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Canadian soldiers drive their Light Armoured Vehicle near the town of Howz-e-Madad in the Panjwaii district of southern Afghanistan in December of 2006.Kevin Van Paassen/The Globe and Mail

Canadian soldiers in Afghanistan were hospitalized for traumatic brain injury between 2006 and 2009 at almost three times the rate of Americans fighting there in earlier years before the war escalated, according to a National Defence study obtained by The Globe and Mail.

The military attributed the "significantly higher" hospitalization rate to "the risky nature of our Kandahar operation" in a report acquired under Access to Information.

"It has often been called 'the invisible wound,' " said Alain Ptito, a researcher at Montreal's Neurological Institute who sat on a Canadian Forces Health Sciences Advisory Panel on TBI. "The numbers may be higher."

Considered to be a disturbing hallmark of casualties in Afghanistan and Iraq because of the many roadside bombs that forcefully rattle the brain, TBI can result in severe concussions, long-term memory loss, depression and changes in behaviour.

While the proportion of Canadian soldiers injured - what the study called "a small but important minority" of about 6 per cent of all personnel in Afghanistan -- has been previously disclosed, the comparison to American numbers is new.

The total number of Canadian soldiers diagnosed with TBI was only 83; seventeen of those were classified with a "more serious forms of brain injury."

Still, the study found the hospitalization numbers taken from the trauma registry database at Kandahar were "significantly higher than the expected rate," amounting to a hospitalization rate of 71 per 10,000 deployed person-years of all Canadians serving in Afghanistan for the three years ending in 2009.

That compares with a rate of only 25 per 10,000 for U.S. troops in Afghanistan between 2001 and 2007 - before the increased fighting in recent years and last year's surge of American troops in heavy combat regions.

It was also much higher than the American hospitalization rate of 42 per 10,000 for brain injuries during the same six-year period of intense fighting in Iraq.

The National Defence study offered several reasons for the differences.

It noted that U.S. personnel were initially deployed in a "mixture of both high-risk and low-risk areas" in Afghanistan and Iraq, while Canadian soldiers concentrated in the dangerous region in and around Kandahar suffered "high casualty rates."

The report suggested American figures may also be low because TBI was not as recognized early on in the war.

(More recent statistics from the U.S. Department of Defense up until 2009 indicate that about 140,000 American soldiers in Iraq and Afghanistan have been diagnosed with traumatic brain injury since 2001 -- about 7 per cent of U.S. personnel deployed in those countries.)

But if anything, experts such as Dr. Ptito fear officials may be underestimating the seriousness of brain injuries among Canadian troops.

The military reported that 40 per cent of soldiers initially diagnosed with TBI were returned to duty, but Dr. Ptito cautioned that standard MRI tests may miss important symptoms. More sophisticated scans called "functional" MRIs, available in Canada, measure blood flow in the brain while the patient is active.

"If you use more sensitive tools, you find there are cerebral dysfunctions and the numbers will go up," Dr. Ptito said.

In addition, the 6-per-cent injury rate cited in the military study apparently applies to all personnel "deployed in support of the current mission in Afghanistan," which would include many non-combat positions.

The rate would presumably be higher among soldiers who are sent outside the base.

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