Twin 4-year-old girls Addison and Cassidy Hempel are similar to their contemporaries in many ways: They love nursery rhymes; "Humpty Dumpty" is the family favorite; they go to school and play house.
But the identical siblings differ from other children in one major way. The girls' world of childhood fantasy is haunted by a heartbreaking reality that they are fighting a disease that seeks to steal their memories.
Like a pediatric version of Alzheimer's disease, Niemann-Pick Disease Type C (NPC), from which the girls suffer, causes patients to slowly lose their motor skills and then their minds. Rarely do children with NPC live past their teenage years.
In fact, the National Niemann-Pick Disease Foundation says NPC always is fatal and the vast majority of children die before age 20 -- with many not making it to the age of 10.
"You can't tell your kids they're dying," said a sobbing Chris Hempel, Addison's and Cassidy's mother. "They'll never know."
The Hempel family didn't always carry this burden. When the girls were born, they were healthy and hitting all the major milestones.
By their first birthday, they knew how to walk, but soon the girls' parents knew something was wrong.
When Addison and Cassidy were 2 years old, they caught a viral infection they couldn't shake. Scans showed each girl had an enlarged spleen, but doctors couldn't determine the cause. Then, things began to get worse.
"I started seeing increasing symptoms with balance and coordination — bumping into walls and falling down," Hempel said. "And when they were around other kids, I noticed they weren't running as quickly or jumping. I started getting really concerned."
She remembered thinking, ''We have to get to the bottom of this quickly.'"
It took months before Hempel and her husband, Hugh Hempel, received the rare NPC diagnosis.
"Affected individuals gradually lose their reasoning abilities, perceptual abilities, ability to make decisions," said Dr. Marc C. Patterson of the Mayo Clinic. "But in addition to that, they also have physical difficulties, particularly with balance, coordination."
The disease stems from a genetic mutation that causes cholesterol to build up in the body, according to the National Niemann-Pick Disease Foundation.
NPC patients are unable to metabolize the cholesterol properly within their cells so the excessive amounts accumulate within the liver, spleen and brain.
The illness is so rare that only about 500 cases have been diagnosed worldwide, according to foundation.
Even with the grim news, the Hempels refuse to give up hope. In fact, Chris Hempel has become a self-described "medical detective," searching for any tidbit of information that could help her family.
"We have this death sentence hanging over us, but we want to do something about it," she said. "I'm just turning over every stone. It's like a forensic expedition is really what it's like. You're on this expedition for your children. Get other people to get a fresh look. See if they can solve it."
Chris and Hugh started a Web site about their girls and also began talking with other parents to compare notes with researchers. Addison and Cassidy even had their brains scanned by Alzheimer's experts, all in the hope someone would be able to help them.
"It's a fundamental belief we have that scientists working on neurodegenerative diseases can learn from each other," Hugh Hempel said.