There are several reasons why a child can have GER. There has been more research on what happens in a child with GER and how to treat it than research into why it happens in the first place.
PAGER parents have several theories. Some are supported by research and some are just ideas.
Immaturity may be the most common cause of "garden variety" acid reflux. Acid reflux is particularly common in premature babies - so common that they are often assumed to have reflux. This seems mainly to be an issue of brain maturity. The brain is supposed to signal the top of the stomach to open when the baby swallows or needs to burp. If the brain signals the stomach to open at other times, the stomach contents bubble up into the esophagus. These inappropriate relaxations are called transient lower esophageal sphincter relaxation's (TLESR's). The part of the brain that controls digestion also controls breathing and temperature control. Newborns, and especially preemies, simply don't have great control over their own bodies. It is not surprising that babies have trouble regulating something as complicated as the digestive system or coordinating all the intricate actions required for eating and breating at the same time.
2. Food intolerance, reactions, allergies
Another problem arises because mammals are not born with fully functional digestive systems. In fact, even if they had teeth and could chew adult foods, babies probably wouldn't be able to absorb the nutrients properly. Some babies just have difficulty digesting some foods and other babies clearly react badly to certain foods. (There doesn't seem to be total agreement on the definition of the word allergy so we usually just say it this way.) Is "difficulty" just a mild "reaction" or is something different happening? We don't know, but we do know that foods do seem to bother many children with acid reflux. Food problems seem to be particularly common in kids who don't grow out of their acid reflux at 12-18 months. In fact, we find that our chronic refluxers often have a strong family history of allergies. There is a series of articles on the topic of food reactions that is available from Scientific Hospital Suppy in Gaithersburg, MD, 301-840-0408. The publication is called "Focus on Food Allergies," but it covers other types of reactions and explains why allergy testing doesn't work very well. Unfortunately, the publication is very technical so you will have to ask your gastroenterologist to translate it. Order a second copy for your doctor.
3. Slow digestion
Any digestive slowness will cause food to hang around in the stomach longer than the typical 30-45 minutes. When food is in the stomach longer, it tends to get very acidic. It also tends to eventually go up if it doesn't go down. The stomach can empty slowly for a variety of reasons. If the child has a very high fat meal or tends not chew food well, the food may stay in the stomach longer for processing. Sometimes the valve at the bottom of the stomach doesn't seem to be letting food through at a normal rate. In some children, the intestines are slow and backing up the whole system. A simple case of constipation can cause or aggravate acid reflux. People with diseases of the intestines may end up with severe acid reflux because their whole system is so slow.
4. Hernias and birth defects of the stomach or esophagus
Children with stomach malformations, esophageal birth defects and hiatus hernias almost always have wicked cases of acid reflux. These conditions are rare but it is routine to check for them. This is the reason that barium studies are usually the first test done on a child with acid reflux. Children with these conditions usually require corrective surgery. Hernia repairs are pretty successful but, unfortunately, it is difficult to get a defective stomach or esophagus fixed well enough to prevent reflux in the future. Kids with esophageal birth defects may be left with chronic acid reflux. Esophageal defects are usually discovered when the baby is only a few hours old, although occasionally, a small hole that lets food into the wind pipe may be missed and the child will be very susceptible to aspiration.
5. Other medical conditions
Many other medical problems have acid reflux as a symptom. Most of them are not subtle. Most kids with these disorders are very sick, indeed. It is very unlikely that a rare disease is causing your child's acid reflux.
PAGER documented families with inherited GERD in 1994 and contacted a team of researcher that proved this crazy idea. It appears that children born to families with a strong history of pediatric acid reflux may have more severe symptoms. This is particularly true if there are other babies with reflux in both mom and dad's families.
There is a common myth that a mother's tension can cause acid reflux. This is simply not true. A mother's tension doesn't help a child who already has acid reflux, but it won't cause acid reflux in a child who doesn't already have it. Since the tension is usually caused by the stress of caring for the child, there is absolutely no benefit from accusing a mother who is already stressed to the limit. A much more useful strategy is to treat the acid reflux more aggressively.
Another common myth is that the LES is floppy or incompetent. This is sometimes the case in older adults, but in children, it is much more likely that the LES simply opens spontaneously. Many doctors still explain reflux as a non-functioning flap over the top of the stomach. This explanation isn't correct, but it gets the point across so simply that many doctors still use it. In reality, there isn't even a flap. The LES is a ring of muscles circling the bottom of the esophagus.
Children do not cause their own GERD. Adults sometimes do through poor lifestyle choices, but not children.