The volleyball thudded off the back of Hannah Zarnich’s head, producing an instant, searing headache. It was a hard shot, in part because Hannah was in her usual position along the front row, her back to the server, and hadn’t seen it coming.

She shook it off and kept playing. After the February game, the freshman at Chantilly High School took two Tylenol and finished the tournament, playing two more matches that day, though she felt lousy and didn’t play well.

But that night the scary stuff started. Over the next four weeks, Hannah at times couldn’t do her geometry homework because the symbols made no sense to her. She was intensely nauseated. She was constantly tired but had trouble falling asleep at night. She couldn’t understand the crawling words on her family’s TV screen. And she would become irritable at odd moments (yes, even more unpredictably than an average teenager).

One morning “it was my shower head,” she said with a smile. “I just wanted to kill my shower head.”

Unless you’ve been hiding the sports pages for the past few years, you know where this is headed. Hannah had a concussion — one whose symptoms are still with her in milder form more than a month later — though the slim 15-year-old has never strapped on a football helmet or swung a lacrosse stick.

Hannah Zarnich, 15, suffered a concussion when she was hit in the back of the head with a volleyball serve in late February. She plans to rejoin on the team after she fully recovers. (SUSAN BIDDLE/FOR THE WASHINGTON POST)

In fact, when a recent study examined the three high school sports that are most similar for boys and girls — basketball, soccer and baseball vs. softball — girls consistently suffered twice as many concussions as boys.

“It is somewhat surprising, because we have not seen this in this age group,” says Andrew Lincoln, director of the MedStar Sports Medicine Research Center at Union Memorial Hospital in Baltimore, who conducted the research using 11 years of data collected in Fairfax County schools.

We have come a long way in the recognition, treatment and prevention of concussions in the last few years. The NFL, prompted by congressional scrutiny of the terrible long-term injuries suffered by some players, has cracked down on blows to the head. The NHL is developing its own measures. High schools and colleges have devoted more attention and resources to the problem.

But one area where we still need to increase our efforts is in spotting concussions among high school girls, especially those who play sports we don’t naturally associate with head injuries.

“You’d think it’s always boys and it’s always football, but it’s across the board,” says Eric Castor, a certified athletic trainer at Chantilly High. Hannah’s nausea and sleepiness caused Elaine Zarnich to think at first that her daughter had caught a stomach bug. A day after the volleyball game, a visit to the doctor confirmed Hannah’s concussion. A few days later, when the on-screen TV guide made no sense to her teen, the Zarniches contacted the school and were put in touch with Castor.

Not only football

Football still dominates the high school head-injury landscape, according to Lincoln’s study. The sport was responsible for more than half the 2,651 concussions recorded in Fairfax County between 1997 and 2008.

But girls’ soccer players had the second-highest concussion rate of participants in the 12 sports studied, accounting for 7 percent of all concussions. Cheerleading, which had the lowest rate, still accounted for 5 percent of all concussions.

No one really knows why girls seem more prone, proportionately, to concussions. Girls may simply report injuries more readily than boys. Heavier muscles in boys’ necks and torsos may allow them to better absorb hits to the head.

Whatever the reason, parents, coaches and teammates need to step up their vigilance. There are 15 to 20 symptoms of a concussion, the most common of which are headache, dizziness and confusion.

But spotting head injuries is not always easy. Teen athletes may try to hide them, and reaction to a concussion can vary from victim to victim. A January study also revealed that girls may show different symptoms than boys. While both complained most often of headaches, boys were more likely to report amnesia and confusion, while girls cited drowsiness and sensitivity to noise.

Concussions “may present differently in girls and boys, differently in one girl from another girl, depending on the history of concussions and where in the brain the impact is,” Lincoln says.

A useful tool

Hannah enjoyed one lucky break in this frightening saga: She goes to Chantilly High. The school five years ago adopted the ImPACT program, under which every athlete, from football linemen to tennis players, takes a computerized base line test of cognitive functioning at the beginning of the season. Even though Hannah sustained her concussion playing for her club team, Castor was able to compare her memory, processing speed and reaction time against the results of the 20-minute test she took as a member of the school’s freshman volleyball team.

He also stepped in to vigorously manage her treatment. He had her excused from classes and homework, sending her home to sleep as much as she needed. He told her to stay off her cellphone, Facebook and video games. Her one job, he explained, was to rest her brain the way she would an injured muscle.

As Hannah slowly returns to school, Castor is testing her every other day. He can tell she’s improving, though some symptoms persist. Her prognosis is good, he says, but recovery from concussions can be unpredictable.

“He’s her advocate to the teachers,” says Elaine Zarnich. He tells Hannah, ‘if anyone gives you a hard time, I’m going to go in and talk to them.’ ”