How Stephan Hawking lived so long using ALS?

you can expect life with ALS three to 10 years, but Stephen Hawking lived with the disease for more than five decades.
He lost his capacity to speak in 1985, but a voice synthesizer allowed him to continue communicating - from conversations to interviews - until his death in the early hours of Wednesday morning at home in Cambridge.
Despite being confined to a wheelchair since his early 20s, that didn't hold back the esteemed theoretical physicist, who earned comparisons to Albert Einstein and Sir Isaac Newton for his research on the cosmos.
Hawking, who wrote 15 books and starred in the Simpsons and Star Trek, was the second very high profile ALS sufferer after Lou Gehrig, the baseball player who catapulted the rare disease into the public eye.
And yet, scientists still cannot explain what causes the disease, nor how to treat it, slow it, prevent it or cure it.

WHAT IS ALS?
Amyotrophic lateral sclerosis (ALS) — otherwise known as Lou Gehrig's disease or motor neuron disease — can kill someone within a few months of diagnosis, and there's only a 10% chance patients live more than a decade, the ALS Association says.

The organization claims most people are diagnosed between 40 and 70. Hawking was diagnosed at age 21.
The thing is The older you are the quicker the disease course tends to be, but we don't really have a handle on why some people survive for longer periods than others

SOME OF THE REASONS WHY DID HE LIVE SO LONG WITH ALS
It's usually diagnosed in people (mainly men) over 40, but most people with the condition first develop symptoms in their 60s.
Juvenile ALS, however, progresses at a much, much slower rate than later-in-life cases.
A lot of it, however, may be down do his genetics. Hawking famously did not use a ventilator, unlike the vast majority of other sufferers, suggesting the disease affected him in a way unlike most others.
Experts also believe much of his longevity may also be attributable to his high quality care.

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