Scala Vestibuli Research Articles

Hematogenous Pneumococcal Meningitis in the Infant Rat: Description of a Model

Sensorineural deafness occurs in 20%-30% of children after Streptococcus pneumoniae meningitis. An infant rat model of S. pneumoniae meningitis was developed to study the pathogenesis of inner ear invasion by S. pneumoniae. S. pneumoniae type 6 was administered intraperitoneally (inoculum: 1-10 x 10(8) cfu) every 24 h for 3 days to 5-day-old rats. Bacteremia (12 [50%] of 24) and meningitis (11 [46%] of 24) were detected most frequently 4 days after the three doses. The mean cerebrospinal fluid (CSF) white blood count for rats with positive CSF cultures was 7271/mm3 (range, 81-20,475). Hematoxylin-eosin-stained brain tissue from the 11 rats with positive CSF cultures showed inflammation in the meninges and scala tympani in 9 each (82%), and scala vestibuli in 6 (55%), but none in the scala media. Gram's-stained brain and inner ear sections from the same 11 rats showed organisms in the meninges in 5 (45%) and scala tympani or vestibuli in 2 (18%). Perilymphatic inflammation occurred significantly (P less than .001) more than did endolymphatic inflammation.

Radial communication between the perilymphatic scalae of the cochlea. I: Estimation by tracer perfusion

The degree to which radial exchange between scala tympani (ST) and scala vestibuli (SV) can occur has not previously been quantified. We have measured the amount of cross-communication in the third turn of the guinea pig cochlea using an ionic tracer, trimethylphenylammonium (TMPA). TMPA was perfused through one scala while TMPA concentrations were measured simultaneously in the perfused and non-perfused scalae. On the basis of the time course of TMPA increase recorded in the non-perfused scala, we were able to calculate rate constants for cross-leak and clearance. Cross-leak in the third turn occurred remarkably rapidly with a rate constant from ST to SV of 0.049 min −1 and from SV to ST of 0.031 min −1. These correspond to transfer half times of 14.2 and 22.4 min respectively. This result demonstrates that ST and SV in the third turn cannot be regarded as independent compartments.

Radial communication between the perilymphatic scalae of the cochlea. II: Estimation by bolus injection of tracer into the sealed cochlea

Radial communication between ST and SV was measured in the sealed cochlea by monitoring the dispersal of an ionic tracer, trimethylphenyl-ammonium (TMPA) injected in the form of a minute bolus. Tracer movements were recorded by a pair of ion-selective electrodes sealed into the injected and non-injected scalae close to the injection site. Measurements were made in the basal or third turn of the guinea pig cochlea. In the third turn, radial communication occurred rapidly with a ST half time from ST to SV of 25 min and from SV to ST of 26 min. In the basal turn the communication was markedly slower, with a ST half time from ST to SV of 170 min and from SV to ST of 240 min. However, the difference between the basal and third turns can be shown to arise almost totally from differences in cross-sectional area of the perilymphatic scalae. When normalized with respect to scala cross-section, the process of tracer movement across the spiral ligament is similar in the basal and third turns. These results demonstrate that radial communication between scala tympani and scala vestibuli is an important route which must be considered in studies involving perilymph.

Electrode Insertion Trauma in Cochlear Implantation

Residual hearing conservation may be important in cochlear implantation of children and of adults with disabling tinnitus responsive to extracochlear stimulation. Damage to the neural elements of the cochlea during electrode insertion may have a negative impact on residual hearing conservation. Histologic analysis of eight temporal bones with cochlear implants reveals trauma at essentially two locations: the round window insertion site and along the basal turn of the cochlea. In four of the bones, insertion at the round window resulted in damage to the osseous spiral lamina and the electrode was inserted through the scala media into the scala vestibuli. Evidence of secondary reactive osseous formation was also noted in these bones. This paper relates the surgical anatomy of the round window to histologic findings and microanatomical dissections. Recommendations for implantation surgery include creation of a cochleostomy by removal of the floor of the round window niche and a superior-to-inferior angle of electrode insertion.

Microstructures of the Bony Modiolus in the Human Cochlea: A Scanning Electron Microscopic Study

The structures of the bony modiolus of the human cochlea are studied by scanning electron microscopy after macerating the organic material with a NaOCl solution. The modiolus on the scala vestibuli side had a canal for the spiral modiolar artery. Ducts for the radiating arterioles spread radially from the canal, and opened to furrows in the roof of the scala vestibuli. The surface of the modiolus facing the scala tympani was lace-like with numerous pores. Just beneath the bone surface were a modiolar canal for the spiral ganglion and spirally-arranged spaces for the anterior and posterior spiral veins. The spiral spaces for the veins had windows (fenestrations) leading to the scala tympani. The openings in the bone on the scala vestibuli and tympani sides of the modiolus are considered to be communication routes for the transport of fluids from the perineural and perivascular spaces in the modiolus to the perilymph in the scalae.

Old theme and new reflections: Hearing impairment associated with endolymphatic hydrops

The cause(s) of the hearing impairment associated with Menières's disease are not understood but are undoubtedly associated with the inner ear endolymphatic hydrops. Two major hypotheses have been proposed and widely received: endolymphatic overpressure followed by leaky membranes and subsequently the mixing of high K + endolymph with perilymph. Our recent data on an experimental model of endolymphatic hydrops have provided grounds for renewed reflections on the pathology. Indeed our data might be interpreted without involving either of the above hypotheses and suggests that the symptoms of Menière's disease might be accounted for by a flow of perilymph from scala vestibuli towards scala tympani with the mixing of the two perilymphs which are similar but not identical in composition. The higher K + concentration arriving from the scala vestibuli into the scala tympani at the apex of the cochlea via the helicotrema is likely to be toxic to hair cell and auditory nerve fiber function. The mixing of the two perilymphs could result in deterioration of low frequency sensitivity, provoke low frequency tinnitus and in the long term cause spiral ganglion cell degeneration at the apex of the cochlea. The feeling of fullness in the ear might be the result of the decreasing perilymph volume in the scala vestibuli which could give rise to inner ear conductive losses. The patency of the cochlear aqueduct might play a role in determining the high risk group of individuals likely to manifest the symptoms of endolymphatic hydrops.

High-Resolution Magnetic Resonance Imaging of the Human Temporal Bone

A magnetic resonance imaging (MRI) system (Hitachi, Naka, Japan) with a superconductive magnet running at 2.11 T was used to obtain 2-mm-thick slices of fixed, decalcified and celloidin-embedded human temporal bone. The temporal bone was then sectioned and stained for routine histological evaluation. Both the MR images and the histological sections were in the mid-modiolar slice plane, and comparable images and sections were analyzed to confirm the identity of the inner-ear structures visualized on the MR images. The cochlear duct, scala tympani, scala vestibuli and basement membrane of all three cochlear turns were clearly imaged on MRI. In addition, the vestibule and three semicircular ducts were also clearly seen. This study raises the possibility of some day using MRI for the diagnosis of inner-ear diseases.

Pathogenesis and Pathophysiology of Meniere's Disease

Meniére's disease, neither spontaneous nor inducible in animals, is studied only in patients. Natural history (epidemiology) shows a triad of major symptoms: vestibular, auditory, and aural pressure. One in 3 patients has bilateral Meniére's; over full lifespans, bilaterality approaches 50%. Aural pressure (74.1%) and positional vertigo during/between attacks (85.9%) are common. Clinical variants can persist for 25+ years. All forms have delayed onset and can occur years after incitement, after otosclerosis, infections like otitis media, syphilis, or trauma. Endolymphatic hydrops is found in all, most importantly in pars inferior (cochlear duct and saccule). Some (not most) cases show ruptures. The saccule can distend into the lateral semicircular canal. Symptomatic attacks are explained on physical/biochemical bases. Both longitudinal (slow) and radial (fast) flow seem operational, longitudinal in advanced Meniére's where membranous labyrinth replaces perilymph in scala vestibuli and vestibule. All forms result from endolymphatic absorptive dysfunction (in duct and sac), with mastoid and periaqueductal hypocellularity, hypodevelopment of Trautmann's triangle, and anterior displacement of lateral sinus. Secondary obstructions in ductus reuniens or utricolo-endolymphatic valve may explain atypical Meniére's (vestibular or cochlear alone).

Role of Middle Ear Endotoxin in Inner Ear Inflammatory Response and Hydrops: Long-Term Study

The permeability of the round window membrane for Salmonella typhimurium-derived endotoxin was examined with use of a total of 33 chinchillas. One milligram of each endotoxin was instilled into the tympanic cavities via the superior bullae. The endotoxin activities in middle ear effusions (MEEs), perilymph, and sera were determined by limulus amebocyte lysate assay. Endotoxin was detected in perilymph on the inoculated side by 12 hours after endotoxin instillation and persisted for up to 3 weeks. Endotoxin level peaked at 24 to 48 hours postinstillation, and it steadily declined afterward. This result suggests that the maximum penetration occurred during the active inflammatory stage. Histologic evidence demonstrated remarkable pathologic changes in the inner ear, including bleeding and inflammatory cell recruitment, mostly in the perilymphatic spaces (eg, scalae tympani, scalae vestibuli, spiral ligament), strial swelling, and sensory cell degeneration. This result suggests that endotoxin present in the middle ear can permeate the round window membrane, causing inner ear tissue damage in this animal model.

Influence of contralateral noise stimulation on wave I of the ABR, elicited with condensation and rarefaction clicks in humans

Sound-evoked efferent effects can be examined by comparing the response amplitudes of the auditory nerve with and without contralateral stimulation. It has been suggested that the efferent system modifies the mechanical characteristics of the cochlear partition by varying outer hair cell tonus and regulating the mean position of the basilar membrane [E. L. LePage, Hear. Res. 38, 177–198 (1989)]. If this is the case, the effect of contralateral stimulation may be different for responses elicited with clicks of opposite polarity (condensation versus rarefaction), as such stimuli cause initial movement of the basilar membrane either toward scala tympani or scala vestibuli. Furthermore, such stimuli are known to elicit dissimilar ABR waveforms [V. Rawool and S. Zerlin, Stand. Audiol. 17 (1988)]. Results of the surface recorded wave of the ABR obtained from human subjects, with and without contralateral noise, for rarefaction and condensation clicks as well as the possible underlying mechanisms will be presented.

Normal and pathologic features of the limbus spiralis and its functional significance

The normal and pathologic morphology of the limbus spiralis was studied by light and electron microscopy. Beneath the interdental cells are stellate, mesothelial, osmiophilic, and light types of fibrocytes. Morphologic evidence suggests that the stellate type cells occupying the main body of the limbus are actively engaged in the recycling of ground substance. The loosely interwoven mesothelial cells (dark cells) continuous with those of Reissner's membrane may participate in the secretion and maintenance of perilymph. Below this superficial cell lining are the osmiophilic and light cells, which appear to serve as a partial barrier to the exchange of substances between the scala vestibuli and the limbus proper. The collective data from experiments on vascular occlusion, drug toxicity, acoustic trauma, and perilymph and endolymph fistulae indicate that these connective tissue cells, particularly the stellate type of cells, are highly susceptible to injury. The lesions are often total or nonexistents, and are common in the basal and apical turns. When the lesion is small, the region adjacent to the inner sulcus cells is most frequently affected. In severe lesions, the adjacent interdental cells and sensory cells often remain normal. When the organ of Corti is concomitantly affected, the outer hair cells are usually missing. A common factor associated with limbus cell atrophy appears to be changes in ground substance due to the contamination of perilymph. Another factor associated with fluid contamination is the permanent or temporary cessation of blood flow to the limbus capillary arcades. The limbus spiralis may serve as one of the most sensitive indicators of alterations in the chemical composition of perilymph.

The effect of adenylate cyclase stimulation on endocochlear potential in the guinea pig

Forskolin, a diterpene extracted from Coleus forskohlii, is potentially an important tool for studying the modulation of ionic currents by cAMP because it stimulates adenylate cyclase in a variety of cells. We studied the effect of forskolin on cochlear potentials and found that its perfusion of the scala vestibuli (SV) to a concentration more than 10(-5) M and the scala tympani (ST) to more than 10(-4) M produced a reversible elevation of the endocochlear potential (EP) in a dose-dependent manner. The cochlear microphonics recorded simultaneously with the EP was not depressed during the EP elevation. A large negative EP was induced by anoxia following the SV perfusion with forskolin (2 X 10(-4) M). The results suggest that the EP elevation produced by forskolin does not result from the decrease in the negative component of EP but from the increase in the positive component of EP.

Cerebrospinal Fluid Absorption in the Rabbit: Inner Ear Pathways

Fifteen adult rabbits were perfused intrathecally with horseradish peroxidase (HRP) for 20-30 min under conditions that prevented any increase in cerebrospinal fluid (CSF) pressure. Histologic and ultrastructural examination of the cochlea disclosed HRP deposits along the cochlear and vestibular branches of the auditory nerve and beyond their ganglia, in a) epineural and perineural spaces; b) intraneural spaces reaching the membrane of myelinated axons via nodes of Ranvier; and c) extending beyond the epineurium into area lymphatics. HRP was also found in the basilar membrane, along with deposits in the scalae tympani, vestibuli, media and the spiral ligament. The endolymph also received HRP which followed vestibular nerve fibers and penetrated between sustentacular and hair cells of the cristae ampullaris and both maculae. HRP permeated interendothelial spaces lining the modiolus to reach the scala vestibuli lymphatics close to all the above areas were also permeated by HRP, but the inner tunnel was devoid of the marker.

Characterization of an EPSP-like potential recorded remotely from the round window.

The whole-nerve cochlear action potential (CAP), to tone burst stimulation, was recorded before and after application of tetrodotoxin (TTX) to the intact round window (RW) membrane. TTX abolished the CAP leaving a residual negative potential without altering the summating potential (SP) or the cochlear microphonic (CM). The residual potential retained its polarity when recorded from scala vestibuli. The peak latency, amplitude, and tuning properties of the residual potential showed features similar to the CAP. Application of kainic acid to the RW membrane eliminated the residual potential, leaving the SP and CM unaltered. It is hypothesized that the sources of the residual potential are the excitatory post-synaptic potentials from the peripheral processes of afferent dendrites under the inner hair cells.

Perilymph composition in scala tympani of the cochlea: Influence of cerebrospinal fluid

A commonly used technique to obtain cochlear perilymph for analysis has been the aspiration of samples through the round window membrane. The present study has investigated the influence of the volume withdrawn on sample composition in the guinea pig. Samples of less than 200 nl in volume taken through the round window showed relatively high glycine content, comparable to the level found in samples taken from scala vestibuli. If larger volumes are withdrawn, lower glycine levels are observed. This is consistent with cerebrospinal fluid (having a low glycine content) being drawn into scala tympani through the cochlear aqueduct and contaminating the sample. The existence of a concentration difference for glycine between scala tympani perilymph and cerebrospinal fluid suggests the physiologic communication across the cochlear aqueduct is relatively small in this species. The observation of considerable exchange between cerebrospinal fluid and perilymph, as reported in some studies, is more likely to be an artifact of the experimental procedures, rather than of physiologic significance. Alternative sampling procedures have been evaluated which allow larger volumes of uncontaminated scala tympani perilymph to be collected.

Regeneration of the eighth cranial nerve. III. Central projections of the primary afferent fibers from individual vestibular receptors in the bullfrog.

The projection site in the central nervous system of individual end-organ branches of regenerated vestibular nerves was documented in bullfrogs. The eighth cranial nerve was surgically sectioned in 12 bullfrogs (Rana catesbeiana) and allowed to regenerate. Horseradish peroxidase was used to label the primary vestibular afferent fibers from the whole nerve (one frog) and the whole anterior branch (two frogs), and the individual receptor end-organ branches to the anterior semicircular canal (three frogs), the horizontal semicircular canal (three frogs), and the sacculus (three frogs). A sufficient number of fibers from the individual receptors were labeled in nine specimens to allow a description of their central projection pattern. The overall pattern of central projections of the regenerated nerves was similar to the pattern of normal nerves. In the eighth nerve root proper and in the area proximal to the vestibular nuclei, fibers from the anterior semicircular canal were ventral, fibers from the sacculus and posterior branch were dorsal, and fibers from the horizontal semicircular canal were in an intermediate position. In ventral, superior, and descending vestibular nuclei, a projection pattern was also identifiable in some specimens. The fibers of the anterior semicircular canal projected ventral to the fibers of the horizontal semicircular canal, with fibers from the posterior branch more dorsal. The saccular fibers projected to the dorsal aspect of the ventral nucleus and the ventral aspect of the dorsal nucleus. For the most part, projections in the regenerated specimens, as verified histologically, were comparable to individual end-organ projections previously identified in normal frogs. The only major discrepancy between the findings from the regenerated specimens and those from normal frogs was in the position of the vestibular tract and the innervation to the medial nucleus. The regenerated vestibular tract was shifted laterally in the brain stem. Regenerated thick and thin fibers were intermixed in the tract and projected to the medial nucleus. Normally, the thin fibers are lateral to the thick fibers in the tract. Therefore, the regenerated afferent fibers of the vestibular nerve selectively reinnervated the vestibular nuclei, and the fibers to the individual end-organs selectively reinnervated particular parts of the nuclei. In addition, the thick and thin regenerated fibers preserved the type of bouton endings normally found. The regenerated thick fibers had rare boutons en passant and a great number of collaterals, while the thin fibers had numerous boutons en passant and only a few collaterals.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrochemical Composition of the Cochlear Fluids in the Early Experimental Hydrops:Preliminary Results

The composition of endolymph and perilymph was studied in the guinea pig cochlea after 2 and 6 weeks of blockage of the vestibular aqueduct in an experimental model of hydrops. Compound action potential was monitored several times in the observation period. The endocochlear potential was measured and the endolymph was sampled at the first and third turns of the scala media. The Na, K, and Cl concentrations were determined in nanolitre aliquots of endolymph and of perilymph, the latter sampled from the basal scala vestibuli. After 2 weeks, no change in endolymphatic electrochemical composition was observed. After 6 weeks, endocochlear potential was decreased by 25% at both cochlear turns; K concentration was decreased in endolymph of the basal turn and Cl concentration was decreased in both turns; the calculated osmolality (Na + K + Cl) was decreased in both turns. These results indicate that the blockage of the vestibular aqueduct induced early auditory dysfunction whereas alterations of the electrochemical composition of endolymph occurred later after a time lag of more than 2 and less than 6 weeks.

Effects of a Calcium Channel Blocker and Calcium Chelating Agents on Cochlear Electrical Activity in the Guinea Pig

Endocochlear potential (EP) in the guinea pig was investigated under the influence of nifedipine and calcium chelating agents. The area from the scala tympani to the scala vestibuli was perfused with control and test solutions. EP decreased gradually after perfusion with the nifedipine solution, when the concentration was higher than 100 ng/ml. At a concentration of 200 ng/ml, EP decreased from +70 mV to +14 mV. At 1000 ng/ml, it decreased from +70 mV to +54 mV and stayed at this level during perfusion. It began to return to the initial level when perfusion with the control solution was recommenced. Cochlear microphonics (CM) also decreased with 200 ng/ml of nifedipine, but less markedly than EP. EGTA (10 mM) decreased EP from +80 mV to +67 mV. This change, however, was only transient and EP returned to the control level. EDTA (4 mM) had no significant effect on EP. These results suggest that Ca ions play a role in maintaining EP in a manner similar to that by which Ca modulates Na-K pump activity of the marginal cells in stria vascularis.

Otic pathology of caprine beta-mannosidosis.

Caprine beta-mannosidosis is an autosomal recessive defect of glycoprotein catabolism with a deficiency of tissue and plasma beta-mannosidase activity and tissue accumulation of oligosaccharides within lysosomes. This rapidly fatal genetic disorder of Nubian goats is expressed at birth by a variety of clinical signs including deafness. Affected goats had folded pinnas, and the tympanic cavity was decreased due to multiple, polypoid projections of bone covered by middle ear mucosa which obstructed the view of the cochlear promontory. Numerous cells of the cochlear duct including mesothelial and epithelial cells of Reissner's membrane, mesothelial cells lining the scala tympani, cells of the stria vascularis, numerous supportive cells of the organ of Corti, cochlear hair cells, endothelial cells, perithelial cells, fibroblasts, macrophages, and neurons of the spiral ganglion contained numerous nonstaining intracytoplasmic vacuoles which resulted in distention of affected cells and caused thickening of involved structures. Ultrastructurally, the vacuoles were membrane-bound and consistent with lysosomes. Vacuolated cells were desquamated into the scala vestibuli and scala tympani. This is one of few reports describing light and electron microscopic otic alterations of a storage disease. Goats with beta-mannosidosis appear to be good models of hearing loss in patients with storage disease.

Biochemical considerations in presbycusis

Virtually no direct information about biochemical changes in the peripheral auditory system due to aging is available. However, numerous predictions about biochemical changes due to characteristic degeneration patterns can be made indirectly. (1) Since glycogen and calmodulin are extremely high in outer hair cells, significant degeneration of these cells due to aging will result in lowered average levels of both substances in whole organ of Corti. Analogous considerations apply if major longitudinal gradients are present (e.g., glycogen, P-creatine, cyclic GMP). (2) Degeneration of stria vascularis, the so-called “metabolic” type of presbycusis, will result in a lowered endolymphatic potential and low endolymph K, while Na and Ca will be elevated. (3) The “inner ear conduction” type of presbycusis is associated with massive degeneration of the spiral ligament (SL). Apart from mechanical effects on basilar membrane motion, this should result in significant changes in perilymph chemistry. The normal SL exhibits extremely high carbonic anhydrase, which is thought to be essential in perilymph regulation. The SL also appears to be responsible for the markedly different kinetics of K and furosemide in scala vestibuli and tympani. The pronounced gradients of these substances between the scalae can no longer be maintained in the absence of a functional SL.