I just read a good overview article covering all the basics of orthostatic intolerance:
Common Syndromes of Orthostatic Intolerance (Stewart, 2013)
https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC3639459/pdf/peds.2012-2610.pdf
(or https://dx.doi.org/10.1542/peds.2012-2610)
This paper starts...