Cochlear Limited, Sydney, New South Wales 2066, Australia.
Ear Hear. 2010 Jun;31(3):367-79. doi: 10.1097/AUD.0b013e3181c9fd86.
For more than a decade, Neural Response Telemetry (NRT) has provided direct access to the electrically evoked compound action potential (ECAP) as elicited by the Nucleus cochlear implant. When used clinically during fitting, ECAP threshold profiles are applied by shifting the profile to the audible threshold and comfort level boundaries (the T- and C-level profiles, respectively). The resulting profiles, to date, have matched the curvature of the ECAP threshold profile exactly. When compared with psychophysical profiles, previous studies have shown that this approach incurs errors in program levels that are no better than flat or population mean profiles. However, C-level profiles are observed to be flatter than T-level profiles. Accordingly, ECAP threshold profiles are flattened in this study when applied at increasing stimulus levels, and the effectiveness of this approach is evaluated among other methods.
In phase I, ECAP thresholds (via AutoNRT) and T- and C-levels were measured from 15 adult Nucleus Freedom implantees. Psychophysical levels were measured using pulse train stimuli at six different stimulation rates, spanning 80 to 3500 Hz. The different rates spread T- and C-levels across a range of stimulus levels. At each of these levels, a scaling factor of best fit was calculated such that the shifted ECAP threshold profile, when scaled (0 giving a flat profile, 1 giving an unmodified profile), gave the best fit to the corresponding psychophysical profile. From the 148 such T- and C-level profiles, a single profile scaling model was determined by a simple linear regression. In phase II, the model was tested on data using three separate stimulation rates (250, 900, and 2400 Hz) and 14 additional subjects. The root mean square psychophysical level mismatch of the ECAP threshold profile, the scaled ECAP threshold profile, a flat profile, and a mean population profile was calculated per subject and per stimulation rate, and the differences in the means of these calculations were compared. In phase III, 13 separate subjects evaluated the scaled ECAP-based program during a 2 wk trial, comparing the new program to a flat program and a conventional ECAP-based program with unmodified ECAP threshold profiles. A questionnaire captured their subjective preferences.
In phase I, the profile scaling model constructed from the data prescribed a flattening of the ECAP threshold profile with increasing mean T- or C-level (in CL units): scale = 1.38 - 0.0043 PsychoMean. In phase II, the scaled ECAP-based profiles were found to fit the psychophysical profiles significantly better in all test configurations (typically of the order of 5% dynamic range) compared with all other profiles. In phase III, 62% of subjects preferred the scaled ECAP-based program, whereas 8% preferred the conventional ECAP-based program, 15% the flat program and 15% had no preference. Analyses of the questionnaires revealed significantly higher ratings for the scaled ECAP-based programs, whereas the conventional ECAP-based programs were not rated differently than the flat programs.
The scaled ECAP threshold profile method provides a clinically significant enhancement to ECAP-based fitting methods, confirming the value of the ECAP threshold profile to cochlear implant fitting.
十多年来，神经反应遥测（NRT）一直提供对核耳蜗植入物诱发的电诱发复合动作电位（ECAP）的直接访问。在临床拟合过程中使用时，ECAP 阈值曲线通过将曲线移动到可听阈值和舒适水平边界（分别为 T 级和 C 级曲线）来应用。迄今为止，所得曲线完全符合 ECAP 阈值曲线的曲率。与心理物理曲线相比，先前的研究表明，这种方法在程序水平上的误差并不比平坦或群体平均值曲线好。然而，观察到 C 级曲线比 T 级曲线平坦。因此，在本研究中，当以递增的刺激水平应用 ECAP 阈值曲线时，使 ECAP 阈值曲线变平，并评估了这种方法与其他方法的有效性。
在第一阶段，通过 AutoNRT 从 15 名成年 Nucleus Freedom 植入者中测量 ECAP 阈值（T-和 C-级）。使用脉冲串刺激在六个不同的刺激率下测量心理物理水平，范围为 80 至 3500 Hz。不同的速率在不同的刺激水平上传播 T-和 C-级。在每个这些水平，计算了最佳拟合的比例因子，使得当缩放（0 给出平坦的轮廓，1 给出未修改的轮廓）时，缩放的 ECAP 阈值轮廓与相应的心理物理轮廓拟合最佳。从 148 个这样的 T-和 C-级曲线中，通过简单的线性回归确定了单个曲线缩放模型。在第二阶段，使用三个单独的刺激率（250、900 和 2400 Hz）和 14 个额外的受试者来测试模型上的数据。计算每个受试者和每个刺激率的 ECAP 阈值轮廓的均方根心理物理水平不匹配、缩放的 ECAP 阈值轮廓、平坦的轮廓和平均群体轮廓，并比较这些计算的平均值之间的差异。在第三阶段，13 名受试者在为期 2 周的试验中评估了基于 ECAP 的缩放程序，将新程序与基于平坦的程序和基于未修改的 ECAP 阈值轮廓的常规 ECAP 程序进行比较。调查问卷捕捉了他们的主观偏好。
在第一阶段，从数据构建的轮廓缩放模型规定了 ECAP 阈值轮廓随着平均 T 或 C 级（以 CL 单位）的增加而变平：比例= 1.38-0.0043 PsychoMean。在第二阶段，与所有其他曲线相比，基于缩放的 ECAP 的曲线在所有测试配置中都发现与心理物理曲线拟合得更好（通常在大约 5%的动态范围内）。在第三阶段，62%的受试者更喜欢基于缩放的 ECAP 的程序，而 8%的受试者更喜欢基于常规 ECAP 的程序，15%的受试者更喜欢平坦的程序，15%的受试者没有偏好。对问卷的分析表明，基于缩放的 ECAP 的程序得到了显著更高的评分，而基于常规 ECAP 的程序的评分与基于平坦的程序没有不同。
缩放的 ECAP 阈值曲线方法为基于 ECAP 的拟合方法提供了显著的增强，证实了 ECAP 阈值曲线对耳蜗植入物拟合的价值。
