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A concussion isn't like a sprained ankle. You can't see any swelling. Days later, there is no limp. No need for crutches.

While an MRI will show disrupted ligaments in the damaged ankle and grade its severity, it is almost always negative when done on a concussed brain. In fact, if the MRI does reveal damage, the injury is more than a concussion.

As for grading the severity of a concussion, for years, neurologists used various systems based largely on whether or not there was loss of consciousness and then its duration. More recent research has demonstrated that amnesia and its duration is a better indication of concussion severity. Even so, most experts who care for athletes at the professional and collegiate level have done away with "grading" concussions.

They are more focused on the brain's need for rest to recover just like the ankle's, and in many cases, it is a much longer period of time. There is certainly no rushing the recovery from concussion, no tape or brace to wear which will make an expedited return relatively safe.

In the immediate aftermath of a concussion, there may be some structural damage at the microscopic level with some cell death. However, the primary problem is a chemical one in the cells that have survived the incident.

We know this thanks to a man who started hitting rats in the head and giving them concussions. David Hovda, PhD, is the Director of the Brain Injury Research Center at UCLA. Since 1989, he and his colleagues have been regularly publishing the results of their work.

When there is a sufficient blow to the brain, the membranes of the affected nerve cells in the brain are stretched or twisted, allowing potassium to exit those cells, which triggers those cells to depolarize, thus the phenomenon of seeing stars if the affected area is involved with sight or ringing in the ears if the affected area is involved with hearing. The exit of potassium (K+) peaks approximately two minutes after the incident but continues for another 3-4 minutes.

Then, until the chemical balance is somewhat restored, those neurons (nerve cells) are unable to fire again. Furthermore, in a protective reflex of sorts, surrounding cells begin to shut down, a process Hovda calls "spreading depression."

If enough cells become depressed, confusion, amnesia, and even loss of consciousness result.

Meanwhile, in an attempt to recover, the brain starts using up massive amounts of blood sugar and will continue to do so for as long as 30 minutes. This overuse of this glucose results in the production of lactic acid which, in excess amounts, inhibits brain function.

A demand for glucose by the brain, such as when one is studying, logically causes an increase in blood flow to the brain. However for reasons not entirely clear, within two minutes of a concussion, the body decreases blood flow to the brain by up to 50 percent. This process continues unabated for 3-4 days but fully normal blood flow may not resume until an average of 10 days have passed. Consequently, at precisely the time the brain needs extra fuel to repair itself, it ends up getting less.

Meanwhile, the exit of potassium allows calcium to enter the neuron. And while the exit of potassium ceases in minutes, it takes the cell 2-4 days to rid itself of the energy-inhibiting calcium.

While the immediate chemical reaction of the brain to the concussion is brief and generally completed in 30 minutes, it takes days for the individual cell and the brain as a whole to restore that chemical balance which was lost so quickly. Until that balance is restored, the brain doesn't work as well and is particularly vulnerable to re-injury.

The epidemiological studies performed by Kevin Guskiewicz, PhD, at the University of North Carolina would seem to agree with Hovda's laboratory work. Guskiewicz has found that 90 percent of second concussions suffered by athletes in the same season as the first occur within 10 days of the first, precisely within that window of decreased cerebral blood flow identified by Hovda.

Neuropsychological testing studies done at the University of Pittsburgh also are consistent with Hovda's findings. There, the developers of the ImPACT test have found memory and reaction time recover in collegiate athletes, on average, within 7-10 days. Teenagers, with their immature brain chemistry, take slightly longer. Some much longer, with 25 percent of them taking up to four weeks.

John Doherty is the sports medicine columnist for The Times. Reach him at ptatcsport@sbcglobal.net.

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