The flash you describe is call a "flasher" - which is caused by detachment of vitreous from the retina. As it pull on or away from the retina, it can cause the release of floaters. Most people over the age of 65 experience them. I have been experiencing them since I was in my 30s. The greater concern is when a flasher persists, indication a more serious situation. this could include a retinal tear, possibly leading to a detachment. A persistent flasher should be considered to be an emergent condition that requires immediate attention. I have had two retinal tears that requires surgical intervention.

You may find that this floater will resolve on its own; however, it is worth following up with an eye doctor promptly, particularly if it is a new symptom.

Another question is whether or not you have had a fall recently or a blow to the head. In that case, all of the good information from Steve applies, which could indicate a retinal tear or detachment because floaters and/or flashes commonly occur. I also have experienced both in those situations, but not with an MPN.

I have them also in my right eye. Not constant but seem to worsen depending on the lighting and sometimes my stress level.

The flash has been described as a zig-zag flash like in a comic strip. The flash might last 20 or 30 minutes and then develop a migraine. This may be an ocular aura. But continuous flashes with floaters that last longer may be a retinal tear. If treated within 24 to 48 hours you can prevent extensive surgery. The treatment for a retinal tear shortly after it occurs, is to repair the tear with laser treatment. We had this repaired on a holiday weekend by a talented ophthalmologist. When in doubt call your doctor asap.

The vitreous in one of my eyes detached without any consequences, as Steve describes, this is due to the vitreous becoming dehydrated. But in the other eye, the vitreous detached itself and pulled on the retina, causing a detachment, which is serious, and I was operated on immediately, applying cryotherapy to the retina again.