A new Viewpoint in the New England Journal of Medicine this week looks at the thorny question of how we distinguish the living from the dead. As the author, bioethicist Dr Robert Truog, points out, being able to clearly make this distinction is an essential function of any society, determining who can be buried, whose wills can be executed, when people can be taken off a ventilator or when their organs can be donated. At first blush, the question of knowing whether someone is alive or dead may seem relatively straightforward – but there’s a surprising amount of disagreement and variation around how death is actually defined.

Take the recent case of Jahi McMath, a 13-year-old girl from California. In 2013, after undergoing a complex tonsillectomy, she experienced a massive haemorrhage which left her with severe, irreversible brain damage. Her doctors proclaimed her brain dead and notified a transplant donor organisation. But her parents didn’t agree, and went to court to oblige Jahi’s doctors to keep her on a ventilator. They argued that whatever the state of her brain, Jahi’s still beating heart meant that she was still alive.

Eventually the courts let Jahi’s parents remove her from the hospital, which issued a death certificate. Jahi was moved to New Jersey whose laws prohibit clinicians from declaring death by neurological criteria if this violates the religious beliefs of the patient. The upshot is that Jahi is legally dead in California, and legally living in New Jersey. She is kept biologically alive with a ventilator, feeding tubes and hormone supplementation, and her body continues to develop. Now aged 17, Jahi has  passed through puberty since her brain injury.

Jahi’s case is rare, but certainly not unprecedented. She is one of what are known as “beating heart cadavers” – people who, after a devastating brain injury, no longer have any brain function, although their hearts can be kept beating with medical intervention. Some of these beating heart cadavers can live on for months or years or, in one case, for two decades.

In most jurisdictions, beating heart cadavers are considered dead for legal purposes. But definitions of brain death vary from country to country and, as can be seen in the Jahi McMath case, even from state to state within a country.

In the UK, for instance, brain death can be declared when there is no activity in the brain stem, even though there could still be activity in the cortex. We are stricter in Australia, where brain death can only be declared when the whole brain is dead. But Australian law leaves it up to the doctors to determine “the creation and prescription of techniques of diagnosis” of death, without further defining how that may be done.

This vagueness has led to controversy in the past. In 2013, Melbourne-based bioethicist Professor Nicholas Tonti-Filippini claimed that patients in ICU were being declared brain dead and having their organs harvested even though they had not been properly found to be brain dead.

“They are diagnosing brain death while there is still some midbrain function. They say. ‘So what?’, because the person is not conscious.” he told Fairfax Media.

But the claims were strongly denied by the Australian and New Zealand Intensive Care Society, which claimed its brain death criteria were “an exemplar of rigour”.

Dr Truog, in his Viewpoint, says the problem with definitions of death stems at least in part from a variance in the way things are defined legally and biologically. Legal definitions are typically defined by “bright lines”, whereas biology tends to be continuous. He gives the example of how people acquire a completely new legal status in society on their 18th birthday, despite the fact that they are biologically and psychologically barely any different from the day before.

Death, he suggests, may be like that. There may be a legal point at which one is dead, but death itself exists on a continuous biological spectrum. There may be some people who have barely any more brain function than Jahi McMath, but who are considered by most jurisdictions as alive. And these patients may be able to live for many years.

The legal definition of death, like so many other legal definitions, is a social construction, Dr Truog says, “based on biological reality but not completely defined by it.”

Jahi McMath’s case remains relatively rare, which prompts Dr Truog to wonder why there aren’t more cases like her. The answer, he says, is that diagnosis of brain death acts as a self-fulfilling prophecy.

“In almost all cases, the diagnosis of brain death is quickly followed by removal of the ventilator or by organ donation, which invariably leads to cardiorespiratory death,” he writes.

You can access the full Viewpoint here.