Shin-ichi Hatsushika

Electrical stimulation of the auditory nerve: The effect of electrode position on neural excitation

Histological studies have shown that the Melbourne/Cochlear electrode array lies along the outer wall of the scala tympani and is therefore some distance from the residual VIIIth nerve elements. In order to investigate the influence of electrode position on neural excitation we systematically varied the position of the electrode array within the cat scala tympani while recording electrically evoked auditory brainstem responses (EABRs). Using both normal hearing and long-term deafened animals, we observed significant reductions in EABR thresholds as the electrode array was moved from the outer wall towards the modiolus. Further threshold reductions were observed when the array was placed underneath the osseous spiral lamina (OSL) close to the peripheral dendrites. These changes were independent of the bipolar inter-electrode separation, and were observed over a wide range of cochlear pathologies varying from normal to a moderate spiral ganglion cell loss. Interestingly, the one animal exhibiting extensive neural loss showed no correlation between EABR threshold and electrode position. There was also a general decrease in the gradient of the EABR input-output function as the electrode array was moved closer to the neural elements. This was, however, only statistically significant when the electrode was positioned adjacent to the peripheral dendrites. Significant reductions in EABR threshold were also observed as the inter-electrode spacing of the bipolar electrodes was increased. The gradient of the EABR input-output function also increased with increasing inter-electrode spacing, although again, this was only significant when the electrode array was positioned close to the neural elements. The present results indicate that the optimum placement of a Melbourne/Cochlear electrode array is adjacent to the peripheral dendrites. However, such a site would be difficult to achieve in practice while minimizing insertion trauma. An array lying adjacent to the modiolus would be a safe alternative while ensuring a significant reduction in threshold compared with the existing site (outer wall). This placement should result in more localized neural excitation patterns, an increase in the number of bipolar electrodes available, together with an increase in their dynamic range. These changes may lead to further improvements in speech perception among cochlear implant patients.

Dimensions of the scala tympani in the human and cat with reference to cochlear implants

The width, height, and cross-sectional area of the scala tympani in both the human and cat were measured to provide dimensional information relevant to the design of scala tympani electrode arrays. Both the height and width of the human scala tympani decrease rapidly within the first 1.5 mm from the round window. Thereafter, they exhibit a gradual reduction in their dimension with increasing distance from the round window. The cross-sectional area of the human scala tympani reflects the changes observed in both the height and width. In contrast, the cat scala tympani exhibits a rapid decrease in its dimensions over the first 6 to 8 mm from the round window. However, beyond this point the cat scala tympani also exhibits a more gradual decrease in its dimensions. Finally, the width of the scala tympani, in both human and cat, is consistently greater than the height.

Cochlear implant. The difference of response based on electrode position on neural excitation.

Multichannel intracochlear implantの発展のために, 鼓室階内の電極の位置および刺激電極間距離の違いに基づく聴神経の反応の差について, 聾の猫を用いてEABRを記録し, それらの閾値とIV波の入出力曲線とを蝸牛の組織学的評価と比較検討した。人工内耳の電極は刺激電極間距離が短く, ラセン神経節に近く設置するのが有利である。 また人工内耳の適応であるsensorineural deafnessの患者においては, ラセン神経節神経終末の強い変性や電極挿入による蝸牛内組織障害の危険性のため, 電極はラセン神経節細胞に近く置いた方が良いと結論された。