Sudden permanent incremental reductions in brightness levels

Hello,

For many years now, I have sudden events every few months to every few years that suddenly reduce my brightness levels in both eyes at the same time by a small amount, while leaving everything else (visual acuity, colour perception, etc) working normally without any change. The pattern is that there is one distinct event every few years, followed by a series of smaller events in the following months before it stabilises again.

This new brightness level becomes the new permanent brightness level. There is also strong tiredness for a few days after an event. There is no change in my behaviour or activity/eating patterns before one of these event sequences start.

Hospital eye clinics and neurology doctors cannot find a reason. There is nothing obvious in the eyes themselves. VEP tests have not revealed anything. I am due for another MRI but previous MRIs have not found anything.

This is also noticable in various other ways, such as going outside while pupils are dilated, which used to be painful. However, due to the reduction in brightness levels, that has not been true for years.

Has anyone ever seen anything like this before, with just a sudden change in brightness levels in both eyes at the same time (usually overnight while sleeping) without any other detectable changes ? If so, was there a confirmed diagnosis made ?

I am looking for viable suggestions I can run past the doctors in case there is something rare they have not considered, so thanks for any hints or suggestions.

Given the lack of noticable structural changes, I am now wondering if this might be some brain processing or neurotransmitter problem, but I am mostly healthy in other respects, and do things like long day walks (10-15 miles) several times a month without any problems.

Thank you for any ideas you may have. There is clearly something very unusual going on and my own research has not turned up anything so far.