Have you seen a player get hit in the head, go stiff for a few seconds (known as the fencing response), and wonder what happened?
Like Joe Flacco after getting hit by Kiko Alonso:
Or Morocco’s Nordin Amrabat at the 2018 World Cup (not even tested for a concussion/MTBI but that’s a topic for a different day):
How about the Celtics’ Jaylen Brown after a nasty fall:
Or Ulf Sammuelsson after getting sucker punched by Tie Domi and hitting the ground:
Or Siarhei Liakhovich after getting hit by Deontay Wilder:
And lastly, Jahvid Best vs Oregon State (I was at this game so this is what I immediately associate with the fencing response):
As you can see, this fencing response to brain injury doesn’t discriminate by sport. In the following piece, I’ll answer why it happens and a few more questions, including:
- Is there a specific type of hit or force that causes a fencing response?
- Is it a telltale sign of concussion/MTBI and does it indicate greater severity?
Let’s start with the why….
I. Why Does a Fencing Response Happen?
During a concussion (also known as a mild traumatic brain injury, or MTBI) forces are imparted into your brain tissue which results in damage to the brain cells (neurons). For a more in-depth explanation, click here.
With enough force, the brainstem (the lower part of your brain) can be affected as well.
Here’s what the brainstem looks like (it’s sub-divided into 3 different parts but you can ignore that. Just focus on the red square):
A key role of the brainstem is controlling reflexes. As infants, we all have built in, “primitive” reflexes that aide with survival. These reflexes go away (“are integrated”) as we pass certain developmental milestones and don’t need them anymore.
Here are some examples (didn’t think you’d be learning about babies today? Wrong!):
- The rooting reflex is when a baby turns its head and opens its mouth in response to a touch on the cheek. This reflex is essential for feeding and nutrition and goes away around 3-4 months:
- The palmar grasp reflex is when a baby closes its hand around anything that is placed in its palm. This reflex helps develop the baby’s hand muscles for voluntary grasping and goes away around 2-3 months:
- The stepping reflex is when a baby moves its legs in a walking pattern when held upright with its feet almost touching the surface. The reflex commonly goes away around 2-3 months (that’s long before a baby can walk which is around a year, plus or minus a few months) but helps develop leg muscles and coordination:
Ok I think that’s enough, but there’s one more reflex that really matters in this fencing response discussion. It’s called the asymmetrical tonic reflex (ATNR) and looks like this:
Notice any similarities….
The ATNR commonly disappears after about the 4th month of life but the “code” still exists in the brainstem. A concussion/MTBI with enough force to the brainstem will momentarily re-activate the code and trigger that fencing response posture.
So now that we know why the fencing response happens, is there a specific type of force that’s more likely to affect the brainstem and cause the fencing response?
II. Rotational Force and the Fencing Response
To understand this, we have have to understand a little about about why concussions/MTBIs happen in the first place.
Studies have shown that that concussions/MTBIs can occur with either direct or indirect contact to the head. For example, getting your head whipped around violently in a car accident (without any other contact with say the steering wheel, dashboard, etc) can cause a concussion/MTBI.
That’s because the key contributors to concussions/MTBIs are the starting, stopping, and rotational forces imparted onto the brain from the start/stop movement of the head and neck. (For more info, click here, here, here, and/or here).
Rotational forces pull the brain tissue in multiple directions (this is called a shearing force) and potentially twist and torque the brainstem. For that reason, concussions/MTBI’s with associated rotational forces are far more likely to reverberate into the brainstem, cause the ATNR code to get triggered, and cause the momentary fencing response.
Got it. But does that fencing response indicate anything more grim…
III. The Fencing Response as an Indicator of Concussion/MTBI and Severity
I’m gonna use the good ole transitive property to explain this one.
The fencing response is an indicator of a concussion/MTBI that has rotational forces. Well, it turns out that brain tissue and the brainstem are particularly vulnerable to the shearing and torque caused by rotational forces. This results in more more widespread damage to the brain tissue and a higher severity of concussion/MTBI. (For more info, you can click here, here, here, here, or here)
Therefore, a fencing response posture is an indicator of a higher severity of concussion/MTBI. It’s considered a frank neurological sign and “the observation of posturing in a patient, if made by a clinician trained in the identification of this physical sign, may also be used to reach the level of clinical certainty needed for a definite concussion determination”.
Further, this shearing damage can result in something known as diffuse axonal injury (DAI). Here’s a short video that explains DAI simply and succinctly:
If the shearing and axonal damage is widespread enough, you will lose consciousness.
Additionally, if a part of the brainstem known as the reticular activating system (RAS) takes damage, you will also lose consciousness because the RAS is responsible for sleep/wake cycles. (Click here or here for more info).
If you recall, the ATNR code for the fencing response is also housed in the brainstem. It’s no coincidence that over 2/3 of concussions/MTBI’s that result in a loss of consciousness also involve a fencing response – when the brainstem gets hit with enough force, both are likely to get impacted simultaneously.
IV. All in All
To sum it all up:
That weird stiffness you see after some concussion/MTBIs is because the brainstem gets hit with enough force to momentarily activate the code of an otherwise dormant infant reflex called the asymmetrical tonic reflex (ATNR).
This most commonly occurs when the concussion/MTBI involves rotational forces, and because rotational forces are more damaging to the brain, the fencing response also indicates a higher severity of concussion/MTBI.
You’ll never look at babies or fencing the same way.
Thanks for reading and until next time.
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