Medical mystery: Genetic clue solves toddler's debilitating illness

Two year-old Cara Greene was a healthy toddler until last November, when her parents, Kristen and Clayton, noticed something wasn't right.

"Her eye movements were just completely erratic and that was really scary," Kristen said. "You would look at her and her eyes would just move all over."

Within weeks, Cara lost muscle control and became unsteady on her feet. Her arms grew limp and even keeping her head up was a struggle.

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Two year-old Cara Greene plays with her parents, Kristen and Clayton.

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"We started noticing her dropping toys and then we started noticing her falling on her face, not catching herself with her arms," Kristen said.

Nine doctors tested Cara for a range of terrifying possibilities, including a brain tumor. Her parents feared they could lose her.

"Yeah, I mean there were... it almost seemed inevitable," Clayton said.

Cara was just days from starting chemotherapy for a presumed autoimmune disorder when she saw Dr. Vandana Shashi, a geneticist at Duke University Medical Center.

"It was a mystery," Shashi said, adding that she suspected a genetic problem.

Using new technology, Duke researchers sped up a process that typically takes three months.

In just three weeks, they pinpointed a genetic mutation so rare only about 100 cases have been diagnosed.

"That morning, before we gave them results, my heart was racing," Shashi said. "I was so excited, I wanted to rush into that room and tell them, 'We have a treatment for your daughter.'"

Cara has Brown Vialetto Van Laere Syndrome, or BVVL. As complicated as it sounds, the treatment is simple. Her parents said they were in shock.

"So, you're telling us if we give her a vitamin that she's gonna stop getting worse and she might get better?" Clayton asked Dr. Shashi.

Massive doses of the Vitamin B2 that she lacked led to dramatic improvements. Now, Cara can feed herself and lift her arms.

A medical mystery solved by cracking her own genetic code.