How likely is iih without optic nerve swelling . normal bmi . normal mri but 6 week constant throbbing headache . non stop . worse lying down doctor thinks migraine variant but only way to rule out iih is spinal tap . dont want to perform if unlik?

Yes, it would be unusual to have significantly elevated intracranial pressure in absence of papilloedema, and it is correct that a direct pressure measurement via LP is the most direct and objective test. But maybe have an ophthalmologist double check eye grounds using a red free light to check maculopapillar bundle fibers, a subtle sign. Chronic daily migraine can be broken up with CGRP's.