By what mechanism does diazepam (Valium) help with the symptoms of migraine and migraine-associated vertigo? Are there any natural alternatives (one or a mixture) that would work with the same mechanism to help migraine sufferers?The biochemical action of Diazepam (Valium) occurs by enhancement of neuronal GABA activity (gamma-Aminobutyric acid). This mechanism of action undoubtedly underlies its benefits for suppression of migraine-related vertigo, but not much more can be said, because the discovery of its benefit was empirical.
Some of the key work on this subject occurred at the University of Pittsburgh and was conducted by Dr. Joseph Furman and Dr. Rolf Jacob (MVRC: Team; Page on google.com). They observed a high degree of anxiety in subjects with prominent migraine-related vertigo and documented significant improvement of vertigo with use of diazepam for treatment of the anxiety. In other words, treatment of the anxiety also benefitted the vertigo.
Note that their finding does not imply that migraine-related vertigo is just another term for anxiety. In fact, anxiety is common with severe migraine whether or not vertigo is present. Moreover, motion-sickness is very common in people with migraine (Michael Soso's answer to Why as I get older have I become more susceptible to motion sickness?). Additionally, sensory triggers and excessive sensory sensitivities are common with migraine. The common denominator of these phenomena appears to be an impairment of sensory adaptation in many people with migraine (Neural adaptation; Michael Soso's answer to Why do some people get a headache when smelling certain smells?).
As a consequence, the benefit of diazepam on migraine-related dizziness could occur indirectly through effects on anxiety (not likely), through direct effects on the vestibular organs and brainstem vestibular nuclei, through effects on the reticular system and thalamic relay nuclei (thought to be important for sensory adaptation), and through other yet-to-be discovered mechanisms.
Given what we know of the mechanisms of diazepam's benefits, no rational method exists to look for "natural alternatives" that do not themselves potentiate GABA neurotransmission. The only "natural alternative" is vestibular rehabilitation (furman migraine vestibular rehabilitiation). Much of the research on this topic also was developed by Dr. Furman and his associates.
I had the pleasure of knowing all these individuals and of collaborating in a small way with Drs. Furman, Jacob, Sparto and Marcus. Additionally, I referred many patients to Dr. Furman for evaluation and management. Consequently, I am well aware of the years of effort required to develop this body of knowledge. I am pessimistic that further advances will develop in settings less technologically advanced than the extremely sophisticated facilities which Dr. Furman has constructed over his very productive career, not to mention the extraordinary network of colleagues and collaborators he also has cultivated.