Kidney Stones 03: Imaging

This is part of the four part PEMBlog series on kidney stones. Throughout you’ll learn about diagnosis and management of an increasingly recognized problem in the Pediatric Emergency Department.

I’m going to cut to the chase here and recognize the fact that there is an increasing amount of controversy over non-contrast CT or ultrasound as the imaging modality of choice in the identification of stones. Before I get to that I wanted to make sure I answered the following question.

Who should get imaging?

In short, any patient in whom the diagnosis is uncertain and in any patient in whom you are diagnosing a stone for the first time.

Imaging studies

Plain Radiography

Though stones can be seen on plain films (calcium, strive and cysteine stones are radiopaque) some can’t (rate stones are radiolucent). Small stones can lie atop bony structures in the image plane and as such the sensitivity does not exceed 60%.

Non-contrast CT

Let’s get it out of the way, CT is the most sensitive test to detect stones, even those not able to be seen by ultrasound (stones ≤1mm) and detection of obstruction or anatomic abnormalities. CT is fast (2 minutes) and usually won’t require sedation. Even with ALARA the risk of radiation exposure is increasingly recognized. Protocols exist, and are now used in many centers.

Renal stone on CT

Ureteral stone on CT

Ultrasound

The ultrasound should include the kidney, filled bladder (Iv fluids, PO and wait, or holy fill – your choice!), and adjoining portions of the ureter. In general, it is operator dependent and it isn’t as good as CT at detecting ureteral stones or those <5mm. Passerotti et al. noted that the sensitivity was 76% and the specificity 100%, which makes sense since if you see a stone you’ve made your diagnosis naturally. In their analysis the mean size of missed stones (needed to get a CT to make the diagnosis) was a little larger than 2 mm. In general U/S missed stones in up to 40% of pediatric patients. Obviously more studies have been performed in adults, and thus in a comparative effectiveness study from Smith-Bindman in NEJM the argument was made that U/S could be used in ⅔ patients with suspected stone. In lieu of reading the article I recommend you check out this live video chat with the authors posted on Academic Life in Emergency Medicine.

Is identification of hydronephrosis a proxy to the unseen ureteral stone in ultrasonography?

If your pre-test probability is high then yes. However, it isn’t necessarily an admission criteria. As noted on a post entitled Top 10 reasons NOT to order a CT scan for suspected renal colic on Academic Life in Emergency Medicine the authors noted:

Hydronephrosis on acute presentation in the ED does not mandate an intervention. The evidence supports that hydronephrosis does not lead to irreversible loss of renal function if corrected within a few weeks. Ultrasound may not be as sensitive as CT for finding a patient’s kidney stone, but it is does very well at diagnosing hydronephrosis.

So, tell me, which test is better?

Well, technically it is the CT according to the test characteristics. However, ultrasound is safer and the best first imaging choice in pediatric patients. And ultimately, you need to ask your self the following question? Am I looking to identify the stone – or do I want to predict those patients that need urologic interventions? Interestingly up to 1/12 patients that have a urologic intervention have a complication – sepsis, steinstrasse (stone street), stricture, ureteral injury, and UTI. If the patient has had stones before, and you suspect that this subsequent presentation in front of you is also a stone then you can possibly eschew imaging anyway. After all, the rate of spontaneous stone passage (this includes patients that get IV fluids and meds) for those <5mm (most stones in kids) is near 100%.