Studies of inner ear blood flow in animals and human beings.

This article reviews current studies on inner ear blood flow, discusses their relevance to the maintenance of normal homeostasis of the inner ear, reports for the first time clear changes in fundamental properties of cochlear blood flow in the chronic hydropic ear, and describes the potential of applying laser Doppler flowmetry technology to the measurement of inner ear blood flow in human beings. Studies of the guinea pig in which perfusion pressure is varied demonstrate a broad range of autoregulatory capabilities of the inner ear vasculature. Gain factors range from 0.76 and higher for recovery for less than 1 minute of modified perfusion pressure. This is significantly greater than reports obtained for brain autoregulation. In a series of four investigations of cochlear blood flow in the hydropic ear in guinea pigs, a decreased responsiveness to electrical stimulation and direct stimulation of the superior cervical ganglia was found, indicating a change in sympathetic control of cochlear tone. Reduced vasomotion was observed, and autoregulatory capabilities were reduced. In human investigations, changes in cochlear blood flow were demonstrated with direct electrical stimulation of the round window and warm water irrigation of the ear canal, but not with carbogen breathing. Increased cochlear blood flow was observed with increased systemic blood pressure, and a remarkable decrease in cochlear blood flow was observed with the application of 1:10,000 epinephrine to the round window. These observations indicate the potential for development of laser Doppler flowmetry technology in the diagnosis and treatment of inner ear vascular disorders, and the animal investigations suggest that changes may occur in the chronic hydropic ear that compromise autoregulation and thus increase the sensitivity of the hydropic ear to other stress factors. Treatments can be found to modify such changes in vascular tone.