Lindsay Simpson still has questions about her concussions.

Her first one came when she was 16. At the time, she was playing for a club soccer team in Atlanta, as a promising high school goalkeeper with aspirations of a Division I scholarship and – in her biggest dreams – a professional playing career.

"I dove for the ball, and my body hit funny. I slammed the back of my head on the ground," Simpson said. "You train very well not to do that, but mistakes happen."

When she was injured in 2002, "concussion" was not the household word that it is today. For weeks, Simpson experienced mysterious symptoms and struggled at school, leaving her and her parents wondering what was wrong. "I had this horrible, excruciating headache. I couldn't be in the bright lights. I was just really not myself," she said.

Her parents took her to doctor after doctor, trying to understand what was wrong. She saw chiropractors, orthopedists, even an orthodontist.

Eventually, they saw a neurologist who suggested it could be a concussion. "He said, 'Have you had any knocks to your head lately?'" Simpson recalled.

The field of concussion research has come a long way since the early 2000s. Greater awareness among the American public – who watch every weekend as college and professional football players undergo concussion evaluations – could be the reason why U.S. teens are reporting concussions in growing numbers.

But many questions still abound, especially when it comes to the differences between men and women. Some studies have suggested that women and girls might be more susceptible to concussions, and they may need longer to recover.

Those disparities are one subject researchers are planning to discuss at this week's International Conference on Concussion in Sport. Some concussion experts hope that the major meeting in Amsterdam could lead to a consensus statement that could improve research on how concussions impact women.

Concussions took her out of the game

Outside of American football, no high school sport causes concussions at a higher rate than girls' soccer.

Nine months after her first concussion, Simpson was able to return to the soccer field. But for the rest of her career – from high school through her time as an NCAA Division I player, Simpson experienced concussion after concussion, some mild, some more serious.

Finally, in the final game of her sophomore spring season at the University of Maryland, she ran out of the box to clear a ball and collided with a player on the opposing team. Simpson's head hit the other player's shoulder. The next thing she remembers is her teammate helping her up off the field.

The symptoms – nausea, dizziness, light sensitivity, memory issues – were so intense that she had to withdraw from her courses.

Afterward, a neurologist told her that she'd never play competitive soccer again. All these years later, she said, "it still hurts, it still stings."

Now, at age 36, Simpson still has questions about the concussions that have altered the course of her life.

"The million dollar question, to me, is why? Why did I get that impact and it affected me that way – and yet you watch someone take a hit every Sunday on TV, and they're fine?" she said. "That's the question I want answered in my lifetime."

Many women get concussions, but studies have historically focused on men

Every year, millions of Americans get concussions. In a 2017 CDC survey, 2.5 million high school students – 15% of all high school students nationwide – reported having experienced a concussion in the previous year. More than 40% were girls.

Yet much of the most consequential concussion research has focused on men and boys, according to a new analysis published in the September issue of the British Journal of Sports Medicine.

Schools, teams and medical professionals nationwide rely on the guidance of three important organizations – including the International Conference on Concussion in Sport – to help guide concussion diagnosis and protocols.

Researchers looked at the studies cited by those organizations' influential concussion statements. The participants, they found, were 80.1% male to only 19.9% female.

"The startling part was just how male it was," said Julianne Schmidt, a concussion researcher at the University of Georgia and one of the study's authors.

Worse, about 40% of the studies cited included no women or girls at all. "Putting a number to it made it very clear that this is a huge imbalance, and it's going to take a lot of work to bring it back into balance," she said.

Schmidt is in Amsterdam for the latest International Conference on Concussion in Sport, where a panel of experts will begin to draft a consensus statement that reflects the latest in concussion research to make recommendations about diagnosis and treatment.

The conference normally meets every four years. But because of pandemic delays, the latest statement now dates from the 2016 meeting – a very long time ago for a rapidly evolving field in which hundreds of studies are published each year.

Because the ICCS statement is so influential, an update that reflects the imbalance of research is needed, Schmidt said – even something as simple as an acknowledgment of the gender gap and a call for more research funding.

"The doctor that's treating an NFL player on the sideline is using the same statements and guidelines that a pediatrician is using to treat a 14-year-old soccer player who's female," Schmidt said. "We can't just assume that women are miniature males."

What the research says about women and concussions

The gender imbalance stems from the origins of concussion research – studies of high-impact sports, namely American football and ice hockey, both of which are predominantly played by men.

Research access has been a factor, too: Professional sports teams and high-level college programs have dedicated medical teams, making it easier to track and diagnose concussions among those athletes than in other populations.

The gender balance has improved over the past decade as researchers have examined groups more diverse than just high-level male athletes, said Dr. Christina Lin Master, a pediatrician and concussion specialist at the Children's Hospital of Philadelphia and the University of Pennsylvania.

Researchers know that women report more concussion symptoms than men after a head impact. Now, Master said, studies are needed to figure out why.

"Is that because they have a more severe injury and have more symptoms, or is that something about gendered behavior where they are reporting more symptoms, or more likely to disclose, as opposed to hide, symptoms?" said Master.

The differences could also be biological, she said. The hypotheses there are many: One factor could be that men have stronger neck muscles than women. Another could be hormones – even menstrual cycles. It could be that women have more delicate axons, the slender fibers that connect neurons to each other.

"There's probably a little bit of 'both-and' and not 'either-or,'" Master said. And some of it may not be related to women themselves, but rather the medical staff around them: A study she led suggested that recovery times disparities among male and female college athletes could be explained by access to medical training staff.

That's where influential groups – like the ICCS panel of experts – come in, she said. An updated consensus statement could help nudge the field toward answering these questions by highlighting the gaps. "If they make a statement on what research is needed, I do think that the funding bodies pay attention," she said.