A little over a year ago, a 25-year-old woman visited University Hospital Birmingham in England complaining of frequent 10-second bouts of nausea and lightheadedness, which was sometimes so intense it caused her to pass out.

"She was very thin, she was pale, a very sensible young woman," says Una Martin, a clinical pharmacologist at the hospital assigned to the case shortly after the woman's first visit. She had no history of smoking, heavy drinking or psychiatric disorders. Strangely, her fainting episodes coincided with eating sandwiches and drinking fizzy beverages.

By the time Martin saw her, the woman had been bounced from one physician to the next and hospitalized twice, once in 2001 and once in 2007—but her condition remained a mystery.

"She felt she had been passed from doctor to doctor [and] was beginning to feel it was all in her head," Martin says. Some of the physicians she had visited thought she was suffering from petit mal seizures—brief epileptic brain episodes characterized by loss of consciousness but without the violent shaking associated with the larger grand mal seizures. Yet neurological tests showed no evidence of epilepsy.

Martin suspected there might be something wrong with the patient's heart, so she sent her patient home with an external loop electrocardiogram recorder, a device worn on the wrist or waist to monitor heart rhythm for extended periods of time. Whenever the young woman felt lightheaded, she would press a button on the device, causing it to record one to two minutes of heart activity. After monitoring her heart for a week, the woman returned the device to Martin who interpreted the results.

Her findings: during dizzy spells (following eating or drinking) the patient experienced complete atrioventricular block, a condition in which the electrical current traveling from the top two chambers of the heart (atria) to the bottom two chambers (ventricles) is disrupted. "I picked this up and referred her to the cardiologists," Martin says.

The cardiologists kept the young woman in the hospital where her heart rate and blood pressure were continuously monitored: "We offered her a sandwich. Atrioventricular block, lasting longer than two seconds, ensued; the patient felt lightheaded, as though she were about to faint," they reported in The Lancet. "We diagnosed swallow syncope."

Swallow syncope? ScientificAmerican.com asked Douglas Zipes, an Indiana University School of Medicine cardiologist and editor in chief of Heart Rhythm, to explain the bizarre condition.

What is swallow syncope?

It is when swallowing induces loss of consciousness. The swallowing triggers a vagal [nerve] reflex from the esophagus to the brain and back to the heart [that] is excessive in its reflex strength. The heart is very sensitive to that reflex response…[and this leads to] a slowing of the heart rate so as to create insufficient blood flow to the brain, causing someone to black out. No one knows why they have these very unusual reflexes.

You said that the swallowing triggers a vagal reflex? What is the vagus nerve?
The vagus nerve is a nerve that generally slows the heart beat, reduces blood pressure and, in general, opposes the sympathetic nerves, the characteristic "fight or flight" reaction [aka stress response]. The vagus is important in the GI [gastrointestinal] tract to increase motility [movement]. Ordinarily, when you swallow a message is sent to the brain, [which then sends] a swallowing message to the esophagus saying to increase esophageal motility. For some reason, this message from the brain is also going to the heart and telling it to slow. And why that cross wiring happens, I don't know.

What is happening in the heart when it receives this slowing message from the brain?
The electrical signal doesn't make it [from the heart's top chambers] to the bottom [ones]; it's called heart block…. Heart block occurs because the vagus sends its impulse to the AV node [the atrioventricular node, a specialized section of tissue that relays electrical signals through the heart], causing it to block conduction from the top chamber to the bottom chambers. … Or, the vagal impulse can stop the normal pacemaker in the heart called the sinus node.

Why would this reflex be triggered by certain foods and not others?
It may take greater distension [expansion] of the esophagus [to stimulate the response, which may occur] with bulk food rather than liquids.

How common is the condition? Do women get it more than men?
It's uncommon but not unheard of. There have been many isolated reports. … It can [also] occur with coughing—it is probably a similarly induced vagal mechanism. … It's so infrequent that I don't know if there is a preference [for women or men].

For several years, doctors could not get the correct diagnosis for this woman. How do you explain that?
All you need to do is to record an electrocardiogram while she's swallowing and you'll get the answer. [Referring to the electrocardiogram results published in The Lancet, Zipes pointed to a large gap between spikes on the top line of the diagram. This gap indicates a pause in the heartbeat lasting 2.129 seconds. The line below represents the woman's heart rhythm after cardiologists gave her an artificial pacemaker to maintain a steady beat].