ABSTRACT

In the infant, a simple reflex controls bladder function without input from the supraspinal centers. A full bladder elicits a detrusor contraction via a signal that is sent along afferent pathways to the spinal cord, which then triggers the appropriate motor response and simultaneous relaxation of the external sphincter muscles. In the absence of an anatomic obstruction, this allows for low-pressure bladder emptying without resistance to flow. The adult pattern of urinary control evolves over time and continence is first achieved by voluntary constriction of the external striated sphincter and later by supraspinal inhibition of the voiding reflex. The adult pattern of voiding is characterized by the absence of bladder contractions during bladder filling and relaxation of the external sphincter just before and throughout bladder emptying. Bladder capacity increases over time to permit the bladder to function as an adequately sized reservoir. In the first 2 years of life, one can predict the bladder’s capacity by multiplying the patient’s weight in kilograms by 8 to estimate the capacity in milliliters. After 2 years of age, bladder capacity can be accurately estimated and expressed by the formula:

Multiplying this value by 30 gives the amount in milliliters.