Clinical outcomes in pediatric patients with ureteral stones are correlated with the posterior acoustic shadow measurement on ultrasound: a pilot study
Jessica C. Dai, MD, Barbrina Dunmire, MS, Tony Chen, MD, Jason Reynolds, MD, Michael R. Bailey, PhD, Mathew D. Sorensen, MD, MS, Jonathan D. Harper, MD, Jonathan S. Ellison, MD.
University of Washington, Seattle, WA, USA.
BACKGROUND: The posterior acoustic shadow measurement has been proposed as a more accurate measure of kidney stone size on ultrasound (US) than stone size measured from the echogenic focus. To date, no studies have evaluated this measurement in children or correlated this measurement with clinical outcomes. We assessed the value of the posterior acoustic shadow with respect to need for surgical intervention in pediatric patients with ureteral stones.
METHODS: We performed a retrospective cohort study of pediatric patients presenting to the Emergency Room from 10/2013 - 9/2017 with a discharge diagnosis of nephrolithiasis. Inclusion criteria included at least 1 ureteral stone visualized on clinical US and at least 60 days of documented clinical follow-up. Exclusion criteria included solitary kidney and fevers, sepsis, acute kidney injury, or positive urine culture at presentation. For each patient, US stone size in the largest dimension was captured from radiology reports. All images were assessed by two blinded reviewers for posterior acoustic shadow. Discrepancies in shadow size >3 mm or in the presence of the shadow were arbitrated by a third blinded reviewer. Shadow size was averaged from the 2 most similar measurements. Primary outcomes were need for intervention (“immediate” or “delayed,” within 60 days) vs. spontaneous passage. US stone size and shadow size were compared for all subjects using the sign test and compared across groups using ANOVA. Univariate and multivariate logistic regression analysis was used to identify factors associated with need for intervention. ROC curves were generated to compare the prognostic value of US stone size and shadow size. Classification of stones into clinically relevant size categories (<5 mm, 5-10 mm, ≥10 mm) based on reported US stone size or shadow size were compared using χ2 test.
RESULTS: 44 ureteral stones in 43 patients were included. 20.5% required immediate intervention (average time 0.2 days±0.4), 22.7% underwent delayed intervention (average time 14.8 days ±8.4), and 56.8% spontaneously passed their calculi (average time 16.2 days ±12.7). 56.8% of stones demonstrated a shadow. Reported US stone sizes were significantly larger than shadow sizes (p<0.001). Compared to reported US stone size, the shadow size resulted in down-sizing of 68% of stones by at least 1 size category, (p<0.001). Average reported US stone size and shadow sizes by outcome group are listed in Table 1. Both US stone size and shadow size were correlated with need for immediate intervention on univariate analysis (p=0.018 and p=0.046, respectively) and ROC curves (AUC 0.915 and 0.87, respectively, p=0.63).
| Immediate intervention on admission | Subsequent intervention within 60 days | Spontaneous stone passage | p-value | |
| Number of stones | 9 | 10 | 25 | N/A |
| Average reported stone size on US | 9.58 mm ± 5.09 | 6.25 mm ± 2.97 | 5.5 mm ± 2.09 | p=0.006 |
| % with no shadow | 55.6 (n=5) | 30 (n=3) | 48 (n=12) | p=0.497 |
| Average measured shadow size | 7.33 mm ± 4.76 | 3.38 mm ± 0.87 | 3.63 mm ± 1.02 | p=0.009 |
CONCLUSIONS: Posterior acoustic shadow sizes can be reliably assessed and measured in pediatric ultrasound images, and measure smaller than reported US stone sizes. The larger measure of echogenic focus size may influence initial treatment decisions. Shadow size was an equivalent predictor of need for immediate surgical intervention. Future larger, prospective studies emphasizing imaging the shadow may potentially provide further evidence for the clinical utility of this measurement.
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