Just was reported Johnson had an Arteriovenous Malformation. The AV malformation is a congenital congealing of capillaries and small blood vessels. They may manifest themselves as a bleed, and depending on the locaition and magnitude of the bleed, will declare the degree of debilitation. The surgery would be designed to stop the bleeding and hopefully stop the advancement of neurological damage. I have not heard whether or not the AV malformation was able to be resected. Sometimes they are so extensive they cannot be resected and pose a persistant problem.
I believe Joe Biden had this same problem in the late 80's.
Hope that Johnsons family and Senator Johnson the best wishes in recovery.