Rapid oral hydration results in hydronephrosis as demonstrated by bedside ultrasound

To determine whether vigorous oral hydration (20 mL/kg) causes hydronephrosis as determined by bedside ultrasound.

We conducted a prospective laboratory trial in 35 healthy volunteers weighing less than 90 kg and between the ages of 18 and 50 years. The right kidney of the volunteers was scanned by emergency physicians at time 0 both before and after voiding, and the volunteers then drank 20 mL/kg of bottled water. The kidney was scanned in the transverse and sagittal planes both before and after voiding at 60 and 90 minutes after completion of the water load. The scans were interpreted by a physician trained and credentialed in emergency ultrasound, blinded to the volunteers' identity, the time of the scan, and the volume of urine voided by the subject. Images were rated as to the degree of hydronephrosis according to literature-established criteria, as follows: grade 0=no hydronephrosis, grade 1=mild, grade 2=moderate, and grade 3=severe hydronephrosis.

Hydronephrosis was present in 3 (8.6%) of the 35 subjects at time 0 (prehydration), 24 (68.6%) at 60 minutes, and 20 (57.1%) at 90 minutes. Overall, hydronephrosis occurred at least once in 28 (80%) of the 35 subjects after oral hydration compared with 3 (8.6%) of the 35 subjects before hydration. Hydronephrosis was found to be significantly related to forced hydration for all posthydration times (60 minutes, 90 minutes, and 60+90 minutes combined) versus prehydration time 0 (P <.001).

Without prior fluid intake, even mild degrees of hydronephrosis were relatively uncommon, and seen in only 8.6% of study patients. In the presence of vigorous oral hydration, however, mild or moderate hydronephrosis is a frequent occurrence seen at least once in 80% of our study of healthy volunteers after hydration. Caution is warranted in this setting when interpreting mild or moderate hydronephrosis found on bedside ultrasound by emergency physicians.