Circadian variation of voided volume in normal school-age children

Functional bladder capacity (FBC) and urine output are important variables in the management of incontinence and nocturnal enuresis. The lack of reference ranges for FBC vs. age, and the arbitrarily defined time-windows for measuring urine output, impede the clinical use of these variables in children. To solve these impediments, we had 26 girls and 28 boys, between 6 and 12 years of age, collect, measure, time, and sample every voiding, using 72-h frequency-volume charts; all samples were analysed for osmolality and creatinine concentration. Voided volumes show a very wide range (10-550 ml) and a subset that is significantly larger than all other voidings: early morning voidings (EMV). The individual maximum voided volume (MVV) belongs to the category of EMV in 74% of the children. MVV, the measure for FBC, fits the 5-95% centiles that have been published for cystographic bladder capacity for age in normal children; all other voiding are mostly below the 5% centile. Voided volume plotted vs. corresponding urine output rate shows that, with output rates below 50 ml/h, rest-phase bladder filling always results in significantly larger voidings (EMV) than activity-phase bladder filling. Two circadian rhythms seem to be involved, one for urine output, and another for inhibition of bladder contractility. With hourly population averages of individual urine and osmole output rates plotted on a time scale, circadian patterns appear; these patterns are masked when urine output is collected in blocks of 6, 8, or 12 h. Both plots are promising tools for studying the pathophysiology of voided volume vs. urine output, e.g. in children with nocturnal enuresis.