Sudden restriction of blood flow to the brain. Scientists have traditionally explained why people faint this way.
However, some mysteries remain. What causes spontaneous changes in a person’s blood flow, and which parts of the brain play a key role?
new the study A study in mice published this week in Nature provides a deeper understanding of the underlying mechanisms behind syncope.
Researchers theorize that activation of neurons that connect the heart and brain may cause syncope.
“This is the first step in showing that there is more to syncope than reduced blood flow,” said Vineet Augustine, assistant professor of neurobiology at the University of California, San Diego, and one of the study authors. .
“Reduced blood flow certainly plays a role, but there are other brain circuits at play here. It’s not as simple as it is in cardiology textbooks,” he added.
Specifically, the researchers discovered that neurons at the base of the skull send signals from the heart to the brain, causing a decrease in heart rate, blood pressure, and breathing rate. This can lead to the most common type of fainting, what scientists call “reflex syncope,” which can be caused by things like dehydration, blood in the eyes, or standing for long periods of time. There is a gender.
“A lot of times we just wonder what to do about it,” said Dr. Zachary Goldberger, a cardiologist at the University of Wisconsin-Madison School of Medicine and Public Health who was not involved in the new study. ” he said.
“Thanks to these scientists, we are now able to understand the potential of that mechanism, so we can potentially imagine a treatment on the horizon,” he said.
This finding does not apply to syncope caused by underlying heart problems, such as an extremely slow or fast heart rhythm. Those cases are rarer and more deadly, Goldberger said.
Mouse syncope experiment
Further research led the researchers to speculate that syncope may be caused by activation of the vagus nerve, a superhighway of neurons or nerve cells that connect the brain to other organs.
The vagus nerve is part of the parasympathetic nervous system and helps the body rest and relax. Scientists believe that when a person faints, the parasympathetic nervous system becomes overactive, causing heart rate, blood pressure, and breathing to slow excessively.
To find out what role the vagus nerve plays in syncope, researchers examined the internal organs of mice under a microscope. From there, they identified specific neuronal extensions of the vagus nerve that run from the lower chambers of the heart to the brainstem, the lower part of the brain.
“Each neuron has two branches,” Augustine said. “One branch goes to the heart and the other branch goes to the brainstem. So it takes signals from the heart and relays them to the brainstem. It forms a really nice bridge.”
The researchers then tested whether stimulating these specific neurons in mice could induce syncope.
“We’re inserting these tiny fibers into their brains, and we can shine just a little bit of blue light to activate these neurons,” said John, a staff researcher in Augustin’s lab. said study co-author Jonathan Lovelace.
Once the neurons are activated, “these mice will probably walk around for five seconds and then spontaneously collapse and become still,” Loveless said. “After a few more seconds, they get up and start walking.”
The mice showed other typical signs of human syncope. My blood pressure, breathing rate, and heart rate decreased, my pupils dilated, and my eyes rolled back into my head.
“We were able to recreate this classic, iconic eye rotation,” Augustin said. “It also correlated very closely with the time when the syncope episode occurred.”
Researchers also noticed that the activation of neurons caused blood vessels to constrict, reducing blood flow from the heart to the brain. These are two other symptoms of syncope.
But Augustine said there is still more to learn. His team still doesn’t know what activates the neural pathways in real life, or whether their findings translate well to humans.
“There may be more nuance to human syncope,” he says.
What causes fainting in humans?
Prior to the new study, scientists understood that people faint when there isn’t enough blood flowing in and out of the heart, cutting off blood flow to the brain.
“Blood is no longer flowing to the rest of the body, including the brain, and you pass out,” said Dr. Shamay Grossman, an associate professor of emergency medicine at Harvard Medical School who is not involved in the disease. the study.
In most cases, unless you pass out while driving or fall and injure yourself, fainting is not a cause for concern, Grossman said.almost 40% of people Syncope may occur at some point in a person’s life, but it is often triggered by something trivial.
“Blood pools in the legs when you stand for long periods of time or when you are thirsty, and in pregnant women, when the fetus presses on the chest, the backflow of blood to the heart is blocked,” Augustine said. . “The same is true when beginners play musical instruments. There is a lot of pressure on the chest, and there is a tendency to faint.”
Some people faint because of their emotional reaction to hearing bad news or something funny. This is probably because your body is overactivating your parasympathetic nervous system to help you feel calm.
Doctors currently have no way to prevent spontaneous blackout attacks, but targeting the neurons identified in the study could be one approach, Augustin said. For example, doctors can remove or replace certain genes involved in the vagus nerve pathway.
“In the future, as things like gene therapy and targeted nerve stimulation become more prominent in humans, there will be potential targets for exploration,” he said.
