Surgical implications of perimodiolar cochlear implant electrode design: avoiding intracochlear damage and scala vestibuli insertion

OBJECTIVE

To review the mechanisms and nature of intracochlear damage associated with cochlear implant electrode array insertion, in particular, the various perimodiolar electrode designs. Make recommendations regarding surgical techniques for the Nucleus Contour electrode to ensure correct position and minimal insertion trauma.

BACKGROUND

The potential advantages of increased modiolar proximity of intracochlear multichannel electrode arrays are a reduction in stimulation thresholds, an increase in dynamic range and more localized neural excitation. This may improve speech perception and reduce power consumption. These advantages may be negated if increased intracochlear damage results from the method used to position the electrodes close to the modiolus.

METHOD

A review of the University of Melbourne Department of Otolaryngology experience with temporal bone safety studies using the Nucleus standard straight electrode array and a variety of perimodiolar electrode array designs; comparison with temporal bone insertion studies from other centres and postmortem histopathology studies reported in the literature. Review of our initial clinical experience using the Nucleus Contour electrode array.

RESULTS

The nature of intracochlear damage resulting from electrode insertion trauma ranges from minor, localized, spiral ligament tear to diffuse organ of Corti disruption and osseous spiral lamina fracture. The type of damage depends on the mechanical characteristics of the electrode array, the stiffness, curvature and size of the electrode in relation to the scala, and the surgical technique. The narrow, flexible, straight arrays are the least traumatic. Pre-curved or stiffer arrays are associated with an incidence of basilar membrane perforation. The cochleostomy must be correctly sited in relation to the round window to ensure scala tympani insertion. A cochleostomy anterior to the round window rather than inferior may lead to scala media or scala vestibuli insertion.

CONCLUSION

Proximity of electrodes to the modiolus can be achieved without intracochlear damage provided the electrode array is a free fit within the scala, of appropriate size and shape, and accurate scala tympani insertion is performed.

The development of speech perception in children using cochlear implants: effects of etiologic factors and delayed milestones

HYPOTHESIS

Speech perception outcomes for cochlear implantation of children vary over a wide range, and it is hypothesized that central pathologic states associated with certain causes of hearing impairment account for a substantial part of the variance.

STUDY DESIGN

A retrospective analysis was carried out to ascertain the relationships between speech perception, etiologic factors, and central pathologic states as indicated by preoperative delayed motor milestones and/or cognitive delays.

SETTING

Data were obtained from the pre- and postoperative records of patients attending a hospital cochlear implant clinic.

PATIENTS

Results for 75 consecutive patients up to age 5 years who underwent implantation were included in the study.

INTERVENTION

Patients received a 22-electrode cochlear prosthesis and were seen by the clinic for regular tune-up and assessments. Home- and school-based habilitation was recommended by the clinic.

MAIN OUTCOME MEASURES

Speech perception measures were classified on a five-point scale to allow for different evaluation procedures at different ages and developmental stages.

RESULTS

The incidence of motor and cognitive delays were fairly evenly spread across etiologic factors, except for cytomegalovirus, which had a much higher than average incidence. Children with motor and/or cognitive delays were significantly slower than other children in the development of speech perception skills after implantation. Etiologic factors did not have a statistically significant effect on speech perception outcome.

CONCLUSIONS

It is likely that central pathologic states account for a substantial part of the variance among children using cochlear implants. Specific indicators of central pathologic states should be used to assess a child's prognosis in preference to less specific information based on etiologic factors alone.

Speech perception results for children with implants with different levels of preoperative residual hearing

OBJECTIVE

Many reports have established that hearing-impaired children using the Nucleus 22-channel cochlear implant may show both significant benefits to lipreading and significant scores on open-set words and sentences using electrical stimulation only. These findings have raised questions about whether severely or severely-to-profoundly deaf children should be candidates for cochlear implants. To study this question, postoperative results for implanted children with different levels of preoperative residual hearing were evaluated in terms of speech perception benefits.

STUDY DESIGN/SETTING

A retrospective study of the first 117 children, sequentially, to undergo implantation in the Melbourne and Sydney Cochlear Implant Clinics was undertaken. All children had been assessed by and received their implants in a tertiary referral centre.

MAIN OUTCOME MEASURES

To assess aided residual hearing, the children were grouped into four categories of hearing on the basis of their aided residual hearing thresholds measured preoperatively. To assess benefits, the scores of children on standard speech perception tests were reviewed. As different tests were used for children with different ages and language skills, children were grouped into categories according to the level of postoperative speech perception benefit.

RESULTS

The results showed that children in the higher categories of aided preoperative residual hearing showed significant scores on open-set word and sentence perception tests using the implant alone. For children in lower categories of aided residual hearing, results were variable within the groups. More than 90% of children with implants with aided residual hearing thresholds in the speech range above 1 kHz achieved open-set understanding of words and sentences.

CONCLUSION

While the results of this preliminary study confirm previous findings of differential outcomes for children with different levels of preoperative residual hearing, they suggest that children with severe to profound hearing impairments should be considered for cochlear implantation.