Betahistine effects on cochlear blood flow: from the laboratory to the clinic.

The development of laser Doppler flowmetry techniques has contributed greatly to the study of cochlear blood flow (CBF). In animal models, intravenous betahistine dihydrochloride clearly increased CBF in a dose-dependent manner. This effect was greater in the cochlear vasculature than in the systemic vascular bed. The effects of betahistine were blocked by the alpha 2-antagonist idazoxan, thus suggesting an interaction between histaminergic and presynaptic adrenergic receptors. This was further supported by studies investigating the effects of electrical stimulation on CBF. Local (round window membrane) application of betahistine did not affect CBF, but had a non-specific effect on cochlear electrophysiology. This indicates that the receptors for betahistine vascular effects in the inner ear are most likely located in the modiolar artery. More recently, laser Doppler flowmetry techniques have been applied to human subjects. It has been shown that intratympanic application of adrenaline affects CBF and that this blood flow is under vigorous sympathetic control. Electrical stimulation has also been used to obtain measures of dynamic responsiveness in human subjects. This results in an increase in CBF, which is dependent on the intensity of the stimulation. Preliminary evidence indicates that this procedure can provide a standardized measure of the dynamic properties of CBF and may provide a means to differentially identify patients with compromised vasculature.