March 14, 2018 03:07 PM

The world lost one of its most brilliant scientific minds Wednesday, when legendary physicist Stephen Hawking died at age 76.

Hawking’s cause of death was likely amyotrophic lateral sclerosis, or ALS, a neurodegenerative disease that wears away at nerve and muscle function over time. He was first diagnosed with ALS more than five decades ago, at age 21, and was initially given just a few years to live — making the very nature of his long, illustrious career as much of a scientific marvel as the theories and discoveries it yielded.

Here’s what to know about ALS, and what may have allowed Hawking to beat the odds for so long.

What is ALS?

ALS, also known as Lou Gehrig’s disease, is a progressive neurodegenerative disease that erodes motor neurons — cells in the brain and spinal cord that control muscular function — until it becomes difficult or impossible for a person to walk, talk, speak, swallow and breathe, according to the ALS Association. ALS symptoms often begin with slurred speech or muscle weakness and twitching, according to the Mayo Clinic, and get worse over time. The rate at which a person’s condition degrades can vary quite a bit, though the average survival time is three years after diagnosis, the ALS Association says.

While there’s no cure for ALS, the condition can be managed with medication, physical therapy, occupational therapy and speech therapy. Some patients also use ventilators to assist with breathing.

Who gets ALS?

Doctors aren’t totally sure what causes ALS in most cases, though it does appear to have a genetic component in some people, according to the Mayo Clinic. It’s usually diagnosed in people between ages 40 and 60, and men seem to be more likely than women to develop the condition, at least before the age of 65, the Mayo Clinic says.

How rare is Hawking’s longevity?

Quite rare. Just 5% of ALS patients live longer than 20 years, according to the ALS Association, and it’s virtually unheard of to survive for 50 years or more — though North America’s longest-living ALS patient, a Canadian named Steven Wells, has had the condition for almost 40 years.

“This is fairly untypical,” says Lucie Bruijn, the ALS Association’s chief scientist, adding that she is not aware of anyone who has survived ALS longer than Hawking did. Hawking was also able to evade the dementia that some people with ALS experience toward the later phases of the disease, she says.

How did Hawking live so long with ALS?

Researchers aren’t sure, Bruijn says. “ALS is a complex disorder, and every journey is so incredibly variable,” she says. “We are working with teams globally on these very big data efforts, trying to understand each person’s clinical journey, what their genetics look like and what they were exposed to.” From that, she says, they’re “trying to figure out the puzzle.”

With only a few cases of extreme longevity on record, Bruijn says the sample size is too small to draw concrete conclusions about the factors that allow people like Hawking and Wells to live so long, though it’s probably some combination of genes, environment and clinical care. The kind of motor neurons affected by a person’s disease may also matter, Bruijn says, noting that motor neurons that control eye movement often resist ALS far longer than those in the brain and spinal cord. Understanding how those cells stave off death may help scientists understand long-term survival, she says.

“What is different about those motor neurons compared to those in the spinal cord and brain?” Bruijn asks. “We don’t have the answers yet, but I think those are the kinds of things that can give us clues.”

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