Mr Stephenson was diagnosed in 2003 when, after leaving the armed forces, he began to struggle to exercise, close his eyes or smile.
“Food would get stuck in my throat and I would have to reach down and pull it out,” he said.
“When I was drinking, I was choking, and water was pouring out of my nose.
“You worry, ‘how am I going to live like this, what am I going to do?'”
The father-of-four’s symptoms fitted in with a recent car crash, but physiotherapists referred him to a neurologist who tested for the condition.
Mr Stephenson said: “The diagnosis was an immense relief.
“If you have a chronic illness, it’s good to learn about it, because you’re going to be stuck with it forever.”
Adults who took part in a trial for efgartigimod reported regaining the ability to walk daily, dress independently, and some returned to work or education.
“Anybody who is on the drug at the minute can stay on it, but for new patients, it’s not going to be available to them.
“I feel so lucky to be on it,” Mr Stephenson said.
“I feel honoured and privileged to have been on the trial.”
Doctors will be able to privately prescribe the drug, also known as Vyvgart, meaning patients who can pay for it will still be able to benefit.
According to drug company Argenx, the price for one 400mg infusion vial is £6,569.73.
The number of vials used for one infusion, which Mr Stephenson said he has fortnightly, depends on the individual.