Cochlear blood flow in response to dilating agents

Advances in diagnosis and treatment of vestibular migraine and the vestibular disorders it mimics

Dizziness is one of the most common chief complaints in both the ambulatory care setting and the emergency department. These symptoms may be representative of a broad range of entities. Therefore, any attempt at treatment must first start with determining the etiology. In this current perspective, we focus specifically on the diagnosis of and treatment of vestibular migraine, which is common and overlaps clinically with a variety of other diagnoses. We discuss the traditional treatments for vestibular migraine in addition to the recent explosion of novel migraine therapeutics. Because vestibular migraine can mimic, or co-exist with, a variety of other vestibular diseases, we discuss several of these disorders including persistent postural-perceptual dizziness, benign paroxysmal positional vertigo, post-concussive syndrome, Ménière's disease, and cerebrovascular etiologies. We discuss the diagnosis of each, as well as overlapping and distinguishing clinical features of which the reader should be aware. Finally, we conclude with evidence based as well as expert commentary on management, with a particular emphasis on vestibular migraine.

Influence of the Electrode Array Design on Incidence of Vertigo Symptoms and Vestibular Function After Cochlear Implantation

PURPOSE

To evaluate if a specific type of cochlear implant (CI) electrode array (EA) reveals higher rates/prevalence of vestibular symptoms and to characterize their respective relationship to intracochlear position and objective vestibular function.

METHODS

This retrospective study included 71 cochlear implantations in patients older than 18 years. The electrode position within the cochlea, electrode insertion angle, and cochlear coverage were determined from postoperative multiplanar reconstructed cone-beam computed tomography scans. All device manufacturers were represented. Data related to preoperative and postoperative PTA as well as vestibular symptoms in the preoperative and postoperative stages were collected from the patient's records.

RESULTS

Twelve of the 71 (16.9%) CI patients experienced vertigo symptoms in the early postoperative period. In 5 (7.0%) patients, the vertigo complaints lasted until the time of the first activation (5-6 weeks postoperative). Postoperative onset of vestibular symptoms was more often seen in patients receiving lateral wall (LW)/straight EAs (19%) compared to perimodiolar/precurved EAs (7%), but this was only a trend and no statistical significance was observed. Moreover, preoperative pathologic caloric responses (CRs) better predicted the postoperative onset of vestibular symptoms.

CONCLUSION

The preoperative consideration of a complicated CI-induced vertigo is important in the counseling particularly of elderly patients. We identified some risk factors for post-CI vertigo that should be considered in the patient's counseling: preoperative pathologic CRs, the extent of surgical trauma, and possibly the use of an LW EA, regardless of the length.

Vascular Supply of the Human Spiral Ganglion: Novel Three-Dimensional Analysis Using Synchrotron Phase-Contrast Imaging and Histology

Human spiral ganglion (HSG) cell bodies located in the bony cochlea depend on a rich vascular supply to maintain excitability. These neurons are targeted by cochlear implantation (CI) to treat deafness, and their viability is critical to ensure successful clinical outcomes. The blood supply of the HSG is difficult to study due to its helical structure and encasement in hard bone. The objective of this study was to present the first three-dimensional (3D) reconstruction and analysis of the HSG blood supply using synchrotron radiation phase-contrast imaging (SR-PCI) in combination with histological analyses of archival human cochlear sections. Twenty-six human temporal bones underwent SR-PCI. Data were processed using volume-rendering software, and a representative three-dimensional (3D) model was created to allow visualization of the vascular anatomy. Histologic analysis was used to verify the segmentations. Results revealed that the HSG is supplied by radial vascular twigs which are separate from the rest of the inner ear and encased in bone. Unlike with most organs, the arteries and veins in the human cochlea do not follow the same conduits. There is a dual venous outflow and a modiolar arterial supply. This organization may explain why the HSG may endure even in cases of advanced cochlear pathology.

[Vascular mechanisms in Meniere's disease]

Meniere's disease (MD) is chronic multifactorial medical condition caused by endolymphatic hydrops, which etiology is unclear. This review highlights possible vascular mechanisms of MD. Impairment of vascular regulation, further ischemic damage of labyrinth and venous drainage pathology could lead to endolymphatic hydrops. Epidemiologic studies reveal high comorbidity of MD and migraine. Both diseases could be the result of trigeminovascular dysfunction. Betahistine, the medication with vascular effect, is widely used in treatment of MD, the effectiveness of calcium channel blockers is evaluated. Keywords: vertigo, Meniere's disease, endolymphatichydrops, migraine, vascular mechanisms, betahistine.

Human inner ear blood supply revisited: the Uppsala collection of temporal bone-an international resource of education and collaboration

BACKGROUND

The Uppsala collection of human temporal bones and molds is a unique resource for education and international research collaboration. Micro-computerized tomography (micro-CT) and synchrotron imaging are used to investigate the complex anatomy of the inner ear. Impaired microcirculation is etiologically linked to various inner ear disorders, and recent developments in inner ear surgery promote examination of the vascular system. Here, for the first time, we present three-dimensional (3D) data from investigations of the major vascular pathways and corresponding bone channels.

METHODS

We used the archival Uppsala collection of temporal bones and molds consisting of 324 inner ear casts and 113 macerated temporal bones. Micro-CT was used to investigate vascular bone channels, and 26 fresh human temporal bones underwent synchrotron radiation phase contrast imaging (SR-PCI). Data were processed by volume-rendering software to create 3D reconstructions allowing orthogonal sectioning, cropping, and soft tissue analyses.

RESULTS

Micro-CT with 3D rendering was superior in reproducing the anatomy of the vascular bone channels, while SR-PCI replicated soft tissues. Arterial bone channels were traced from scala vestibuli (SV) arterioles to the fundus, cochlea, and vestibular apparatus. Drainage routes along the aqueducts were examined.

CONCLUSION

Human inner ear vessels are difficult to study due to the adjoining hard bone. Micro-CT and SR-PCI with 3D reconstructions revealed large portions of the micro-vascular system in un-decalcified specimens. The results increase our understanding of the organization of the vascular system in humans and how altered microcirculation may relate to inner ear disorders. The findings may also have surgical implications.

Effects of perilymphatic pressure, sodium nitroprusside, and bupivacaine on cochlear fluid pH of guinea pigs

CONCLUSIONS

Hydrostatic positive pressure and vasoconstrictor acidified the cochlear fluids, whereas the vasodilator made the fluids alkaline. CBF might play a role in regulating cochlea fluid pH.

OBJECTIVES

Cochlea fluid pH is highly dependent on the HCO3(-)/CO2 buffer system. Cochlear blood flow (CBF) supplies O2 and removes CO2. It is speculated that cochlear blood flow changes might affect the balance of the HCO3(-)/CO2 buffer system in the cochlea. It is known that the elevation of inner ear pressure decreases the CBF, and local application of vasodilating or vasoconstricting agents directly to the cochlea changes the CBF. The purpose of this study was to elucidate the effect of positive hydrostatic inner ear pressure and application of a vasodilator and vasoconstrictor of cochlear vessels on the pH of the endolymph and perilymph.

METHODS

The authors performed animal physiological experiments on 30 guinea pigs. Hydrostatic positive pressure was infused through a glass capillary tube inserted into the scala tympani of the basal turn. The vasodilator, nitric oxide donor (sodium nitroprusside; SNP), and the vasoconstrictor, bupivacaine, were placed topically onto the round window of the guinea pig cochlea. Endolymph pH (pHe) and endocochlear potential (EP) were monitored by double-barreled ion-selective microelectrodes in the second turn of the guinea pig cochlea. During the topical application study, scala vestibuli perilymph pH (pHv) was also measured simultaneously in the second turn.

RESULTS

The application of hydrostatic positive pressure caused a decrease in pHe and EP. Positive perilymphatic pressure caused the endolymph to become acidic pressure-dependently. Application of 3.0% SNP evoked an increase in both the pHe and pHv, following by a gradual recovery to baseline levels. On the other hand, 0.5% bupivacaine caused a decrease in both the pHe and pHv. The EP during topical application showed slight, non-significant changes.

Changes in Cochlear Blood Flow Due to Endotoxin-Induced Otitis Media

The purpose of this study was to elucidate the influence of otitis media on blood flow in the lateral wall of the cochlea by means of a model of endotoxin-induced otitis media. The cochlear blood flow (CBF) following lipopolysaccharide inoculation into the middle ear cavities of rats was measured by laser-Doppler flowmetry and compared with that of untreated ears. After this evaluation, the influence on CBF of concomitant use of a nitric oxide synthase inhibitor was also investigated. The first day after inoculation, the CBF of treated ears decreased significantly. This decrease recovered gradually between the 7th and 14th days. With concomitant use of a nitric oxide synthase inhibitor, the decrease in CBF was prevented to some extent. The results showed a functional influence upon CBF by endotoxin-induced otitis media. The significance of prophylactic use of the drug is also discussed in regard to the effect on CBF following otitis media.

Gender differences in myogenic regulation along the vascular tree of the gerbil cochlea

Regulation of cochlear blood flow is critical for hearing due to its exquisite sensitivity to ischemia and oxidative stress. Many forms of hearing loss such as sensorineural hearing loss and presbyacusis may involve or be aggravated by blood flow disorders. Animal experiments and clinical outcomes further suggest that there is a gender preference in hearing loss, with males being more susceptible. Autoregulation of cochlear blood flow has been demonstrated in some animal models in vivo, suggesting that similar to the brain, blood vessels supplying the cochlea have the ability to control flow within normal limits, despite variations in systemic blood pressure. Here, we investigated myogenic regulation in the cochlear blood supply of the Mongolian gerbil, a widely used animal model in hearing research. The cochlear blood supply originates at the basilar artery, followed by the anterior inferior cerebellar artery, and inside the inner ear, by the spiral modiolar artery and the radiating arterioles that supply the capillary beds of the spiral ligament and stria vascularis. Arteries from male and female gerbils were isolated and pressurized using a concentric pipette system. Diameter changes in response to increasing luminal pressures were recorded by laser scanning microscopy. Our results show that cochlear vessels from male and female gerbils exhibit myogenic regulation but with important differences. Whereas in male gerbils, both spiral modiolar arteries and radiating arterioles exhibited pressure-dependent tone, in females, only radiating arterioles had this property. Male spiral modiolar arteries responded more to L-NNA than female spiral modiolar arteries, suggesting that NO-dependent mechanisms play a bigger role in the myogenic regulation of male than female gerbil cochlear vessels.

An animal model for the analysis of cochlear blood flow [corrected] disturbance and hearing threshold in vivo

Impairment of cochlear blood flow (CBF) is considered to be important in inner ear pathology. However, direct measurement of CBF is difficult and has not been investigated in combination with hearing function. Six guinea pigs were used to show feasibility of an animal model for the analysis of cochlear microcirculation by intravital microscopy in combination with investigation of the hearing threshold by brainstem response audiometry (ABR). By the application of sodium nitroprusside (SNP), CBF was increased over 30 min. Reproducibility of measurements was shown by retest measurements. Mean baseline velocity of CBF was 109 +/- 19 mum/s. Vessel diameters had a mean value of 9.4 +/- 2.7 mum. Mean hearing threshold was 19 +/- 6 dB. In response to SNP, CBF velocity increased significantly to 161 +/- 26 mum/s. Mean arterial pressure decreased significantly to 36 +/- 11 mmHg. After the end of the application, CBF velocity recovered to a minimum of 123 +/- 17 microm/s. Within the retest, CBF velocity significantly increased to a maximum of 160 +/- 31 microm/s. Second recovery of CBF velocity was 125 +/- 14 mum/s. Within the second retest, CBF increased significantly to 157 +/- 25 microm/s. ABR thresholds did not change significantly. The increase in blood flow velocity occurred in spite of substantial hypotension as induced by a vasodilator. This may explain the fact that ABR threshold remained unchanged reflecting a maintained blood supply in this part of the brain. This technique can be used to evaluate effects of treatments aimed at cochlear microcirculation in inner ear pathologies.

Effect of inner ear blood flow changes in Ménière's model mice

CONCLUSIONS

The endolymphatic sac (ES) is important for inner ear fluid homeostasis. A dysfunctional ES can cause vertigo attacks following additional stress such as a sudden change in endolymphatic volume and/or pressure, or restricted inner ear blood flow.

OBJECTIVE

The purpose of this study was to elucidate the mechanism of vertigo attacks in Ménière's disease.

MATERIALS AND METHODS

Adult CBA/J mice were given an intratympanic injection of lipopolysaccharide and an intraperitoneal injection of aldosterone. These 'model' animals had epinephrine or sodium nitroprusside (SNP) instilled into the middle ear cavity. Cochleae, vestibules, and endolymphatic sacs were studied morphologically by light microscopy.

RESULTS

The injection of epinephrine into the model animals reduced the endolymphatic hydrops in the cochlea, but also produced mild hydrops in the vestibule, which was never observed in untreated (control) animals. The ES did not react to epinephrine in the normal way. Injection of SNP did not cause any changes.

Effect of inner ear blood flow changes on the endolymphatic sac

CONCLUSIONS

That the endolymphatic sac (ES) reacts to changes in inner ear blood flow may be important for homeostasis of the inner ear fluid volume and pressure.

OBJECTIVES

To elucidate the effect of changes in inner ear blood flow on the ES and to learn more about the volume and pressure regulatory function of the ES.

MATERIALS AND METHODS

Epinephrine or sodium nitroprusside (SNP) was injected into the middle ear cavity of adult CBA/J mice. The ES were analyzed morphologically by light microscopy.

RESULTS

Epinephrine reduced the luminal size of the ES leading to an accumulation of intraluminal homogeneous substance. Injection of SNP increased the size of the ES lumen, accompanied by a collapse of the lateral intercellular space (LIS) and dense perisaccular tissue. These changes were almost reversed 4 h after injection.

Adverse effects of topical papaverine on auditory nerve function

BACKGROUND

Papaverine hydrochloride is a direct-acting vasodilator used to manage vasospasm during various neurosurgical operations. Transient cranial nerve dysfunction has been described in a few cases with topical papaverine. This study supports previous reports and provides neurophysiological evidence of an adverse effect on the auditory nerve.

METHODS

We conducted a retrospective review of 70 consecutive microvascular decompression operations and studied those patients who received topical papaverine for vasospasm. Topical papaverine was used as a direct therapeutic action to manage vasospasm in a total of 11 patients. The timing of papaverine application and ongoing operative events was reviewed relative to changes in neurophysiological recordings. Brainstem auditory evoked potentials (BAEPs) were routinely used to monitor cochlear nerve function during these operations.

FINDINGS

A temporal relationship was found between topical papaverine and BAEP changes leading to complete waveform loss. The average temporal delay between papaverine and the onset of an adverse BAEP change was 5 min. In 10 of 11 patients, BAEP waves II/III-V completely disappeared within 2 to 25 min after papaverine. Eight of these 10 patients had complete loss of BAEP waveforms within 10 min. One patient showed no recovery of later waves and a delayed profound sensorineural hearing loss. The average recovery time of BAEP waveforms to pre-papaverine baseline values was 39 min.

CONCLUSIONS

Topical papaverine for the treatment of vasospasm was associated with the onset of a transient disturbance in neurophysiological function of the ascending auditory brainstem pathway. The complete disappearance of BAEP waveforms with a consistent temporal delay suggests a possible adverse effect on the proximal eighth nerve. Recommendations to avoid potential cranial nerve deficits from papaverine are provided.

Bi-phasic intensity-dependent opioid-mediated neural amplitude changes in the chinchilla cochlea: partial blockade by an N-Methyl-D-Aspartate (NMDA)-receptor antagonist

Dynorphins, glutamate, and glutamate-sensitive N-Methyl-D-Aspartate (NMDA) receptors exist in the mammalian cochlea. Dynorphins produce neural excitation and excitotoxic effects in the spinal cord through a kappa-opioid facilitation of NMDA receptor-sensitivity to glutamate. The kappa-opioid receptor drug agonists N-dimethylallyl-normetazocine [(-)-pentazocine (50 mmol)] and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide [U-50488H (100 mmol)] were administered across the cochlear round window membrane in the chinchilla. Each drug produced significant post-baseline amplitude changes in the click-evoked auditory nerve compound action potential. Amplitude changes at threshold amounted to increases in sensitivity that ranged from 4-8 decibels, measured in sound pressure level (dB SPL). The large neural amplitude increases at threshold were accompanied by progressively smaller amplitude changes at 5 and 10 dB above threshold (dB SL). However, at stimulus intensities > or =20 dB SL, post-baseline neural amplitudes were suppressed to levels below baseline and control values. These bi-phasic intensity-dependent neural amplitude changes have never before been observed following i.v. administered (-)-pentazocine in this species. Finally, the bi-phasic neural amplitude changes in U-50488H-treated (100 mmol) animals were partially blocked (except at 20 dB SL), following a round window pre-treatment with the NMDA receptor drug antagonist, dizocilpine hydrogen maleate [(+)-MK-801 (8 mmol)]. Our data suggests that endogenous dynorphins within lateral efferent olivocochlear neurons differentially modulate auditory neural excitation, possibly through cochlear NMDA receptors and glutamate. The role played by lateral efferent opioid neuromodulation at cochlear NMDA receptors, is discussed.

Response of cochlear blood flow to prostaglandin E1 applied topically to the round window

CONCLUSIONS

The increase in cochlear blood flow (CBF) after administration of prostaglandin E1 (PGE1) to the round window depends on increased blood flow through the anterior inferior cerebellar artery (AICA).

OBJECTIVES

To evaluate the response of CBF to PGE1 applied topically to the round window, and to investigate the origin of blood flow changes after this topical application.

MATERIAL AND METHODS

The response of CBF to topically applied PGE1 was measured by placing the tip of a laser Doppler probe on the bony wall of the basal turn of the cochlea after the middle ear mucosa over the cochlea had been removed in guinea pigs and rats. In rats, the CBF response to PGE1 administration was investigated after occlusion of the AICA or stapedial artery.

RESULTS

CBF increased following PGE1 administration in both guinea pigs and rats. In rats, CBF increased from 100% to 132%+/-10% (mean+/-SD) after the topical application of 0.5 microl of a 0.014% PGE1 solution. CBF decreased after occlusion of the AICA or stapedial artery but did not increase after PGE1 administration during occlusion of the AICA. The CBF response to PGE1 administration was similar before and after occlusion of the stapedial artery.

Fatores clínicos, etiológicos e evolutivos da audição na surdez súbita

Existem várias terapias preconizadas para o tratamento da surdez súbita, algumas apresentam riscos significativos necessitando inclusive de internação hospitalar. OBJETIVO: Este estudo prospectivo analisa aspectos clínicos, etiológicos e evolutivos nos casos de surdez súbita (SS) em pacientes tratados ambulatorialmente com medicação oral. FORMA DE ESTUDO: clínico com coorte transversal. MATERIAL E MÉTODO: 40 pacientes com perda súbita da audição submeteram inicialmente a avaliação clínica otorrinolaringológica, testes audiométricos, análise hematológica e ressonância magnética. Confirmado o diagnóstico de SS, todos os pacientes receberam inicialmente prednisona e pentoxifilina sendo acompanhados por pelo menos um ano. RESULTADO: 45% (n=18) apresentaram normalização dos limiares auditivos, 40% (n=16) apresentaram melhoras auditivas, 15% (n=6) mantiveram os mesmos limiares iniciais. Nove casos (22,5%) apresentaram manifestações clínicas que justificaram a perda auditiva (infecção viral, fatores imunomediados, alterações vasculares e outros), três (7,5%) apresentaram tumores na região do ângulo ponto-cerebelar. A evolução auditiva nestes 12 casos com etiologia presumida não apresentou diferença estatística significante em relação aos 28 casos sem etiologia definida. O tratamento clínico instituído nos primeiros sete dias de instalação da perda auditiva, nos pacientes que obtiveram melhora, foi o único parâmetro estatisticamente significante dos fatores prognóstico avaliado. CONCLUSÃO: A pesquisa exaustiva etiológica deve ser realizada em qualquer caso de perda auditiva neurossensorial aguda. A presença de 7,5% de tumores localizados na região do ângulo ponto-cerebelar nos casos de SS juntamente com outras causas tratáveis justifica a investigação clínica nestes pacientes. Nossos pacientes apresentaram uma boa melhora auditiva em 67,5% dos casos, independentemente da etiologia. O início da terapia nos primeiros sete dias de instalação da perda auditiva foi o único fator de melhora significante dos limiares auditivos.

Clinical, etiological and progression factors of hearing in sudden deafness

Out of the many forms of therapy for sudden deafness, some require hospitalization and present significant risks.

AIM

This prospective study analyzes etiology and evolution in cases of sudden deafness (SD) where outpatient oral treatment was used.

STUDY DESIGN

Clinical with transversal cohort.

MATERIAL AND METHOD

Forty cases of sudden hearing loss were followed for at least one year. All were submitted to initial clinical evaluation, auditory tests, routine blood analysis, and magnetic resonance imaging. All received initial treatment with pentoxifylline and prednisone.

RESULTS

45% (n=18) presented normal auditory thresholds, 40% (n=16) showed some improvement in hearing, 15% (n=6) maintained initial hearing level. Nine cases (22.5%) presented clinical conditions possibly implicated in hearing loss (viral infection, immunomediated hearing loss, vascular disorders, and so on); three (7.5%) had cerebellopontine tumors. Evolution of hearing in these 12 cases with presumed etiology presented no differences from hearing in the 28 cases without any known etiological factor. Clinical treatment within the first seven days was the only statistically significantly different condition in patients who improved hearing.

CONCLUSIONS

An objective search for etiological bases should be conducted in any case of acute sensorineural hearing loss. The presence of cerebellopontine tumors in 7.5% of cases of SD, among other treated causes, justifies a thorough clinical investigation in these patients. Overall good evolution of hearing was observed in 67.5% of cases of SD, regardless of its etiology. Therapy within the first seven days of SD was significantly related to better outcomes in hearing.

A comparative study of intratympanic steroid and NO synthase inhibitor for treatment of cochlear lateral wall damage due to acute otitis media

We studied the damage to the cochlear lateral wall induced by otitis media and the therapeutic effects of intratympanic administration of steroid and nitric oxide (NO) synthase inhibitor. In Sprague-Dawley rats, right middle ear cavities were inoculated with lipopolysaccharide, followed after 30 min by intratympanic administration of dexamethasone, NOS-inhibitor or PBS. Twenty-four hours after lipopolysaccharide inoculation, cochlear blood flow was measured by laser-Doppler flowmetry. Prostaglandin E(1) was topically applied to the round window membrane of the right ear and changes in cochlear blood flow were calculated. Changes of cochlear blood flow were significantly different among the three groups. Increases in cochlear blood flow following PGE(1) application were higher in the group that received NOS-inhibitor. Electron microscopic examination revealed that changes in the stria vascularis were less severe in rats treated with dexamethasone or NOS-inhibitor. Our results show the effectiveness of intratympanic dexamethasone or NOS-inhibitor in treating cochlear lateral wall damage caused by otitis media.

Disorders of cochlear blood flow

The cochlea is principally supplied from the inner ear artery (labyrinthine artery), which is usually a branch of the anterior inferior cerebellar artery. Cochlear blood flow is a function of cochlear perfusion pressure, which is calculated as the difference between mean arterial blood pressure and inner ear fluid pressure. Many otologic disorders such as noise-induced hearing loss, endolymphatic hydrops and presbycusis are suspected of being related to alterations in cochlear blood flow. However, the human cochlea is not easily accessible for investigation because this delicate sensory organ is hidden deep in the temporal bone. In patients with sensorineural hearing loss, magnetic resonance imaging, laser-Doppler flowmetry and ultrasonography have been used to investigate the status of cochlear blood flow. There have been many reports of hearing loss that were considered to be caused by blood flow disturbance in the cochlea. However, direct evidence of blood flow disturbance in the cochlea is still lacking in most of the cases.

Preventing internal auditory artery vasospasm using topical papaverine: an animal study

BACKGROUND

Internal auditory artery (IAA) spasm is thought to be one of the causes of postoperative sensory hearing loss after attempted hearing preservation removal of an acoustic neuroma. The use of topical papaverine, a nonspecific vasodilator, to prevent vascular insufficiency to the inner ear and to improve hearing outcomes has been suggested but not proven.

MATERIALS AND METHODS

Vasospasm was mechanically induced by compressing the IAA in the control ears of six rabbits after application of topical saline. The subsequent reduction of cochlear blood flow (CBF) was measured using a laser-Doppler (LD) flow-monitoring technique. Functional loss of cochlear activity was verified with distortion product otoacoustic emissions (DPOAE). The contralateral experimental ears were treated with the topical application of papaverine directly to the IAA and cochleovestibular nerve complex. CBF and DPOAE were compared between the control and papaverine treated ears for 3-minute and 5-minute IAA compressions.

RESULTS

Every control ear demonstrated some degree of postcompression IAA vasospasm (i.e., reduced CBF) and reduction of DPOAE. Nearly complete recovery of CBF and DPOAE to baseline was observed in all of the papaverine treated ears.

CONCLUSIONS

An animal model of IAA vasospasm was described. Mechanically induced vasospasm of the IAA was prevented by the topical application of papaverine. These findings have clinical implications for surgical procedures involving the internal auditory canal/cerebellopontine angle such as acoustic neuroma removal.

Comparison of the vascular innervation of the rat cochlea and vestibular system

In order to gain a better understanding of the neuronal and local control of inner ear blood flow, the vascular innervation to the rat cochlea and vestibular system was examined. Specimens were removed in toto beginning at the basilar artery extending to the anterior inferior cerebellar artery, labyrinthine artery, common cochlear artery, modiolar artery and anterior vestibular artery. When possible the vessels were dissected in continuity through the cribrose area. The vestibular endorgans were also removed. Specimens were examined using immunohistochemical techniques for the presence of vasoactive intestinal peptide, neuronal nitric oxide synthase, neuropeptide-Y, substance P and calcitonin gene related peptide. Results show that the vasculature to the cochlea and vestibular portion of the inner ear receive similar types of nonadrenergic innervation, that within the vestibular endorgans, only CGRP and SP were found in the neuroepithelium or in association with vessels, and that within the vestibular system, the majority of the vascular innervation appears to stop at or near the cribrose area. In the cochlea however, it extends to include the radiating arterioles. These findings suggest that cochlear blood flow is under finer control and that neuronally induced changes in blood flow may have a more global effect in the vestibular periphery.

Ototoxicity of sodium nitroprusside is not due to nitric oxide

Sodium nitroprusside (SNP) has been used as a donor for nitric oxide (NO) to study the effects of NO on the mammalian cochlea. In the present study, we set out to determine whether NO was the chemical responsible for the ototoxic effects seen after the application of SNP at the round window membrane of the adult guinea pig cochlea. In the first instance, NO released from S-nitrosocysteine, a compound not related to cyanide, has no toxic effect on the hair cells of the cochlea. Light-exposed SNP that could no longer produce NO, light-exposed SNP to which acetylcysteine (ATC) or hydroxycobalamin (HCL) was added to eliminate cyanide, and freshly prepared SNP to which ATC or HCL was added were also tested. Six groups of animals consisting of three animals in each group were used. The single chemical or combination of chemicals stated above was soaked in a piece of gelfoam that was then applied to the round window membrane of the animal under ketamine-xylasine anesthesia. The animals were reanesthetized 3 days later and perfused for scanning electron microscopy and hair cell quantitative analysis. The results showed that, in animals given S-nitrosocysteine, no hair cell loss was noted, while light-exposed SNP led to severe hair cell damage similar to that seen after the administration of fresh SNP. In animals treated with the mixture of light-exposed SNP and ATC or HCL, or fresh SNP with ATC or HCL, ototoxicity was significantly attenuated. These results have convincingly demonstrated that NO at a certain level is not destructive to auditory hair cells and the hair cell loss observed after SNP application is most likely due to the cyanide released from the SNP instead of NO.

Expression of inducible nitric oxide synthase (iNOS/NOS II) in the cochlea of guinea pigs after intratympanical endotoxin-treatment

Since NO is believed to be involved in cochlear physiology, presence of the constitutive isoforms of nitric oxide synthase (NOS), and the target enzyme of NO, soluble guanylyl cyclase (sGC) in structures of the mammalian cochlea have been demonstrated. To date, no reports have been published regarding the detection of the inducible isoform (NOS II) in the cochlea. In order to show the capability of iNOS expression in cochlear tissue, a mixture of proinflammatory bacterial lipopolysaccharides (LPS) and tumor necrosis factor alpha (TNF-alpha) was injected into the tympanic cavity of guinea pigs, vs. saline-solution as control. Paraffin sections of LPS/TNF-alpha treated and saline-treated cochleae (6 h) were examined immunohistochemically with specific antibodies to neuronal, endothelial and inducible NOS and to sGC. Initiated expression of iNOS in the cochlea was observed in the wall of blood vessels of the spiral ligament (SL) and the modiolus, in supporting cells of the organ of Corti, in the limbus, in nerve fibers and in a part of the perikarya of the spiral ganglion after LPS/TNFalpha-treatment. iNOS was not detected in saline-treated control tissue. Expression of both constitutive NOS-isoforms (endothelial and neuronal NOS) and of sGC showed no significant differences in both experimental groups. Endothelial eNOS and neuronal bNOS were detected co-localized in ganglion cells, in nerve fibers, in cells of the SL and in supporting cells of the organ of Corti, but not in sensory cells. Strong labeling for bNOS became evident in the endosteum of the cochlea, while in the endothelium of blood vessels and in the epithelium of the limbus only eNOS could be labeled. sGC could be detected in SL, in supporting and sensory cells of the organ of Corti, in nerve fibers, ganglion cells, in the wall of blood vessels and in the limbus-epithelium. While small amounts of NO, generated by bNOS and eNOS, seem to support the cochlear blood flow and auditory function as well as neurotransmission, high amounts of iNOS-generated NO could have dysregulative and neurotoxic effects on the inner ear during bacterial and viral infections of the middle and inner ear.

The nitric oxide/cyclic GMP pathway: a potential major regulator of cochlear physiology

The nitric oxide (NO)/cyclic guanosine monophosphate (GMP) pathway is now recognized as a major regulatory system in cell physiology and tissue homeostasis. This pathway may control processes as diverse as muscle relaxation, gut peristalsis, neurotransmission and hormonal secretion. It is also involved in the development and function of sensory systems such as vision and olfaction. This review will detail the NO/cyclic GMP pathway, evaluate studies in the auditory system and discuss its potential participation in cochlear blood flow, supporting cell physiology and excitotoxicity.

Ototoxicity of sodium nitroprusside

Nitric oxide (NO) not only has normal physiological roles like vasodilation and neurotransmission in the living organism, it could also have possible neurodestructive effects under certain pathological conditions. The present study aimed to determine whether direct exposure of guinea pig cochlea to a NO donor like sodium nitroprusside (SNP), or a nitric oxide synthase (NOS) inhibitor like N(G)-nitro-L-arginine methyl ester (L-NAME), would cause damage to the auditory hair cells. A piece of gelfoam was placed on the round window of the right ear of adult albino guinea pigs. It was then soaked with 0.1 ml of SNP (3.4 microM), 0.1 ml of L-NAME (9.3 microM or 18.5 microM) or 0.1 ml of injection water, the vehicle used to dissolve the above chemicals. Twelve animals receiving SNP were perfused 1 day, 2, 3 and 7 days later, with three animals being used for each survival period. Six animals receiving L-NAME were allowed to survive up to 7 days before perfusion. Eight animals receiving injection water or 0.45% saline were used as controls. With the scanning electron microscope, the inner and outer hair cells were counted over a 1 mm length of the basilar membrane in each turn of every cochlea. The results showed that, in animals treated with L-NAME at both concentrations stated, no significant loss of either inner or outer hair cells was noted in any part of the cochlea studied. However, as early as 1 day after SNP treatment, a striking loss of inner and outer hair cells was observed in the three lower turns of the cochlea. Damage to the outer hair cells was extended to the apical turn with increasing survival period, but no significant loss of inner hair cells was evident in the apical turn at any of the survival periods studied. To rule out the possibility that the effects were due to the presence of cyanide, a metabolite of SNP, hydroxycobalamin was introduced into the scala tympani of three animals through a cannula-osmotic pump device during SNP treatment. There was no significant difference in the results between the groups with and without hydroxycobalamin infusion 7 days after SNP treatment. The present study suggests that an excessive production of NO in the inner ear could lead to extensive loss of hair cells.

Localization of soluble guanylate cyclase activity in the guinea pig cochlea suggests involvement in regulation of blood flow and supporting cell physiology

Although the nitric oxide/cGMP pathway has many important roles in biology, studies of this system in the mammalian cochlea have focused on the first enzyme in the pathway, nitric oxide synthase (NOS). However, characterization of the NO receptor, soluble guanylate cyclase (sGC), is crucial to determine the cells targeted by NO and to develop rational hypotheses of the function of this pathway in auditory processing. In this study we characterized guinea pig cochlear sGC by determining its enzymatic activity and cellular localization. In cytosolic fractions of auditory nerve, lateral wall tissues, and cochlear neuroepithelium, addition of NO donors resulted in three- to 15-fold increases in cGMP formation. NO-stimulated sGC activity was not detected in particulate fractions. We also localized cochlear sGC activity through immunocytochemical detection of NO-stimulated cGMP. sGC activity was detected in Hensen's and Deiters' cells of the organ of Corti, as well as in vascular pericytes surrounding small capillaries in the lateral wall tissues and sensory neuroepithelium. sGC activity was not observed in sensory cells. Using NADPH-diaphorase histochemistry, NOS was localized to pillar cells and nerve fibers underlying hair cells. These results indicate that the NO/cGMP pathway may influence diverse elements of the auditory system, including cochlear blood flow and supporting cell physiology.

Endothelin-A receptors mediate vasoconstriction of capillaries in the spiral ligament

The purpose of this study was to determine whether endothelin-1 (ET-1), endothelin-2 (ET-2) or endothelin-3 (ET-3) alter the vascular diameter of capillaries in the spiral ligament. Changes in vascular tone were measured in capillaries from the isolated spiral ligament in vitro. Capillaries were occluded on one end and opened on the other end. Red blood cells trapped in the capillaries served as markers for a luminal volume defined by the red cell itself, the capillary wall and the occluder. Movement of the red cell toward the open end was taken as evidence for vasoconstriction and movement of the red cell toward the occluder was taken as evidence for vasodilation. The inner diameter of the capillaries was 7.0 microm and decreased maximally by a factor of 0.8 in response to ET-1 and ET-2 (both 10(-8) M). Vasoconstriction induced by ET-1 and ET-2 was concentration-dependent in the range between 10(-12) and 10(-8) M whereas ET-3 (10(-8) M) had no effect. The EC50s for ET-1 and ET-2 were 1.2 x 10(-10) M and 1.4 x 10(-9) M, respectively. Thus, the potency order was ET-1 > ET-2 >> ET-3. Vasoconstriction induced by ET-1 and ET-2 was completely inhibited by the competitive antagonist 10(-6) M BQ-123 (cyclic D-Asp-L-Pro-D-Val-L-Leu-D-Trp). Vasoconstriction induced by ET-1 or ET-2 continued for more than 1 min after removal of agonist from the perfusate. Rapid vasodilation of capillaries preconstricted by ET-1 was observed in response to 10(-3) M sodium nitroprusside. Sodium nitroprusside, however, had no significant effect on the vascular diameter of resting capillaries. These results demonstrate that capillaries in the spiral ligament can constrict and the endothelin-mediated vasoconstriction occurs via ET(A) receptors.

Blood flow-independent accumulation of cisplatin in the guinea pig cochlea

Considerable interindividual variability in the ototoxic effect of cisplatin has become the unpredictable dose-limiting factor in its use as curative as well as palliative therapy. The drug accumulates in highly vascular areas in the cochlea, causing dose-related hair cell loss. The purpose of this study was to assess blood flow-dependent aspects of cisplatin absorption in the cochlea in order to better understand factors that may influence cisplatin-induced ototoxicity. The effect of reduced cochlear blood flow on the ototoxic action of cisplatin was studied in guinea pigs. Before cisplatin administration the cochlear vasculature in each animal was unilaterally pre-constricted, by the application of 2% epinephrine to the round window. A 20-30% reduction in cochlear blood flow, assessed by laser Doppler flowmetry, was maintained before and after intravenous infusion of 0.1% cisplatin. Cisplatin infusion affected cochlear blood flow but not vessel conductivity. The cochlear blood flow decrease, maintained by local epinephrine application to the round window during cisplatin infusion, did not alter the cisplatin-induced hearing loss. In addition, the concentration of free cisplatin in scala tympani perilymph did not differ between epinephrine-treated and non-treated ears. Our results indicate that cisplatin transport into the cochlea is not an energy-dependent process in the lateral wall vasculature.

Blockade of opioid-induced changes in auditory function at the level of the cochlea

OBJECTIVE

We have previously investigated the auditory neural effects of the kappa-opioid receptor agonist, (-)pentazocine. When administered intravenously (i.v.), this drug temporarily alters auditory nerve compound action potential (CAP) amplitudes. To test the hypothesis that the observed neural effects of i.v. (-)pentazocine occur via kappa-receptor interactions within the cochlea, we attempted to block these effects by employing a specific kappa-opioid receptor antagonist applied directly to the cochlear round window (RW) membrane.

DESIGN

In 31 normal-hearing, male pigmented chinchillas, amplitude changes in the click-evoked auditory CAP (N1) were tracked at six stimulus intensities during a baseline and a postbaseline period in which i.v. (-)pentazocine (8 mg/kg) was administered. (-)Pentazocine administration was preceded by the delivery to the cochlear RW membrane of an artificial perilymph solution given alone or containing the kappa-opioid receptor selective antagonist, norbinaltorphimine (Nor-BNI), which was administered at two concentrations in separate groups of animals.

RESULTS

The amplitude increase in the CAP after (-)pentazocine was significantly reduced when i.v. (-)pentazocine was preceded by RW-administered Nor-BNI (4 mM).

CONCLUSIONS

The reversibility of agonist effects by Nor-BNI indicates direct or indirect opioid kappa-receptor-mediated auditory neural effects at the level of the cochlea and suggests a connection between kappa-receptors and auditory neural function.

Naloxone blockade of (-)pentazocine-induced changes in auditory function

OBJECTIVE

In a previous report, we found that intravenous (i.v.) (-)pentazocine improved auditory sensitivity and significantly altered compound action potential (CAP) amplitudes. Its sigma (sigma)-receptor-selective optical isomer (+)pentazocine administered at the same dose was without effect, suggesting that the observed auditory neural effects might be mediated by an opioid receptor. To directly test this hypothesis, in the present investigation we attempted to antagonize the auditory neural effects of (-)pentazocine using the pure, nonspecific drug antagonist naloxone.

DESIGN

In 25 normal-hearing, male, pigmented chinchillas, amplitude and latency changes in the click-evoked auditory nerve CAP (N1) and cochlear microphonic (CM) were tracked at six stimulus intensities during a baseline period and after the postbaseline administration of the opioid drug agonist (-)pentazocine (16 mg/kg; i.v.). In separate groups of chinchillas, (-)pentazocine was given alone or administered in combination with the standard opioid receptor antagonist naloxone administered at two doses.

RESULTS

Robust changes in CAP amplitudes after (-)pentazocine occurred in the absence of measurable alterations in CAP response latencies, CM amplitudes, or blood chemistries and were significantly antagonized when naloxone (5 mg/kg) was added to the i.v. infusion.

CONCLUSIONS

The observed blockade clearly indicates that the agonist effects of (-)pentazocine are opioid receptor-mediated and suggests a connection between opioid receptors and auditory neural function. Mechanisms of action and the connection between an opioid modulation of auditory function and stress, hyperacusis, and tinnitus are discussed.

Age-related changes in cochlear vascular conductance in mice

Vascular changes contribute to age-related hearing loss but the mechanisms involved in microvascular reactivity, particularly in the aged ear, are still incompletely understood. In this study, possible age-related changes in cochlear blood flow (CBF) and vascular reactivity were studied in presbycusic mice (C57BL/6) and young, age-matched, and old controls (CBA/J) without presbycusis. Reactivity was monitored by laser Doppler flowmetry and assessed by change in cochlear vascular conductance (VC) (defined as the ratio of CBF to blood pressure) in response to round window-applied sodium nitroprusside, a vasodilating agent. Mean VC response of C57BL/6 mice differed from controls both in maximum response and in post-drug recovery time. In C57BL/6 mice, mean VC increased about 28%, in contrast to an increase of over 40% in young and age-matched CBA/J controls. A less elevated VC response, similar to that of the presbycusic mice, was shown by aged (20-21 month) controls. Also, VC response in C57BL/6 mice was sustained throughout the 60 min observation period, while response of most CBA/J controls recovered in 50 min or less. These changes suggest age-dependent, pathologically-related altered responsiveness in cochlear vascular reactivity.

Effect of topical application of nitroglycerin on cochlear blood flow

PURPOSE

To investigate whether topical application of the vasodilator nitroglycerin would increase cochlear blood flow without adversely affecting inner ear function.

MATERIALS AND METHODS

The effect of topical application of nitroglycerin to the round window membrane on cochlear blood flow was measured in 33 guinea pigs with a laser Doppler flowmeter. Endocochlear potential, as an indicator of inner ear function, was recorded by a glass microelectrode inserted through the round window membrane in a second series of 27 guinea pigs. Blood pressure was also monitored in both experiments.

RESULTS

Low doses (0.0001 to 1 microgram) of nitroglycerin induced an increase in cochlear blood flow with no change in blood pressure. The cochlear blood flow was maximally increased by approximately 50%. A high dose (50 micrograms) of nitroglycerin induced a significant decrease in blood pressure but did not significantly affect endocochlear potential.

CONCLUSIONS

Topical application of nitroglycerin may be useful in increasing cochlear blood flow in various inner ear diseases.

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.

Effect of hemorrhagic hypotension on blood flow to the basal and upper turns of the cochlea

The effect of hemorrhagic hypotension on blood flow in the cochlea was studied in guinea pigs. Using two laser Doppler probes, simultaneous measurements were made from the basal and upper turns of the cochlea. Hypotension induced by removal of blood resulted in reduction of cochlear blood flow. Cochlear blood flow, however, did not reduce the same extent as the extent of hypotension and the reduction of blood flow was observed to be more remarkable in the upper turns than in the basal turn.

Effects of vasodilating agents on cochlear blood flow in mice

The mouse has several distinct advantages as an experimental model for study of the cochlear blood flow (CBF) in the mammal, particularly for studies involving genetic manipulation, development and aging factors. In this investigation we evaluated the effects of selected vasodilating agents on CBF using laser-Doppler flowmetry. Sodium nitroprusside, hydralazine and pentoxifylline were applied topically on the round window (RW) or systemically by a subcutaneous injection. Topical application of sodium nitroprusside and hydralazine elevated CBF significantly, while systemic administrations of these two drugs failed to elevate CBF consistently. Pentoxifylline did not change CBF following topical application but increased CBF slightly following systemic administration. Age and gender did not influence the CBF response to topical application of sodium nitroprusside.

Neuronal regulation of cochlear blood flow in the guinea-pig

1. Previous studies have shown that electrical stimulation (ES) of the guinea-pig cochlea causes a neurally mediated increase in cochlear blood flow (CBF). It is known that the centrifugal neuronal input to the cochlea comes through the perivascular sympathetic plexus from the cervical sympathetic chain and along the vestibular nerve (VN) from the periolivary area of the brainstem. Both of these neuronal systems are distributed topographically in the cochlea. 2. In order to study the neural origins of ES-evoked CBF increase, laser Doppler flowmetry was used to test the following hypotheses. (a) The response is regional, that is, limited to the area of the cochlea stimulated. To test this we performed differential ES of the cochlear turns. CBF was measured from either the third or the first turn. (b) The response is mediated via autonomic receptors within the cochlea. To study this, we applied atropine, succinylcholine and idazoxan locally to the cochlea. (c) The response is influenced by neuronal input via the sympathetic cervical chain (SC) and components of the VN. We stimulated and sectioned the SC, and sectioned the VN, to test this hypothesis. 3. We observed that the CBF response was topographically restricted to the stimulated region. Locally applied muscarinic or nicotinic antagonists (atropine and succinylcholine respectively) did not affect the response. However, local idazoxan (an alpha 2-blocker) eliminated the response. Locally applied adrenaline and SC stimulation modified the dynamic range of the response. SC sectioning enhanced the responsiveness of the cochlear vasculature to ES. The VN section caused a temporary decrease in CBF and elimination of the ES-evoked CBF response. 4. We conclude that the release of dilating agents is topographical with respect to ES current flow, the ES-evoked CBF increase is peripherally mediated via alpha 2-receptors, and the response is influenced by input via the SC. The elimination of the response by VN sectioning proximal to the brainstem indicated that fibres of the VN mediate the CBF increase during direct cochlear ES. The data suggest that these fibres may be the efferent limb of a neural loop involved with the regulation of CBF. Such a system could provide a mechanism for the rapid increase in CBF with organ stress.

Amplitude enhancement is seen in the cochlear nerve but not at, or before, the afferent synapse

The amplitude of the cochlear nerve compound action potential (CAP) produced by a moderate intensity tonal stimulus (S2) can be enhanced when S2 is preceded by a low intensity S1 of the same frequency. The presence of S1 had no observable influence on the threshold of the CAP to S2. Enhancement was not observed in the cochlear microphonics or summating potentials. Deactivation of the contralateral olivocochlear bundle did not influence enhancement. Tetrodotoxin (TTX) was applied to the round window to block cochlear nerve spike activity, resulting in a residual EPSP-like potential, as described in the guinea pig by Dolan et al. (1989). Kainic acid, in turn, eliminated this EPSP-like response. Even though some differences were found in the responses of the gerbil and their guinea pig preparation to TTX and kainic acid, enhancement was not observed in this residual potential. When enhancement was observed at the level of the CAP, it was observed at brainstem levels. It is suggested that enhancement originates within the cochlear nerve axons.

The cochlear blood flow: a comparison between the laser Doppler and the microsphere surface methods

Since the introduction in the early 1980s, of the laser Doppler (LD) method for measuring cochlear blood flow (CBF) it has been debated whether the measured changes reflect the total or regional blood flow and whether the method per se influences the CBF. In order to answer those questions, the effect of one vasodilating drug, sodium nitroprusside, was investigated after topical application on the round window membrane (RWM) with respect to its influence on CBF. Two different techniques, the microspheres surface method and the LD method, were used. Untreated animals and animals which received saline or nicotine acid on the RWM were used as controls. The effects on CBF and blood pressure (BP) were continuously registered with LD. When a maximal flow had stabilized, 6 x 10(6) microspheres were injected into the left side of the heart. After the microspheres had been distributed within the body, the animals were killed. Both cochleae were microdissected and the microspheres counted turn by turn in the lateral wall. The number of spheres in the two ears was compared and the difference was recorded as the increase caused by the drug. The percentage change in CBF measured using the LD was compared with that obtained by using the microsphere surface method (MSM). No change in CBF measured by the two techniques was registered in the untreated animals, or after saline or nicotinic acid, while sodium nitroprusside induced a substantial increase in CBF. The mean percentage change of CBF measured with the LD method was compared with the calculated mean percentage change of microspheres for all turns in the cochlea, and in the first turn. Student's t-test and the linear correlation coefficient were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)