Washington (CNN) Frank Larkin expressed gratitude to President Donald Trump at the White House last year when he stood with the President for the signing of an executive order creating a task force to work on a plan for preventing veteran suicides.

Frank's son Ryan, who followed in his father's footsteps to become a Navy SEAL, took his own life in 2017. At the signing event, Larkin described his son's struggles after suffering from what was later shown to be traumatic brain injury.

So it was a "hit to the gut," according to Larkin, when Trump said in January that he did not conside r TBIs to be "very serious injuries relative to other injuries that I have seen," referring to the injuries suffered by US service members in the Iranian missile attack against the al-Asad air base in Iraq.

Larkin, a former US Senate sergeant-at-arms, wrote an open letter to the President in which he described Trump's comments as "an undeserved punch felt by every person suffering from a TBI, their shattered families, and supporting communities who struggle everyday with the consequences of insidious brain injuries."

"I didn't want it to be political," Larkin said to CNN about his letter. "I didn't want it to be a jab at the President. I just want people to understand that TBI is serious business. It's very disruptive."

400,000 troops have been diagnosed

The highest levels of the Defense Department share Larkin's concerns about the seriousness of TBIs, an injury that more than 400,000 US troops have been diagnosed with in the past two decades. "We take this issue very seriously," Defense Secretary Mark Esper said in January. "This is an injury we need to keep educating everybody about ... it's a learning process for many of us."

"It can become post-traumatic stress," Chairman of the Joint Chiefs of Staff Gen. Mark Milley said at the same news conference. "It can become a wide variety of behavioral/health issues when you do damage to the brain."

There are now 112 service members who have been diagnosed with TBI from the al-Asad attack, according to the Defense Department. "We'll continue to monitor them the rest of their lives," said Milley, "and continue to provide whatever treatment is necessary."

The Pentagon is searching for crucial new answers on rapid diagnosis, treatment and protection from blast pressures. Field testing is underway for small gauges that soldiers wear on their helmets, chests and shoulders to measure the impact of a blast, in the hope of improving the speed and accuracy of diagnosis.

The Defense Department and the Department of Veterans Affairs have earmarked $50 million for research on the links between combat concussions, dementia, Parkinson's disease and suicide risk.

But it's not just enemy fire that can cause TBI. Many service members are subjected to blast pressures from their own weapons, in training and in the field, that can cause damage. And certain kinds of TBI currently cannot be detected in a living person.

'Something's wrong with my head, but nobody believes me'

Ryan Larkin entered the Navy in 2006 and volunteered for the SEAL program, according to his father. He deployed all over the world, including multiple times to Iraq and Afghanistan.

"Ryan was a quiet guy. He was very smart," Frank Larkin told CNN. "He was always laughing. Little bit of a jokester at times. But very dedicated to being a SEAL. He loved being a SEAL. Loved his teammates. It was a tight group."

Ryan Larkin

But after back-to-back six-month tours in Iraq and Afghanistan, Ryan's personality began changing. "He stopped smiling. His emotions became flat," Frank Larkin said. "He became more short-fused. Quick to anger, frustration. His predominant complaint was he couldn't sleep."

Things got worse after Ryan's tours, when he became a SEAL instructor. "He started to develop bouts of anxiety. Depression would ebb and flow," said Larkin. "His executive function started to deteriorate. It was hard for him to organize through the day. He had memory issues, problems with balance. His vision started to change."

Ryan was diagnosed with severe post-traumatic stress disorder, but despite his complaints that something was wrong with his head, Larkin says, it didn't show up on any of the tests that were run. Eventually, Ryan was honorably discharged. "He couldn't do his job anymore," said Larkin.

But Ryan maintained that there was something else amiss. "Ryan always said, 'Something's wrong with my head, but nobody believes me,' " said Larkin. "And when he took his own life, he knew exactly what he was doing."

Larkin said his son asked him to donate his body to TBI research if anything ever happened to him. "So that's what happened when, on that horrible morning ... when we found him in the basement of our home," said Larkin. "He had taken his life in a way that actually preserved his brain for this research."

Ryan Larkin was vindicated posthumously, according to his father. "They called us in and said, your son had an undiagnosed severe level of microscopic brain injury directly related to blast exposure, called interface astroglial scarring." Larkin, who has researched the issue extensively, explains that this kind of microscopic damage cannot yet be detected in a living brain.

It's caused by the kind of blast pressure that service members can be subjected to not just from enemy fire, but also from firing their own weapons. Larkin estimates his son was exposed to these kinds of pressures hundreds of times, "firing high-caliber weapons, the .50-caliber sniper rifle, firing the shoulder-fired rockets, mortars, [and] in combat being exposed to enemy indirect fires and IEDs."

"It's estimated 85% or more of this blast pressure exposure occurs in the training environment," said Larkin.

That's what makes the technology currently being tested to measure the blast exposures service members are subjected to so crucial.

"We're on a mission now, on his behalf," said Larkin of his son. "And that's to illuminate TBI, invisible wounds and the nexus to suicide, and see if we can generate an urgent level of research that can help provide the answers as to what we're dealing with and hopefully guide us down the right path."

'I wake up with headaches every morning'

Ryan Britch is a veteran of Afghanistan who now works for the Iraq and Afghanistan Veterans of America, a veterans advocacy group. He says he didn't even realize it when he suffered a TBI in Afghanistan.

"I was within 10 meters of an RPG blast while we were on dismounted patrol," he told CNN. "I remember the RPG exploding and I felt this blast wave kind of push me back. But it wasn't until later that evening when I started experiencing symptoms of being dizzy and having headaches. I think when you're having that adrenaline rush, your body doesn't quite recognize what's going on."

He says he didn't realize the seriousness of his injury until he had "this unbelievable wave of dizziness" while he was grocery shopping. "That's when I knew something was very wrong, and that's when I reached out to VA to figure out what was going on."

It took two years before he was diagnosed with a TBI, and he says he is still feeling the effects. "I wake up with headaches every morning. Concentration issues to this day."

Like Frank Larkin, Britch is trying to illuminate the public understanding of the seriousness of the issue. "The public needs to know that traumatic brain injury is a serious injury and it's physiological," he says. "There's no denying that it's a physical injury."

That's why there is a worry when the President of the United States downplays the issue.

"When you have somebody as prominent as the commander in chief dismissing the significance of traumatic brain injuries," said Britch, "it's going to discourage people from getting the treatment that they need."