What causes nausea and vomiting?

Here’s another excerpt from my new book, How Your Child Heals. It’s from the chapter on symptoms, and it’s about what causes nausea and vomiting.

Most of us are familiar with nausea, that queasy feeling experience has taught us may soon be followed by vomiting. When that happens, we begin to feel a quiver at the base of our tongue and in the back of our throat. At this point we may be able to suppress the feeling enough to keep from vomiting by swallowing a few times or taking some deep breaths. If none of that works, we soon toss whatever is inside our stomach out through our mouths, after which the nausea is typically improved, at least for a short time. If there is nothing in our stomachs, we may still go through the vomiting reflex–the dry heaves.

Vomiting differs from mere spitting up, what parents of a baby often call a wet burp. Vomiting is a very forceful act involving contraction of powerful muscles in the stomach and abdomen. When a baby spits up it is because the muscular tissue at the junction between her stomach and the lower part of her esophagus is too lax to keep the food inside. We call that regurgitation or reflux of stomach contents. An older child or adult with heartburn is experiencing a version of the same thing, only usually the stomach contents do not make it all the way up into the mouth. Spitting up is simply a local event in the lower esophagus, with the stomach contents running back up the wrong way for a moment. In contrast, vomiting is a complex reflex in which several parts of the brain and the digestive system need to communicate with each other and coordinate what they are doing.

Both nausea and vomiting are controlled by a place in the lower part of the brain in the region we call the brain stem. Regulatory centers for many of our basic reflexes, like the one that keeps us breathing, are located nearby. This fact tells us that vomiting is an ancient and primitive reflex that has been with us for a very long time. Doctors are notorious for devising esoteric and fancy names for anatomic places, but this spot in the brain is called by a very practical term–the vomiting center.

Many things can awaken the vomiting center and cause it to do its job. Signals from the higher centers in the brain where we do our thinking can do it. Anyone who has had a queasy thought after seeing something distasteful can attest to this connection. The links between the vomiting center and the parts of the brain that regulate balance are especially close, which is why a ride in a roller coaster or a bumpy airplane can make you throw up. The vomiting center also is sensitive to mechanical pressure on it, so vomiting is a common symptom when people have increased pressure inside their brain.

The vomiting center also quickly responds to a whole host of things it detects in the bloodstream. Many medications have nausea and vomiting as a side effect. This is a particular problem with some of the drugs we use to treat cancer. We even have drugs we can give to provoke vomiting as their intended effect. Changes in the body’s hormones, such as occurs with pregnancy, can activate the center. The majority of woman will have at least some problems with nausea and vomiting when they are pregnant, especially during the early months.

For a parent with a sick child, the most important things that tickle the vomiting center are those that happen in the digestive tract, since many disorders of the stomach and intestines lead to vomiting. There are nerves located throughout the digestive tract, especially in the upper portions of it, which run back to the vomiting center. These even begin in the mouth, which is why a person who gags when the back of the throat is touched may quickly vomit. For some people, even brushing their teeth can bring this on if they are not careful.

For the stomach and small intestines, any inflammation there sends messages back up the neural network to the vomiting center. If the signals are strong enough, the person will vomit. For children, the most common cause of this is a viral infection, the stomach flu. Intestinal nerves are especially sensitive to stretching. This also applies to the nerves that control nausea and vomiting, so a digestive tract that is stretched full of air and food that is not going anywhere can do more than hurt; it is also primed for the vomiting reflex. We know this is so because, in such a situation, often the simple technique of slipping a tube down into the stagnant lake of stuff in the stomach and upper intestines and sucking it back out will relieve a person’s nausea and vomiting.

The vomiting act itself, though it happens quickly, is an intricate series of events. When the vomiting center sends out the go signal, the stomach muscles first relax, halting any further movement of its contents. The next stage is what is properly called retching, which is several sharp, jerky spasms of the muscles in the chest and of the diaphragm, the powerful muscle sheet that spans the floor of the chest and separates the heart and lungs from the stomach, intestines, and other organs in the abdomen. Part of the retching reflex is to close the vocal cords tightly together. Then comes the actual vomiting. The abdominal muscles squeeze the stomach, the esophagus opens, and whatever is in the stomach comes back out. The vocal cords stay shut, preventing any of the vomited material from getting into the lungs. This is an important protective reflex; when it does not function, stomach contents with all their acid can cause serious injury to the lungs.

We know a lot about what things trigger the vomiting center and how they do it. The particular molecular signals themselves are even known. This information has allowed researchers to fashion drugs that block these signals. These drugs are most effective for the vomiting caused by extremely powerful signals to the vomiting center, such as those that come from cancer treatment drugs. A drug called ondansetron (brand-named Zofran) is an example.

Most parents deal with vomiting children in the context of the stomach flu. For these children, whose vomiting is less severe, doctors generally do not recommend using any of the drugs that suppress the vomiting center. There are several good reasons for this recommendation. The anti-vomiting drugs work on the brain by blocking the action of several molecules that brain cells use to talk to one another, called neurotransmitters. The drugs target neurotransmitters that are particularly abundant in the vomiting center. But these neurotransmitters work elsewhere in the brain, too, and blocking them can cause unwanted side effects, especially in children. There are exceptions to everything in medicine, but since the vomiting from stomach flu is not severe and passes in a day or so, the risk of side effects from these medications generally outweighs the potential benefit of using them.

Is vomiting of any use, and does it help healing when your child is sick? Certainly it is helpful for the body to have a way to get unwanted and toxic material quickly out of the digestive system, and vomiting accomplishes that. Nausea seems a useful thing to have, too, as a way of notifying us to get ready because vomiting is likely to follow.

Until recently doctors deliberately provoked vomiting in children who had eaten something potentially dangerous, and we advised parents to keep ipecac, a drug that does this, handy for such an occasion. We no longer recommend this because the risk of all the retching and throwing up outweighs any benefit of bringing it on. For parents, it is logical to regard vomiting as a natural reflex that may be doing some good in spite of the brief misery it can cause a child. Because the drugs that either block or provoke vomiting can have significant side effects, in nearly all situations it is best to let nature decide when she is going to make use of the reflex.