Topography of neurovascular structures in relation to round window and how it relates to cochlear implantation.

PURPOSE: The purpose of this investigation was to evaluate the distances and angles on basal turn of cochlea in relation to round window at which the jugular bulb, internal carotid artery and facial nerve are at maximal risk and their implications in cochlear implantation (CI). METHODS: Fifty-four cadaveric temporal bones were microdissected to expose the basal turn of cochlea, the carotid canal, the facial canal and the jugular fossa. The points were marked on the basal turn of cochlea, where there was minimum distance of basal turn of cochlea from the roof of the jugular fossa (point a), carotid canal (point b) and facial canal (point c). The distances and angles of these points from the round window were measured. RESULTS: The points a, b and c were at mean (range) distances of 2.8 mm (1.3-4.1 mm), 8.4 mm (6.5-10.4 mm) and 16.4 mm (12.5-20.5 mm) and at mean angles of 30° (15°-45°), 111° (71°-136°) and 284° (255°-315°), respectively, from the round window. CONCLUSIONS: This study highlights that 2.8 ± 0.5 mm (30 ± 5.40), 8.4 ± 1 mm (111 ± 12.70) and 16.4 ± 1.7 mm (284 ± 13.5) from the round window are the high-risk points on the basal turn of the cochlea for the jugular bulb, internal carotid artery and facial nerve, respectively. A wide range found for each parameter indicates that it is mandatory to evaluate these distances in each CI patient on preoperative radiographs to avoid intraoperative injury to these vital structures.

Simultaneous measurement of electrocochleography and cochlear blood flow during cochlear hypoxia in rabbits.

In this study, a new monitoring system is developed to measure cochlear blood flow (CBF) and electrocochleography (ECochG) during transient ischemic episodes of the cochlea. A newly designed otic probe was used for the simultaneous recordings of laser-Doppler CBF and ECochG directly from the round window (RW). The probe enabled the recording of high amplitude compound action potentials (CAP) and cochlear microphonics (CM) with few averages. Experiments were conducted on rabbits to generate episodes of cochlear ischemia by using timed compressions of the internal auditory artery (IAA). The computer monitoring system extracted and measured CAP and CM components from ECochG in real-time. Results indicate that CM and CAP generally followed CBF during compressions and releases of IAA. Both CBF values and CAP amplitudes showed an overshoot following the reperfusion. CAP amplitude measures were found to be very sensitive to ischemia showing very rapid amplitude, latency and morphological changes. CM amplitude decreased more slowly than the CAP and CBF. Simultaneous recordings of CBF and ECochG using the otic probe provide a valuable neuromonitoring tool to investigate the dynamic behavior of the cochlea during ischemia.

Effects of leukotrienes and prostaglandins on cochlear blood flow in the chinchilla.

Prostaglandins (PGs) such as PGE2 and PGI2 are vasodilators, and leukotrienes (LTs) such as LTB4 and LTC4 are vasoconstrictors. Our previous studies have shown that salicylate ototoxicity is associated with decreased levels of PGs and increased levels of LTs. We hypothesized that vasodilating PGs increase cochlear blood flow and vasoconstricting LTs decrease cochlear blood flow. PGE2, Iloprost (a PGI2 analog), LTB4, and LTC4 were applied to the round window membranes of chinchillas and cochlear blood flow was measured with a laser Doppler flowmeter. PGE2 increased cochlear blood flow, while LTC4 decreased cochlear blood flow. This findings show that vasodilating PGs may have therapeutic implications for sensorineural hearing loss and/or vertigo by increasing cochlear blood flow. Vasoconstricting LTs may cause hearing loss by decreasing cochlear blood flow.

Improvement in inner ear blood flow by nitric oxide following experimentally induced cochlear thrombosis in anesthetized guinea pigs.

The nitric oxide (NO) donor sodium nitropruside (SNP) applied to the round window membrane has recently been found to increase cochlear blood flow (CoBF) in normal guinea pigs and in normal and presbyacusic mice. This study examined the effect of topical applications of SNP on experimentally impaired CoBF in anesthetized guinea pigs. Small (3 microliters) portions of 3% SNP were applied to the round window niche of both normal and thrombosed cochleas. Local vascular impairment was produced by ferromagnetic thrombosis of cochlear blood vessels and the microcirculation measured using laser Doppler flowmetry. Ferromagnetic thrombosis resulted in a mean decrease of CoBF to 52% of baseline. There was a clear improvement in mean CoBF to 84% of baseline by the topical application of SNP that depended on the degree of ischemic damage produced. Under neuroleptanalgesia and ketamine-xylazine anesthesia, significant increases in CoBF were measured in normal ears as well as in the thrombosed ones. However, several SNP applications were needed to improve the impaired CoBF, while a single portion was sufficient in the normal cochlea to cause a drastic increase in mean CoBF to 234% of baseline. In urethane-anesthetized animals, no flow increase was found despite repeated drug administration. Careful evaluation of the laser Doppler signals was necessary to accurately determine the concentrations of the moving blood cells and their mean velocities.

[Microlight-guided spectrophotometry of the cochlea]. / Mikrolichtleiter-Spektrophotometrie an der Cochlea.

BACKGROUND: Intraoperative measurements of local intracapillary hemoglobin oxygenation of the human cochlea via the round window membrane have been shown to be possible using the Erlangen microlight-guided spectrophotometer. The aim of the present study was to develop a new microlight guide suitable for measurements in the round window niche and to evaluate electrophysiologically the possible impact of the procedure on the cochlea. METHODS: Measurements were made for wavelengths of 450 to 900 nm at a total power of 5.3 mW. The exit diameter of the light guide was 200 microns and the angle used was 20 degrees. The recording depth was about 250 microns. The atraumatic character of the spectrophotometry was demonstrated by monitoring the compound action potential (CAP) threshold tuning curve from 1 to 34 kHz in ketanest-anesthetized guinea pigs. RESULTS: CAP thresholds remained constant (0.3 +/- 3.9 dB SPL) during 3 to 30 min of exposure to the light. CONCLUSIONS: These results suggest that the spectrophotometry may be useful as a new technique for intraoperative monitoring of intracapillary hemoglobin without causing physiological deterioration of the cochlea.

Effect of leukotriene inhibitor on cochlear blood flow in salicylate ototoxicity.

Our previous studies showed that salicylate ototoxicity is associated with decreased levels of vasodilating prostaglandins (PGs) and increased vasoconstricting leukotrienes (LTs) in the perilymph and reduced cochlear blood flow (CoBF). The purpose of this study was to test the hypothesis that leukotriene inhibitor prevents salicylate ototoxicity by preventing abnormal elevation of LT levels in the inner ear, thus averting a decrease in CoBF resulting from abnormal levels of arachidonic acid metabolites in the inner ear. Ototoxicity was induced in chinchillas by either local round window membrane (RWM) application or systemic treatment with salicylate both with and without pretreatment with leukotriene inhibitor (Sch 37224). A moderate reduction in CoBF was documented with both local RWM and systemic treatment with salicylate. Salicylate induced hearing loss and reduction in CoBF were prevented by pretreatment with a leukotriene inhibitor. This study suggests that leukotriene inhibitor prevents salicylate ototoxicity by averting a decrease in CoBF mediated by abnormal levels of arachidonic acid metabolites in the inner ear.

Scanning electron microscopic studies of the capillaries crossing the scala tympani.

Scanning electron microscopic examinations were carried out on the perilymphatic space bordering the round window in guinea pigs. A number of small vessels were found crossing free through the lumen of the scala tympani in this area. The larger number (5-10) of these capillaries are suspended between the bony cochlear wall and the terminal net of fibrocytes which covers the cochlear opening of the cochlear aqueduct and spreads onto the basal third of the round window membrane. Other capillaries (3-5) cross the lumen of the scala tympani from the outer cochlear wall to the modiolus. All of these capillaries have a thin endothelium and a very thin pericytic covering. These findings suggest that the capillaries crossing the perilymphatic space may give rise to a different chemical composition of the perilymph bordering the round window in comparison with the more upper parts of the scala tympani.

Thickness of the human round window membrane in different forms of otitis media.

The thickness and morphologic characteristics of the round window membrane were evaluated in temporal bones from normal subjects as well as those with serous otitis media, purulent otitis media, and chronic otitis media. Temporal bones were studied in chronological order in six age ranges to determine the possibility of age-related differences. No significant difference in the mean thickness of the round window membrane was observed in terms of age groups in normal temporal bones or temporal bones from patients with otitis media; however, a significant difference in the mean thickness was observed in the various forms of otitis media compared with the normal round window membrane in all age groups. The membrane was thickest in patients with chronic otitis media when compared with that in normal subjects or those with serous or purulent otitis media. The epithelial layer (including the subepithelial space) and the fibrous layer were measured individually to determine in which layer the change in mean thickness occurred. These measurements showed an involvement of all layers of the round window membrane in those groups with otitis media, with maximal involvement of the combined epithelial layer and subepithelial space.

Microvasculature of the stria vascularis in the round window area in the rat. A scanning electron microscopy study.

This paper reports the results of a scanning electron microscopy study of corrosion cast preparation of the vessels in the cochlea of the rat. This method confirms the general findings about the cochlear vascularization, but also gives the opportunity to study the detailed pattern of the vascular supply of the stria vascularis in the round window area. The basal end of the stria vascularis has a remarkably different vascular anatomy compared with the other areas of the cochlea. The possible role of the vessels in the round area is discussed.

Blood flow of the round window.

Motion pictures of the blood flow of the round window in guinea pigs were taken. In this part the blood flow of the inner ear could be distinguished from that of the middle ear. Our motion pictures show that the blood flow of the inner ear can be observed through the round window membrane without an operation on the inner ear.