Michelle Hunter-Behrend MD and Laleh Gharahbaghian MD, FACEP
Leading to approximately 1.2 million annual visits, renal colic is a common presentation amongst emergency department patients (1). Nephrolithiasis remains the most common cause of renal colic. As computed tomography (CT) imaging is highly sensitive and specific for renal stones, many practitioners have adopted the routine use of CT in the initial evaluation of renal colic patients. However, in the younger patient population presenting with renal colic, the ionizing radiation associated with CT imaging is not necessary as an initial assessment.
Although renal ultrasound (US) has a lower sensitivity and specificity for nephrolithiasis, recent studies comparing the use of US versus CT in the initial patient diagnostics show no difference in complications, adverse events, pain scores, or hospitalizations (2). Furthermore, patients with a normal renal ultrasound are at low risk for hospitalization or urologic intervention (1,3). Thus, the adoption of renal ultrasound in the initial work-up of patients with renal colic is an important imaging modality that the emergency medicine physician should be comfortable performing. The complete renal exam consists of evaluation of both kidneys (long and short axis views) as well as the bladder (sagittal and transverse views) (Figure 1).
Here we will review how to optimize acquisition and interpretation of renal ultrasound images.
1. Oblique your probe. It may be particularly difficult to locate and obtain adequate images of the left kidney, often due to bowel gas and rib shadows obstructing the image. To navigate through these rib shadows, rotate the probe so that the probe marker is slightly posterior (towards the gurney). By using this oblique angle, the probe is in plane with the ribs in the intercostal space, avoiding the rib shadows and allowing you to visualize between rib spaces more effectively. (Figure: 2)
2. Go back… way back. Remember anatomy? The kidneys are retroperitoneal organs, and as such, they sit all the way in the back, posterior to all the intra- abdominal organs. The rib spaces are more wide posteriorly. For difficulty assessing the kidneys from a mid-axillary approach, consider a posterior approach. Have the patient roll to their side to expose the posterior axillary line. This may help avoid bowel gas and ribs that can obstruct the view.
3. A deep breath! For a cooperative patient, it may be as simple as having the patient take and hold a deep breath in order to widen the rib space and move the kidney into view. This action is especially important in evaluating the left kidney, which often lies more superior than the right kidney.
4. Use color! Don’t confuse renal pelvis dilation (pelviectasis) with renal vessels. Sometimes when assessing the renal hilum, a prominent renal artery and vein may be mistaken as mild hydronephrosis. These vessels, adjacent to the renal pelvis, should be differentiated from hydronephrosis by the application of color Doppler. The renal pelvis will not have color flow, but the renal vessels will show flow.
5. Compare sides. While it may be satisfying to see hydronephrosis on the side of the patient’s pain, do not stop your search. Hydronephrosis is highly suggestive of an obstructing pathology, like a renal stone, in the setting of renal colic. However, it is important to ensure whether hydronephrosis is seen unilaterally or bilaterally. Bilateral hydronephrosis changes the differential diagnosis. While it may be as simple as letting the patient urinate (then repeating the renal US), bilateral hydronephrosis should prompt the practitioner to think of other etiologies of renal colic than a renal stone.
6. Distinguishing renal cysts from hydronephrosis. When evaluating the kidneys, occasionally renal cysts may be found. These should be differentiated from hydronephrosis by assessing their location and architecture. Renal cysts tend to be located more peripherally, in the cortex. (Figure 3). Furthermore, renal cysts will have a round appearance and not be continuous with the renal collecting system and pelvis (Figure 4). In contrast, hydronephrosis that has extended to the pyramids will connect to the renal pelvis. (Clips One, Two & Three)
7. If at first you do not succeed, give IV fluids and try again. Many patients may come to the ED dehydrated, especially if their renal colic has been associated with vomiting. This may create a false-negative exam when the initial ultrasound is performed early in the patient’s assessment. To increase renal ultrasound sensitivity for hydronephrosis, try hydrating the patient with 1-2 liters of IV fluids then repeat the (4).
8. Look for ureteral jets. When hydronephrosis is seen, an evaluation of ureteral flow from each side should be done to assess for signs of complete obstruction. This is best done when IV fluids are flowing and using color Doppler over the bladder. Ureteral jets will be seen from both the left and right side as color rays from each side. (Figure 6)
In conclusion, instead of ordering a CT for each patient with renal colic, consider trying these tips and tricks to optimize your renal ultrasound imaging and avoid the radiation. Emergency medicine physicians should feel comfortable performing and interpreting the renal ultrasound exam, as this has the potential to greatly reduce the number of CT scans obtained each year.