What is the role of Doppler ultrasonography in the evaluation of nephrolithiasis?

Updated: Jun 21, 2018
  • Author: Chirag N Dave, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Answer

The intrarenal resistive index, as measured on Doppler studies, has been proposed as one way to diagnose acute renal obstruction using ultrasound. Under normal conditions, renal vascular resistance is relatively low and renal blood flow is excellent throughout the cardiac cycle, with a reasonable flow continuing even during diastole. During conditions associated with increased vascular resistance (eg acute ureteric obstruction), the decrease in renal blood flow during diastole is proportionately of greater magnitude than that during systole.

The resistive index is calculated as peak systolic velocity minus end-diastolic velocity divided by peak systolic velocity. An elevated resistive index of 0.7 or more is considered indicative of an acute ureteral obstruction. A change in the resistive index between the affected and contralateral (healthy) kidney of 0.04 or more also suggests a ureteral obstruction. (The affected kidney has the higher resistive index value.)

This study may be particularly useful in pregnancy (when exposure to ionizing radiation must be minimized), severe contrast media allergy, and azotemia. For best results, measure the intrarenal resistive index during a pain attack but before any nonsteroidal anti-inflammatory drugs (NSAIDs) or other anti-inflammatory medications are administered.

However, the intrarenal resistive index does not identify partial or intermittent obstructions and is less helpful in the early phase of even complete ureteral blockage. It also does not provide any information about the radiolucency, size, shape, or position of any stone and cannot be used to differentiate between intrinsic and extrinsic urinary obstructions.

Pyelosinus extravasation or fornix rupture, which occurs in up to 20% of patients with acute ureteral obstructions, leads to a loss of dilation and may be responsible for false-negative findings from studies. Other nonobstructive renal problems, such as renal failure, diabetic nephropathy, and renal compression, can affect the readings.

Considering that up to perhaps 35% of patients with documented acute ureteral obstruction do not demonstrate any significant hydroureteronephrosis, the use of a noninvasive study such as Doppler ultrasonography and intrarenal resistive index, which does not depend on visual ureteral or renal pelvic dilation, may eventually prove very useful. For now, additional studies on this technique are needed before the intrarenal resistive index can be reliably used for diagnosing acute renal colic and ureteric obstruction.

Future studies may utilize 2-dimensional ultrasonography in combination with color Doppler analysis of the ureteral jets to enhance sensitivity of ultrasonography in patients with ureteral colic. [29]


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