THE ULTRASOUND ZOOM
Twinkling renal stones Improving the Ultrasound Exam for Suspected Urolithiasis
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
Edited by Leila Posaw, MD, MPH, Dept. of Emergency Medicine, Jackson Memorial Hospital
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 Fig. 1: Use of the acoustic shadow width to determine kidney stone size with of stones. If the discerning clinician ultrasound.4 wishes to more accurately measure a stone, one in vitro study found that measuring the acoustic shadow width led to 78% of the measurements to be accurate to within 1 mm, similar to CT resolution.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
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. 28
By Grant Barker, BS, MD Candidate at University of Miami, Miller School of Medicine
EMPULSE WINTER 2019
