Anyone know what this is called?

That feeling when you stand up and get struck by a sudden dizziness for no apparent reason. Usually it kicks in about a second or two after standing in and is quite severe dizziness.

 

Orthostatic hypotension, aka fainting.

You say dizziness though, sometimes that's just the fluid in your inner ear taking time to adjust. It is the mechanism by which you know up from down and is called proprioception.

In that case it would be orthostatic vertigo, a less commonly heard term.

Here's how the two are related:
Benign paroxysmal positional vertigo and orthostatic hypotension.

Hmm, translated that means probably orthostatic hypotension is at the root cause of orthostatic vertigo. Look for OH (by testing positional blood pressure) before you start worrying about brain tumors and other serious things.

 

Thanks! That helps a lot!

 

You could also just say "light headed". As in "He felt a bit light headed when he stood up."

 

Or if you're going for the vernacular, he got a head rush when when he stood up.

 

"Head rush" is the term I hear for this. "Light-headed" works as well though.