Monitoring longitudinal behaviour of impedance and Neural Response Telemetry measurements in a group of young cochlear implant users

Abstract

Electrophysiological measures such as impedance telemetry and Neural Response Telemetry (NRT™) were developed by Cochlear™ in 1992 as clinical tools allowing the objective setting of stimulus levels for cochlear implant users. Extensive research proved the usefulness of NRT™’s as an aid in the programming process of audible and comfortable stimulus levels for children younger than six years. The Nucleus® Freedom™, launched in 2005, introduced new developments in cochlear implantation. Approval from the FDA for this system was obtained in March 2005 and for the first time included children from age 12 months with profound hearing loss. The Joint Committee on Infant Hearing suggested that children be diagnosed and that treatment commenced by the age of six months. The new features of the Nucleus Freedom™ give clinicians the necessary tools to treat this challenging population. An urgent need exists to ascertain the stability and accuracy of the new features introduced by this system, especially the Auto-NRT™ software, to validate its use within the paediatric population. A longitudinal descriptive design was utilized implementing quantitative research methods to critically describe the behaviour of impedance telemetry and NRT™’s in a group of young cochlear implant users. The quantitative method included the application of the Custom Sound™ software and the Auto-NRT™ feature for this group at implantation, device activation, and then at determined follow-up visits. Nine young children between nine months and five years and eleven months were used as participants during the twelve months of research. Impedance telemetry was described in terms of the mean Common Ground (CG) and Monopolar 1+2 (MP1+2) values calculated from measurement data collected on the basal, medial, and apical electrodes of the electrode array. The electrodes identified for statistical procedures for both measurement types were E3, E6, E8, E11, E13, E16, E19 and E21. Friedman’s ANOVA was used as a statistical measure to determine the level of significance in changes among the measurement modes and conditions. The Wilcoxon signed-rank test was indicated in the presence of significant changes identified by Friedman’s ANOVA to calculate the level of significance in a pair-wise comparison. Results indicate that impedance telemetry remained consistent over the electrode array and over time in both measurement modes. A slight increase in mean values was observed during the first three months, followed by a gradual decrease at the six months interval. These changes were statistically nonsignificant. No specific trends were evident in impedance telemetry over time. NRT™-measurements remained consistent across the electrode array over time. Significant changes were present between the intra-operative to device activation measurement intervals. This trend is also described in studies of adult cochlear implant users. NRT™-measurements were stable during the first year postimplantation within the paediatric population. A comparison between the mean impedance telemetry and NRT™’s disclosed an inverse trend during the first six months post-implantation. Most changes were non-significant, indicating that these measures can be used effectively in the new semi-automated fitting software. The implementation of these measurements can lead to streamlined and accountable service delivery to young cochlear implant users. AFRIKAANS : In 1992 is elektrofisiologiese metings soos impedanstelemetrie en Neurale Respons Telemetrie (NRT™) deur Cochlear™ ontwikkel as kliniese hulpmiddels om objektiewe instelling van stimulasievlakke vir kogleêre gebruikers moontlik te maak. Navorsing het bewys dat NRT™’s ‘n effektiewe hulpmiddel is tydens programmering van hoorbare en gemaklike stimulasievlakke by kinders jonger as ses jaar. Die Nucleus® Freedom™ met nuwe ontwikkelings ten opsigte van kogleêre inplantings is in 2005 bekendgestel. Die FDA het in Maart 2005 hierdie sisteem goedgekeur vir gebruik by kinders selfs so jonk as 12 maande met uitermatige gehoorverlies. Die Joint Committee on Infant Hearing het voorgestel dat diagnose en aanvang van rehabilitasie teen ses maande ouderdom moet plaasvind. Die nuwe funksies van die Nucleus® Freedom™ stel oudioloë in staat om hierdie uitdagende bevolking te hanteer. ‘n Dringende behoefte bestaan om te bepaal of hierdie sagteware, veral Auto-NRT™ wat saam met hierdie sisteem bekendgestel is, oor voldoende akkuraatheid en stabiliteit beskik om in die hantering van die pediatriese bevolking te gebruik. ‘n Longitudinale, beskrywende ontwerp, wat kwantitatiewe metodes implementeer, is aangewend om die beweging van impedanstelemetrie en NRT™’s by ‘n groep jong gebruikers van kogleêre inplantings krities te beskryf. Dit het die gebruik van die Custom Sound™ sagteware en die ingeslote Auto- NRT™ funksie behels. Dit is tydens inplantering, by aktivering van die toestel, en bepaalde opvolgsessies uitgevoer. Nege jong kinders tussen die ouderdomme van nege maande en vyf jaar en 11 maande is tydens die 12 maande navorsingsperiode as proefpersone benut. Die impedansmetings is beskryf in terme van die Common Ground (CG) en Monopolar 1+2 (MP1+2) stimulasiemodaliteite. Data is verkry vanaf geselekteerde elektrodes op die basale, mediale en apikale gedeeltes van die elektrode. Vir statistiese ontledings van impedans en NRT™ is hierdie elektrodes geselekteer: E3, E6, E8, E11, E13 E16, E19 en E21. As statistiese ontledingsmetode, is Friedman se ANOVA toegepas om die vlakke van beduidenheid van beweging tussen die verskillende toetsmodaliteite en -omstandighede te bepaal. Die Wilcoxon signed-rank toets is aangedui in die teenwoordigheid van statisties beduidende veranderinge. Die doel van hierdie toets was om die vlak van beduidenheid paarsgewys te verifieer. Resultate dui op konstante impedansmetings oor die elektrode asook oor tyd in beide toetsmodaliteite. ‘n Geringe, statisties nie-beduidende, verhoging in gemiddelde waardes is waargeneem tydens die eerste drie maande na inplantering, waarna die waardes weer geleidelik afgeneem het tot en met die ses maande opvolginterval. Geen spesifieke neiging kon vir impedanstelemetrie bepaal word nie. NRT™-metings het konstant gebly oor die elektrode en met tyd. Statisties beduidende veranderinge is gemeet tussen die intra-operatiewe en aktiveringsintervalle. Hierdie neiging is ook beskryf in studies van volwasse gebruikers van kogleêre inplantings. NRT™-metings, binne die pediatriese populasie, het dus stabiel gebly oor die 12 maande periode post-inplantering. ‘n Vergelyking tussen die gemiddelde impedans- en NRT™-metings het ‘n inverse neiging geïdentifiseer gedurende die eerste ses maande na inplantering. Veranderinge was oor die algemeen statisties nie-beduidend, wat daarop dui dat hierdie metings effektief gebruik kan word. Die implementering van hierdie metings kan meer doeltreffende dienslewering aan die jong gebruikers van kogleêre inplantings tot gevolg hê.