Maybe I could contribute something: Dr Peter Novak mentioned two forms of cerebral hypoperfusion in one of his lectures. (Note that he was talking about people who have OI but who do not have a specific diagnosis of POTS, OH, etc. He calls this OCHOS, and he has diagnosed several people with CFS with OCHOS.)
One form is not enough blood getting to the brain for classical OI reasons: low blood volume, preload failure, blood pooling in abdomen and lower extremities, lack of vasoconstriction in lower part of body, generally low blood pressure. This could be helped by volume expansion, volume redistribution, and vasoconstrictors.
The other form is the arterioles in the brain being abnormally constricted, so actively preventing enough blood getting into the brain. (Arterioles are the first branch after arteries.) He thinks this is from an autoimmune problem causing abnormal cerebral vasoconstriction. This could be helped by vasoconstrictors and immune treatment (when we eventually find out which one to give). People in this group tend to have normal or generally high blood pressure.
None of the OCHOS group has marked changes in heart rate or blood pressure on TTT, but when their cerebral blood flow is measured with transcranial Doppler during a TTT it is seen to fall.
Just thought I’d mention this as one possible reason different people respond well or badly to vasodilation/vasoconstriction.
