Cochlear implantation: strategies to protect the implanted cochlea from middle ear infection

Pneumococcal meningitis post-cochlear implantation: Preventative measures

Objective

Both clinical data and laboratory studies demonstrated the risk of pneumococcal meningitis post-cochlear implantation. This review examines strategies to prevent post-implant meningitis.

Data Sources

Medline/PubMed database; English articles after 1980. Search terms: cochlear implants, pneumococcus meningitis, streptococcus pneumonia, immunization, prevention.

Review Methods

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

Results

The presence of inner ear trauma as a result of surgical technique or cochlear implant electrode array design was associated with a higher risk of post-implant meningitis. Laboratory data demonstrated the effectiveness of pneumococcal vaccination in preventing meningitis induced via the hematogenous route of infection. Fibrous sealing around the electrode array at the cochleostomy site, and the use of antibiotic-coated electrode array reduced the risk of meningitis induced via an otogenic route.

Conclusion

The recent scientific data support the U.S. Food and Drug Administration recommendation of pneumococcal vaccination for the prevention of meningitis in implant recipients. Nontraumatic cochlear implant design, surgical technique, and an adequate fibrous seal around the cochleostomy site further reduce the risk of meningitis.

Pneumococcal meningitis threshold model: A potential tool to assess infectious risk of new or existing inner ear surgical interventions

HYPOTHESIS

A minimal threshold of Streptococcus pneumoniae is required to induce meningitis in healthy animals for intraperitoneal (hematogenous), middle ear, and inner ear inoculations, and this threshold may be altered via recent inner ear surgery.

BACKGROUND

There has been an increase in the number of reported cases of cochlear implant-related pneumococcal meningitis since 2002. The pathogenesis of pneumococcal meningitis is complex and not completely understood. The bacteria can reach the central nervous system (CNS) from the upper respiratory tract mucosa via either hematogenous route or via the inner ear. The establishment of a threshold model for all potential routes of infection to the CNS in animals without cochlear implantation is an important first step to help us understand the pathogenesis of the disease in animals with cochlear implantation.

METHODS

Fifty-four otologically normal adult Hooded Wistar rats (27 receiving cochleostomy and 27 controls) were inoculated with different amounts of bacterial counts via three different routes (intraperitoneal, middle ear, and inner ear). Rats were monitored during 5 days for signs of meningitis. Blood, cerebrospinal fluid, and middle ear swabs were taken for bacterial culture, and brains and cochleae were examined for signs of infection.

RESULTS

The threshold of bacterial counts required to induce meningitis is lowest in rats receiving direct inner ear inoculation compared with both intraperitoneal and middle ear inoculation. There is no change in threshold between the group of rats with cochleostomy and the control (Fisher's exact test, p < 0.05).

CONCLUSION

A minimal threshold of bacteria is required to induce meningitis in healthy animals and is different for three different routes of infection (intraperitoneal, middle ear, and inner ear). Cochleostomy performed 4 weeks before the inoculation did not reduce the threshold of bacteria required for meningitis in all three infectious routes. This threshold model will also serve as a valuable tool, assisting clinicians to quantitatively analyze if the presence of a cochlear implant or other CNS prostheses alter the risk of meningitis.

Cochlear implantation in the presence of chronic suppurative otitis media

Nine patients are presented who underwent cochlear implantation in the presence of chronic suppurative otitis media. Four had a simple tympanic membrane perforation, four had a pre-existing mastoid cavity and one had cholesteatoma in the ear chosen for implantation. Patients with a simple perforation had a staged procedure with myringoplasty followed by cochlear implantation after an interval of three months. Patients with cholesteatoma or with an unstable mastoid cavity were also staged. A mastoidectomy or revision mastoidectomy was performed with obliteration of the middle ear and mastoid using a superiorly pedicled temporalis muscle flap and blind sac closure of the external meatal skin. After a further six months a second stage procedure was performed to confirm that the middle-ear cleft was healthy and to insert the implant. Patients presenting with a stable mastoid cavity underwent obliteration of the cavity and implantation of the electrode as a one-staged procedure. To date there have been no serious problems such as graft breakdown, recurrence of disease or implant extrusion, and all patients are performing well.

Otitis media in children with cochlear implants

Fifty children who received a cochlear implant between 1991 and 1995 were evaluated for incidence of acute otitis media (AOM). Thirty-seven (74%) children had AOM before implantation and 8 (16%) after implantation. All children who had AOM after implantation had a history of AOM. A subgroup of 14 children required ventilating tubes for recurrent AOM before implantation. Five (35.7%) in this group had AOM after implantation. The incidence and severity of AOM decreased after implantation. All episodes of postimplant AOM were successfully treated with routine oral antibiotics, and no infectious complications occurred. A history of recurrent AOM should not inordinately delay cochlear implantation.

Surgical management of advanced ear disease

With careful surgical technique and attention to underlying medical problems, most cases of advanced ear disease in small animals can be managed successfully. Specific techniques and therapy are described.

Histological study of chronic electrode implantation through the round window of the guinea pig

A platinum-iridium electrode with a Teflon cover tube was chronically implanted through the round window into the scala tympani of the guinea pig. Animals were sacrificed at 30 to 40 weeks after electrode implantation, and the cochleae were observed by scanning electron microscopy or with light microscopy. The results revealed that the electrode was firmly enclosed with fibrous tissue on the inside of the round window and with epithelial tissue on the outside. These tissues were considered to act as a barrier to the entry of infection while preventing leakage of the perilymph.