An expert on Lou Gehrig's disease explains what we know about this debilitating condition and how Hawking has beaten the odds

Editor’s Note: Renowned theoretical physicist Stephen Hawking died on March 14, 2018, at age 76. This story, originally published on his 70th birthday on January 7, 2012, is being resurfaced to explain how he beat the odds and lived so long with the disease amyotrophic lateral sclerosis (ALS).

Stephen Hawking turns 70 on Sunday, beating the odds of a daunting diagnosis by nearly half a century.

The famous theoretical physicist has helped to bring his ideas about black holes and quantum gravity to a broad public audience. For much of his time in the public eye, though, he has been confined to a wheelchair by a form of the motor-neuron disease amyotrophic lateral sclerosis (ALS). And since 1985 he has had to speak through his trademark computer system—which he operates with his cheek—and have around-the-clock care.

But his disease seems hardly to have slowed him down. Hawking spent 30 years as a full professor of mathematics at the University of Cambridge. And he is currently the director of research at the school's Center for Theoretical Cosmology.

But like his mind, Hawking's illness seems to be singular. Most patients with ALS—also known as Lou Gehrig's disease, for the famous baseball player who succumbed to the disease—are diagnosed after the age of 50 and die within five years of their diagnosis. Hawking's condition was first diagnosed when he was 21, and he was not expected to see his 25th birthday.

Why has Hawking lived so long with this malady when so many other people die so soon after diagnosis? We spoke with Leo McCluskey, an associate professor of neurology and medical director of the ALS Center at the University of Pennsylvania, to find out more about the disease and why it has spared Hawking and his amazing brain.

What is ALS—and is there more than one form of it?

ALS, which is also known as a motor-neuron disease—and colloquially as Lou Gehrig's disease in the U.S.—is a neurodegenerative disease. Each muscle is controlled by motor neurons that reside in the brain in the frontal lobe. These are controlled electrically and are synaptically connected to motor neurons that reside lower down in the brain—as well as motor neurons that reside in the spinal cord. The guys in the brain are called the upper motor neurons, and the guys in the spine are called the lower motor neurons. The disease causes weakness of either upper motor neurons or lower motor neurons or both.

It's been known for quite some time that there are variants of ALS. One is referred to as progressive muscular atrophy, or PMA. It appears to be an isolated illness of the lower motor neurons. However, pathologically, if you do an autopsy of a patient, they will have evidence of deterioration of upper motor neurons.

There is also primary lateral sclerosis—PLS—and clinically it looks like an isolated upper motor-neuron disorder. However, pathologically they also have lower motor-neuron disorder.

The other classic syndrome is called progressive baldor palsy—or progressive supranuclear palsy—which is weakening of cranial muscles, like the tongue, face and swallowing muscles. But it pretty much always spreads to limb muscles.

Those are the four classic motor-neuron disorders that have been described. And it was thought for quite some time that these disorders were limited to motor neurons. It's now clear that that's not true. It's now well recognized that 10 percent of these patients can develop degeneration in another part of the brain, such as other parts of the frontal lobe that don't contain the motor neurons or the temporal lobe. So some of these patients can actually develop dementia, called frontal-temporal lobe dementia.

One of the misconceptions about ALS is that it's only a motor-neuron disease, and that's not true.

Life expectancy turns on two things: the motor neurons running the diaphragm—the breathing muscles. So the common way people die is of respiratory failure. And the other thing is the deterioration of swallowing muscles, and that can lead to malnutrition and dehydration. If you don't have these two things, you could potentially live for a long time—even though you're getting worse. What's happened to him is just astounding. He's certainly an outlier.

Has he lived so long because he got the disease when he was young and had the juvenile-onset type?
Juvenile-onset is diagnosed in the teenage years, and I don't know enough about his course to say. But it's probably something similar to juvenile-onset disorder, which is something that progresses very, very, very slowly. I have patients in my clinic who were diagnosed in their teens and are still alive in their 40s, 50s or 60s. But not having ever examined him or taken a history, it's a little hard for me to say.

He's a very good example of the sparing of the non-motor parts of the brain that can occur.

How frequent are these cases of very slow-progressing forms of ALS?
I would say probably less than a few percent.

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