University of Virginia Health System

 Why do children become constipated?

Many parents have been told their child became constipated because he or she drinks too much milk or eats too much cheese. While what we eat clearly effects our bowel habits, diet alone is usually not the cause of chronic constipation. As a group, children who develop chronic constipation tend not to drink as much fluid as children who do not become constipated, however in most cases, it is not until the child begins to associate pain with passing bowel movements that constipation develops.

While there are certainly other reasons that children may become constipated, they are all quite uncommon.

1. Hirschsprung's disease
2. Neurological abnormalities
3. Hypothyroidism

Most of the time, the pain with bowel movements starts when a child passes a very big or hard bowel movement. There are lots of reasons this might happen:

• In young infants, a severe case of diaper rash may cause some small tears or rips to develop at the anus. These are called anal fissures. These rips can be extremely painful - like "paper cuts" on your fingers. Older children may develop anal fissures following a bout of diarrhea or when they pass a very large bowel movement that forces the anal sphincter to open too wide.
• In young infants, when their diet changes from breast milk to formula, or from formula to cow's milk, their bowel movements often become much harder. The same thing often happens in older children when when they begin eating solid foods.
• As toddlers go through toilet training, they may hold back too long causing their bowel movements to become large and/or hard. The same thing may occur when children enter pre-school or kindergarten because many children will not go to the bathroom at school (lots of adults won't go to the bathroom at work or in a public place).
• Following a bout of diarrhea, some children get a little dehydrated and as the diarrhea goes away, the intestine does what it is supposed to do and absorbs salt and water. This causes the bowel movement to become more harder. This can also happen following surgery or some other traumatic event.

It really doesn't matter why the pain starts. . . what is important is that the child passes a big and/or hard bowel movement and it hurts! Children are smart people, and if something hurts, they at least wonder whether they should do it again.

Once children begin to be afraid of passing bowel movements, the cycle of chronic constipation has begun. While that child may continue to pass bowel movements fairly regularly, if it hurts when they go, they often hold back . . . that is, they don't completely empty themselves when they go to the bathroom. Some people call this "with-holding". Because the child is holding back, they very slowly fill up their large intestine with stool and stretch it out of shape. If you stretch your intestine suddenly, it hurts! Most people call this type of pain "cramps". However, if you stretch your intestine slowly, it doesn't hurt and you don't get cramps. When you stretch the intestine slowly, the walls of the intestine relax by doing something we call "accomodating". As the large intestine slowly stretches, it gets bigger and bigger. Doctors often call this "megacolon" meaning the large intestine is too big. This explains why children with chronic constipation can pass bowel movements that are extremely large . . . often bigger than bowel movements that fully grown adults pass; their large intestine has gotten stretched out of shape.

Since children with chronic constipation chronically "hold-back", they don't regularly empty the lower part of their intestine. This means that their rectum is usually full. Normally, when we pass a bowel movement, we completely empty the bottom of our large intestine (rectum). This means that most of the time, the very bottom part of the large intestine (the rectum) is empty. Once a day, twice a day, or every other day, some stool moves into the empty rectum and stretches it. It is this stretching that gives us the "feeling" or "urge" to go to the bathroom (figure at right). When things are working correctly, the "urge" to go to the bathroom comes on slowly and it doesn't hurt. You respond to the "urge" by going to the bathroom and passing a bowel movement which empties your rectum.

Since children with chronic constipation almost always have stool in the rectum, the nerves that send the signal to the brain are constantly being stimulated; they are constantly getting the signal to go to the bathroom. Over time, the child "learns" to ignore this signal. This is not a conscious decision, but rather something that just happens. It is much like sitting in a room with a buzzing light. For the first several days, you hear the buzzing, but after a while, you learn to ignore the buzzing . . . you just tune it out. After a while, children tune out the signal to pass a bowel movement. Once this happens, the urge to go to the bathroom comes in a very different way. It comes when the large intestine gets critically stretched. It gets stretched so much that it basically says "I can't take it any more!" - this feeling hurts, it is cramps.

In most children, this type of urge comes on quite suddenly and is VERY uncomfortable. When young children get this feeling, they may become very unhappy, begin sweating or become pale, disappear into a quiet room or closet, or grab hold of the back of a chair or another piece of the furniture and stand on their tip toes. These are all responses to the pain they are experiencing. They are having cramps and a tremendous urge to pass a bowel movement, but because of the pain associated with passing bowel movements, they are holding back. They are not holding back because of spite but rather, out of fear!

Eventually, the child will pass a bowel movement, but it often is very large and very hard, so there will be lots of pain. The pain will just reinforce the child's fear of passing bowel movements so the problem will go on and on and on and on . . . . as the cycle of pain and fear continues, passing bowel movements becomes more and more abnormal.

People who don't have trouble passing bowel movements don't think about it much, but passing bowel movements is a very complicated process.   Basically, three things must happen for someone to successfully pass a bowel movement:

1. We must get the urge to pass a bowel movement.
2. Once we feel the urge and decide to go, we increase the pressure in the large intestine to push the bowel movement out. To increase the pressure we must use many different muscles working together. We take in a breath and push down with our diaphragms. We tighten all the different muscles of the abdominal wall and push downward.  At the same time we are pushing downwards, we are lifting and relaxing a number of muscles in the pelvis.
3. At the same time we are pushing or straining, we must relax the external anal sphincter. This is the muscle at the very bottom of the rectum that keeps stool inside the intestine.

The next time you go to the bathroom, pay attention to what you are doing - you might be surprised!

Not surprisingly, in children suffering from chronic constipation, the coordination of these processes often gets very confused. First, they often do not get the urge to pass a bowel movement in the normal way, but rather, their urge often comes as pain or cramps. Second, when they start straining, rather than using all of their muscles together to push the bowel movement out, they often push with some muscles and pull with others - they grunt and strain and seem to push as hard as they possibly can, but they are usually fighting against themselves. Finally, because they are pushing and straining so hard, rather than relaxing the external sphincter to let the bowel movement out, they often squeeze or tighten the sphincter. As a result, they must generate enough pressure to force the bowel movement through the closed sphincter. This explains why some children with chronic constipation sometimes pass very thin bowel movements.