Twinkling Renal Stones: Improving the Ultrasound Exam for Suspected Urolithiasis

THE ULTRASOUND ZOOM:

Twinkling Renal Stones: Improving the Ultrasound Exam for Suspected Urolithiasis

By Grant Barker, BS, MD Candidate at University of Miami, Miller School of Medicine

Edited by Leila Posaw, MD, MPH, Dept. of Emergency Medicine, Jackson Memorial Hospital

I am a fourth-year medical student with an interest in emergency medicine and point-of-care ultrasound (POCUS). While attempting to hone my humble practice and POCUS skills, I learned about an ultrasound technique called the twinkling artifact (TA), or the color comet tail, that has been prospectively validated as being highly sensitive and specific for the diagnosis of renal calculi. I believe this technique—when added to any emergency physician’s ultrasound toolbox—will prove very useful in the fast and efficacious diagnosis and management of kidney stones.

Renal colic is a commonly encountered condition in the ED. Renal POCUS is a useful tool for diagnosing urolithiasis in patients with a low-to-moderate risk of important alternative diagnoses. POCUS may also be used to quickly assess the aorta and exclude an aortic aneurysm or dissection. Since the ACEP Choosing Wisely campaign, providers have been much more cautious of exposing patients with renal colic to unnecessary radiation from computed tomography (CT). However, CT is still recommended for patients at high risk of alternative diagnoses: those with fever, abnormal vitals, intractable pain, vomiting or abnormal renal function. [1]

The kidney’s echogenic central area can make identifying stones (especially small stones less than 5 mm) difficult. Recently, techniques have been described to increase the sensitivity of POCUS in stone detection and measurement, which adds significantly to its utility in directing management of renal colic. The TA is one such technique that provides EPs with a comparable level of sensitivity and specificity of stone detection as non-contrast CT, while preserving the safety and efficiency of POCUS.

Gray Scale (B-mode) Imaging

With traditional B-mode ultrasound, only about 64% of stones are in the “field of view,” consisting of the area proximal to the ureteropelvic junction and distal to the ureterovesical junction, due difficulty in assessing the ureter. [2] Of the visualized stones, ultrasound is only about 16% sensitive for those <7mm and 75% sensitive for those ≥7mm. [3] Ultrasound has been criticized as overestimating the size of stones. If the discerning clinician wishes to more accurately measure a stone, one in vitro study found thatmeasuring the acoustic shadow width led to 78% of the measurements to be accurate to within 1 mm, similar to CT resolution. [4]

Fig. 1: Use of the acoustic shadow width to determine kidney stone size with ultrasound. [4]

Hydronephrosis

Hydronephrosis can be used to guide management due to the association between stone size and hydronephrosis. Patients with mild/no hydronephrosis are less likely to have calculi >5mm. 5 Renal cysts may be mistaken for hydronephrosis but are typically single and peripheral. Mild hydronephrosis may be seen in pregnant patients and the overhydrated patient. An under-hydrated patient may also not have hydronephrosis on scanning, even in presence of obstruction and colic.

Fig. 2: Grading of hydronephrosis. [6]

Twinkling Artifact

Color Doppler (CD) ultrasound is an old technology that may have a new role in the diagnosis of urolithiasis. This is due to a phenomenon known as TA, first described in 1996 by Rahmouni et al., which is displayed as rapidly alternating colors of the Doppler signal, possibly due to the reflective nature of the calculi. A large prospective study comparing CT to TA in patients presenting to the ED with acute flank pain reported a sensitivity and specificity of 97.2% and 99%, respectively. [7] This is far better than traditional scanning in B-mode alone. However, it is best to regard the TA with CD as complementary to gray scale imaging. The few false negatives on CD were stones <5mm with no hydronephrosis, and passed spontaneously.

Renal POCUS is performed with a 3.5- 5 MHz probe (curvilinear or phased array). The right kidney is best visualized in the coronal view. The probe is placed on the right side of the patient, in the lower intercostal space and mid axillary line, and the acoustic beam is directed posteriorly. The left kidney is best visualized with the probe on the left side of the patient, in the coronal plane, in the low intercostal space and the posterior axillary line, and with the beam directed posteriorly.

First, the renal pelvis and upper ureters are evaluated with B-mode. If an echogenic focus is detected, the presumed stone is interrogated with CD.

Second, the bladder and vesicoureteral junction are evaluated with B-mode (and echogenic foci with CD). If no echogenic focus/stone is detected, and there is a high clinical suspicion, an effort should be made to trace the ureter from the renal pelvis to the vesicoureteral junction.

With CD, the TA (Fig. 4) will be observed as a series of alternating color signals in a triangular shape under an echogenic focus. This technique is best performed with a low-frequency probe and high pulse repetition frequency (tip: reduce depth of view and increase velocity scale to over 60 cm/s). To avoid being fooled with flashy mimics caused by motion, the technique should be performed and hold true for over 5 seconds. [8]

Fig. 4A: Kidney stone in B mode.

Fig. 4B: Twinkling artifact from same kidney stone in color mode.

Increasing EP confidence in POCUS for confirming the presence of renal calculi in patients with flank pain will lead to a decrease in the ordering of CT scans and spare our patients the consequences of radiation exposure. I am determined to use POCUS and the TA in my practice; this can only make me a more effective clinician. ■

References & Suggested Reading

1. Wang RC. Managing Urolithiasis. Ann Emerg Med. 2016;67(4):449-54.

2. Cullen I, Cafferty F, Oon SF, et al. Evaluation of suspected renal colic with non-contrast CT in the emergency department: a single institution study J Endourol 2008;22:2441e5.

4. Dunmire B, Harper JD, Cunitz BW, et al. Use of the Acoustic Shadow Width to Determine Kidney Stone Size with Ultrasound. J Urol. 2016;195(1):171-7.

6. Timberlake MD, Herndon CD. Mild to moderate postnatal hydronephrosis--grading systems and management. Nat Rev Urol. 2013;10(11):649-56.

7. Abdel-gawad M, Kadasne RD, Elsobky E, Ali-el-dein B, Monga M. A Prospective Comparative Study of Color Doppler Ultrasound with Twinkling and Noncontrast Computerized Tomography for the Evaluation of Acute Renal Colic. J Urol. 2016;196(3):757-62.

8. Ania Z. Kielar, MD, FRCPC, Wael Shabana, MD, PhD, Maryam Vakili, MD, Jonathan Rubin, MD, PhD Prospective Evaluation of Doppler Sonography to Detect the Twinkling Artifact Versus Unenhanced Computed Tomography for Identifying Urinary Tract Calculi J Ultrasound Med 2012; 31:1619–1625.