Small functional bladder capacity (FBC) is now known to play a role in the pathogenesis of enuresis. For some time, it was considered a less likely explanation for enuresis in children without daytime symptoms, but studies confirmed that children without daytime symptoms may have a low nocturnal bladder capacity and that this is a very common factor in enuresis.
In a study by Mattsson and Lindstrom, FBC was positively correlated with nighttime urine output. [10] It has been theorized that children with enuresis may maintain a smaller nocturnal bladder volume and that this situation may condition the detrusor muscle to contract at a lower volume. According to this theory, the low nocturnal bladder capacity is a consequence of enuresis rather than a cause.
Bloom et al suggested a problem with the external urethral sphincter as a possible cause of low nocturnal bladder capacity, [11] noting that the control of voiding rests at the external urethral sphincter, where constant activity is present as a guarding reflex to preserve continence. They speculated that the activity of the external urethral sphincter might fall below a critical level during sleep and thereby trigger a detrusor contraction.
Chronic constipation may also lead to reduced bladder capacity due to accumulation of stool in the distal colon.
