Supine versus prone positioning for ultrasound evaluation of postnatal urinary tract dilation in children

Ultrasound (US) is used in the initial evaluation and surveillance of urinary tract dilation in children. Urinary tract dilation is diagnosed in 1-2% of all pregnancies during routine prenatal sonography with technological advances in US imaging. Urinary tract dilation classification systems, including the 2014 multidisciplinary consensus, assess anterior-posterior renal pelvic diameter and calyceal dilation. There is no consensus regarding optimal patient positioning - supine versus prone - during US assessment of urinary tract dilation.

We performed this study to determine whether there is a significant difference in the measurement of the anterior-posterior renal pelvic diameter, presence of calyceal dilation, or resulting urinary tract dilation consensus score obtained between supine and prone positions.

Two raters retrospectively reviewed renal bladder US exams of children with urinary tract dilation of one or both kidneys. We included technically adequate US examinations of orthotopic kidneys that were imaged in both supine and prone positions; we excluded children with renal anomalies or prior surgery. Anterior-posterior renal pelvic diameter measurements as well as central and peripheral calyceal dilation were documented in both supine and prone positions. A postnatal urinary tract dilation consensus score was assigned to each kidney based only on these features.

Urinary tract dilation in either the supine or prone position was performed in 146 kidneys (69 right, 77 left) in 89 children. Median age was 0.26 years (interquartile ratio [IQR] 0.08-0.61 years). Female-to-male ratio was 1:3 (21/89). The interclass correlations of the anterior-posterior renal pelvic diameter were 0.88 and 0.87 in the supine and prone positions, respectively, without significant differences (P=0.1). When comparing all kidneys together, the mean anterior-posterior renal pelvic diameter was 7.1 mm (95% confidence interval [CI] 6.4-7.8) in supine and 7.9 (95% CI 7.1-8.7) in prone, with a mean difference between the measurements of 0.83 mm (95% CI 0.3-1.4; P=0.16). Central calyceal and peripheral calyceal dilation were more commonly found in the prone position versus the supine position in both kidneys. Central calyceal dilation was observed in 15 cases in the prone but not in the supine position. In one kidney, it was seen only in the supine but not in the prone position. Overall the urinary tract dilation score differed between positions in 10.3% (15/146) of cases, most of them (14/15) with a higher score in prone compared to the supine position.

Scanning in prone position tends to more frequently show calyceal dilation and a greater size of the anterior-posterior renal pelvic diameter, resulting in higher urinary tract dilation classification scores, with almost perfect interobserver agreement.