Rephrase* is it possible to have a spinal csf leak without evidence of sagging on brain mri? if so, how do i press issue with neuro md who refuses to chase further despite classic symptoms (postural headache, nausea, lightheadedness)?

Occasionally POTS ( postural orthostatic tachycardia syndrome) is confused with CSF leak. Spinal imaging is used to detect spinal CSF leaks. Do you have a history of spinal pathology which predisposes you to CSF leakage? If not, I can understand why your neurologist or neurosurgeon proceed with the testing. You could get a second option

Recommend a second opinion, as it appears that you have reached loggerheads with current provider.