Tackling concussion, beyond Hollywood

When Hollywood tackles a medical issue, such as in the film Concussion, it is worth pausing to consider the issue in question. In practice, the terms concussion and mild traumatic brain injury are used interchangeably. However, for the 10% or more of patients demonstrating persisting neurocognitive dysfunction after concussion, there is nothing mild about it.¹ Furthermore, a history of multiple concussions has been associated with increased risk of neurodegenerative disease, in particular, chronic traumatic encephalopathy,² which is the subject of Concussion. Nevertheless, despite Hollywood films and endless news headlines about this illness, there remains remarkable confusion over the underlying pathology.

Contrary to the all too common animations illustrating the brain slamming back and forth inside the skull, the principal mechanical basis of concussion is likely to be head rotational acceleration.³ A consequence of these rotational forces is rapid deformation of the brain resulting in tissue damage, particularly to vulnerable white matter axons. During normal movement, axons can stretch to at least twice their resting length and relax back, unharmed. However, very rapid stretching, such as that which occurs with concussion, results in components of the axon becoming stiffer, resulting in breakage of axonal microtubules, a pathology known as diffuse axonal injury. Intriguingly, computational modelling suggests that the viscoelastic Achilles’ heel in axons is the microtubule stabilising protein tau.⁴

A consequence of microtubule damage is interruption of axonal transport, leading to protein accumulation in axonal swellings at sites of injury. However, although axonal swelling is evidence of diffuse axonal injury, the vast majority of injured axons appear morphologically normal after traumatic brain injury, even in severe cases. Nevertheless, in many of these normal-appearing axons, there is likely dysregulation of sodium channels resulting in impaired action potential, a potential physiological substrate for the common symptoms of concussion, such as decreased processing speed, memory disturbance, and loss of consciousness. Accompanying this sodium influx is an increase in intra-axonal calcium, leading to activation of proteases and inevitable axonal degeneration.⁵ As such, the so called mild injury of concussion could result in permanent axonal loss.

The script for Concussion has the hero, Dr Omalu (played by Will Smith), proclaiming “I have found a disease that no one has ever seen.” The reality is that chronic traumatic encephalopathy had been recognised for many decades before being discovered in former American footballers. In these earlier accounts, largely in former boxers, neuropathology examinations at autopsy described a range of abnormalities in tau and amyloid β contributing to dementia pugilistica, or chronic traumatic encephalopathy.²

Interestingly, accumulation of amyloid precursor proteins in injured axons triggers amyloid β deposition. Furthermore, axonal tau protein can become phosphorylated after traumatic brain injury, leading to pathological aggregation. Thus, diffuse axonal injury-associated proteins, namely tau and amyloid precursor protein, are implicated in both the acute injury of concussion and also the late neurodegeneration of chronic traumatic encephalopathy.

Despite the Hollywood attention and increasing hyperbole regarding concussion and its long-term consequences, remarkable knowledge gaps remain. Most notably, we are really only at the beginning of deciphering the underlying causes, one important candidate being diffuse axonal injury.