I’m writing here because the doctors have no idea and just wanted to see if anyone else has had this before. 6 years ago I was mountain biking and it was toward the beginning of the ride. I suddenly felt that feeling when you stand up to quick and lose consciousness a little. I eventually woke up on the ground and my friend said I was having a seizure. By the time I got to hospital I felt 
fine and ready to go. Just yesterday I was on a road ride, 1.5 hours a bit of climbing it was cool, stopped at the cafe for a bit drank lots of fluid and ate cookies. On the way down on a really steep long section I picked up 75kph ish and the speed seems to trigger this light headless and micro seconds of losing consciousness. I pulled over on the flat section and stretched a bit and thought it will go away, i’m just light headed. No recollection cycling much further. Woke up on the ground in a daze with another cyclist calling an ambulance. Gashed my eyelid needed stitches and some road rash etc. What happened? No one can guess, just say heart problem maybe. Why the 6 year gap? What should I get tested and has anyone had anything like this before?!
This sounds like an issue that needs to be discussed with either a cardiologist or a neurologist (or both)…I would not rely just on your general practitioner for something like this.
Hope you figure it out quickly!
@aprimrose85 search for Vagal Response, specifically Vasovegal Syncope, to see if any of the symptoms match your experiences. If so I’ll send some info. Also let me know if anyone else in your family has these episodes - they can be hereditary.
It happened to me twice playing rugby years ago when I was in my twenties. Fainted for just a few seconds both times - brief enough where only one other player on my team noticed and said something to me afterwards.
I don’t know the cause. Possibly something to do with heat and over-hydration / low sodium. As both times it happened was in Texas where it was hot+humid, and I remember both times drinking a lot of water before the games to hydrate.
Went to a cardiologist and didn’t find anything. Hasn’t happened since.
Several years before Covid a friend had a blackout which caused a nasty crash but touch wood post covid and several years, and extensive medical testing it has never reoccurred. As a kid I also suffered from severe migraines, which led to black outs and seizures but touch wood since I started wearing glasses in my early 20s (that might just be coincidence); 16 years later and extensive medical testing which found nowt, it has never happened again. I hope its the same for you.
Have a search for “exercise associated postural hypotension”
From the US NIH :
Exercise-associated collapse (EAC), also known as exercise-associated postural hypotension (EAPH), is defined as the inability to stand or walk unaided during or after the completion of strenuous exercise.
I had an episode where I got up from bed and passed out 3 steps later. Searching led to the conclusion that it’s surprisingly common in endurance athletes.
-Tim
I used to get the blackout feeling all the time when riding, especially during the start of MTB races. I never actually passed out though. I always had the mindset of get through that, stay upright, then settle in. Kind of dumb. Then a nurse pointed out that I probably have exercise induced asthma, got an inhaler, and haven’t really had issues with it again. The asthma made complete sense given my history in sports and wish I would have been diagnosed with it when I was kid instead of 35. If I forget my inhaler, I feel it within 2-3 minutes of starting a ride and turn around to get it.
This. If all the tests come back negative, there is also value in an Apple watch with the ECG/EKG recorder which may pick up an occasional arrhythmia the 24hr etc tapes miss.
This seems unlikely but if your seat cuts off your circulation that can make you pass out. I was riding with a guy who sat on a log after a really hard effort and it unknowingly blocked the circulation and out of nowhere he passed out. It happened so fast. When he fainted he fell off the log which reopened the circulation and woke back up. This guy regularly also experiences orthostatic hypotension - the feeling you described where you stand up suddenly and almost or do pass out. While orthostatic hypotension is not uncommon, just an FYI that it’s also not normal. (Sometimes people who experience things a lot tend to think everyone else feels that way too). You could get a stress test done.
There’s a well-known cyclist from Pittsburgh, PA who experienced something similar to what you described and ended up paralyzed. Wikipedia says he experienced vertigo but who knows what really caused it. But my understanding from what I heard from others at the time was he got dizzy or blacked out or something during a ride and fell into a ditch. So I’d be wary of riding outdoors without knowing what is triggering this. Danny Chew - Wikipedia
Yes it was after the strenuous exercise even a cafe break 30 minutes and downhill all the way. It was when I reached a high speed and braking seemed to trigger it…I did stop but not for long…all I remember.
I have one nothing detected 
Interesting because it was the high speed (75kph) that seemed to trigger the lightheadedness. When I stopped and stated again I fainted within 1 minute of starting again. 5 years ago I was mountain biking. It’s possible the ‘fear of bodily injury’ triggered vasovagal syncope. I kept my speed below 50kph on the descent today.
@aprimrose85 Vasovegal Syncope can be caused by any number of factors including physical, emotional, and even environmental (eg flashing lights, tunnel vision from riding long distances along a tree-lined path, etc).
The onset can feel similar to that light-headed feeling some people get when standing up quickly (ie drop in blood pressure). Clearly if biking stop immediately. Forcing yourself to cough will push blood back to your head and can prevent a fainting episode. Also boosting your blood sugar (ie gel) can help, even if you think your blood sugar is already high from a recent coffee stop. Finally, pay attention to hydration as being dehydrated can make an episode more likely.
are you smoking the devil’s lettuce ?
It was only on a high speed descent that I got the feeling. I think it was 80kph, think my new tyres got me feeling cocky. It was exactly like when you stand up too quick. I have an eeg test tomorrow and had a full spectrum blood work done. Will post if find anything here. Not sure will.
No drugs barely any alcohol. How boring right? 
Are you a doctor because you and others seem to be more helpful and know more than actual doctors!
Apple Watch and hospital ecg passed also disk a cardio stress test.
@aprimrose85 I’m not a doctor but my family has a history of Vasovegal Syncope.
Heart-wise I would be expecting a 24 hr tape and echocardiogram as well since you blacked out during exercise which is not normal.