Why Do People Hiccup?

The hiccup is not only a persistent annoyance of human life, but one that affects other mammals as well. Despite centuries of medical research, the exact causes of hiccups are still not very well understood.

While hiccups usually pass quickly - after a few minutes, some bouts can last for hours or longer. In rare cases, persistent hiccups can last for several days or even months. The longest recorded bout of hiccups lasted for almost seven decades, from 1922 to 1990.

One "hic" in a bout of hiccups is actually a rapid sequence of events. First, a misfiring signal from one of the nerves that control breathing causes the muscles of the diaphragm to rapidly contract. This contraction usually occurs on just one half of the diaphragm, but occasionally affects both sides.

This contraction expands the lungs, causing a quick intake of air. However, the airflow is interrupted 35 milliseconds later by a sudden closure of the glottis - the flap of tissue in the back of our throats that covers the windpipe to prevent choking. It is this intake of air and sudden interruption that produces the "hic" sound.

The Cause of Hiccups

Although the exact cause of hiccups is not fully understood, the most likely explanation is an irritation of either the vagus or phrenic nerve that prompts the nerve to misfire. This misfiring signal to the diaphragm then sets the hiccup reflex in motion. Since these nerves attach to the diaphragm and stomach, many cases of hiccups are thought to be caused by overeating, eating of fatty foods, alcohol consumption, and carbonated beverages.

Hiccups generally go away on their own. This process can sometimes be expedited either by stimulating the vagus nerve to the stomach (by drinking water or putting sugar on the tongue, for example) or by building up CO2 in the bloodstream (by holding one's breath or breathing into a paper bag). Though many home remedies exist, there is little conclusive evidence to indicate whether these are indeed hiccup cures or merely placebos.

Why an air-breathing animal such as humans would have a reflex that expands the lungs and then closes the airway certainly seems mysterious. The hiccup serves no beneficial purpose, functioning only as an occasional annoyance. The hiccup only makes sense, to paraphrase Theodosius Dobzhansky, in light of evolution.

In all vertebrate animals, from fish to frogs to cats to humans, the sequence of muscle contractions that produce breathing is controlled by the brain stem. In gilled fish, the signals are carried by short nerves from the head to the gills. In humans, the signal must travel down the vagus and phrenic nerves from the base of the skull, through the spinal column, and into the chest cavity before reaching the diaphragm. This long path creates ample opportunity for a misfire somewhere along the line, creating the nerve spasm that triggers the hiccup.

The "hic" itself is also most likely a vestige of our ancestors, dating back to the evolution of lungs in gilled fish and amphibians. According to Neil Shubin, paleontologist and author of Your Inner Fish, the hiccup pattern of lung intake followed by closure of the glottis is virtually identical to that seen in tadpoles today, which can breathe with either lungs or gills. When breathing water, tadpoles gulp the water in through their mouths, then shut the glottis to force the water through their gills. The hiccup in mammals may be the body attempting to breathe through gills that haven't existed for hundreds of millions of years.

Though hiccups are still largely a medical mystery - and are quite often an annoyance - they may just hold one of the keys to understanding who we are and how our species evolved.