Diameter of the external urethral sphincter as a predictor of detrusor-sphincter incoordination in children: Comparative study of voiding cystourethrography

Purpose: Voiding cystourethrography is a diagnostic procedure widely used to evaluate lower urinary tract abnormalities in children. In children with and without suspected voiding dysfunction we measured the internal diameter of the external urethral sphincter on voiding cystourethrography to evaluate its diagnostic accuracy as a predictor of detrusor-sphincter incoordination. Materials and Methods: In part 1 of the study 59 boys and 37 girls with normal voiding function underwent voiding cystourethrography. In each child I to 6 voiding cystourethrograms (mean 2.1) were performed as a part of urological evaluation. Thus, a total of 200 voiding cystourethrograms were obtained. Underlying urological disease was primary vesicoureteral reflux in 57 cases, congenital hydronephrosis in 9, urinary tract infection in 6, ureteral anomalies in 11 and other in 13. To determine external urethral sphincter inner diameter on a given voiding cystourethrogram the sphincter was measured on consecutive images and the widest diameter was chosen. In part 2 of the study 43 children with suspected voiding dysfunction underwent voiding cystourethrography and external urethral sphincter electromyography. Electromyography results were comparatively analyzed with external urethral sphincter data. Results: In part 1 an age dependent increase in external urethral sphincter inner diameter was noted in children with normal voiding function. External urethral sphincter inner diameter in mm. was determined by the formula, 0.166 X age in years + 4.31 in boys (p = 0.0001, r = 0.374) and 0.222 X age in years + 2.73 in girls (p < 0.0001, r = 0.595). Overall the incidence of an external urethral sphincter inner diameter of less than 3 mm. was only 4% (8 of 200 patients). In part 2 external urethral sphincter electromyography documented detrusor-sphincter incoordination in 15 children (35%). When we defined an external urethral sphincter inner diameter of less than 3 mm. as a cutoff value for predicting detrusor-sphincter incoordination, the sensitivity, specificity, positive and negative predictive values were 93%, 89%, 82% and 96%, respectively. Conclusions: External urethral sphincter inner diameter has satisfactory sensitivity and specificity as a predictor of detrusor-sphincter incoordination in children. Thus, simple measurement of the inner diameter of the external urethral sphincter on voiding cystourethrography should be recommended in children with suspected voiding dysfunction before performing rather invasive urodynamic studies.