I want to do some research on why typical migraine presents with one sided symptoms, but I think it has to do with the brain having two hemispheres, and how cortical spreading depression works in the brain. That would make some sense, as when migraine occurs in the brainstem region, the symptoms are different and more global.
While the classic definition of migraine is a one sided headache, it is expected that migraines should at least occasionally switch sides. A truly side-locked headache is actually a red flag symptom for a neurologist, prompting a brain MRI. That is my situation. Facial pain/pressure, eye tearing, tinnitus, stuffy nose all on the left. My symptoms have progressively gotten low grade chronic, and I was found to have cervical spine major issues on the left side only. But that’s just my specific case.
With unilateral ear symptoms that persist in migraine patients, it’s hypothesized that migraine altering blood flow to the inner ear may cause low level damage that persists. Some think migraine may cause a “mini stroke” in the inner ear. But I agree that if these were the case, one would see the damage on audiograms or VNG/Caloric testing, which are usually normal. My own theory for unilateral tinnitus/hearing symptoms that persist or fluctuate with normal inner ear testing/normal MRI (and no frank Menieres patterns), is that it is a form of somatic tinnitus, irritated cranial nerves - triggered by muscle spasm, which can be caused by migraine, amongst other conditions.