Does KUB play a role in the diagnosis of bladder bowel dysfunction?

Kidney ureter bladder radiography (KUB) is widely used for the evaluation of constipation in children with bladder and bowel dysfunction (BBD); however, there is varying evidence to support its routine diagnostic use. One drawback to KUB is radiation exposure. The dangers of radiation in children are well-documented, and per As Low As Reasonably Achievable, non-beneficial radiation should be avoided. This risk is especially high in children who undergo repeated imaging in the follow up of constipation treatment.

We sought to assess the utility of KUB in diagnosing children with BBD by comparing it to four diagnostic tests and/or validated instruments: the Dysfunctional Voiding Symptom Score (DVSS), Rome IV criteria, rectal diameter on ultrasound (RD), and the Bristol Stool Form Score (BSFS).

We prospectively enrolled a cohort of patients presenting to an academic pediatric urology practice with symptoms of BBD. Severity of stool burden on KUB (mild, moderate, or severe), RD on ultrasound (≥3.4 cm), DVSS, Rome IV, and BSFS were obtained for each patient. All imaging was interpreted by a pediatric radiologist and pediatric urologist. Primary outcomes were the association between the four diagnostic tests and KUB stool burden. Bivariate analysis of all individual variables versus KUB was performed, as well as multivariate regressions to determine if multiple measures were predictive of KUB stool burden when combined.

Between October 2020 and May 2022, 50 patients were enrolled. All children were under the age of 18, with a median age of 8 years (IQR 3-13). 38 % were male. Median BMI-for-age-percentile was 80.8 (IQR 50.3-98.3). When comparing individual variables to KUB in bivariate analyses, it was found that RD on ultrasound is predictive of significant stool burden on KUB (p = 0.03). No other individual variables were predictive. In the multivariate analyses, no combination of tests was found to be predictive of KUB.

We compared the effectiveness of four commonly used diagnostic tests in children with BBD to validate the use of KUB. In conclusion, our results support the use of RD on ultrasound as a non-radiating alternative to KUB to assess stool burden. Data also suggest that KUB for fecal load does not correlate with urinary (DVSS) or bowel (Rome IV, BSFS) symptoms in BBD, and that symptoms scores should still be used independently for diagnosis and monitoring of treatment response.

In conclusion, KUB has a limited role in the diagnosis of BBD.