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DEADLY BITE: For people suffering from the rare condition, swallow syncope, eating a sandwich could cause cardiac arrest.
iStockphoto.com/JoanVicent

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.