Are we accurately predicting bladder capacity in infants?

Estimating bladder capacity is an important component in the evaluation of many urological disorders. For estimates to be of clinical value, precise reference ranges are needed. While accepted reference ranges have been established in adults and older children, none have been validated in infants. We endeavour to determine the normal bladder capacity of children less than 1 year of age.

We retrospectively reviewed the charts of children aged 0 to 12 months with cutaneous stigmata of spinal dysraphism who were referred to the urology clinic to rule out tethered cord between October 2004 and July 2011. Patients with normal urologic assessment, who did not have surgery during the time they were followed, were included in the study cohort. Urodynamic studies were performed using the Laborie Medical Technologies UDS-600. Bladder filling occurred via a catheter at a rate of 10% of the expected total bladder capacity/minute. Bladder capacity was defined as the volume of filling when the child voided around the catheter. We collected data, including age at urodynamics, bladder capacity, detrusor pressure at capacity, bladder compliance and length of follow-up.

In total, 46% (84/183) of patients had a normal urologic assessment and met the inclusion criteria. The median age was 9.0 months (interquartile range [IQR] 6.8-11.0). The average bladder capacity was 48.9 mL (standard deviation [SD] 32.8) and the mean detrusor pressure at capacity was 8.5 cmH2O (SD 10.0). Mean compliance was 14.1 mL/cmH2O (SD 13.6). The average length of follow-up was 40.7 months (SD 26.2) and during this interval no patients were found to have urologic or neurologic abnormalities and none underwent tethered cord release.

Bladder capacity in infants with a median age of 9.0 months was found to be 48.9 mL. This is less than half of the volume predicted by a commonly employed formula. A novel method of estimating bladder capacity in infants is required.