Stephen Hawking death: How did physicist live so long with motor neurone disease?

When he was diagnosed at 21 with a type of motor neurone disease, amyotrophic lateral sclerosis (ALS), Professor Stephen Hawking was given two years to live.

But his death 55 years later has sparked an outpouring of grief from the scientific community and the wider public. It is testament to how he defied the odds, and helped define our understanding of the universe.

Hawking’s intellectual achievements have become almost mythical in the face of the disease which progressively shut down his limbs, confining him to a wheelchair, paralysing his muscles and rendering him unable to speak.

ALS is more accurately a group of conditions, which have the shared characteristic of the gradual degeneration nerve cells in the brain and body – the cells which coordinate our voluntary movement.

This includes walking and moving the arms, but also extends to the core muscles and the diaphragm, which enables us to breathe, and the muscles used for speech and swallowing.

In most cases diagnosis comes much later in life – typically between 55 and 75 – and patients typically die within three to five years.

Death generally comes from respiratory failure, as breathing shuts down, though malnutrition and dehydration from difficulty swallowing also make it harder to stay healthy.

Only around 10 per cent of people survive more than 10 years with the disease. But there are cases at the extreme end of the statistical curve – as in all diseases – who defy all expectations. There is no evidence that more intelligent people live longer with ALS.

Hawking is thought to be one of its longest survivors.

Hawking has been cared for by his wives and children through the years, and has repeatedly credited the NHS for its part in his care, saying: “I would not be here without it.”

Later on, he was receiving 24-hour care from three nurses who would manually feed him, and he had a respirator to help him breath.

The neurones that make up the brain are grouped into specialised regions.

In people without ALS, upper motor neurones that are found in the brain send messages along the spinal cord to the lower motor neurones in the spine. Those stimulate muscle movement.

In ALS the upper and lower motor neurones both die. This cell death been linked with the accumulation of abnormal clumps of protein molecules within the nerve cells.

As the neurons die off, the ability to coordinate movements is impaired and weakens. That initially causes involuntary twitching, and eventually sees them waste away unused.

In Hawking’s case, he communicated through a voice computer for much of his life, initially using his hands to control it.

As his condition progressed, he was left communicating through movement in his cheek, holding conversation at a rate of six words a minute.

He was also left with his incredible mind, though not all patients are spared the death of nerve cells responsible for non-motor function.

There is no definitive test for ALS. It is instead diagnosed from the myriad symptoms and progression of the condition.

While there are shared hallmarks, it comes in many forms: early and late onset, fast and slow progressing, around 10 per cent of cases are thought to be due to an inherited genetic link.

Some experts have speculated that Hawking’s may have had a juvenile onset form – though this is typically diagnosed in the teenage years – and this type can progress very slowly, with patients alive today in their sixties.

But they are a just a small minority of all cases, and that reality has been something which spurred Hawking on.

“I have lived most of my life in the expectation of an early death, so time has always been precious to me,” he said in 2006. “I have so much that I want to do. I hate wasting time.”

But in his disease, as in his scientific endeavours and his work campaigning for other causes, Hawking’s life was a remarkable outlier.