Master Cpl. Mike Blois emerged from his final battle in Afghanistan with no gunshot wounds, no shrapnel hits, and no burns. And yet four years later, he still grapples with the effects of a devastating injury he managed to conceal from everyone around him for more than 18 months.

On the night of Jan. 29, 2007, Blois and a comrade became trapped for four hours in their light armoured vehicle, fighting off a double-sided ambush from Taliban rebels armed with rocket propelled grenades. Throughout the firefight, Blois was exposed from the chest up as he manned the LAV's pintle-mounted machine gun, with night-vision goggles strapped to his head.

As each Taliban grenade landed nearby, he absorbed the full blow of their explosions. His head was rattled so badly, he lost the hearing in one ear, as well as all memory of the night.

But when Blois was sent home to recover, he did what many soldiers do: he lied about his pain. Though he suffered from intense headaches and dizzy spells that often caused him to vomit, he told his doctors he was feeling great.

"I pulled a ruse on everybody and said I was okay. Because I wanted to go back overseas," Blois explained to CTV.ca in an interview. "I wanted to go back and fight."

Blois assumed the symptoms would clear up on their own. And after a time, the dizziness did get better. But other symptoms didn't, and even today, he still has near-daily headaches despite medications meant to fend them off. He often feels dizzy when he sits up and sometimes has trouble with his memory, his mood and with fatigue.

Blois is part of a flood of Canadian soldiers coming home from Afghanistan with what's being called "the signature wound" of the war: traumatic brain injuries caused by powerful explosions.

In a conflict in which most deaths of Canadian soldiers have been the result of improvised explosive devices, many of those who survive the roadside bombs or grenade attacks are being diagnosed with head injuries that doctors are still trying to understand.

Brain injuries can affect mood, behaviour

It's difficult to know how many others there are like Blois. The Canadian Forces has released precious little information about the nature of the injuries that combat soldiers have incurred in nearly 10 years of conflict in Afghanistan. The U.S., which has been more upfront about the physical toll borne by its troops, estimates that as many as 20 per cent of combat troops who regularly worked "outside the wire" away from bases, have suffered at least one concussion.

Most of these soldiers recover within a few days, particularly if they were far from the blast or were injured only once. But others, like Blois, have longer-term problems that affect their health, mood, and ability to work.

It's still not well understood what happens to the brain in an explosion. Most traumatic brain injuries are caused by a blow to one side of the head. But an explosion appears to cause damage in a completely different way, sending out energy waves and atmospheric overpressures that can penetrate Kevlar helmets. As these waves pass through the brain's jelly-like structure, researchers are learning they cause tiny cells deep within the brain to die.

Dr. Andrew Baker, chief of critical care at St. Michael's Hospital in Toronto and a researcher at the hospital's Brain Injury Laboratory, has been working to understand how explosions affect brain structure. He recently published a study in the Journal of Neurotrauma on the effect of low-intensity blast waves – the kind of energy waves that wouldn't even knock a soldier out or blow out their eardrums.

As Baker explains, if one thinks of the brain as a computer network, the "grey matter" on the brain's surface would be the computers themselves while the "white matter" below would be the cables connecting them. Doctors have found that patients whose white matter has been damaged in some way often have lingering difficulties with complex thinking and decision making.

"They have trouble with memory and concentration, and it's very frustrating because they're trying to make these connections to retrieve things from their memory, but can't," he explains.

In his lab studies, Baker found that even low-intensity blast waves damage white matter cells or cause them to transmit signals more slowly. What's more, these changes appear to continue for days after the blast.

For now, the research is preliminary and even Dr. Baker isn't sure what it might mean in terms of both short-term and long-term effects. But the study comes amid other disturbing research about the cumulative effects of brain injuries.

Effects of repeated concussions still unclear

Among athletes such as boxers, football and hockey players, repeated concussions have been linked to a serious, degenerative brain condition called chronic traumatic encephalopathy. Once called "punch-drunk syndrome," CTE causes dementia-like symptoms, as well as depression, aggression and other behaviour changes. Whether blast-induced brain injuries could lead to similar problems is still an open-ended question.

Baker notes that concussions, or mild traumatic brain injuries as doctors prefer to call them, can be hard to spot. They don't cause bleeding in the brain, so they can't be detected with CT scans. And their symptoms – headaches, mood changes, memory problems – can often overlap with the symptoms of PTSD, another condition that escapes detection with typical brain scans.

In fact, some research suggests the two conditions often go hand in hand. A landmark study in 2008 found that about one in six U.S. combat troops returning from Iraq had suffered at least one concussion during their service. And these soldiers were more likely to later be diagnosed with PTSD.

Nearly 44 per cent of those who had concussions that caused them to lose consciousness went on to be diagnosed with PTSD, compared to nine per cent of those who weren't injured.

That study raised some important questions that are a long way from being answered. Is it possible that a mild traumatic brain injury itself increases the chance of developing PTSD later on? Or maybe enduring a traumatic event make it more difficult for the brain to recover from a concussion?

Blois is one of those soldiers who has both a brain injury and PTSD. For him, the incident that sparked his traumatic stress came when he watched a close comrade step on a massive land mine and then die in his arms. The incident haunts Blois with guilt and finds its way into his thoughts and dreams more often than he'd like.

While the Canadian Forces now takes PTSD much more seriously than it once did, regularly screening soldiers for the condition, the same cannot be said of mild brain injuries.

Brain injuries misdiagnosed as PTSD?

Dr. Donna Ouchterlony, the head of the brain injury clinic at St. Michael's worries that PTSD is being overdiagnosed in soldiers who actually have brain injuries from explosions. The problem is that many of these soldiers may not realize they have a head injury, particularly if they never experienced a blow to the head, and so they're being sent for the wrong kind of treatment.

"They're lumping head injury into PTSD right now," Ouchterlony believes.

"PTSD and brain injuries are very different things. And they're not noting that distinction," she says of the Canadian military.

Ouchterlony explains that while treating PTSD often means therapy with a psychologist, treating a brain injury means addressing each physical symptom one by one. There are specific treatments for dizziness, for example, that can clear up the condition quickly.

Headaches are typically treated with the same medications given to those with migraines. But the key treatment for headaches is to avoid physical exertion – just the opposite of what Master Cpl. Blois was doing before he was finally diagnosed.

For close to two years after the LAV attack, Blois ignored the head pain and other symptoms while he returned to work, this time as a trainer at the Land Force Central Area Training Centre in Meaford, Ont. While he hoped that he'd be called back into combat overseas, his body had other plans.

"It all came to a head in Meaford when I was training soldiers. I was running with a bunch of soldiers and collapsed," Blois remembers. "And that was pretty much the end of my big ruse."

Blois was taken to hospital and later diagnosed with post-concussion syndrome. He was told that his instructor days were over and that he'd have to move into a desk job.

Blois' symptoms are much improved now that he's finally under treatment for his injury. But after taking some time to review what he wanted to do with his future, he's decided to leave military life. He's taking a medical discharge from the Canadian Forces and has decided to go to law school, perhaps to study criminal law. He starts this September.