A study of Vietnam war veterans who suffered brain injuries during the conflict has found that the men show a faster decline in their cognitive functioning as they grow older than veterans without such injuries.
Published in the journal Brain , the study has found that the rate of decline can be predicted by how intelligent the veterans were before they were injured, their level of education, the size and location of the injury, and certain genetic markers that have been linked to brain injury and neurodegeneration. Greater intelligence and a higher level of education before the injury was sustained were associated with a lesser decline in cognitive functioning in the years afterwards.
Dr Jordan Grafman, one of the authors of the study, says that governments need to bear these findings in mind, particularly in the light of the head injuries being sustained in the on-going conflicts in Iraq and Afghanistan. "Nearly two-thirds of injured US soldiers sent from Iraq to the US army medical center have been diagnosed with traumatic brain injuries," he said. "The additional burden of accelerated cognitive decline to brain-injured veterans should be considered when estimating their future healthcare needs. These veterans would benefit from life-long care by neurologists and specialists in head injury. Particularly as they age, given their injury, they may have greater needs than others and the healthcare system, if prepared for this, can essentially shadow these soldiers.
"Clinicians treating veterans with brain injuries should evaluate any changes in their neurobehavioural status carefully so as not to confuse an exacerbated decline in function with dementia." 
Dr Grafman, the senior investigator in the Cognitive Neuroscience Section at the National Institute of Neurological Disorders and Stroke, National Institute of Health, Maryland, USA, and his colleagues studied a group of Vietnam veterans with penetrating head injury (PHI) 36-39 years after the injury. The veterans were part of the Vietnam Head Injury Study, a prospective follow-up study of about 2,000 Vietnam veterans that has been running since 1967. This most recent report is on the third phase of the study, and it looks at a total of 199 veterans who are now in their mid to late 50s.
The researchers used the Armed Forces Qualification Test (AFQT) as the means of measuring intelligence. The men would have taken the test at the time they joined the armed forces, giving a baseline against which subsequent test scores could be measured. It has a range from 0-100.
By the time of the third phase of the study, men with head injuries had a median average AFQT score of 54, while a control group of Vietnam veterans without head injuries had an average score of 74. When the researchers looked at the rate of decline between the second phase of the study (carried out between 1981-1984) and the third phase, they found a faster rate of decline in the brain injured group: the AFQT score declined by an average of seven in the men with head injuries and four in the control group.
However, when the researchers analysed the results more closely they found that a higher AFQT score and a higher level of education before the injury were both significant predictors of a slower decline in post-injury cognitive decline up to phase two of the study, but at phase three, only a higher preinjury AFQT score was protective in terms of later decline.
"Greater levels of preinjury intelligence were associated with lesser decline in long term AFQT scores," said Dr Grafman. "A higher AFQT score before injury acted in a protective manner and even predicted a higher AFQT score over 30 years after the PHI. Change in AFQT score in the first two decades after the injury was most associated with preinjury intelligence, and to a lesser extent educational duration. Thus, it appears that educational level has an impact earlier in the process of recovery from head injury. However, AFQT score prior to injury remained the greatest forecaster of overall cognitive outcome in the study's third phase, almost four decades after PHI."
He believes that greater intelligence is an indicator of the brain's ability to establish multiple neural connections and that this enables the brain to recover better from injury. "In essence, the more you dive into intellectual pursuits, the richer the intra- and inter-connectivity in a brain region leading to greater protection following brain damage," he explained.
When the researchers looked at the size and position of the brain injury they found that the size (measured as total volume loss by a CT scan) was associated with a decline in intelligence in the earlier phases of the study, and if the left ventricle and right frontal regions of the brain were injured, this also caused a greater decline in intelligence by phase three of study.
They investigated the links between genetic markers and cognitive decline and found that one marker, a receptor for a glutamate neurotransmitter (called GRIN2A), did predict the change in intelligence over the years. GRIN2A is linked to the age of onset of Huntingdon's Disease and to the brain's ability to re-establish neural connections. Dr Grafman said: "This is the first study to examine genetic factors in the long term outcome following PHI and it suggests that genetic markers may play a small but significant role in different stages of cognitive recovery or decline after head injury. This is an interesting but tentative result, which requires further investigation to establish its significance both in terms of degree and timing of its impact following a head injury."
He said the decline in cognitive functioning seen in the head injured veterans could mean that they might have to take early retirement from employment and might be less able to manage domestic responsibilities. However, the research could be useful for the men, their families and their doctors. "By telling patients and their families that later in life they will be at risk for a more rapid decline in certain abilities compared to unimpaired individuals can be difficult to hear," he said. "But it can be helpful in terms of long term planning and, potentially, practices could be developed that might protect against it, like staying physically fit and intellectually active."
 Demographic, structural and genetic predictors of late cognitive decline after penetrating head injury. Brain. doi:10.1093/brain/awm300
 The researchers stress that penetrating head injury (PHI) is a different type of brain injury to blast injuries, which are also being experienced in current conflicts. Little is known about the long term effects of blast injuries and further research needs to be carried out on this.
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