Some studies have indicated that the cause is collapse of the internal jugular veins by external tissue pressure when standing, but the exact tissues causing this have not been described.
Gisolf, standing CVP vs standing internal jugular venous pressure. The pressure increase in the internal jugular veins is due to the tissue pressure.
Now we need to figure out what tissues are causing the pressure. In doing so I performed numerous ultrasound studies, many on myself, which made it clear where the obstruction was and what tissues were causing it.