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Initially I looked up Google for severe pain behind the eye. I then discussed it with my GP and was referred to a neurologist.
Only neurologists understand what you are going through. I have had GP's say to me "so you get migraines". They are unaware that a migraine can be classified as a starter motor for a cluster attack.
The most useful information can be found on forums - Cluster Headache, Patients Like Me etc. You get better responses from fellow sufferers that out of the medical journals. If cluster headaches weren't so complex there would be a cure for them and sufferers would have to go to hell and back with every episode.