Olivia Hoffmann was a normal 2-year-old — smiling, chattering, crawling all over the furniture. But there was something strange. “She never started walking,” said Christine Hoffmann, Olivia’s mother. “She was so close, but never got it.”

That prompted Scottsdale residents Christine Hoffmann and her husband, Eric, to take Olivia to a doctor.

They figured out why their first child couldn’t walk in August 2004: Olivia had metachromatic leukodystrohpy, or MLD, a rare genetic disorder that usually kills its youngest victims before they turn 10.

From the time of diagnosis, Olivia went downhill fast. She lost muscle control within a few months and soon couldn’t eat on her own. By 2005, the family had to have a feeding tube put in.

“It was absolutely devastating. It was so hard to believe, because at that point, when she was first diagnosed, she was a normal 2-year-old, chatting up a storm, happy as can be,” Hoffmann said. “When your child is born, you assume you’re going to have a healthy child.”

But despite the disease, Olivia passed a milestone this week — Thursday was her fifth birthday.

To celebrate, the Hoffmanns are organizing Olivia’s Walk on Sunday at Kiwanis Park in Tempe. The 5K walk and run will benefit the MLD Foundation and Olivia’s Fund, which helps the Hoffmann’s pay for some medical needs.

MLD is a genetic metabolic disorder. According to the MLD Foundation, those with the disease are missing an enzyme in their blood, the absence of which allows a material to build up in the brain that destroys the myelin sheath, a protective covering on nerves.

Infantile onset, which is the type of MLD Olivia has, shows itself between 18 months and 5 years old. Children start having problems with walking, eventually lose control of their muscles and have frequent seizures and congestion.

While there aren’t reliable figures on how many people have MLD, the chances of two people who carry the gene meeting and having children are rare, about one in 40,000, said Teryn Suhr, executive director of the foundation, which helps provide support for families with the disease and encourages awareness and research.

It’s so rare that the Hoffmanns believe Olivia could be the only person in Arizona with the disease.

While there is no cure, there has been some promising research, Suhr said. Bone marrow transplants can help slow the disease in children who haven’t shown symptoms, and there are some researchers studying enzyme replacement.

The MLD Foundation is also trying to raise awareness of the disease, which in older children or adults can be mistaken at first for cerebral palsy or attention deficit disorder, Suhr said.

“People don’t think of MLD when they see a child who is not progressing or has not hit their milestones,” Suhr said. “They’ll see this child as slow.”

Meanwhile, Olivia’s disease has progressed. She has to have help turning her head, and her eyesight is mostly gone.

“She doesn’t smile anymore,” Hoffmann said. “Maybe once every three months we’ll get a little bit of a grin on her face. Not only do we have this awful terminal illness, we’ve lost communication with her.”

The family has found ways to help Olivia enjoy her time. She usually smiles when she hears music, and she has a teacher who comes in once a week to read and help her with art projects and science lessons.

“Olivia seems to like it,” Hoffmann said. “She can be having a really tough day and a lot of congestion, but she always pulls it together for Miss Evelyn.”

Seven months ago, Olivia became a big sister. She loves spending time with her twin brothers, who, through testing and in vitro fertilization, the Hoffmanns made sure didn’t have the disease.

“We never knew if we’d see her fifth birthday, so we feel really blessed we’ve got this far,” Hoffmann said. “She truly is our little angel.”

Olivia’s Walk

What: A 5K run/walk to benefit the MLD Foundation and Olivia’s Fund, which helps with Olivia Hoffman’s medical costs

When: 7:30 a.m. Sunday

Where: Kiwanis Park, 6111 S. All-American Way, Tempe

Cost: $40 registration fee

Register online at http://www.caringbridge.org/az/olivia

For more information on metachromatic leukodystrophy, visit the MLD Foundation’s Web site at www.mldfamily.org