Brazil’s jailed former president Jair Bolsonaro was recently reported to have undergone three medical treatments to stop a bout of persistent hiccups. While hiccups are usually harmless and short-lived, his case highlights a condition that can, in rare circumstances, become medically significant.

Hiccups are one of the few bodily functions named after the sound they make. The sound is caused by a sudden, involuntary spasm of the diaphragm – a large, dome-shaped muscle that separates the chest from the abdomen and plays an essential role in breathing.

When the diaphragm contracts unexpectedly, air is drawn rapidly into the lungs. At the same time, the vocal cords snap shut, producing the characteristic “hic” sound.

Most people hiccup between two and 60 times per minute. Episodes typically last a few minutes, sometimes a few hours, before resolving on their own. They are usually only a minor irritation.

Doctors become more concerned when hiccups persist. Those lasting longer than 48 hours are classified as “persistent”. If they continue for more than a month, they are described as “intractable hiccups” – a rare and often debilitating condition.

The causes of common, short-term hiccups are usually straightforward. They can be triggered by an overfull stomach, alcoholic or fizzy drinks, very hot or very cold drinks, or emotional states such as anxiety and excitement or stress. These factors irritate or stimulate the nerves involved in the hiccup reflex, briefly disrupting the normal control of breathing.

Given how common hiccups are, it is not surprising that there are countless suggested cures. Some are supported by scientific evidence, while others rely on anecdote and personal experience.

Many of the techniques that appear to work best have one thing in common: they affect how the diaphragm functions. By altering breathing patterns or briefly increasing carbon dioxide levels in the blood, they may help reset the nerves that control the muscle.

Several small studies have examined structured........

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