TECUMSEH, Neb. -- A Nebraska girl with a rare syndrome that causes her to grow faster than her peers is beginning to have heart failure.

Mackenzi Berglund, of Tecumseh, Neb., will soon turn 3, and she's already 108 pounds -- the average weight of a 13-year-old -- and 42 inches tall. She's much bigger than her older sister, Kaleigh, who is 5.

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Video: Mackenzi's Heart Is Failing
Video: December 2004: Mackenzi Towers Over Peers

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Television station KETV in Omaha reported in December that Mackenzi jumped from a children's size 6 to a size 16 within one year.

The family has taken many trips to the Mayo Clinic in Rochester, Minn., the Nebraska Medical Center and Children's Hospital in Omaha, but doctors have yet to provide a firm diagnosis. The current focus is on a possible genetic disorder.

Mackenzi's mother, Dana Harbin, got some discouraging news recently. Her daughter's size has contributed to the development of severe sleep apnea. As a result, her heart is failing.

"She tries not to let anything slow her down, but she's getting to the point where she'll take a couple steps and she's completely out of breath," Harbin said. "It's just putting too much strain on her heart. The left side is working too hard. It's beating too hard for her to keep up with, so it's slowly failing. If we don't do anything about it, then she could die."

Harbin hopes new tests will provide answers.

"It's very overwhelming," Harbin said. "We did bone aging and genetics listing and stuff like that, and they pinpointed what it was, but it's so rare, and the research is so new they really don't have anything to do for it."

Physically, Mackenzi is confronted by hardships around every corner. In addition to her heart strain, walking and balancing are more difficult for her. The extremely rare condition, which has yet to be named, will most likely cause arthritis in her joints before she is 7 years old, the television station reported.

As she matures and is incorporated into a social network with her peers, her emotional well being will also be at stake.

Harbin quit her job recently to focus on Mackenzi's care. She said a lack of income has put their family under financial strain. If you'd like to help, the Mackenzi Berglund Benefit Fund has been set up at Auburn State Bank.