I recently started to consider that my POIS may be related to migraine (even though I don't feel it as a headache, but more like tension and cognitive problems / speaking problems).
I found this page about sex headaches:
https://www.mayoclinic.org/diseases-conditions/sex-headaches/symptoms-causes/syc-20377477Sex headaches are obviously a kind of POIS, but the mayo clinic doesn't refer to it as such. So POIS may be actually more common than we think.
It turns out that sex headaches are more frequent with men.
I had migraines in the past as a child (the kind of migraines with visual aura/halo), but they stopped at age 20 or so. Now I have POIS. My grandmother suffered from migraine a lot.
My probably overly simplistic theory is that the cerebral veins have tiny muscles which contract/release to control cerebral blood circulation. Something might be wrong with the control of muscular tension causing tension headache (migraines), cognitive problems, anxiety, and tight muscles.
I've had some modest success with home-remedies for migraines, such as magnesium, riboflavin, and butterbur. Also, the Wim Hof breathing exercise helps, perhaps because holding breath for a while causes the veins to dilate (?)
Any ideas what to try next?