Vascular trauma during cochlear implantation: a contributor to residual hearing loss?

OBJECTIVE: This study was performed to investigate the vascular structures of the cochlea that are potentially vulnerable to mechanical trauma during cochlear implant surgery. BACKGROUND: Despite improvements in surgical technique and electrode design, residual hearing is lost in a significant percentage of cochlear implant patients. Although a variety of factors may contribute, it is widely believed that mechanical trauma plays an important role. This study focused on the vasculature of scala tympani and its potential susceptibility to injury during implant surgery. METHODS: Anatomic study of normal human temporal bones prepared by either conventional cross sectioning or by microdissection for examination by light or scanning electron microscopy. RESULTS: Blood vessels located at or near the perilymphatic surface of scala tympani are predominately of the venous type, and they are situated so as to be at risk for trauma during implantation. Those vessels include veins on the lateral wall and floor of scala tympani, the modiolar wall and the undersurface of the osseous lamina and basilar membrane. CONCLUSION: Injury or occlusion of blood vessels associated with scala tympani may adversely affect inner ear function, potentially contributing to hearing loss following cochlear implantation.

Ultrastructural aspects of the microvasculature of the cochlea: the internal spiral network.

We performed a study of the microvasculature of the cochlea with scanning electron microscopy on corrosion casts in the guinea pig. This study was performed in 140 cochleas from healthy adult guinea pigs. Different microvascular techniques were used, including injection-microdissection in 53 cases, injection-diaphanization in 27 cases, a histologic technique in 34 cases, injection-microdissection-diaphanization in 32 cases, and scanning electron microscopy on corrosion casts in 14 cases. The internal radiating arterioles branch off as collateral branches of the spiral modiolar artery, as first-order collateral branches. We detail the morphology, caliber, trajectory, and collateral branches. Among their collateral branches, are third-order arterioles, the arterioles to the spiral ganglia, and the arterioles of the tympanic lip. The arterioles of the tympanic lip form, through their anastomosis, a rich capillary network at the edge of the spiral lamina, called the internal spiral network. We emphasize the glomeruli of Schwalbe, which arise near the scala tympani as third-order arterioles with a medium caliber of 14 microm. The upper glomeruli, situated in the bony wall, and the lower glomeruli, situated in the width of the osseous spiral lamina, form vascular loops made of anastomosed capillaries with a caliber of less than 10 microm. These loops play an important role as efficient devices, or "relay stations," for regulation of cochlear blood flow. The comparison of results obtained with each technique gave the perspective of the cochlear microvasculature with great accuracy.

Changes in immunostaining of inner ears after antigen challenge into the scala tympani.

To study the mechanisms of immune responses and immune injuries in inner ears, labyrinthitis was induced by inoculation of keyhole limpet hemocyanin (KLH) into the scala tympani of systemically sensitized guinea pigs. Inner ears were then immunostained for KLH, immunoglobulin G (IgG), albumin, connexin26 (Cx26), and sodium-potassium adenosine triphosphate (Na,K-ATPase). Inflammatory cells containing KLH were observed in the scala tympani and in the collecting venule of the spiral modiolar vein (SMV). Spiral ligament, spiral limbus, and blood vessels including the SMV were diffusely positive for IgG and albumin. Immunoreactivity for Cx26 and Na,K-ATPase was decreased compared with the normal ears in the fibrocytes of the spiral ligament. These results suggest that inflammatory cells and blood constituents could extravasate into the cochlea from blood vessels and that fibrocyte damage in the spiral ligament could cause cochlear dysfunction.

Effects of hypercapnia on cochlear and cerebral blood flow in rabbits.

Cochlear blood flow (CoBF) and cerebral blood flow (CBF) were measured using laser Doppler flowmetry and the microsphere method during CO2 and O2 gas mixture inhalation in rabbits. Both methods revealed that CBF was increased by CO2 inhalation. CoBF measured by laser Doppler measurement decreased during CO2 inhalation while that measured by the microsphere method showed a slight increase, but lateral wall blood flow in the cochlea measured by the microsphere method showed no significant change. Our results obtained with laser Doppler flowmetry were not inconsistent with those reported in humans. It is considered that the rabbit, in which thick bone surrounds the cochlea, is a good model for the experimental evaluation of the laser Doppler technique used in the clinical measurement of CoBF.

The blood vessels in Reissner's membrane of the rat.

Blood vessels on Reissner's membrane were electron-microscopically found in 24 out of 30 rats of the Wistar strain. The vessels were noted in the upper turns, mainly in the third turn of the cochlea. They originated from radiating arterioles in the upper spiral ligament and took straight courses radially down to the spiral limbus, and further to the modiolus, running along the surface of the scala vestibuli side of the membrane. Their diameter was approximately 4 microns. Bifurcated blood vessels were occasionally detected in 3 out of the 24 rats that possessed the vessels in the membrane. From the morphological point of view the blood vessels in Reissner's membrane may possibly have some function in the inner ear of the rat.

A study on the vasculature of the internal auditory artery in humans by casts.

Vascular casting has so far been used mainly for observation of the vascular system of the inner ear in experimental animals. In the present study, this method was applied to analysis of the three-dimensional vascular structure in the inner ear in humans. To date there has been no comparative study of overall pictures of blood vessels in the human inner ear. Seven cases were observed mainly by means of scanning electron microscopy. Even with this new method, the results in many respects obtained confirmed those of previous reports. However, the following findings which have not been previously reported or are currently in dispute, were observed: i) In two cases, spring coil-like structures of blood vessels were found in the internal auditory canal; ii) We corroborated the pattern of arrangement of cochlear arteries previously described in words and schemata, i.e., the following two types were confirmed: the first type showing the cochlear branch of vestibulocochlear artery and the spiral modiolar artery running into the cochlea, and the second type showing the cochlea supported by arteries derived from the artery corresponding to the vestibulocochlear artery, alone; and iii) A right-angled arteriolar branch of the spiral modiolar artery was found to further bifurcate to form radiating arterioles extending in two directions: both over the scala vestibuli and along the spiral lamina.

The cochlear glomeruli in the modiolus of the guinea pig.

The cochlear glomeruli were studied in guinea pigs using scanning electron microscopy of vascular corrosion casts and transmission electron microscopy of tissue sections. Two types of coiled vessels forming the cochlear glomeruli were found in the bony wall of the modiolus. First, upper glomeruli were seen situated in the bony wall next to the scala vestibuli; second, lower glomeruli were located in the osseous spiral lamina just above the spiral ganglion. Upper glomeruli gave rise to radiating arterioles which supplied capillaries of the stria the spiral lamina and limbus. Unlike the main supplying arteries, smooth muscle cells were not present in the walls of the arterioles forming the glomeruli and a peculiar layer of lamellar pericytes was found. The arterioles were strikingly longer than their parent vessels and no autonomic nerves were found in close spatial relationship. Hence, these findings indicate that the cochlear glomeruli serve as efficient devices for reducing cochlear blood pressure.

Spiral modiolar vein: its importance in inner ear inflammation.

The inner ear responds immunologically to foreign proteins and pathogens introduced into the cochlea. The route of entry of the cells participating in the inner ear inflammatory process is the spiral modiolar vein with its collecting venules. Since this vein is located adjacent to the scala tympani, the end result of cellular infiltration is fibro-osseous obliteration of this scala preferentially. This observation has implications for the placement of cochlear implant electrodes of patients with labyrinthitis as the cause of hearing loss.

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.

A morphometric technique for analysis of cochlear vessels.

A computer based system was developed for morphometric analysis of the cochlear lateral wall vasculature. Variables measured were vascular density, RBC density, vessel width, aggregation density, lumen compression count, WBC count and pigment density. Vessels were divided into three systems based on the lateral wall structures that they supplied. The results indicated that these three vessel systems were morphologically distinct in terms of their vascular density, RBC density, and vessel width. How these differences relate to functional needs of the lateral wall is discussed.

The vascular anatomy of the gerbil cochlea.

The vascular pattern of the cochlea was studied in 12 adult gerbils (Meriones unguiculatus) using the Berlin blue (Prussian blue) contrast injection technique. The capillary areas of the gerbil cochlea are similar to those of other mammals studied. As in some other mammals, the vessel of the basilar membrane was inconsistently identified in the basal turn. It was usually replaced by a larger uninjected channel, supporting the suggestion that the vessel of the basilar membrane may be more functionally important in fetal life than in adulthood. The vascular pattern of the external cochlear wall is well maintained from base to apex, although a simplification of vasculature can be observed apically.

The microsphere surface technique for evaluation of cochlear vessels and circulation. A preliminary report.

A new technique for the evaluation of cochlear microcirculation and cochlear vessels is described. This method is a modification of two previously used techniques, the "soft-surface-specimen method" and the "microsphere method". In this preliminary paper it was found that the microspheres were easily visualised in all cochlear tissues. The absolute and relative distribution of microspheres was calculated. The method appears promising for experimental investigations of cochlear microcirculation.

Surface view of strial vessel, prominence vessel, and external sulcus cells as revealed by scanning electron microscopy.

The stria vascularis and the spiral prominence in the lateral cochlear wall were observed with scanning electron microscopy (SEM) after dissolving out the epithelial layer and the basement membrane by tryptic digestion and HCl hydrolysis. The external sulcus cells have regularly arranged pegs and a fine basal interdigitation on the cell surface. The strial vessel and the spiral prominence vessel could be visualized well and the characteristic surface view of their periendothelial cells presented. The former has cytoplasmic processes with a fine branching appearance similar to red coral and the latter has bulged popcorn like processes. Their functional significance is discussed.

A cochlear vascular anomaly in a patient with hearing loss and tinnitus.

An unusual blood vessel in the cochlea of a patient is reported. The blood vessel derives from the osseous lamina spiralis and crosses straight through the scala tympani toward the lateral wall of the perilymphatic duct. In its course a branch derives from this vessel toward the other spiral vessel. In the region of this abberrant vessel a complete hair-cell loss is present. A high tone perceptive loss with a relative dip and tinnitus was found in the same ear during life. The possible cause and effects of this abberrant vessel are discussed.

Methodological aspects of some inner ear vascular techniques.

Modifications of preparation technique assessing cochlear vasculature together with suggestions regarding their applicability to various types of research projects are presented. Detailed information is given regarding the 'soft-surface' dissection and preparation technique along with selected parameters for evaluating the cochlear vasculature. Some basic requirements for any histological technique studying the inner ear blood supply are suggested.