The role of the renal resistive index ratio in diagnosing obstruction and in the follow-up of children with unilateral hydronephrosis

OBJECTIVE: To assess the role of the renal resistive index ratio (RIR) in discriminating equivocal upper urinary tract dilatation in children, and thus in establishing the need for surgery, in comparison with traditional diagnostic tools. PATIENTS AND METHODS: The study comprised 40 children with unilateral hydronephrosis unrelated to vesico-ureteric reflux, posterior urethral valves, megaureter or a duplex system. In all patients one or more of the following 'indices of obstruction' were positive; an anteroposterior renal pelvic diameter of >20 mm, a half-time diuretic 'washout' (T/2) of > 20 min, a separate renal function of < 40%, and symptoms of obstruction (pain, sepsis). All these variables were measured on admission and after a mean (range) follow-up of 9 (2-24) months. After this period, all patients who were symptomatic or with two or more of the variables above the normal range were considered as obstructed and underwent a dismembered pyeloplasty. The variables were then re-assessed 6 months after surgery. The RIR was evaluated using duplex Doppler ultrasonography with a 3.5-5 MHz transducer, by the same operator. Differences between obstructive and unobstructive unilateral hydronephrosis were estimated from the mean values of the variables assessed and Student's t-test used to determine significant differences. The correlation between the T/2 and RIR before and after surgery was also evaluated. RESULTS: During follow-up the RIR was abnormal in 27 of 30 patients with hydronephrosis who were considered to be 'obstructed'. Twenty-three of these patients, selected for surgery, had a positive diuretic renogram; 11 had loss of differential renal function and 16 had recurrent clinical symptoms. There were significant differences in the mean RIR and T/2 between obstructed and unobstructed patients. Six months after dismembered pyeloplasty, the RIR returned to normal in all patients except three in whom it was previously > 1.1. The diuretic renogram, if initially showing pathology, always became normal. The RIR did not change in patients with an unchanged and severe loss of differential renal function before and after surgery. CONCLUSIONS: In this study the RIR was a good index of obstruction in children with unilateral hydronephrosis and it correlated well with the results of diuretic renography. The specificity of the RIR was reduced whenever there was severe renal damage.