“It can mess up what you can do or how you go about what you do, but she doesn’t let it mess up who she is,” Ryan Zimmerman said of his mother, Cheryl, and her battle with multiple sclerosis. (Toni L. Sandys/WASHINGTON POST)

The worst part is not knowing. Ryan Zimmerman came to grips with that long ago, before the disease confined his mother to a wheelchair and slowly, insidiously and relentlessly stole the ability of her muscles to function. Cheryl Zimmerman can no longer move any part of her body below her shoulders.

On the baseball diamond, playing for the Washington Nationals, where he has grown into one of the best third basemen in the world, Zimmerman has complete control. He is powerless in his mother’s daily battle against multiple sclerosis. He can only wonder what might happen next.

Cheryl Zimmerman learned she had multiple sclerosis in 1995, when Zimmerman was 11. He has lived more of his life with the knowledge that his mother is under siege than without, which is hard for him to believe. The hardest part isn’t seeing her diminished physical condition — Cheryl Zimmerman is strong, and she demands no special treatment from her family. The hardest part is that any day her condition might be the same as the day before, or it might not.

“That’s the most frustrating thing,” Zimmerman said. “You really don’t know what’s going to happen.”

Zimmerman has taken into his hands what he can, what the resources his baseball skills have allowed him to. Thursday night, Zimmerman will own Nationals Park.

He had it written into his latest contract that one day a year he has full use of the stadium for an event called Night at the Park. It benefits the ziMS Foundation, the charity he conceived at age 20 and has run with the help of his family.

Multiple sclerosis, “affects everybody differently,” said Keith Zimmerman, Ryan’s father. “You ask a doctor, ‘What does that mean?’ They don’t have an answer for you.”

Cheryl Zimmerman and her husband still come to Nationals Park a few weekends each season to watch their son play, never when it becomes too hot or too cold. She watches most Nationals games from home in Virginia Beach, yelling and screaming at the television. She leaves angry voicemails for Zimmerman when he cusses during an interview. Her health has changed, but she hasn’t.

“It can mess up what you can do or how you go about what you do, but she doesn’t let it mess up who she is,” Zimmerman said. “No matter what happens to your mom, she’s always going to look the same. She’s always been your mom.”

Cheryl Zimmerman was an athlete. She played lacrosse and field hockey in college. Keith, her high school sweetheart, was an athlete, too, a high school football star and a college baseball player. The family jokes Ryan got his athletic genes from Cheryl.

“I would say that’s probably true,” Keith said. “Actually, both of us were athletes. . . . ”

In the background, Cheryl heard her husband waffled on the speakerphone. “Yes,” she confirmed. “He did.”

Early indications

Around 1991, Cheryl Zimmerman started to lose feeling in the left side of her body. Her left leg would drag behind as she walked. She could not tolerate very hot and very cold temperatures. She would come home from teaching elementary school special education completely exhausted.

The Zimmermans went to doctor after doctor trying to figure out what was wrong. She had spinal taps and MRI exams. “It was years and years and years of, ‘This is what we think it might be,’ ” Keith Zimmerman said. “It wasn’t, you just went to the doctor and they prick your finger and tell you, ‘You have MS.’ ”

The diagnosis finally came in 1995. “You think it’s unfair,” Zimmerman said. “That’s kind of what you go through at first. You still don’t know why it happened.”

She continued teaching for four years, but started to tire more quickly in 1999 and retired the following year.

She began using a cane. Then a walker. By the end of the year, she needed a wheelchair. Zimmerman was 16.

As the MS progressed, Zimmerman and his brother, Shawn, played a crucial role in running the house. They did laundry and cooked meals. Ryan drove Shawn and Cheryl wherever they needed to go.

“You realize that it’s not going to change,” Zimmerman said. “So you might as well make the best out of it. She does a really good job of that. If anything like that brought her down, it would bring everyone around her down.”

Foundation started

Zimmerman came home during the all-star break in 2006, his first full season in the major leagues. The past year had been a whirlwind — drafted in June 2005, promoted to the big leagues in September, his first spring training in February, his rookie season. The break gave him a chance to exhale, and to bring something up to his parents.

“I’ve been thinking about doing a foundation,” Zimmerman told them.

That’s how it started. His parents recruited friends as volunteers. Zimmerman’s agent contacted lawyers. Keith and Cheryl took the lead. By that fall, they held their first event, a charity golf tournament in Virginia Beach.

They still hold the golf tournament. They created a gala at the University of Virgina that supports the school’s neurological department. Zimmerman also wanted to extend the foundation into Washington. When Zimmerman and the Nationals engaged in negotiations for a contract extension before the 2009 season, he would not sign a new deal without one particular stipulation.

“In every contract proposal that we ever made, Ryan included the use of the park for his charitable foundation,” said Brodie Van Wagenen of CAA Sports, Zimmerman’s agent.

This year’s “A Night at the Park” attracted 14 new corporate sponsors. The event raised $200,000 last year, and organizers expect to raise even more this year. Since ziMS was formed, it has raised roughly $500,000 for various charities benefiting the fight against multiple sclerosis.

“I think it’s great,” Cheryl said.

Life’s real challenges

The uncertainty and worry and responsibility hoisted upon Zimmerman as a teenager shaped him, of course. “It made him grow up a lot faster,” Cheryl said. More, though, he was shaped by his family’s reaction to their situation.

“You’d be kidding yourself if you didn’t think it affected you mentally or psychologically or any of those things,” Keith Zimmerman said. “You do the best you can with a crappy situation. Cheryl and I have often talked about, we try to stay positive and set a good example for our boys and other people suffering with MS. One of our goals was to try to let our boys have as much a normal life as possible.”

Ryan Zimmerman understands the difference between an injury or a slump and real challenges, real problems. A disease stole his mother’s career, her ability to walk and her independence. But it did not take away her grace.

“You look at someone like that, who obviously can’t do things they want, for her to think of other people before her still is one of those things where you sit back and think, ‘Man, there’s a lot of people in this world who wouldn’t do that,’ ” Zimmerman said.

“You never know what’s going to happen with my mom,” he added. “If you don’t take time to enjoy it and realize life is good, you’re going to look back and be disappointed.”