Cochlear blood flow related to hyperosmotic solution

The Effect of Endolymphatic Hydrops and Mannitol Dehydration Treatment on Guinea Pigs

Background

Endolymphatic hydrops (EH) is considered as the pathological correlate of Menière’s disease (MD) and cause of hearing loss. The mechanism of EH, remaining unrevealed, poses challenges for formalized clinical trials.

Objective

This study aims to investigate the development of hearing loss, as well as the effect of dehydration treatment on EH animal models.

Methods

In this study, different severity EH animal models were created. The laser Doppler vibrometer (LDV) and auditory brainstem responses (ABR) were used to study the effects of EH and the dehydration effects of mannitol. The LDV was used to measure the vibration of the round window membrane (RWM) reflecting the changes in inner ear impedance. ABR was used to evaluate the hearing changes. Furthermore, tissue section and scanning electron microscopy (SEM) observations were used to analyze the anatomical change to the cochlea and outer hair cells.

Results

The RWM vibrations decreased with the severity of EH, indicating an increase in the cochlear impedance. The dehydration therapy lowered the impedance to restore acoustic transduction in EH 10- and 20-day animal models. Simultaneously, the ABR thresholds increased in EH models and were restored after dehydration. Moreover, a difference in the hearing was found between ABR and LDV results in severe EH animal models, and the dehydration therapy was less effective, indicating a sensorineural hearing loss (SNHL).

Conclusion

Endolymphatic hydrops causes hearing loss by increasing the cochlear impedance in all tested groups, and mannitol dehydration is an effective therapy to restore hearing. However, SNHL occurs for the EH 30-day animal models, limiting the effectiveness of dehydration. Our results suggest the use of dehydrating agents in the early stage of EH.

Bone conduction experiments in animals - evidence for a non-osseous mechanism

Bone conducted stimuli are used to differentiate between conductive and sensori-neural hearing loss. It has been thought that the main route for the transfer of vibratory energy from the point of application of the bone vibrator on the skull to the inner ear is completely osseous. An additional mechanism may play a prominent role. In rats, a bone vibrator was applied to the skull and also directly on the brain, after removing bone (a craniotomy), exposing the brain. Auditory nerve-brainstem evoked response (ABR) could be elicited not only with the vibrator on bone, but also with the vibrator directly on the brain. Similar results were obtained in guinea-pigs and fat sand rats. Noise masked this ABR. Extensive removal of skull bone did not alter the ABR to bone-conducted stimuli delivered to the exposed brain. Experimental elimination of the ossicular chain inertial mechanism and of the occlusion effect did not greatly alter the bone conduction response. A reduction in the fluid volume of the cranial cavity induced threshold elevations of the bone conducted ABR but not of the air conducted ABR. These findings can be interpreted as evidence that the 'classical' bone conduction mechanisms should be modified to include a major pathway for cochlear excitation which is non-osseous: when a bone vibrator is applied to the skull, the bone vibrations may induce audio-frequency sound pressures in the skull contents (brain and cerebro-spinal fluid) which are then communicated by fluid channels to the fluids of the inner ear.

Medical treatment of Menière's disease: a review of literature

A review of the literature on drug therapy for Menière's disease was made using the Medline on-line biomedical database to screen publications between January 1978 and April 1995. In total there were only 118 publications in this 17-year period which specifically dealt with medical therapy for Menière's disease. It appears that only betahistine and diuretics have a proven effect in double-blind studies on long-term control of vertigo in Menière's disease. No medical therapy, however, has a proven effect on hearing or long-term evolution of the disease. There are multiple reports, especially in the recent literature, regarding the intratympanal use of aminoglycosides for vertigo control. Based on this literature review and on empirical findings, a strategy for medical treatment of Menière's disease is proposed.

Basal nitric oxide production in regulation of cochlear blood flow

Nitric oxide (NO), recently identified as endothelium-derived relaxing factor, has been shown to influence both vascular and neural function. In blood vessels, NO is produced by endothelial and smooth muscle cells and may play a role in regulation of cochlear blood flow. In the central nervous system, NO functions as a neurotransmitter involved in long term potentiation. The principle hypothesis tested in this study was that basal NO production in the cochlear blood vessels contributes to regulation of CBF. Since NO is a vasodilator, diminished NO synthesis may decrease the level of CBF. Application of a competitive inhibitor of NO synthase either intravenously or to the round window membrane caused a reduction in CBF. The application to the round window membrane did not affect compound action potential thresholds. With intravenous administration, the effect on CBF was dose-related and could be reversed with the physiologic substrate, L-arginine. These data indicate that NO is produced in the cochlear blood vessels and contributes to the regulation of CBF.

The effect of topical application of vasodilating agents on cochlear electrophysiology

The aim of this investigation was to study whether increased blood flow has beneficial effects on the ear or whether it is damaging to the auditory function, expressed as the auditory gross neural response. Four vasodilating agents were examined after topical application with respect to their influence on cochlear blood flow (CBF), blood pressure, and auditory function in the normal guinea pig. CBF was recorded with laser-Doppler. The drugs used were sodium nitroprusside, hydralazine, nicotinic acid and histamine. Sodium nitroprusside and hydralazine increased CBF and induced concentration dependent loss of auditory function. Neither nicotinic acid nor histamine had any effect on CBF. Nicotinic acid had varying effects: in some cases a negative influence on threshold shifts was seen, and in others a positive one. In most cases histamine caused improvement of the auditory function. The data indicate that topical administration of drugs to the inner ear is effective to influence both CBF and auditory function.

Stellate ganglion drives sympathetic regulation of cochlear blood flow

The functional properties of the sympathetic fibers innervating the cochlea are not well understood. Adrenergic fibers supplying lateral wall structures of the cochlea have been observed terminating on radiating arterioles and collecting venules. Adrenergic fibers also terminate as 'free' endings in the spiral osseous lamina. Stimulation or transection of sympathetic fibers originating from superior cervical chain and supplying the cochlea have yielded mixed results concerning many aspects of cochlea physiology. In order to clarify the origin of sympathetic fibers and their role in control of cochlear blood flow (CBF), we examined the effect of electrical stimulation of the stellate ganglion (ESS) and transection of postganglionic fibers from the stellate on CBF measured by laser Doppler flowmetry and on systemic blood pressure (BP) in the guinea pig. ESS produced a 20-35% increase in BP and 10-15% decrease in CBF. The decrease in CBF presumably reflects the net result of increased perfusion pressure, local autoregulatory mechanisms, and a direct sympathetic-induced vasoconstriction. Section of the immediate postganglionic sympathetic trunk had little or no effect on the ESS-related change in BP; however, it eliminated the CBF reduction. Intravenously infused beta 1-blocker diminished the BP increase due to ESS, while the electrically-evoked reduction in CBF remained. Local application of an alpha-blocker on the round window blocked ESS evoked CBF reductions without altering the BP increase. These data confirm the functional role of sympathetic projections from the stellate ganglion in CBF regulation in guinea pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Cochlear blood flow and microvascular resistance changes in response to hypertonic glycerol, urea, and mannitol infusions

The effect of hyperosmotic agents on cochlear blood flow (CBF) was tested in normal guinea pigs and in guinea pigs having prior unilateral operations to ablate the endolymphatic duct. Laser-Doppler-measured CBF was normalized to remove apparent changes related directly to systemic blood pressure. Hyperosmotic fluids were given via venous infusion: glycerol (20% and 40% solutions), urea (10%, 30%, and 40% solutions), and mannitol (40% solution). All agents were dissolved in 0.9% saline and the mixtures were given at a rate of 0.3 to 0.6 mL/min for 5 minutes. Control infusions were of 0.9% saline and isotonic dextran 70 (Pharmacia). All hyperosmotic infusions resulted in similar increases in normalized cochlear blood flow (nCBF) that extended to a maximum of 300% of the baseline value in a dose-dependent way during the infusion time period. Within approximately 30 minutes following infusions, nCBF had returned to baseline levels. Saline infusion alone had little effect on nCBF, but isotonic dextran 70 gave a sustained increase to 122% of the baseline levels. There was no difference between the responses of nCBF in hydropic and normal cochleas for either control or hyperosmotic solutions. Measurements of systemic hematocrit at time intervals during and following the infusions showed that transient reductions of up to approximately 8% (for the maximum osmotic challenge) occurred during the infusion. It is concluded that the hyperosmotic treatments tested here are equally effective for short-term enhancements of nCBF in both normal and hydropic cochleas. The basis of the flow increase is partially rheologic and partially due to a local vasodilation.

Measuring cochlear blood flow by laser Doppler spectroscopy

Cochlear blood flow (CBF) was studied with a commercially available laser Doppler system in 20 guinea pigs. The cochlea was exposed to permit placement of the laser Doppler probe over the intact lateral wall of the basal turn. Ketamine and xylazine were used for anesthesia, and blood pressure was monitored from the femoral artery. In some cases, skin blood flow was monitored with a second laser Doppler system, and cardiac output was monitored with an ultrasonic Doppler system placed over the right brachiocephalic artery. We found that the laser Doppler signal is composed primarily of blood flow supplied by the internal auditory artery. Local pressure on the contents of the internal auditory canal after occipital craniotomy was found to reduce CBF to 15% of its original value in a reversible fashion. There was no change in CBF after bilateral occlusion of the common carotid arteries. There appears to be a mechanism governing CBF that stabilizes its value in the face of changes in blood pressure and cardiac output. This is similar to the vascular behavior of the central nervous system. Through the use of positive airway pressure and blood removal at different rates, cardiac output could be depressed to varying degrees. The magnitude of decrease in CBF was clearly related to the rate at which cardiac output and blood pressure dropped. This was confirmed when intravenous phenylephrine was given in sequential and increasing doses. CBF increased as blood viscosity decreased, as expected according to the vascular behavior of the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of glycerol on cochlear function and ionic concentration

The use of glycerol continues to be a popular clinical test for diagnosing reversible hearing loss in patients with Meniere's disease, although its mechanism of action remains obscure. The purpose of this investigation was to study experimentally the alterations in the ionic composition and function of the cochlea which occur following glycerol administration. Immediate decreases in inner ear pressure and increases in AP threshold were seen. Delayed decreases in the endocochlear potential with increases in inner ear electrolytes occurred. However, we were unable to find any substantial changes in inner ear oxygen concentrations. Our findings support the concept that the principal action of glycerol is in osmotic reduction of inner ear pressure.

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.