Be careful regarding upright MRIs as they are over-read and issues are found on films that really don’t exist. A carefully performed MRI scanning Occiput-C2 (which have to be specifically ordered) and a CT scan of this region would be the imaging of choice. Then, possibly a diagnostic injection by a well-experienced injectionist to confirm and treat the disorder could be considered. Don’t forget that vascular phenomenon (vertebral arteries) can also cause your symptoms.
Dr. Corenman