It happens in the blink of an eye: Your breathing muscles contract, your vocal cords clamp shut, and out comes that unmistakable sound.
We all get hiccups from time to time. So do cats, rats and human fetuses. Perhaps you ate too quickly, got too excited or drank something carbonated. Or you are coming out of anesthesia after an operation. But often there's no clear trigger. Doctors don't know what purpose they serve, nor do they know how to make them go away.
In other words, the humble hiccup remains largely a mystery. And just as theories abound on causes and cures, so do home remedies.
"People have very interesting interventions: Scare me, hurt me, tickle me, drink for 90 seconds, drink upside down for 90 seconds," said internist Tyler Cymet. "But these aren't based in science."
Cymet, head of medical education at the American Association of Colleges of Osteo pathic Medicine, performed a five-year study involving 54 hospital patients with hiccups. Beginning in 1995, he tried a wide range of treatments, including breath holding and strong medications, but he ended up with a null result: None of the techniques proved effective in ridding patients of their hiccup spells.
"I think the jury is in that nothing works: It starts and stops on its own, and that's about it," he said.
But why do we do it in the first place? Some researchers propose it is a fetal digestive reflex that guards against breathing in amniotic fluid while in the womb.
Another theory posits that hiccups date all the way back to our amphibian ancestors. The classic pattern of breathing in followed by an abrupt closing of the glottis is seen particularly in tadpoles when they use their gills rather than their lungs to breathe. The tadpole's brainstem tells a flap to close the glottis upon inspiration to prevent water from entering the lungs. This allows the water to pass through the gills. The hiccup reflex may have persisted up the tree of life even though it no longer serves any purpose.
What is agreed upon and well known, however, is the mechanism of a hiccup. Referred to in medicine as singultus, it is defined by a sudden contraction of the diaphragm and intercostal muscles -- located within the spacing between the ribs -- followed by snapping shut of the glottis, the space between the vocal folds within the larynx. The quick spasm of inhalation colliding with the closed larynx causes the characteristic sound and bodily jerk.
Most experts also agree that hiccups involve a neuronal circuit starting with the phrenic and vagus nerves. The vagus nerve extends from the brainstem to the abdomen, while the phrenic nerves send signals from the brain to the diaphragm.
While hiccups are little more than a temporary annoyance for most of us, they can become all-consuming. Coleen O'Lear, a home page editor at The Washington Post, hiccups nearly every day -- sometimes just occasionally, other times in fits.
"When I get them rapid-fire, they are pretty high-pitched and sound ridiculous," O'Lear said. "It is physically uncomfortable."
The 29-year-old has had hiccup spells for as long as she can remember; doctors say they may be linked to acid reflux disease, which she has had since she was a baby. She has tried every home remedy she has heard of: spoonfuls of sugar, holding her breath, deep breaths, drinking upside down, sucking on lemons. Some may work temporarily; most fail completely.
But one thing does work: eating a spoonful of creamy peanut butter very slowly. O'Lear thinks it has something to do with coating her esophagus, but Cymet believes it has more of a mental calming effect.
"The peanut butter is a type of cognitive behavioral therapy, where you're controlling your breathing and thinking about what you're doing instead of getting anxious," he said.
Although her newsroom colleagues are entirely accustomed to them, O'Lear still finds the hiccups frustrating.
"People don't know how to react to it," she said. "Most of the time, they bless me."
The chronic hiccuper who stepped into psychologist Duane Hurst's office two years ago got so fed up with her five-year-run of daily hiccup spells that she initially requested an invasive procedure to crush the phrenic nerve, effectively paralyzing her diaphragm. Hurst, of the Mayo Clinic in Scottsdale, Ariz., thought such a procedure was far too extreme and instead offered to try a technique called heart rate variability biofeedback.
Our nervous system controls involuntary bodily functions, such as heartbeat and blood vessel contraction, via two complementary branches. One branch stimulates responses related to our fight-or-flight instinct -- increased heart rate, dilated pupils, sweat secretion -- while the other initiates the rest-and-restore mode.
Some people are oversensitive to stress stimuli and flip too easily back and forth between the two -- perhaps a beneficial trait when man was a target of predators, but not as helpful in today's world, where it can lead to unwanted anxiety. A quick fight-or-flight response that once saved man from cave bears and saber-toothed cats can kick in at inopportune moments, such as at a meeting with the boss or while merging on the highway.
By measuring the interval between heartbeats, doctors can track the interplay between the two branches and the extent to which they are battling each other: One ups the heart rate, while the other slows it down.
For the woman who had considered having her phrenic nerve crushed, Hurst used an electrocardiogram to detect variability in her heart rate and instructed her to take carefully measured breaths. The idea was to help her find a respiration rate that would activate her rest-and-restore mode, rebalance the nervous system and ease stress.
"Not many people are aware that they can use their breathing in a systematic way," Hurst said. "Each one of us individuals has a breathing sweet spot."
Once she started paced respiration, she began to calm down -- and then, suddenly, no more hiccups. One biofeed back session was all she needed.
"Her hiccups literally stopped when she was in the chair," Hurst said. "She is going on two years of being symptom-free."
This was the first hiccups case for Hurst, who typically uses heart rate variability biofeedback to treat migraines, tension headaches, fibromyalgia, anxiety disorders and irritable bowel syndrome. Similarly, Cymet has used breathing exercises, cognitive behavioral therapy and even yoga or Pilates for the hundreds of hiccupers he has seen over the years. Overall, he estimates a 20 to 25 percent success rate.
But he says most people don't need to worry unless the hiccups interfere with respiration or eating.
"Everybody gets them, but we don't know why -- we don't know if [in terms of evolution] it's adaptive or maladaptive, "Cymet said. "We're still in the dark ages of understanding hiccups."