"I was terrified to be blind," McCullough said.
But rather than sit back and wait for the worst, this feisty 79-year-old is fighting back, reports CBS News Correspondent Elizabeth Kaledin.
Carla is doing something no one has ever done before. She's the first human to undergo a brand new technology, and if it works it could change medicine forever.
It's called gene silencing, an effort to turn off problem genes before they cause disease.
Scientists have discovered that when they inject genetic material known as "double stranded RNA" into the body it splits in two, attacking defective genes and silencing them.
Phil Zamore is experimenting with gene silencing in fruit flies at the University of Massachusetts Medical School.
"I'm very optimistic it can work," he says.
Unlike other experiments that aim to change a person's genetic make-up, Zamore says gene silencing is really just drug development using the body's genetic material as medicine.
"Potentially this would work for any gene," he says. "But it only works for turning things off, not on."
Consensus is it has the potential to shut down any malfunctioning gene causing any disease, from Huntington's to Alzheimer's.
The power of gene silencing first took seed in the greenhouse. Botanists trying to make petunias more purple accidentally turned off the purple gene and made them white instead. Light bulbs went off in labs around the world.
And that's where Carla McCullough comes back in.
"Well, we know it works in human cells growing in a dish," Zamore says. "The question is: Will it work in a complete human being like you and me?"
And will it be safe? To find out, researchers have injected gene-silencing drugs into Carla's eyes in an attempt to turn off the gene causing her macular degeneration.
Carla is a pioneer. "I'm very proud of that," she says, "and I hope I can help other people too."
If it works, we may all have Carla McCullough to thank.