Less concussions are generally better than more concussions but, when should an athlete retire? The answer to how many concussions are too many, is not that simple.
To sum it up, concussion is a brain injury that results from acceleration and deceleration of the brain tissue. The acceleration causes the brain to stretch and sheer, which produces an excitatory and stimulatory response. When this happens, on a cellular level, millions of brain cells start to fire and burn energy. The end result is an energy crisis in the affected cells of the brain. Over the next few hours to days, someone who has suffered from a concussion will experience a continued drop in cellular energy.
During this energy low, the brain is extremely vulnerable. Any additional trauma to the head – even if the force is less – can cause another concussion. These second concussions are much more serious than the first and can potentially lead to permanent or fatal outcomes.
But here’s the tricky part – how you feel, has nothing to do with the recovery of the brain. You might feel great with minimal symptoms, but in reality, your energy levels are still low, and your brain is incredibly vulnerable to additional trauma. This is why proper management, and not simply relying on symptoms to guide return to sport decisions, is so important.
It may not be the number of concussions that you get, but rather how you recover between each one that matters most.
Research on animals has demonstrated that if subsequent concussion injuries are sustained AFTER full recovery from the energy deficit, the cumulative effect of these injuries is lessened or even abolished all together (1,2).
Studies on this metabolic phenomenon was introduced with Christopher Giza’s landmark work, The Neurometabolic Cascade of Concussion (3) . Since then, much of this research has been pioneered by a group in Italy who started with mice (2,4) . They found that in a mouse, concussion results peak at an energy low at about six to fifteen hours after injury, and it takes about five days to get back up to baseline levels (5).
The next study by Vagnozzi et al., then looked at multiple concussions in mice. This study found that if there was a full five-day recovery between concussions, there was no cumulative or additive effect(2). However, if a second concussion occurred on day three, the two brain injuries added up, and the result was the same energy lows as a severe TBI. In addition, 10% of the mice died as a result of Second Impact Syndrome at the 3 day interval.
Next, researchers moved on to human studies (6,7). It was discovered that humans went through the same process of energy reduction and restoration however this process took about six-times longer in humans than in mice – between 22 and 45 days to recover. Interestingly, and concerningly, symptoms had all resolved within the first 2 weeks of injury, but it took between 3 & 6 weeks for the brain to actually recover significantly and get out of the vulnerable period.
This, once again, highlights the fact that relying on symptoms to determine when you are safe to return to your sport, is completely inadequate!
Feeling better does not signify that your brain has fully recovered.
A few athletes in this last study didn’t wait for the study to be over, and instead returned to their sport early – and got another concussion during their vulnerability period. In these cases, it now took three to four months to recover. In other words, after one concussion, brain recovery happens between 3 and 6 weeks however, sustaining a second concussion prior to this full recovery resulted in a 3 to 4 MONTH recovery.
Do we have a concussion problem? Or, do we have a concussion management problem?
So back to our original question, how many concussions is too many? To provide the most appropriate answer, further questions must first be asked.
As a rule of thumb when making a retirement decision for an athlete, ask two main questions:
1) Is each concussion that someone is getting, resulting in longer and longer recoveries?
2) Is each concussion happening with less and less force?
If the answers to either of these questions are ‘yes’, then an athlete should likely consider stopping contact sports. Potentially, they are caught in a perpetual period of vulnerability, where previous concussions may have been mismanaged or they were cleared to return to play far too soon. This could explain, as discussed above, reasoning for becoming concussed with less force and for taking longer to recover.
If the answers to these questions are no, there are again further questions to be asked. In 2018 Davis-Hayes at al., published an algorithm that works through a decision tree, which can be a helpful guide to discussions with athletes and patients when deciding to retire (8). If you are having these conversations with parents and athletes, please be sure to check out this algorithm.
As you can see, the question of how many concussions is too many, is a difficult one to answer as there is no magic number. It may depend on a variety of factors that all need to be taken into consideration. Please consult with your healthcare professional to have these conversations.
Complete Concussion Management is a network of clinics and trained practitioners that provide evidence-informed concussion care for all those impacted by concussion. We may have a clinic near you.
1. Meehan WP III, Zhang J, Mannix R, Whalen MJ. Increasing Recovery Time Between Injuries Improves Cognitive Outcome After Repetitive Mild Concussive Brain Injuries in Mice. Neurosurgery. 2012 Oct;71(4):885–92.
2. Vagnozzi R, Signoretti S, Tavazzi B, Cimatti M, Amorini AM, Donzelli S, et al. Hypothesis of the Postconcussive Vulnerable Brain: Experimental Evidence of Its Metabolic Occurrence. Neurosurgery. 2005 Jul 1;57(1):164–71.
3. Giza CC, Hovda DA. The Neurometabolic Cascade of Concussion. Journal of Athletic Training. 2001 Sep;36(3):228–35.
4. Vagnozzi R, Tavazzi B, Signoretti S, Amorini AM, Belli A, Cimatti M, et al. Temporal Window of Metabolic Brain Vulnerability to Concussions: Mitochondrial-Related Impairment—Part I. Neurosurgery. 2007 Aug;61(2):379–89.
5. Signoretti S, Marmarou A, Tavazzi B. N-Acetylaspartate reduction as a measure of injury severity and mitochondrial dysfunction following diffuse traumatic brain injury. Journal of …. 2001.
6. Vagnozzi R, Signoretti S, Tavazzi B, Floris R, Ludovici A, Marziali S, et al. Temporal Window of Metabolic Brain Vulnerability to Concussion: A Pilot 1H-Magnetic Resonance Spectroscopic Study in Concussed Athletes—Part III. Neurosurgery. 2008 Jun;62(6):1286–96.
7. Vagnozzi R, Signoretti S, Cristofori L, Alessandrini F, Floris R, Isgro E, et al. Assessment of metabolic brain damage and recovery following mild traumatic brain injury: a multicentre, proton magnetic resonance spectroscopic study in concussed patients. Brain. 2010 Oct 28;133(11):3232–42.
8. Davis-Hayes C, Baker DR, Bottiglieri TS, Levine WN, Desai N, Gossett JD, et al. Medical retirement from sport after concussions. Neurol Clin Pract. 2018 Feb 12;8(1):40–7.