What is the role of imaging studies in the workup of postnatal hydronephrosis and hydroureter?

Updated: Dec 16, 2020
  • Author: Dennis G Lusaya, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Answer

Postnatal radiologic evaluation of a newborn with antenatal hydronephrosis begins with an ultrasonography examination. The timing of ultrasonography and the need for other studies depend on the severity of postnatal hydronephrosis and whether there is bilateral involvement or an affected solitary kidney.

Ultrasonography of kidneys and bladder should be performed in the postnatal period on affected infants. The timing of the study depends on the severity of the antenatal hydronephrosis. In general, examination should be avoided in the first 2 days after birth because hydronephrosis may not be detected because of extracellular fluid shifts that underestimate the degree of hydronephrosis. However, infants with bilateral hydronephrosis and those with a severe hydronephrotic solitary kidney require urgent evaluation on the first postnatal day because of the increased likelihood of significant disease and a possible need for early intervention. For unilateral hydronephrosis without antenatal bladder pathology, performing postnatal sonography 1-4 weeks after birth is recommended.

A voiding cystourethrography (VCUG) is performed to detect VUR and, in boys, to evaluate the posterior urethra. For this procedure, a urinary catheter is inserted into the bladder and contrast material is instilled. Fluoroscopic monitoring is performed while the bladder is filling and during voiding. Infants usually tolerate this procedure well. Although the duration of fluoroscopy is minimized, the gonads, especially the ovaries, are exposed to radiation. [19]

Diuretic renography is used to diagnose urinary tract obstruction in infants with persistent hydronephrosis and is usually ordered after a VCUG has demonstrated no vesicoureteral reflux. [20] It measures the drainage time from the renal pelvis and assesses total and individual kidney renal function. The test requires insertion of a bladder catheter to relieve any pressure that can be transmitted to the ureters and kidneys. Intravenous access is needed for hydration and the administration of the radioisotope and diuretic. The preferred radioisotope is technetium Tc 99m-mercaptoacetyltriglycine (Tc99mMAG3), which is taken up by the renal cortex, filtered across the glomerular basement membrane to the renal tubules, and excreted into the renal pelvis and urinary tract. [21]

Diuretic renography includes two phases. First, radioisotope is injected intravenously and renal parenchymal (cortical) uptake is measured during the first 2-3 minutes. The relative contribution of each kidney to overall renal function (called the split renal function) is assessed quantitatively and is useful as a baseline study. Subsequent studies can be compared to assess whether kidney function remains stable or has deteriorated, suggesting true obstruction. [22]

Second, at peak renal uptake, intravenous furosemide is administered and the excretion of isotope from the kidney is measured, referred to as the washout curve. This phase indicates the extent of obstruction, if present. In a healthy kidney, furosemide administration results in a prompt washout. In a dilated system, if washout occurs rapidly after diuretic administration (< 15 min), the system is not obstructed. If washout is delayed beyond 20 minutes, the pattern is consistent with obstructive uropathy. However, a delayed washout must be interpreted with caution. [23, 24]

In a series of 39 infants with antenatal unilateral hydronephrosis followed without surgery, diuretic renography indicated obstruction in 24 patients whose renal function never decreased and who thus did not have obstruction. [24] These results may be due, in part, to the normally low neonatal GFR that can be refractory to diuretic therapy. If washout is from 15-20 minutes, the study is indeterminate.

Gravity-assisted drainage imaging may assist in the assessment of pediatric hydronephrosis. Unlike customary diuretic renography, in which the patient remains supine, with gravity-assisted renography a single, static image is obtained after positioning the child in the upright position for 5 minutes to promote additional drainage of tracer from the collecting system. Notable improvement in drainage with this maneuver suggests that slowness in drainage is not due to urinary tract obstruction. [25]

Split renal function results are the most useful criteria to evaluate a decrease in renal function. In patients with unilateral hydronephrosis (which is the most common clinical scenario), if the normal nonhydronephrotic kidney and hydronephrotic kidney both have equal function, conservative management without surgery is a safe option. In a cohort of 831 cases of antenatal hydronephrosis, renal scanning performed in 229 newborns demonstrated that 16% of patients had a significant decrease in renal function of one kidney, defined as 35% or less differential renal function. [26] A decrease in differential renal function was associated with severe antenatal hydronephrosis (ie, renal pelvic diameter >10 mm at 20-24 wk gestation and >16 mm at 33 wk gestation).

Magnetic resonance urography (MRU) in children is becoming more commonly used in the diagnosis and management of congenital uropathies such as UPJ obstruction. [27, 28] MRU is especially useful in the management of obstructed kidneys that have rotation or ascent anomalies, or are solitary. MRU can more clearly define the anatomy and delineate the proper surgical approach (ie, retroperitoneal vs transperitoneal). Newer MRU technology may even define obstruction, eliminating the need for diuretic renal scanning.

The disadvantage of MRU is that the study often requires general anesthesia or heavy conscious sedation in children. Furthermore, the contrast agent gadolinium can only be used if renal function is normal (requiring a preprocedure serum creatinine test) because of reports of irreversible renal fibrosis in patients with renal insufficiency.


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