The effect of intracranial hypertension on cochlear blood flow

Effects of perilymph volume adjustments on cochlear blood flow in the guinea pig

Previous research suggests a potential relationship between perilymphatic pressure (Pp) and cochlear blood flow (CBF); however, the alterations in Pp necessary to produce changes in CBF have not been adequately described or quantified. The effects of perilymph volume changes on systemic blood pressure (BP) and CBF were presently investigated in the guinea pig cochlea. Five microliters of perilymph were displaced in each of three conditions: viz. evacuation of 5 microliters from the cochlea; replacement of these 5 microliters; and finally the addition of 5 microliters of artificial perilymph into the cochlea. All perilymph volume adjustments were completed in 1-microliter increments during which changes in CBG and BP were recorded. Significant alterations in CBF were observed during 1-microliter perilymph volume adjustments in each condition with no significant changes in systemic BP. The results from this study support our hypothesis that an inverse relationship exists between CBF and Pp in that decreases in perilymph volume yielded elevations in CBF while increases in perilymph volume yielded reductions in CBF.

Intracranial pressure and auditory evoked responses of the cat

Auditory evoked responses (AERs) were recorded from the primary cortex, medial geniculate body (MG), inferior colliculus (IC) and cochlear nucleus (CN) of the cat anesthetized with sodium pentobarbitone to examine the effects of increased intracranial pressure (ICP) on neural activity in the different levels of auditory centres. ICP was increased by injecting saline solution into the intracranial space and a tone burst was used for activating the auditory centres. Cortical response (ACR) began to decrease in amplitude from about 30-40 mmHg of ICP. A decrease in amplitude of MG response and that of IC response followed in the order with a further increase in ICP. CN response was most resistant and usually remained even when ACR and MG responses were totally abolished. Recovery of the AERs followed a release of the increased pressure in the reversed order to the decrease in the AERs. When an increase was repeated with a short interval of pressure release such as 5 to 10 min, recovery of ACR became much slower and no recovery was sometimes observed 30-60 min after the release of ICP increased to a level below 100 mmHg. A discussion was conducted on the origin of the changes in AERs in response to increased ICP. We concluded from the results that the higher auditory centres are more susceptible to an increase in ICP to suppress the neural activities without apparent influence on the lower centres. A clinical test of ABR may be available to predict the prognosis of the auditory disorders.

The effect of blood pressure on inner ear blood flow

We investigated the effects of blood pressure on the blood flow in the inner ear as compared with other organs in normotensive rats (NR), spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR). The cardiac output and the regional blood flow were measured by a radioactive microsphere technique in conscious rats. The haemodynamics of the inner ear and other organs and the effects of a converting-enzyme inhibitor (SA-446) on the regional blood flow were compared among NR, SHR, and RHR, the blood pressure being 107, 152, and 185 mm Hg, respectively. With the exception of the heart, the organ blood flow was lower in SHR than in NR in all organs, including the inner ear. It was also lower in RHR than in SHR. The blood pressure dropped after i.v. injection of SA-446, while the regional blood flow increased. The effect was strongest in RHR, followed by SHR and finally NR.