Hello @czechmate -- Welcome to Connect. I know it can be disconcerting when the doctors are not able to provide any diagnosis or treatment. You are right to be concerned and advocate for yourself to find an answer. I did find a few links that may provide some more information for you.

Cavernous Sinus Thrombosis
-- https://www.webmd.com/brain/cavernous-sinus-thrombosis

6 ways sinusitis can cause a mysterious 'brain fog'
-- https://www.health24.com/Medical/Sinusitis/about-sinusitis/6-ways-sinusitis-can-cause-a-mysterious-brain-fog-20170725

@czechmate may I ask if the doctors put you on any medications? Sometimes a side effect of some medications can be brain fog.

I'm not sure if Mayo Clinic is an option for you, but they are very good at diagnosing rare and difficult to diagnose health problems. If you would like to seek help from Mayo Clinic, you can one of our appointment offices. The contact information for Minnesota, Arizona and Florida
can be found here:
-- http://mayocl.in/1mtmR63.

John