To compare urine volumes voided and output rates in prepubertal children with and without monosymptomatic nocturnal enuresis (MNE), to investigate the balance between nocturnal urine output and functional bladder capacity.

In 76 prepubertal children with MNE, all voidings were collected over 48 hours: bedwetting volume (BWV), early-morning voiding after a dry night (EMV), and other voided volumes (VV). Output rates were calculated based on volumes voided and time intervals. Data collected in 50 typical prepubertal children were used for comparison. In both populations, holding-exercise volumes (HEV) were also collected, to approximate maximum volume voided (MVV).

Of the 15% total bedwetting events recorded with output rates more than 2 standard deviations above the normal population average, only half met the International Children’s Continence Society criteria for “nocturnal polyuria.” The circadian rhythm of urine output is the same in both populations; during inactivity, low rates and long filling times result in large EMV. BWVs are also produced with low rates, but have shorter filling times. MVV is small for age in MNE, but HEV for age is the same in both populations. Treating MNE with holding exercises needs to be studied prospectively.

The cause of bedwetting might be aborted bladder filling in the circadian inactivity phase rather than nocturnal polyuria.