What causes headache after dialysis?

Hello doctor,

My dad has been on dialysis for 2 years and has only recently been getting pounding headaches 1 hour into dialysis. He says they are pretty painful and it only occurs in his forehead and just to the right of the forehead. It does not make him sensitive to light, sound or any vision changes. He was in the hospital a month ago for a left leg amputation and while in the rehab unit, he had a seizure that we witnessed and it took him 2 days to wake up from it.

The MRI showed Posterior Reversible Encepholopothy, caused by very high blood pressure. Luckily, he is almost 100% neurologically and memory regained. I bring that up because only after the seizure and PERS MRI result is when the headaches started on dialysis. He never had headaches before on dialysis prior to the seizure/headache. The doctors did not put him on any new medications after this and tylenol helps the headache greatly but it still bothers him, and I do not like him taking tylenol almost every day. He is on Metoprolol 50MG Extended Release 1x per day usually in the morning and his blood pressure is fairly controlled (usually in the 130's/70's) however sometimes it gets high during dialysis . I have an appointment with a neurologist in January, as that was the first available. Any answers?