What Do I Do About A Brief Bright Light In His Right Peripheral Vision When Diagnosed As Retinal Migraine?

My brother had a bright light that transversed from the top to the bottom in the peripheral vision of the right eye. Episode was less than 30 minutes. Evaluation by ophthalmologist was negative for PVD/Retinal tear or detachment. By exclusion, the Dx was retinal migraine without HA. Since this problem is rare and occurred in a population outside the parameters of migraines (i.e., a 56 yoa M with no prior Hx of migraines nor a family Hx of migraines), is another differential Dx possible?