowel wall can simulate contrast enhancement in a ureter, and overlying stool and intestinal gas can sufficiently obscure the kidney to prevent identification of lesser degrees of VUR.

Scarring from reflux nephropathy can usually be differentiated from scarring from other etiologies (such as parenchymal infarction) by the fact the calyx underlying the scar will be deformed in reflux nephropathy.

Computed tomography (CT) scanning is not indicated in the evaluation of reflux nephropathy because of radiation dose, the need for intravenously administered contrast material, and the potential need for patient sedation. Although CT scans can demonstrate renal scarring and hydronephrosis, they cannot demonstrate VUR.

Magnetic resonance imaging (MRI) is not routinely indicated in the evaluation of reflux nephropathy because of the procedure's cost and the potential need for patient sedation. As with CT scanning, MRI can demonstrate renal scarring and hydronephrosis, but not VUR.^[4]

Alteration of renal contour and reduced thickness of the renal parenchyma (seen in the images below) are the primary findings of reflux nephropathy. Ultrasonography is excellent for demonstrating upper urinary tract dilatation.^{[5, 6]}

Echo-enhanced cystosonography performed by using ultrasonographic contrast media is a new diagnostic procedure with a major attribute in the exclusion of ionizing radiation.

Degree of confidence

With ultrasonography, the degree of confidence is moderate in the detection of reflux nephropathy. Lesser degrees of renal scarring may not be evident with ultrasonography.

Echo-enhanced cystosonography, in limited experience, is reported to be as accurate as radiographic VCU in the identification of reflux.

False positives/negatives

Fetal lobation and other normal variants of renal development may be mistaken for renal scarring on ultrasonograms.

^99m TC dimercaptosuccinic acid and^99m Tc glucoheptonate are scintigraphic agents with a high affinity for the renal cortex (as demonstrated in the image below), because they are concentrated by renal tubular cells.^{[7, 8]} As a result, they produce a relatively high level of renal detail. Concern for acute pyelonephritis, the assessment of renal cortical scarring, and the identification of a malpositioned kidney are among the indications for cortical scintigraphy. The use of single-photon emission CT (SPECT) scanning increases the sensitivity of the agents to changes of renal scarring and acute pyelonephritis.

Reduced accumulation of a cortical imaging isotope within the renal margins is an indication of renal scarring. Radionuclide cystogram provides an alternative to VCUG in identifying renal scarring. Cystograms have the primary disadvantage of poor anatomic resolution but the advantage of a lower radiation dose.

Degree of confidence

The degree of confidence is high with renal cortical imaging performed with^99m Tc dimercaptosuccinic acid.

False positives/negatives

Patient motion, structural abnormalities of the kidneys, and unsatisfactory isotope administration may result in false-positive and false-negative findings.

Patient motion results in inaccurate imaging depiction of isotope distribution and greater probability of underestimating renal scarring (false-negative findings). Hydronephrosis, anomalies such as ectopic and/or fused kidneys, and renal masses or cysts may result in the overestimation of severity of renal scarring (false-positive findings).