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In the days after Chargers quarterback Stan Humphries received his latest concussion, the team reassured the public that Humphries was consulting neurologists and that a CT-scan and an MRI showed no problems. Humphries' agent chimed in, too, pledging that the team cared about his client and that Humphries would not return to the playing field until he was "100 percent." But experts in the treatment of concussion victims say that this comforting scenario is misleading and incomplete.

Neither a CT-scan nor an MRI can pick up the damage Humphries' brain most likely sustained. This was Humphries' fourth concussion in his pro career, his second in fewer than three weeks. Recovery from second concussions -- much less fourth ones -- is rarely 100 percent. Most neurologists have neither the training nor the inclination to determine whether Humphries has a truly clean bill of health. Such an examination can take hours and be spread out over days. For that, Humphries would have to consult specialists who operate beyond the strictly medical confines of most neurologists' practices.

The knockout blow Humphries received in the November 2 game against the Cincinnati Bengals is well documented from the outside. But what happened inside his brain and the long-term consequence of that trauma are much harder to pinpoint.

"An angular acceleration of the head, whipping backwards or to the side" is how biomechanicist Peter Francis describes what probably occurred when Humphries was thrown to the ground landing on the back of his head on the AstroTurf. That, in turn, "can cause diffuse axonal injury," says Francis, a professor at San Diego State's department of Nutrition and Exercise Science. "It creates lots of tiny, tiny, tiny lesions. The only way to determine they exist," Francis says, "is by a postmortem."

"Axonal injury" refers to damage to the axons, the long fibers that reside inside neurons, which are cells in the brain. When the brain suffers a violent movement beyond what it was designed to sustain, the axons can be stretched, bent, or, in the most extreme cases, torn and broken.

A human brain is a mass with the consistency of Jell-O or thick pudding, surrounded by a bath of shock-absorbing fluid. The fluid is meant to protect the brain from the sharp ridges that line parts of the skull. But when the skull snaps out of control and comes to a sudden halt, as Humphries' did, the bath can't stop the brain from hitting the skull and moving with more violence than the brain can stand. The brain performs a dangerous dance called the coup, contre coup, literally the "hit, counter hit," slogging first one way and rebounding the other.

To replicate what happens in a concussion, Francis says, "Researchers have filled a cavity with fairly dense Jell-O" and subjected it to similarly rapid changes in speed and direction. "You produce shear forces in the brain. The adjacent layers are moving at different speeds, causing [the axons that run through them] to shear off. Something as multidimensional as a football game can cause linear and angular acceleration," which produce those shearing forces.

The full extent of the damage can take days or weeks to make itself known. Axons can't regenerate or heal, so any brain injury is permanent. The brain must devise alternatives to compensate for the lost axons. Recovery can take years.

"You've only got so much in the bank, and every accident is a withdrawal," explains Dr. Barbara Schrock, a neuropsychologist who works with patients with mild traumatic brain injuries at Sharp Memorial Rehabilitation Center.

There are gradations of brain injury, she explains, from impairment to disability to, at worst, handicap. "Your brain is like an entire symphony orchestra," she says. "The back half of the brain is like the instruments, the front part [the frontal lobe] is the conductor of the orchestra. He has the plan and makes sure what the orchestra does is according to the plan, and if it isn't, he makes adjustments."

Schrock calls the collective abilities of the frontal lobe "the executive skills. Those very delicate executive skills are the most fragile in terms of injury."

They are also hard to measure. "People will notice changes in a person's ability to remember what he was doing; people will describe that as short-term memory loss, but that's not short-term memory loss. The frontal lobe's job is to set a goal and to comply with that. When you get dinged, you can't keep that stable intention in your mind.

"For a football player, that may not be that big a deal," Schrock says, because players lead such specialized lives while their careers are active. They have agents and accountants and helpers to handle everyday chores. "But what about afterward?" Schrock asks. "A lot of people don't notice changes in their cognitive abilities until they're under demand to use them. Whether a person can say the date doesn't mean they can figure out how to pay their bills, how to go grocery shopping, or remember what they had for lunch.

"A person whose executive skills have been impaired," she says, will show "what look like personality changes. People don't get the big picture. They don't get what their loved one is trying to tell them; they tend to be irritable and have flash tempers. Family members will describe them as being more childish; that's because children don't have frontal lobes. Frontal lobes don't start getting hooked up until you're 7 and don't finish until you're 18."

Schrock hasn't studied Humphries clinically, but "it's probably no coincidence that his concussions have been more frequent. The brain controls everything, including things like reaction time. Not reacting quick enough, not picking up the signals fast enough" make him more susceptible to being hit.

Medically, he's more likely to be injured if he is hit. "In terms of head injury in general, the odds of having another head injury once you've had one are 4 to 1; once you've had two, they're 8 to 1." Humphries is looking at number five.

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