115 - Emergency Renal Ultrasonography

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 and more accurate in measuring stone size. 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, 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 levels 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. 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, whereas CT has consistently shown sensitivity greater than 90% for the diagnosis of ureteral stones. 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. 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. The goal of bedside ultrasound is to identify unilateral hydronephrosis as an indirect sign of obstructing ureteral • The presence of unilateral hydronephrosis in the correct clinical context is used as an indirect finding for the diagnosis of renal colic. Obstructing stones are not typically visualized directly during bedside sonography. • Identification of hydronephrosis in one kidney should prompt evaluation of the contralateral side to rule out bilateral hydronephrosis as a result of bladder outlet obstruction. • Ultrasound measurements of the urinary bladder can be used to estimate bladder volume and identify a distended bladder in patients with suspected urinary retention. • Abdominal aortic aneurysm should always be included in the differential diagnosis of renal colic. Emergency practitioners should maintain a low threshold for performing screening ultrasound of the aorta in patients with risk factors for an aneurysm. KEY POINTS