Suggest Treatment For Migraine Attacks With Black Spots

I've had frequent migraines for the last 4 years. I had an MRI done 4 years ago and was told that nothing showed up that could have been the cause. Basically I've been treating them with OTC medication but have found very little relief. The last couple of times that I've had a migraine, I've seen black spots. They only go away when the migraine ceases. These are becoming unbearable....why is this happening and what can I do to get relief?

Thank you for your time and attention.