Hiccups are experienced by almost everyone during their lives – bouts of hiccups have even been shown to occur in fetuses in utero and in a wide variety of animals. The medical term given for hiccups is "singultus".
The physiology behind the hiccup is complicated. This motor action is an involuntary reflex controlled by the medulla – it involves the coordinated action of the diaphragm, the muscles that govern opening and closing of the trachea and the nerves that innervate these muscles. It seems that there is a hiccup centre in the medulla from where efferent nerve fibers travel to the diaphragm.
In a hiccup, the firing of this centre leads to the intense activation of the respiratory muscles followed by the rapid closure of the glottis. This closure occurs approximately 35 milliseconds after the activation and with a noise produced at the top of the trachea that is the characteristic hiccup sound.
It is interesting to note that there is not yet a named function for the hiccup. Even though the hiccup reflex results in both repetitive and great stimulation of the inspiratory musculature, it normally does not serve any purpose in respiration because of the prompt closure of the glotus following the intense inspiratory drive.
There are theories that the hiccup has a role in the gastrointestinal tract to remove air from the stomachs from young mammals – this is thought because of the numerous gastrointestinal stimuli observed to elicit them.
It could be that the hiccup is a fetal development tool – although there is a high incidence in utero and infancy, suggestions of its role in suckling, clearance of meconium or strengthening of respiratory musculature do not appear completely satisfactory.
It is just as likely however that the hiccup is a vestigial reflex with a no longer present purpose. The cause of hiccups has been linked to the following:
- alcohol consumption
- excessive smoking
- a temporary swollen stomach – this can be caused by overeating or eating too fast, drinking hot or carbonated drinks, or swallowing air
- eating too quickly or eating spicy foods
- sudden emotions, such as stress, fear or excitement
- any illness that irritates the nerves that control the diaphragm
- hyperventilation (because this is when carbon dioxide levels in the blood decrease)
While hiccups are not harmful and generally only last for minutes, it is possible for them to last longer than 48 hours. In this case, they are referred to as "persistent" hiccups. Hiccups are also termed as “intractable” if they persist for longer than a month.
It is possible for certain disorders to trigger long-term hiccups. These can interrupt with the body’s usual control of the hiccup reflex and can include:
- multiple sclerosis
- traumatic brain injury
- particular drugs such as barbiturates, anesthesia (following procedures particularly if they involve abdominal organs), corticosteroids, benzodiazepines and tranquilizers
It is most common for cases of long-term hiccups to fade away without any medical treatment. In cases where there is an underlying illness, treatment of this can be effective.