Causes & Mechanisms

It’s important to remember what’s normal. Normal people, children and adults, DON’T get up to go to the toilet at night. They urinate before retiring, then hold their urine in until awakening in the morning – at which point they usually need to “relieve themselves” first thing. No one really knows for sure what causes bedwetting to persist beyond age 6, but generally there are three factors felt to contribute to differing degrees in different children.

By far the most prevalent is DEEP SLEEP. Many mothers of enuretics will describe a child who sleeps so soundly they can go in and vacuum the bedroom without waking them up! Theoretically, this leads to bedwetting through the failure to generalize learning to the sleeping state. As we “toilet train”, we gradually “learn” to hold in our urine when it’s not an appropriate time to go– at the dinner table, in the car, on the swingset, i.e. whenever we’re not at the toilet. Eventually our brain “generalizes” this knowledge, and applies it in the sleeping state as well. Most of us have enough “brain activity” even in the sleeping state to accomplish this, but some children don’t.

A second factor which often contributes to bedwetting is a SMALL BLADDER. If your bladder capacity is low, then it’s obviously more difficult to “hold it in” till morning. Bladder capacity can be measured by having a child drink a lot, and try to hold it in. When they can stand it no longer, have them urinate in a measuring cup. Normal capacity is about 10cc per kilogram of body weight, or about 1½ ounces for every 10 pounds. If a child is found to have a capacity more that 20% below this, that is considered abnormal and is likely to be playing a role. In such cases, “bladder stretching exercises” may help -although they usually work best as part of a larger behavioral treatment regimen (see below).

Finally, the least common but still very important (and distinctive) contributing factor is BLADDER SPASTICITY. The bladder is a sack-like smooth muscle, and is under “involuntary control”. It starts to contract or “squeeze” in response to stretch, which essentially is a function of how full the bladder is. If unopposed, this contraction would lead to voiding immediately. However, this is resisted by the urinary sphincter, a ring-like muscle under semi-voluntary control at the bladder outlet which, when contracted, blocks the exit. In most people, all the muscle fibers of the bladder start to contract in unison, in a coordinated fashion, when stretched to a certain point. If they meet with resistance (a closed sphincter) after a few minutes they all “give up”, or relax, again in unison. After some time, when stretched a bit further they “try again”, and the process repeats itself until the person relaxes the sphincter and allows the bladder to empty. This is experienced consciously as an “urge” to urinate which come and goes over time. However, in some children the smooth muscle fibers of the bladder fail to act in unison. Once a certain level of fullness or stretch is reached, some but not all of the fibers start to contract. By the time these fibers relax, others are contracting. There is no coordination. As time goes on, the percentage of fibers contracting at any one moment increases. This is experienced by the child as an urge to go which, once it starts, never subsides. It just grows and grows until they give in to it. Therefore, these children tend to consider the urge irresistible. At night they wet the bed, but during the day it’s still easy to tell who they are. They are the one’s whose parents pull over into the breakdown lane to let them go, rather than telling them to wait until they get to their destination or the next rest area.