I have gone into Hypothermic shock a couple of times but only one time was that serious

 The first time was in 1970 and we had climbed Mt. Shasta the previous August in 1970. However, now it was Christmas Vacation and we three were College Students going to college in Los Angeles, San Diego and Sacramento. So, I left San Diego where I lived then and went to college and drove north to pick up one friend in Los Angeles and one friend in Sacramento. So, we arrived about 14 hours later in a blizzard at Bunny Flat. However, we were all young and feeling immortal and stupid like young men often are. And so we put on our snow shoes to climb up in the snow to Horse camp to stay there a little bit. But, I only brought a world War II sleeping bag because I intended to sleep inside the Horse Camp stone and cement structure so I wasn't prepared for almost freezing to death in the ice Cave or snow Cave we dug with our snow shoes. Also, inside of Horse camp was then a wood stove so we wouldn't freeze to death in 5 feet of snow we were traveling over. However, we never reached Horse Camp because of the white out. We found ourselves going in a circle because we couldn't see anything. So, when we crossed our own tracks in a circle we knew we were in trouble and couldn't get to Horse came that night. Also, in 1970 GPS finders (pocket size weren't invented yet or Cell phones). So, digging a snow cave was the ONLY way we might survive this little adventure of 3 college kids on a lark.

By the way if your survival is at stake like ours was in the blizzard white out, snow shoes can dig a pretty good snow cave which keeps you at least 50 degrees inside of one, especially if you light a candle to warm your hands inside this cave you dig. However, I don't think we brought a candle then only flashlights which is not useful for very long.

So, my first symptoms in the snow cave were convulsive shaking where your whole body sort of is acting almost like you are having a seizure or something except you aren't going unconscious like a person often does in a seizure.

Though we survived all this all my joints (knees, elbows, wrists, fingers hips) all hurt for several years. Today often people would go to the hospital but we as young men in 1970 simply rented a hotel room at what was then called "The Lemurian Lodge" which if it exists is called something else now in Mt. Shasta and took turns getting into a hot bath tub to try to warm up our core temperature again. We stayed and did this for 3 or 4 days because like I said most people now would go to the hospital for exposure for several days. However, we didn't think that way then as young men in college.

So, outside of our joints hurting for several years the only permanent damage for me at least was not being able to sense if I was too hot or too cold often during the rest of my life which can be problematic.