Electron microscopic temporal bone histopathology in experimental pneumococcal meningitis

Role of the Stria Vascularis in the Pathogenesis of Sensorineural Hearing Loss: A Narrative Review

Sensorineural hearing loss is a common sensory impairment in humans caused by abnormalities in the inner ear. The stria vascularis is regarded as a major cochlear structure that can independently degenerate and influence the degree of hearing loss. This review summarizes the current literature on the role of the stria vascularis in the pathogenesis of sensorineural hearing loss resulting from different etiologies, focusing on both molecular events and signaling pathways, and further attempts to explore the underlying mechanisms at the cellular and molecular biological levels. In addition, the deficiencies and limitations of this field are discussed. With the rapid progress in scientific technology, new opportunities are arising to fully understand the role of the stria vascularis in the pathogenesis of sensorineural hearing loss, which, in the future, will hopefully lead to the prevention, early diagnosis, and improved treatment of sensorineural hearing loss.

The Severity of Infection Determines the Localization of Damage and Extent of Sensorineural Hearing Loss in Experimental Pneumococcal Meningitis

Hearing loss is an important sequela of pneumococcal meningitis (PM), occurring in up to 30% of survivors. The role of the severity of infection on hearing function and pathomorphological consequences in the cochlea secondary to PM have not been investigated to date. Using a well-established model of PM, we systematically investigated the functional hearing outcome and the long-term fate of neurosensory cells in the cochlea, i.e., hair cells and spiral ganglion neurons (SGNs), with a focus on their tonotopic distribution. Intracisternal infection of infant rats with increasing inocula of Streptococcus pneumoniae resulted in a dose-dependent increase in CSF levels of interleukin-1β, interleukin-6, tumor necrosis factor α, interleukin-10, and interferon-γ in acute disease. The severity of long-term hearing loss at 3 weeks after infection, measured by auditory brainstem response recordings, correlated to the initial inoculum dose and to the levels of proinflammatory cytokines determined in the acute phase of PM. Quantitative cochlear histomorphology revealed a significant loss of SGNs and outer hair cells that strongly correlated to the level of infection, with the most severe damage occurring in the basal part of the cochlea. Inner hair cells (IHCs) were not significantly affected throughout the entire cochlea. However, surviving IHCs lost synaptic connectivity to remaining SGNs in all cochlear regions. These findings provide evidence that the inoculum concentration, i.e., severity of infection, is the major determinant of long-term morphological cell pathologies in the cochlea and functional hearing loss.

SIGNIFICANCE STATEMENT

Hearing loss is a neurofunctional deficit occurring in up to 30% of patients surviving pneumococcal meningitis (PM). Here, we analyze the correlation between the severity of infection and the inflammatory response in the CSF, the tonotopic distribution of neurosensory pathologies in the cochlea, and the long-term hearing function in a rat model of pneumococcal meningitis. Our study identifies the severity of infection as the key determinant of long-term hearing loss, underlining the importance of the prompt institution of antibiotic therapy in patients suffering from PM. Furthermore, our findings reveal in detail the spatial loss of cochlear neurosensory cells, providing new insights into the pathogenesis of meningitis-associated hearing loss that reveal new starting points for the development of otoprotective therapies.

Streptococcus pneumoniae-induced ototoxicity in organ of Corti explant cultures

Hearing loss remains the most common long-term complication of pneumococcal meningitis (PM) reported in up to 30% of survivors. Streptococcus pneumoniae have been shown to possess different ototoxic properties. Here we present a novel ex vivo experimental setup to examine in detail the pattern of hair cell loss upon exposure to different S. pneumoniae strains, therefore recapitulating pathogen derived aspects of PM-induced hearing loss. Our results show a higher susceptibility towards S. pneumoniae-induced cochlear damage for outer hair cells (OHC) compared to inner hair cells (IHC), which is consistent with in vivo data. S. pneumoniae-induced hair cell loss was both time and dose-dependent. Moreover, we have found significant differences in the level of cell damage between tissue from the basal and the apical turns. This shows that the higher vulnerability of hair cells located at high frequency regions observed in vivo cannot be explained solely by the spatial organisation and bacterial infiltration from the basal portion of the cochlea. Using a wild type D39 strain and a mutant defective for the pneumolysin (PLY) gene, we also have shown that the toxin PLY is an important factor involved in ototoxic damages. The obtained results indicate that PLY can cause both IHC and OHC loss. Finally, we are reporting here for the first time a higher vulnerability of HC located at the basal and middle cochlear region to pneumolysin-induced damage. The detailed description of the susceptibility of hair cells to Streptococcus pneumoniae provided in this report can in the future determine the choice and the development of novel otoprotective therapies during pneumococcal meningitis.

Characteristics of brain stem auditory evoked potentials in children with hearing impairment due to infectious diseases

Among objective audiologic tests, the most important were tests of brain stem auditory evoked potentials. The objective of the study was to test the configuration, degree of hearing loss, and response characteristics of auditory brain stem evoked potentials in children with hearing loss occurred due to infectious disease. A case control study design was used. The study group consisted of 54 patients referred for a hearing test because of infectious diseases caused by other agents or that occurred as congenital infection. Infectious agents have led to the emergence of various forms of sensorineural hearing loss. We have found deviations from the normal values of absolute and interwave latencies in some children in our group. We found that in the group of children who had the diseases such as purulent meningitis, or were born with rubella virus and cytomegalovirus infection, a retrocochlear damage was present in children with and without cochlear damage.

Bacterial invasion of the inner ear in association with pneumococcal meningitis

OBJECTIVE

To examine the pathways of bacterial invasion and subsequent spreading in the inner ear during pneumococcal meningitis.

STUDY DESIGN

A well-established adult rat model of Streptococcus pneumoniae meningitis was used.

METHODS

Thirty rats were inoculated intrathecally with S. pneumoniae serotype 1, 3 or 9 V and received no additional treatment. The rats were sacrificed when reaching terminal illness or on Day 7 and then prepared for serial sectioning and PAS-Alcian blue staining for light microscopy.

RESULTS

During the first few days after inoculation, bacteria invade the inner ear through the cochlear aqueduct, into the scala tympani of the cochlea (perilymphatic space). From here, bacteria spreads apically toward the helicotrema and subsequently basally through the scala vestibuli, toward the vestibule and the vestibular system. When the bacteria after 5 to 6 days had reached scala vestibuli of the basal turn of the cochlea, hematogenous spreading occurred to the spiral ligament and into the cochlear endolymph, subsequently to the vestibular endolymph. We found no evidence of alternative routes for bacterial invasion in the inner ear. Several internal barriers to bacterial spreading were found within the inner ear. Bacterial elimination was evidenced by engulfment by macrophages within the inner ear.

CONCLUSION

From the meninges, pneumococci invade the inner ear through the cochlear aqueduct during the first days of infection, whereas hematogenous invasion via the spiral ligament capillary bed occur at later stages. Although internal barriers exist within the inner ear, the spreading of bacteria occurs via the natural pathways of the fluid compartments. Bacterial elimination occurs by local macrophage engulfment.

Nitrogen and oxygen molecules in meningitis-associated labyrinthitis and hearing impairment

Pneumococcal meningitis remains a serious disease with a case fatality rate of 15%-25%. Furthermore, long-term residues affect up to 50% of survivors. One of the most frequent sequelae is sensorineural hearing loss, which occurs in 26% of survivors of pneumococcal meningitis. Unfortunately, sufficient treatment regimens are still missing. New insights into the pathology and pathophysiology of meningitis-associated hearing loss have come from animal models of bacterial meningitis. Most likely, bacteria reach the cochlea through the cochlear aquaeduct. Once arrived in the perilymphatic spaces, they induce a severe suppurative labyrinthitis. The blood-labyrinth barrier breaks, hair cells are damaged, and neurons in the spiral ganglion undergo cell death, leading to meningitis-associated hearing loss. Reactive oxygen and nitrogen species, in particular peroxynitrite, seem to be among the crucial mediators of cochlear damage and hearing loss during meningitis. In our rat model of pneumococcal meningitis, adjunctive therapy with the antioxidants and peroxynitrite scavengers Mn(III)tetrakis(4-bencoic acid)-porphyrin (MnTBAP) and N-Acetyl-L-Cystein (NAC) significantly attenuated acute and long-term hearing loss. In several other animal studies of pneumococcal meningitis, adjunctive antioxidant therapy also protected infected animals from intracranial complications. Therefore, the use of antioxidants seems to be a promising future treatment option in pneumococcal meningitis.

Doxycycline reduces mortality and injury to the brain and cochlea in experimental pneumococcal meningitis

Bacterial meningitis is characterized by an inflammatory reaction to the invading pathogens that can ultimately lead to sensorineural hearing loss, permanent brain injury, or death. The matrix metalloproteinases (MMPs) and tumor necrosis factor alpha-converting enzyme (TACE) are key mediators that promote inflammation, blood-brain barrier disruption, and brain injury in bacterial meningitis. Doxycycline is a clinically used antibiotic with anti-inflammatory effects that lead to reduced cytokine release and the inhibition of MMPs. Here, doxycycline inhibited TACE with a 50% inhibitory dose of 74 microM in vitro and reduced the amount of tumor necrosis factor alpha released into the cerebrospinal fluid by 90% in vivo. In an infant rat model of pneumococcal meningitis, a single dose of doxycycline (30 mg/kg) given as adjuvant therapy in addition to ceftriaxone 18 h after infection significantly reduced the mortality, the blood-brain barrier disruption, and the extent of cortical brain injury. Adjuvant doxycycline (30 mg/kg given subcutaneously once daily for 4 days) also attenuated hearing loss, as assessed by auditory brainstem response audiometry, and neuronal death in the cochlear spiral ganglion at 3 weeks after infection. Thus, doxycycline, probably as a result of its anti-inflammatory properties, had broad beneficial effects in the brain and the cochlea and improved survival in this model of pneumococcal meningitis in infant rats.

Morphological correlates of acute and permanent hearing loss during experimental pneumococcal meningitis

In patients with acute bacterial meningitis, hearing loss can be transient but is often permanent. The mechanisms underlying meningitis-associated hearing loss are not fully understood. Therefore, we investigated the morphological correlates of hearing loss in a rat model of pneumococcal meningitis. Transcutaneous intracisternal injection of Streptococcus pneumoniae resulted in a dose-dependent hearing loss (determined by auditory brainstem response audiometry), which was partially reversible during the acute stage. Nevertheless, a severe permanent hearing loss persisted until 2 weeks after infection. Suppurative labyrinthitis was accompanied by blood-labyrinth barrier disruption (determined by cochlear Evans blue extravasation), which correlated closely with hearing loss during the acute stage but not after recovery. Two weeks after infection, spiral ganglion neuronal density was markedly decreased and correlated with the severity of permanent hearing loss. Neuronal loss can be explained by the new finding of meningitis-associated spiral ganglion neuronal necrosis rather than apoptosis (determined by morphology, TUNEL staining, and immunohistochemistry).

Neuroprotective effects of brain-derived neurotrophic factor (BDNF) on hearing in experimental pneumococcal meningitis

Bacterial meningitis is still one of the most common causes of acquired profound sensorineural deafness in children despite antibiotic treatment. We investigated the neuroprotective effects of brain-derived neurotrophic factor on hearing function in experimental bacterial meningitis. We implanted stainless steel tubes into both cerebral ventricles of Sprague-Dawley rats aged 21 days. Bacterial meningitis was induced by inoculating a strain of serotype III Streptococcus pneumoniae into the cisterna magna. Six micrograms per day of brain-derived neurotrophic factor (groups 1 and 3) or albumin (groups 2 and 4) was injected into the cerebral ventricles 24 hours after or before infection, respectively, for a duration of 7 days. Additionally, all rats received antibiotic subcutaneous treatment starting 24 hours after infection for 7 days. Brainstem auditory evoked potentials were recorded 24 hours before and 24 hours after infection and after 7 days of treatment with brain-derived neurotrophic factor or placebo and antibiotics, respectively, to determine hearing threshold. Our results showed that the hearing thresholds of animals in each group increased significantly 24 hours after infection compared with the results recorded 24 hours before infection (P < .01). After 7 days of treatment with brain-derived neurotrophic factor, brainstem auditory evoked potential responses recurred in 16 ears when stimulated at 75 dB hearing level in groups 1 and 3. Their hearing thresholds significantly decreased compared with the control group 2 (P < .05) and group 4 (P < .01). However, 13 of 14 ears absent brainstem auditory evoked potential responses could still not be identified at 75 dB hearing level in control groups 2 and 4. The improvement of the hearing thresholds in group 3 (treated before infection) was greater than that of group 1 (treated after infection) (P < .05), but there was no significant difference found between the control groups before and after infection (P > .05). Our study supports the hypothesis that the administration of exogenous brain-derived neurotrophic factor can be effective in preventing or treating hearing loss following bacterial meningitis.

Meningitis-associated hearing loss: Protection by adjunctive antioxidant therapy

Hearing loss is the most frequent long-term complication of pneumococcal meningitis, affecting up to 40% of survivors. Unfortunately, adjuvant therapy with dexamethasone has failed to satisfactorily reduce its incidence. Therefore, we evaluated the use of antioxidants for the adjunctive therapy of meningitis-associated deafness. Eighteen hours after intracisternal injection of 7.5 x 10(5) colony-forming units of Streptococcus pneumoniae, rats were treated systemically either with ceftriaxone and the antioxidants and peroxynitrite scavengers Mn(III)tetrakis(4-benzoic acid)-porphyrin (MnTBAP) or N-acetyl-L-cysteine (NAC) or placebo (1 ml phosphate-buffered saline) for 4 days. Hearing was assessed by auditory brainstem response audiometry. Adjunctive antioxidant therapy significantly reduced the long-term hearing loss (14 days after infection) for square wave impulses (mean hearing loss +/- SD: ceftriaxone and placebo, 45+/-26 dB; ceftriaxone and MnTBAP, 9+/-23 dB; ceftriaxone and NAC, 19+/-30 dB) as well as 1 kHz (ceftriaxone and placebo, 28+/-19 dB; ceftriaxone and MnTBAP, 10+/-16 dB; ceftriaxone and NAC, 10+/-17 dB), and 10 kHz tone bursts (ceftriaxone and placebo, 62+/-27 dB; ceftriaxone and MnTBAP, 16+/-13 dB; ceftriaxone and NAC, 25+/-26 dB). Furthermore, both antioxidants attenuated the morphological correlates of meningogenic hearing loss, namely, long-term blood-labyrinth barrier disruption, spiral ganglion neuronal loss, and fibrous obliteration of the perilymphatic spaces. Adjuvant antioxidant therapy is highly otoprotective in meningitis and therefore is a promising future treatment option.

Isolated vestibular ossification after meningitis associated with sensorineural hearing loss

OBJECTIVE

Sensorineural hearing loss after bacterial meningitis is common. It is thought to be secondary to cochlear inflammation from direct bacterial seeding through the cochlear aqueduct. Although cochlear ossification with or without vestibular involvement is common, isolated vestibular ossification is rare. We review our recent experience with isolated vestibular ossification associated with profound sensorineural hearing loss and the implication for cochlear implantation in this population.

STUDY DESIGN

Retrospective case review.

SETTING

Academic tertiary referral center.

PATIENTS

Patients referred with profound sensorineural hearing loss after bacterial meningitis with evidence of ossified vestibular labyrinth but normal cochleae on computed tomographic scanning were included in the study. INTERVENTIONS We review the history, imaging findings, operative intervention, and operative findings in this series of patients. MAIN OUTCOME MEASURES Clinical history, imaging findings, operative intervention, and operative findings.

RESULTS

Three patients with isolated vestibular ossification without computed tomographic evidence of cochlear osteoneogenesis were identified. Two of these patients subsequently underwent cochlear implantation with the Clarion device with complete insertion of the electrode array. Intraoperatively, one patient had isolated scala tympani ossification and the implant electrode array was placed within the scala vestibuli.

CONCLUSION

A pattern of sensorineural hearing loss and isolated ossification of the vestibular system without radiographic evidence of cochlear involvement may be seen after bacterial meningitis. In these patients, magnetic resonance imaging scanning should be performed, because computed tomographic scanning may not always detect cochlear ossification. Cochlear implantation is feasible in these patients.

Reactive nitrogen species contribute to blood-labyrinth barrier disruption in suppurative labyrinthitis complicating experimental pneumococcal meningitis in the rat

Sensorineural hearing damage is a frequent complication of bacterial meningitis, affecting as many as 30% of survivors of pneumococcal meningitis. There is a substantial body of evidence that oxidants, such as reactive nitrogen species (RNS), are central mediators of brain damage in experimental bacterial meningitis. In the present study, we investigated whether RNS also contribute to the pathophysiology of suppurative labyrinthitis in our well-established rat model of pneumococcal meningitis. In all infected rats, but not in uninfected controls, we observed suppurative labyrinthitis. Cochlear inflammation was accompanied by severe blood-labyrinth barrier (BLB) disruption as evidenced by increased Evans Blue extravasation. Furthermore, increased cochlear expression of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) was detected by immunohistochemistry. Colocalization of iNOS and tyrosine nitration (a marker of RNS attack) indicated that nitric oxide (NO) produced by iNOS contributes to oxidative cochlear damage through the action of RNS. To determine the pathophysiological role of RNS in BLB disruption, rats were treated with peroxynitrite scavengers (MnTBAP and uric acid, UA). Six h after adjunctive treatment with 300 mg/kg i.p. UA or 15 mg/kg i.p. MnTBAP+100 mg/kg i.p. ceftriaxone, BLB disruption was significantly reduced compared with that in infected animals treated only with ceftriaxone. Therefore, we conclude that RNS are involved in the breaching of the BLB during meningogenic pneumococcal labyrinthitis.