Maybe I can be of assistance on this. After getting $30,000 of medical bills as a result (insurance covered 26,000)

I climbed Mt. Dana about 4 years ago. At the 12,000' level I became somewhat dizzy and more important was right on the edge of passing out. My vision darkened on outside. There was no good place to lie down, but I wanted to try that. As soon started lowering down knew for sure would pass out, so stopped.

Tried like 4 times. What worked best was, to keep going head down and forward. This lasted maybe 30-40 minutes. I did summit and felt better up there and made it down no problem. Another symptom was my right arm was not getting enough blood, it was half asleep.

Year later climbed Ruby mountains at 10,000' and same thing with arm, but the passing out part, was not as bad.

Then 6 months later was skiing at 8,000 and was afraid was going to pass out and fall off chair. Told ski buddies that and lowered the bar. After at top and resting a while was talking to one of them at peak and "whited out" Everything turned white, trees everything except dark jacket on who was talking with. Figured it out, my eyes were dilating against my will and was on edge of passing out.

Went to a doctor and he sent me to ER, they thought it was stroke or heart attack, 3 days later had $30k bill and they could find nothing wrong with me after zillion tests. BTW if you every watch your heart beat on sonar thing for 30 minutes it is so darn complicated its a wonder hearts last a week.

I told them it was not a stroke it was lack of blood getting to arm. Figured that out 2 hours after got out, had ski shoulder injury that crushed my rotator cuff tendon, artery runs same place. So at high altitude at extreme exertion the artery gets bigger and pinched by collar bone, which was moved by injury, one issue down.

The near passing out? Figured it out, common denominator is Aspartame. I avoid it but use cough drops at high alt for wet throat. Airline pilots are not allowed to drink aspartame sweetened drinks, their have been seizures at altitude. Aspartame passes the blood brain barrier.

I stopped all Aspartame and this problem has never happened again. After reading your post, thought this could be of value to you and others. Will post it separately with Aspartame in title.

Regards, Darp