Literature review
Pneumococcal meningitis post-cochlear implantation: Potential routes of infection and pathophysiology

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Abstract

Objective

This review describes the current concept of pneumococcal meningitis in cochlear implant recipients based on recent laboratory studies. It examines possible routes of Streptococcus pneumoniae infection to the meninges in cochlear implant recipients. It also provides insights into fundamental questions concerning the pathophysiology of pneumococcal meningitis in implant recipients.

Data Sources

Medline/PubMed database; English articles after 1960. Search terms: cochlear implants, meningitis, pneumococcus, streptococcus pneumonia.

Review Methods

Narrative review. All articles relating to post-implant meningitis without any restriction in study designs were assessed and information extracted.

Results

The incidence of pneumococcal meningitis in cochlear implant recipients is greater than that of an age-matched cohort in the general population. Based on the current clinical literature, it is difficult to determine whether cochlear implantation per se increases the risk of meningitis in subjects with no existing risk factors for acquiring the disease. As this question cannot be answered in humans, the study of implant-related infection must involve the use of laboratory animals in order for the research findings to be applicable to a clinical situation. The laboratory research demonstrated the routes of infection and the effects of the cochlear implant in lowering the threshold for pneumococcal meningitis.

Conclusion

The laboratory data complement the existing clinical data on the risk of pneumococcal meningitis post-cochlear implantation.

Section snippets

Meningitis Post-Implantation: Extent of the Problem

By September 2003, the total number of reported cases of post-implantation meningitis worldwide was 118 (55 cases in the United States and 63 cases in other parts of the world), including a total of 17 deaths.1 The age of patients with cochlear implant-related meningitis ranged between 13 months and 81 years. The majority of U.S. patients were less than five years of age; non-U.S. patients were equally distributed among adults and children. The onset of meningitis ranged from less than 24 hours

The Routes of Pneumococcal Infection

The pathogenesis of pneumococcal meningitis is complex. There may be differences in the pathogenesis of pneumococcal meningitis among subjects with a cochlear implant, depending upon the route of bacterial infection. This is because 40 percent of subjects with post-implant meningitis were found to have concurrent acute otitis media and 80 percent were found to have bacteremia.2

S. pneumoniae can spread to the meninges either directly from the middle ear or from the hematogenous seeding of

Temporal Bone Studies in Patients with Pneumococcal Meningitis

The routes by which S. pneumoniae reaches the meninges have also been examined in a number of temporal bone studies. Some studies of individual subjects' temporal bones with pneumococcal meningitis have provided evidence to support the direct association between otitis media and meningitis and the direct spread of the infection to the meninges via the inner ear.18, 19, 20, 21 However, a hematogenous spread of bacteria from the middle ear or nasopharynx to the meninges with subsequent

Threshold of S. pneumoniae Required for Meningitis

The occurrence of meningitis seems directly related to the duration and the intensity of the bacteremia, because these variables determine how many bacteria reach the subarachnoid space.25 It appears that a threshold of the bacteria with the required virulence factors must be reached in the blood and meninges of a healthy animal to establish meningitis, with any breach of the dura reducing the threshold of the bacteremia required to produce meningitis.26

The quantification of the bacterial

The Effect of Cochlear Implantation on the Threshold

The quantitative threshold model was used to examine the fundamental question: does the presence of a cochlear implant increase the risk of pneumococcal meningitis in healthy subjects who have no existing risk factors? Compared to nonimplanted control cohorts, the presence of a cochlear implant in healthy animals was associated with a reduction in the threshold of bacteria required to induce pneumococcal meningitis, irrespective of the route of infection (Fig 4).30 This threshold shift was

Effects of Inner Trauma on the Risk of Pneumococcal Meningitis

A severe surgical insertion trauma (fracture of osseous spiral laminae [OSL] and modiolus) was also found to be an independent factor for subsequent risk of pneumococcal meningitis.33 In these instances, the threshold for infection was reduced when bacteria were given via the middle or inner ear route but not the hematogenous route.33 Presumably, a more direct communication between the inner ear and the internal auditory meatus (IAM) was created by trauma to the modiolus and OSL. This provided

Recurrent Meningitis in Patients with a Cochlear Malformation

Patients with abnormal cochlear morphology are at increased risk of spontaneous meningitis.35, 36 The increased risk may be due to more open communication between the inner ear and the CNS in these subjects.37, 38, 39, 40, 41 The cochlear aqueduct may be more patent in patients with malformed cochleae, again providing more direct communication between the cochlea and the CNS. However, there are no published data reporting the size and patency of the cochlear aqueduct and vestibular aqueduct in

Biofilms and Meningitis

The role of biofilms on cochlear implants in relation to pneumococcal meningitis is unclear. The bacteria are embedded in a slime-like matrix composed of extracellular polymeric substances and are resistant to the host immune defense and antibiotics.44 There are very few cochlear implant biofilm studies. The presence of S. aureus biofilm on the receiver/stimulator component of the cochlear implant, and not on the electrode array, has been reported.45, 46 No one has yet described the presence of

Histopathology and Routes of Infection Based on the Threshold Model

The histopathological appearance of the cochlear specimens was very distinct and varied between different routes of inoculation.9, 27, 30 Animals that acquired meningitis following hematogenous inoculation showed a bilateral symmetrical distribution of bacteria and inflammatory cells within the cochleae (Fig 5).9, 27, 30 In these animals, bacteria were found predominately in the modiolus and IAM during the early stage of meningitis, and they were not observed in the scalae of the ipsilateral

Conclusion

The most common organism identified in post-implantation meningitis is S. pneumoniae. All potential routes of spread of S. pneumoniae from the middle ear to the meninges should be considered when examining post-implant meningitis. A quantitative pneumococcal meningitis threshold model in rodents was established, and demonstrated that the presence of a foreign body, such as an electrode array in the inner ear, increased the risk of pneumococcal meningitis in healthy animals. The threshold shift

Author Contributions

Benjamin P. C. Wei, main contributions to the conception, design of the review, acquisition of data and interpretation of data and writing and drafting the review and approval of final draft; Robert K. Shepherd, joint contributions to the conception, design of the review, acquisition of data and interpretation of data and writing and drafting the review and approval of final draft; Roy M. Robins-Browne, joint contributions to the conception, design of the review, acquisition of data and

Disclosures

Competing interests: None.

Sponsorships: None.

Acknowledgments

The Garnett Passe and Rodney Williams Memorial Foundation: scholarship in Otolaryngology Head and Neck Surgery. The State Government of Victoria Operational Infrastructure Program and the National Institutes of Health (HHS-N-263-2007-00053-C).

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