This was posted a ways back on HFTE, but seemed applicable given the post this AM of the boxer convulsing on the mat. I'm a touch less confident in this than my other articles because it's a topic still being researched (and not an area I excel in...brainstems are complicated), but I ran it past my neuro prof and got his approval, so good enough for me. An original post on the disaster that is Tim Hudson's ankle is in the works for Monday.

Standout Cal RB Jahvid Best was nearing the end of his 2009 season when he turned the left corner and hurdled an Oregon State defender. There was just enough contact in the air to send him crashing, vulnerably and viciously, onto his shoulders and neck, whiplashing his helmet free. Cal’s celebration was immediately halted and viewers gasped and turned away as the camera zoomed in on an awkwardly angled neck and somehow more awkwardly extended arms.

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This was not Jahvid’s first concussion and wouldn't be his last…in fact, Best’s Lions teammate Nate Burleson said today he thinks Jahvid should retire for the sake of his long-term health (kudos Nate, that’s a rarely expressed opinion these days). There are countless people more qualified than I who have discussed and are researching the long-term effects of concussions and the deeply terrifying implications of chronic traumatic encephalopathy (CTE), so I’ll leave those topics alone for now and attempt to answer a more simple question…what’s up with those arms?

The extension of one or both arms immediately after a concussion is known as the fencing response. The term is due to the extension often being unilateral (one-sided), which causes injured athletes to assume a pose similar to a lunging fencer (“fencer”? Is that right? I feel like there’s probably an unreasonably absurd word for “one who fences”). Anyway, a quick Google search can provide compilation videos of pugilists, hockey players, football players, and countless other athletes demonstrating just how unconscious they've been knocked. The response is also fast enough that this uncomfortable-to-witness posture is often formed and held as the athlete is on the way to the ground. It’s also a transient (brief in nature) condition. In Jahvid’s case, you can see his arms and neck beginning to relax as his teammates are still gesturing wildly for aid from the sideline.

To explain the mechanism behind this we’ll need some background but I’ll attempt to keep this brief. The unconscious extension and/or flexion of upper and lower extremities is a well understood effect of serious brain stem injuries. Pictured below, decerebrate posturing (extension of upper and lower extremities) and decorticate posturing (extension of lower extremities and flexion of distal upper extremities) can be used to form a general prediction of whether a coma patient’s condition is improving or worsening.

These two distinct postures are due to a variety of motor pathways in the brain stem being either severed or left intact and a shift between the two can display whether damage is approaching or receding from the medulla of the brain stem (the seat of much vital function control…an infarct approaching the medulla is a very bad sign). These effects are due to a long lasting interruption of pathway signals and are not transient, like we see during concussions during sports…so while related, don’t likely explain the exact series of events leading to the fencing response.

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A piece on concussions at the Bleacher Report claims the fencing response is a regression back to an infantile reflex of extending arms and reaching toward a care-giver…but I can’t find a single piece of evidence backing this and it doesn’t pass the “that sounds like utter nonsense” test. More helpfully, this is an area of some ongoing research, as the presence of the fencing response in both sports and war zones can serve as a signifier of the severity of a concussive brain injury (especially important since it can be easily identified by non-medical professionals). Animal model research at the University of Kentucky show the effect after moderate but not mild brain trauma, although the laboratory definition of “mild” and “moderate” aren’t given explicitly.

I apologize for not having a definite explanation here, but such is the medicine of brain injuries…this stuff is complicated as all hell. Here’s the most likely mechanism of this response. The traumatic impact itself causes a disturbance in neurotransmitter release. Either an increase or decrease in this release will transiently mimic the more severe responses pictured above. A short-term increase of neurotransmitter in an extension pathway will appear similar to the response caused by the severance of the corresponding flexion pathway, and vice versa. Quickly after impact, neuron function will return to normal and any excessive neurotransmitter present in neuronal synapses will be quickly removed or degraded. These neurons can then return to normal function and the extended limbs will relax, at this point the athlete is probably totally fine and should be reinserted to the game as quickly as possible.