Casey Biddle, a 15-year-old sophomore at Twin Valley High School in Chester County, can't wait to get back on the soccer field next fall.

Never mind the constant headaches, the struggles with her memory, and the two naps a day she takes in the nurse's office - all after suffering as many as three concussions and whiplash playing soccer since August.

And it doesn't matter that her mother, Angela, sometimes looks in her eyes and sees "a blankness, there's no emotion anymore. It's like my sunshine who used to bound into a room is not there."

Casey, who has wavy blond hair and spidery eyelashes, said she "can't think of myself not playing." And so, her mother may let her. "She wants to be out there," her mother said. "I'm scared for her."

Casey is not facing those fears alone. Remarkably, five of her friends also have had their lives turned upside down as a result of a string of head injuries suffered while playing girls soccer since middle school.

Outwardly, they seem like any other teens, wearing tight, low-rise jeans, black eyeliner, and fluorescent nail polish. But at an age when most kids are obsessing over the newest Twilight movie or who's going to the prom, Casey and her friends commiserate about migraines, memory loss, and problems keeping up in school. They can't tolerate the lights and noise of amusement parks, dances, or movies.

"It's overwhelming. An entire life changed," said Allison Kasacavage, a freshman at Downingtown East High School, sounding wearier than most 15-year-olds. She said she had suffered five concussions, three from soccer, starting at age 12.

After getting elbowed in the head, knocked unconscious, and taking too many headers and colliding with another girl in three separate games over 18 months, "I knew I was done with soccer," she said. "I couldn't go back."

The issue of chronic head injuries for big-name, highly paid pro football and hockey players has been in the headlines recently, punctuated by worries of a link to this month's suicide of former NFL superstar Junior Seau. But what several experts have described as a public-health crisis among young female athletes has been flying under the radar.

According to a study of high school soccer players in the Journal of Athletic Training, girls sustained reported concussions 68 percent more often than boys did. Girls soccer trailed only football when it came to the total number of concussions among young athletes, according to the American Journal of Sports Medicine.

As experts labor to understand the reasons for this, they are alarmed at what they see as lax or inadequate rules and poor judgment in letting girls like Casey and her friends back on the field so soon after a serious head injury.

"It was shocking in this day and age to see that kids are playing with overt symptoms after multiple concussions and recent blows to the head," said Philip Schatz, a professor at St. Joseph's University and an expert in head injuries who learned of the cluster of cases in Downingtown after it was reported last week on NBC's Rock Center With Brian Williams.

Schatz noted that most school soccer programs have concussion protocols that require athletes to leave a game, take off a week or longer, and get an independent medical evaluation. Thirty-three states, including New Jersey, have laws mandating concussion education for coaches, but that doesn't always apply to travel and rec programs.

"If you were talking about adults and mild heart attacks, only one would require a complete change in lifestyle," Schatz said. "Here we are subjecting our children to multiple traumatic brain injuries and not changing their behavior."

But some girls are forced to alter their lifestyle.

Jenna Rohr, a ninth grader at Downingtown East High, started playing soccer when she was 6 but had to stop after a string of concussions. She's just getting back to school now after missing much of eighth grade.

"I used to not have to study and I got good grades," said Jenna, who has long blond hair and is tall and slender but who still sits on her father's lap. "Now I try to study, but I still struggle."

Her father, Brad, is worried: "We don't really know what the future is going to hold for these girls."

Robert Cantu, who as codirector of the Center for the Study of Traumatic Encephalopathy at Boston University is considered a leading national expert on head injuries in sports, said the cluster of Chester County cases seemed higher than usual. But he's quick to add that concussions for girls playing soccer is becoming "a national problem." On average, he said, 10 percent of girls will suffer a concussion in the course of one season.

There are several theories why girls have a higher concussion rate than boys playing the same sport.

It is "possibly related to girls' being a little more honest than guys in reporting," Cantu said, "and I also worry about weak necks. When girls take head shots, the brain is accelerated to a greater degree." He believes heading should be banned in girls' soccer under age 14.

Cantu and Schatz agree no amount of damage is good for a brain, but they say in most cases, it's not permanent if given time to clear up.

Said Schatz: "After a concussion, you've got to shut them down. You know how hard it is to shut a kid down?"

The Chester County families certainly do.

Hannah Thomas, 15, a freshman at Downingtown West who looks bigger and stronger than the other girls on her team, got hit with a ball on the side of her face in November 2009. She kept playing but can't remember the rest of the game. At dinner afterward, she didn't feel well and by the end of the week, "everything got 10 times worse. I couldn't focus on school, my eyes were shaking," she said. The doctor said she had suffered a concussion and told her to stay home from school for a week.

She's missed a lot of school since but resumed playing in January, even though she suffers headaches and nausea every day. At the end of last season, she was hit in the head at practice and "most likely" suffered another concussion, the doctor told her.

"I think it's worth it," she said of the risk of reinjuring herself. "I love the sport so much and I've done it so long."

So does her father, Jim Thomas, who has coached most of the girls.

"We feel it's safe for her to play," he said. The fact that she is so dedicated to the game "gives an edge" to his decision, he said.

"What rules is Hannah's love of the game, what she puts into it, and how much she wants to compete," he said.

He cautioned that if she sustained another blow to the head, he could change his mind.

Thomas and the other parents feel coaches need to be better trained in assessing concussions. And when a player gets hit, they need to pull her from the game. Thomas said he had never had formal training in concussions but had read up on the subject.

As for youngsters heading the ball, "you might want to eliminate it," he said. With his younger players, "I never told them to head and never told them not to head."

Matthew Grady, a sports-medicine specialist at Children's Hospital of Philadelphia who has treated three of the girls, said even doctors don't always recognize the signs of concussions, which they used to diagnose only when an athlete was knocked unconsciousness.

"We know that's wrong. Only 10 percent of the time do they lose consciousness," he said, noting that during medical school in the late '90s, he never was taught about concussions.

Kimmie Zeffert, 15, has been sidelined since taking three hits to the head, two when she stopped shots and another from a girl's elbow, during one game in August 2009. Just 5 feet tall, the Downingtown East freshman compensates by being gutsy.

After the third shot, "I don't remember anything else," but she kept playing. Two days later, she blacked out. A CAT scan found two contusions on her brain. Like her friends, she has been in and out of physical and occupational rehab and hasn't been to school full time since. If she goes to a movie on the weekend, "I don't plan anything else. I'm done," she said.

But now she wants to help her older brother coach a girls soccer team, and her mother is worried she could get bonked in the head with a wayward ball.

"I'm freaking out," Wendy Zeffert said.

On the other hand, she said, she hadn't seen her daughter this happy in years.

"She has her identity back. I'm proud of her. Even though I'm probably the worst mother in the world," she said.

Caitlin Murphy, 15, is less candid than the others about her injuries because she has trouble remembering them all. "Sometimes I can't remember my own name," the quiet, dark-haired East freshman said.

Her mother, Denise, said her daughter fell and hit her head twice off the field before taking a hit to the back of the head during soccer practice in October 2009.

Now her daughter keeps reinjuring herself, including falling off the bed and smacking her head on the floor, and tripping at church and hitting her head on a piano. "She's bumped her head so many times," Denise said.

That's a common problem for athletes. According to statistics, people who sustain one concussion are four to six times more likely to suffer a second, Schatz said.

After recovering, Caitlin played again last summer but hit her head while bodyboarding. In August, her symptoms got so bad, the doctor benched her permanently. Her memory is still bad, and she can manage only two half-days of school per week.

Fighting the resulting boredom is almost as hard as treating the symptoms.

"I tried to pick up photography, but it's not as fun," said Allison, looking tired and pale after two hours at her friend's house. She goes to school a few hours a day but can't handle much else because of balance and eye-convergence problems.

Casey's doctor also suggested she look into another hobby to replace contact sports.

"They were like, 'Scrapbooking? No!' " she said and laughed.

She hopes that if she tones down her game, not heading balls and being so aggressive, her parents will allow her to play again.

Her mother is struggling with what to do. She knows that it's a risk and that her daughter "can't foresee long-term effects" of her injuries.

"I don't know," she said, a look of outright fear on her face. "She's right on the line."

Contact Kathy Boccella
at 610-313-8123 or kboccella@phillynews.com.