Sub-concussion & Repetitive Head Impacts: Should we worry about every bang, bump, and boom?
Let's start by making this personal. I played ice hockey from 1999-2012 competitively and finished playing in college (ACHA). Statistically, I sustained approximately 200-500 head impacts per season over 13 years. That's 2,600 to 6,500 head impacts throughout my career, with 3 known concussions in there. So, am I doomed to a demented death by suicide?
While popular media and news outlets may want me to believe so with certainty, the research doesn't show that to be the case.
Why I'm pretty sure I'm going to be okay, and you can be too!
A grand summary of repetitive head impacts.
Defining "sub-concussion."
To step outside the box, we need to understand the box. The current box surrounding concussion has three levels of traumatic brain injury (TBI).
Mild TBI (interchangeably, "concussion")
Moderate TBI
Severe TBI
The categorization is based mainly upon imaging findings, consciousness level, and memory changes. It's important to note that within the current system of TBI-grading, there's no such thing as "mild-" or "severe concussion." There's simply "concussion."
When comparing concussion to other forms of TBI, it's essential to realize:
Mild TBI (concussion) comes with a risk of symptoms in the 12 months following injury.
Severe TBI comes with a risk of death in the 12 months following injury.
As you can see above, a concussion is tricky because, in some cases, the only noticeable findings may be a moment of disorientation followed by symptoms. There's no imaging, blood, or fluid marker (to date) that can currently, definitively tell us if a concussion has occurred or not.
Read that again: There is no gold-standard, definitive way to prove the diagnosis of concussion.
If we can't define what it is (i.e., concussion), how can we define something less than it is (i.e., sub-concussion)?
We can't.
So at the time that I write this, we have no way to objectively define what "subconcussion" means because we have no way to define concussion objectively.
The point of this preamble? We need to stop calling impacts "subconcussion." It would better serve us to call them what they are, "head impacts."
What do we know about repetitive head impacts?
According to a 2018 systematic review of the subconcussion evidence:
"Given the evidence reviewed, we conclude the following: 1) exposure to repetitive hits to the head in sport presents the risk of microstructural and functional changes to the brain in male athletes, and 2) prolonged exposure to repetitive head impacts in sport, for both males and females, should be avoided. Further study is essential to advance our understanding of how exposure to head impacts affects the brains of athletes in the short and long-term."
More specifically, they found:
The structural changes were white matter changes, brain volume changes, and cortical thinning.
However, these repetitive head impacts were not linked to neurocognitive deficits (e.g., memory, attention, executive function).
Now, before you say, "That's it! Sports should be banned!"
Consider the following limitations pointed out by the researchers in the study.
54% of the studies had small sample sizes and didn't report responses/attrition rates, leading to a sampling bias.
These studies also had inadequate or absent control groups. This decreased their validity and increased the risk of Type I ("false positive") errors.
Most of the studies did not screen for substance abuse, which is known to cause white matter changes.
Youth and female athletes were "severely underrepresented" in the studies, making the data less generalizable across various groups of people.
Many of the measures were indirect (e.g., self-reported data). These studies did not account for a history of concussion (mTBI) in the athletes.
Additionally, we have other research that looked at repetitive head impacts from "heading" in adult amateur soccer that found no changes in brain volume or cortical thickness. When looking at balance/vestibular deficits after headers in soccer, they may only last 24hrs.
This research from NCAA football found no changes in plasma Tau protein levels throughout a season (even when compared to swimmers). Plasma tau in football players seemed to change independently of head impacts or concussion.
And when it comes to functional imaging to detect structural changes (e.g., DTI), "non-head" orthopedic injuries show similar DTI patterns to concussion injuries.
Jumping back to blood biomarkers. Keep in mind that single markers are not likely to be valuable by themselves. We'll likely need a panel of markers to make better concussion decisions (e.g., NF-L, miRNAs, Tau, GFAP, UCH-L1, etc...).
So, while we certainly would not recommend that folks go and repeatedly hit their head. We do not have conclusive research to say that these "not concussion" head impacts are undoubtedly detrimental. We just don't know, and the current neurocognitive evidence doesn't seem to show deficits.
Here's why I'm not overly concerned with my 13yrs and thousands of impacts.
Because while concussion and head impact history are non-modifiable, my lifestyle since my playing days is modifiable. And suppose you have followed me anywhere on the internet for any amount of time. In that case, you'll know I'm all about being healthy on purpose.
To keep the rest of this relatively brief, we'll take a look at "lifestyle" and exercise in the context of long-term health and recovery.
In this study of former NFL players, they evaluated cognitive function, anxiety, depression, and emotional regulation - all the concerns around repetitive head impacts and CTE. What they found was that folks who did more of the actions listed below had better outcomes in those categories:
Moderate-to-vigorous aerobic exercise
Resistance training
Diet quality (e.g., Mediterranean Diet)
Sleep quality and duration (>6hrs per night)
So, the NFL players who exercised regularly, consumed plenty of healthy foods, and slept well, had better cognitive function, less anxiety, less depression, and better emotional regulation.
Digging into exercise, in particular, we know that athletes and other folks have trouble regulating their blood flow after concussion(s). Brain blood flow is mainly controlled with 4 mechanisms:
Cerebral autoregulation (e.g., blood pressure)
Cerebrovascular reactivity (e.g., CO2 responses)
Neurovascular coupling (e.g., sending blood to active brain areas)
Neuroautonomic regulation (e.g., sympathetic vs. parasympathetic)
Some research shows changes in cerebral autoregulation (e.g., regulation based on blood pressure) after a season of hockey and football. In contrast, another study on hockey and football players found no changes in neurovascular coupling (e.g., sending blood to active brain regions) after a season.
What's all this jargon mean for the person (like me) with significant repetitive head impact exposure? Whether you've been concussed or not, there may be changes in brain blood flow. Well, regardless of the concussion type (e.g., vestibular vs. cognitive vs. migraine) or the duration (acute vs. PCS), the rehab should always include exercise. This is because exercise helps to improve brain blood flow regulation (e.g., blood pressure, CO2 sensitivity, and sympathetic/parasympathetic balance).
Now, let's take that even further. If you're a person who lives and breathes on the planet today, then you're subject to the same CDC, WHO, and AHA physical activity guidelines: 150min moderate-intensity activity per week (or 75min vigorous-intensity).
What I'm saying is, if repetitive head impacts disrupt blood flow, simply following the standard recommendation of 150min per week should cover our bases of blood flow recovery.
Grand Summary
Many contact athletes are bound to experience numerous head impacts over the courses of their careers. As the research stands today, that may be causing some microstructural changes. However, those changes may not be all that different from orthopedic injuries (e.g., torn rotator cuff). Further, the clinical neurocognitive changes (e.g., memory, attention, and executive function) do not appear to be affected at all by repetitive head impacts.
What the research also shows is that our lifestyle matters. Whether we're sedentary and eat lots of processed foods or exercise regularly and eat plenty of fruits and veggies matters. Simply meeting the relatively unsexy recommendation of 150-300min of exercise per week is enough to meet concussion recovery standards, as well.
For folks who are actively competing, or have children actively competing, in contact sports and would like to minimize the metabolic deficits that are known to occur in concussion, read: Prevention before impact - How to minimize your risk of damage following a mild traumatic brain injury (mTBI).
Dr. Mark Heisig is a licensed naturopathic doctor with continuing mTBI education from The American Academy of Neurology (AAN), Complete Concussion. Management (CCMI) and The Carrick Institute. His office is located in Scottsdale, AZ.