Sudden loss of peripheral vision in left eye followed by headache.?

Possible migraine: The loss of peripheral vision prior to a headache may be a migraine with aura, the aura may be the vision changes. This should be evaluated by a neurologist to make sure there is no other medical problem. The neurologist can also offer treatment options that will help.

Migraine is likely: If this was short term, it is probably an occipital migraine. If longer lasting it might be a stroke. You should seek your ophthalmologist the first time this happens to rule out something serious.