Introduction

Acute flank pain is a common complaint in the emergency department. The presence of unilateral flank or groin pain, especially in the setting of gross or microscopic hematuria, raises clinical suspicion for renal colic secondary to an obstructing ureteral stone. Bedside renal ultrasonography is a rapid and safe imaging modality for evaluation of renal colic. The presence of unilateral hydronephrosis in the correct clinical context is often sufficient to make the diagnosis. Because pain from a rapidly expanding or leaking abdominal aortic aneurysm (AAA) can masquerade as renal colic, additional evaluation of the aorta should be considered, especially in patients with risk factors for AAA.

Acute urinary retention is a common cause of abdominal pain. Bedside ultrasound is a rapid, noninvasive diagnostic tool for the evaluation of suspected urinary retention. Sonographic estimation of urinary bladder volume provides an accurate quantitative assessment of retention in the emergency setting. Additionally, screening ultrasound for the presence of adequate urine volume has been shown to decrease the number of unsuccessful attempts at catheterization in the pediatric population.

Evidence-Based Review

Computed tomography (CT) currently has the greatest sensitivity and specificity of imaging modalities used for the diagnosis of renal colic. When compared with ultrasound, non–contrast-enhanced CT is more sensitive in identifying obstructing ureteral stones^1–4 and more accurate in measuring stone size.^4,5 CT also more consistently locates the specific site of obstruction along the course of the ureter and can identify other causes of abdominal pain that may mimic acute renal colic.

However, routine use of CT for suspected renal colic has significant limitations. Patients are exposed to significant amounts of ionizing radiation when they undergo CT of the abdomen and pelvis,^6,7 which makes CT inadvisable during pregnancy and less appealing in the pediatric population. Because renal colic is a recurrent diagnosis, patients may eventually receive multiple CT scans over time, and cumulative radiation doses may rise to concerning levels⁸ and place patients at risk for cancer later in life. Additionally, radiologic imaging has been shown to be the largest contributor to the cost of hospitalization in patients with renal colic, with CT being the most expensive imaging modality.^9,10

Ultrasound offers a safe, noninvasive means of assessing for renal colic that does not subject the patient to any ionizing radiation. Traditionally, ultrasound has been thought to have only fair sensitivity (37% to 64%) for detecting stones and better sensitivity (74% to 85%) for the diagnosis of acute obstruction,¹ whereas CT has consistently shown sensitivity greater than 90% for the diagnosis of ureteral stones.^1–4 Recent studies using modern ultrasound equipment in the hands of skilled operators have demonstrated comparable sensitivity (76% to 98%) of ultrasound for the detection of ureteral stones.^11–14 Although specific stone location and size cannot always be determined with ultrasound, surrogate findings may have prognostic value in guiding patient management. One study showed that normal renal ultrasound findings in the setting of suspected renal colic predicted a very low risk (0.6%) for subsequent urologic intervention.¹⁵

The goal of bedside ultrasound is to identify unilateral hydronephrosis as an indirect sign of obstructing ureteral stones. In the emergency department setting, Rosen et al. demonstrated a sensitivity of 72% and specificity of 73% for the detection of hydronephrosis with bedside ultrasound.¹⁶ More recently, Gaspari and Horst reported a sensitivity of 87% and specificity of 85% for the diagnosis of hydronephrosis.¹⁷ These results are comparable to a sensitivity of 81% and specificity of 92% for bedside ultrasound performed by on-call urologists; however, this study evaluated other renal findings in addition to hydronephrosis.¹⁸ Although the specific size of the obstructing stone is not typically demonstrated on bedside ultrasound, the degree of hydronephrosis may correlate with the size of the obstructing stone. A 2010 study showed that no or mild hydronephrosis predicted smaller stone size (<5 mm) as opposed to moderate or severe hydronephrosis.¹⁹

Bedside ultrasound for the measurement of urinary bladder volume has been well described in the literature. Bedside ultrasound provides a quantitative assessment of bladder volume when evaluating for urinary retention in the emergency department setting and can guide the need for emergency catheterization.^20,21 Many studies have shown good correlation between sonographic bladder volume calculations and urine volumes obtained via catheterization.^22–24 In the pediatric population, ultrasound has been shown to predict patients with volumes sufficient for successful catheterization of urine samples (>3 mL).^25–28 This may help avoid multiple attempts at urinary catheterization, which can result in unnecessary patient discomfort and the potential for iatrogenic infection. Additionally, bedside ultrasound has demonstrated increased success rates during attempts at suprapubic aspiration when compared with traditional, landmark-based techniques.^29–31

How to Scan

Emergency renal ultrasonography is performed with a 2- to 5-MHz curvilinear transducer. When rib shadows limit adequate sonographic windows for the kidneys, switching to a phased-array probe may be helpful. In general, patients can be positioned supine for renal ultrasound. The right lateral recumbent position may be necessary when imaging the left kidney because this kidney tends to be located more posteriorly and the spleen is not as large an acoustic window as the liver.

For the right kidney, the probe is initially placed along the lower portion of the rib cage in the midaxillary line with the probe marker directed toward the patient’s head. The resulting coronal view is the traditional right upper quadrant view of the focused assessment with sonography for trauma (FAST) examination. The kidney lies in a plane oblique to the long axis of the body, and the probe usually needs to be rotated slightly counterclockwise to obtain a maximally longitudinal view of the kidney (Fig. 115.1, A). This orientation also helps the ultrasound transducer align the image in between the ribs, thus minimizing rib shadows. The transducer should then be angled anterior to posterior so that it fans through the entire kidney. To obtain a transverse view of the kidney, the probe is rotated 90 degrees in a counterclockwise direction (Fig. 115.1, B). The transducer should then be fanned superior to inferior to image the entire kidney in the transverse plane.

Fig. 115.1 Longitudinal (A) and transverse (B) images of the kidney.