Some Factors Inducing Impairment of Blood Circulation in the Inner Ear and Brain

Abstract

The impairment of inner ear circulation may result from abnormalities in either local or systemic parameters including blood pressure (BP), hemodynamics, the autonomic nervous system (ANS), and endolymphatic pressure. In testing this hypothesis, the blood circulation of guinea pigs and rabbits was measured by three different techniques (i. e., laser Doppler, hydrogen clearance, and oxygen tension methods). We found, first, in the normal guinea pig, blood flow (BF) in the brainstem was maintained constantly within BP range of 35-80 mmHg, though the inner ear showed weak autoregulatory functions for BF within the same BP range. The pO₂ decrease following lowered BP was significantly less in the cochlea than in canal. Second, ANS dysfunction induced by denervation of the carotid sinus nerve or administration of amphetamine led to instability of BP, inhibition of baroreflex sensitivity for BP modulation, and poor autoregulatory functions for brainstem BF. Head up tilt stimulation (at 40°) also induced similar results. Third, transient interruption of blood supply to the brain and inner ear regions by microembolism was produced by plasma hyperaggregability after intravertebral arterial injection of adenosine diphosphate. Finally, in guinea pigs, unilaterally obliterated endolymphatic sac and ducts caused no significant difference in cochlear BF between non-operated and operated ears when BP was within normal range. However, cochlear BF autoregulartory functions for lowering BP declined significantly, in the hydropic ear 3 to 7 months after surgery.
We discussed the control mechanisms of inner ear circulation and some local and systemic parameters causing abnormality of autoregulatory functions for the inner ear and brain BF.