Between 7-33% of patients who have ‘mild’ traumatic brain injury (sometimes called concussion) develop persistent post-concussion syndrome, which may last weeks to months after injury. More than 15% have a measurable cognitive deficit at one year. There is growing interest in the syndrome of post-traumatic encephalopathy, which may follow a blast injury or repeated sports-related concussion. However, our knowledge of risk factors that predispose people to sustaining such injury is limited. 

Nordström and colleagues (BMJ, 2013) have examined the associations between a history of concussion, cognitive function, academic achievement and measures of social wellbeing in a cohort of more than 300,000 Swedish conscripts. Given the paucity of data on premorbid neurocognitive testing in traumatic brain injury, this paper draws on an impressively large dataset that allows comparison of neurocognitive function before and after such injury in a nationwide cohort of Swedish men. 

A substantial proportion of patients with mild traumatic brain injury are never admitted to hospital or seen in the outpatient setting. Therefore, this study probably underestimated the incidence of this condition in the study population. 

Despite these caveats related to case ascertainment, the study provides unique insights into the epidemiology of mild traumatic brain injury. Unsurprisingly, poor cognitive function, low educational status and other risk factors were associated with mild traumatic brain injury. However, surprisingly, the association between cognitive function and concussion did not depend on the temporal association between the two and was just as common when poor cognitive performance preceded concussion. In addition, similar cognitive scores were seen before and after injury in twins discordant for mild traumatic brain injury, which suggests that both genetic and environmental influences contributed to the low cognitive function found. Other strong independent (but not unexpected) risk factors for development of mild traumatic brain injury included a previous episode of brain injury, hospital admission for intoxication, and low education and socioeconomic status. Surprisingly, the analysis found no significant differences in cognitive performance before and after the index event in men who sustained an injury. 

These results are important for several reasons. Firstly, they identify potential risk factors for mild traumatic brain injury and could help to guide attempts to investigate prevention strategies, perhaps through education initiatives. Secondly, they provide a context for interpreting studies that measure cognitive function after injury only and compare it with matched controls from the general population, with the assumption that those with brain injury have similar pre-injury characteristics to the general population. The results of this study suggest that such assumptions may be incorrect. 

Finally, those who subsequently sustained a mild traumatic brain injury had similar cognitive performance to that of those who had previously sustained such an injury, which implies that the injury itself may not reduce cognitive function. However, the tests used (word recollection, visuospatial geometric perception, logical and inductive performance, and mathematical and physics problem solving) have not been validated as sensitive measures of changing performance in cognitive areas thought to be affected by mild traumatic brain injury. These tests may therefore have missed important changes.