I have been training Muay Thai for about a year now, and am concerned about the risks I'm taking with regards to brain damage. I spar about once a week, and I really don't get hit that often or hard as I have a style that incorporates a lot of movement and have good training partners. However, there are some individuals in my gym who have a large size advantage on me and are more skilled, and many times they clip me on the chin with punches. From time to time (I can count on one hand how many times it's happened in the past year), these punches "rock" me in the lightest sense of the word (I've never "seen stars" or been unable to hold my balance, only a bit rattled and stunned). We don't really wear head gear at my gym when we are doing Muay Thai sparring (the boxers always do), and I've noticed that when people wear headgear they think they can punch harder thus nullifying the benefit to a great degree. That being said, I own some head gear, and it is in my best interest to wear it so I am going to start. I plan on doing 1 amateur fight at some point in my life for the experience, but that's it. With all this taken to account, am I really posing a threat to my long term mental health and cognitive functioning?
Several sports aside boxing and martial arts (such as soccer, rugby, ice hockey) have the potential to cause damage.
So, the risks are real and clear (for example: Kickboxing sport as a new cause of traumatic brain injury-mediated hypopituitarism), as they are for any sports that allows contact.
That said, how severe the risk is? This is a difficult question to answer without specific details about how often and how hard do you get hit (and even if we did know, I think). From what you say, it seems that you are training reasonably and that your gym is mostly fine in regards to sparring, not a constant competition to see who is the biggest bad-ass, which is a good thing.
In any case, prevention is by far the most important factor: The Cumulative Effect of Repetitive Concussion in Sports
This can be done in several ways, some of which are pretty effective. Head wear is good, although control in sparring is of paramount importance. How hardly you guys hit each other during sparring is crucial, period. It is also very important to develop reasonable defensive skills, which is often pretty much forgotten. A lot of muay thai / kickboxing sparring seems to me like a simultaneous punch-bag practice using the opponent (even when done without power). What I mean that emphasys is often too much on attacks that on any defence beyond basic guard. It's easy to find skilled practitioners or fighters that have poor footwork and basically just stand there and hit.
It's not a critic to the style (which is my main one too). But I personally have began to change my training practice a lot since last year to develop this skills, after noticing how underemphazysed they have been during all my years of training, and how I just kept to be teaching them to my students just as I was trained. It's not like I wasn't able to overpower my students or some sparring partners before, for instance, but I now see how being sometimes hit back was seen as simply an ordinary part of the practice, and it does not have to be. Just think of this: how much of your training is dedicated to practising attacks (either on opponents, punching bag, shadow boxing or pads), as compared to defensive moves? As a teacher or Muay Thai / kickboxing classes myself, I can also tell I understand it. In most basic classes (I live in a small place and do not have too experienced students) that is what people simply expect to find, and they often do not like/care about doing some seemingly boring practice of footwork and elusive movements.
Now, I definitely recommend this different approach: I barely get myself hit in the head any more, and when I do it's a much more moderate entity. Which is not only much safer, but provides a different feeling altogether to the practice! And the best thing about this is that it isn't all that difficult either. Simply put more attention to basic evasive movements (slips, footwork, bob and weave, timing and distancing) than to striking.
The training style and practice I have been developing in the last year focuses also on strategical thinking, which is the crucial component in my opinion, and it goes by preparing moves that just are good in most situation. I do not recommend too much "traditional" training of defensive moves that is technique based (for instance, if the opponent attacks with A you do this, if he attacks with B you do that), but rather on an informed guessing. Certain moves can be applied to almost any attack coming regardless on what that attack is, thus reducing the specificity of the response. For instance, slipping your head to the right when an arm movements happens is going to make your opponent miss as long as what he is throwing anything but a left hook. If you keep moving your head during the fight, even by simply repeating a certain set of positions, you can not only confuse your opponent, but reduce how likely a certain punch is to reach you, even before considering your guard or blocks. It's as simple as that. If you stay, as most do, in a fixed standard position, there is a 100% chance your opponent will punch toward your head, and you must rely on active blocking and a solid guard to deal with it at all the time. If your head is moving around all the time, your opponent must either guess when you are going to move in order to reach you. Sure, sometimes you will still have to deal with an incoming punch by using block or neutralizing it through your guard, but that will happen only if the opponent did indeed guessed correctly or simply, by chance, thrown exactly that one punch that is not too suitable for your technique (in the example above, the left hook). In any case, this is reducing the chance of any attack coming toward your head from 100% to a much much lower number (even as low as 10% or less, I would estimate). Not only it looks good and feels great, but guess is that better for your brain?
PS: When I say that the head is moving, the movement should come from slight adjustments of your legs and hip, NOT your neck or torso, as many do when slipping. That way you can move much faster and be energy efficient, which is the key to any elusive play.
The science is not clear on this yet, no one knows 'how much is too much'. However:
Plenty of legends without (noticeable) brain damage
Boxing, kickboxing and judo all have old, wise, super-fit legends hobbling around. This shows that it's atleast possible to scrap all your life and come out nice and shiny. More recently Saenchai (muai thai) is looking very good for late forties, and so is Mayweather (boxing). Footage of these recent legends shows how their technical, elusive, defensive styles contribute to their long term health.
You've got the right mindset
Consider how many of your training partners are more 'lax' than you when it comes to brain damage. Many have accepted it as a fact of training; they go into sparring ready for whatever beating and rocking in order to 'build experience'.
You've kept track of every stiff punch and every slight rock. You've been very mindful of your defense. You're way ahead of the curve and, with continued effort, you will get touched less and less as you develop.
You have already thought hard about this. You know you love this sport, but you love your brain more. Prove to yourself that you can protect your brain - continue keeping score, set goals like e.g. "I will take 20% fewer significant strikes to the head this month, or I'll quit".