Responsiveness of the Electrically Stimulated Cochlear Nerve in Children With Cochlear Nerve Deficiency

Objectives: This study aimed to (1) investigate the responsiveness of
the cochlear nerve (CN) to a single biphasic-electrical pulse in implanted
children with cochlear nerve deficiency (CND) and (2) compare their
results with those measured in implanted children with normal-size CNs.
Design: Participants included 23 children with CND (CND1 to CND23)
and 18 children with normal-size CNs (S1 to S18). All subjects except
for CND1 used Cochlear Nucleus cochlear implants with contour electrode
arrays in their test ears. CND1 was implanted with a Cochlear
Nucleus Freedom cochlear implant with a straight electrode array in the
test ear. For each subject, the CN input/output (I/O) function and the
refractory recovery function were measured using electrophysiological
measures of the electrically evoked compound action potential (eCAP)
at multiple electrode sites across the electrode array. Dependent variables
included eCAP threshold, the maximum eCAP amplitude, slope of
the I/O function, and time-constants of the refractory recovery function.
Slopes of I/O functions were estimated using statistical modeling with
a sigmoidal function. Recovery time-constants, including measures of
the absolute refractory period and the relative refractory period, were
estimated using statistical modeling with an exponential decay function.
Generalized linear mixed-effect models were used to evaluate the
effects of electrode site on the dependent variables measured in children
with CND and to compare results of these dependent variables
between subject groups.
Results: The eCAP was recorded at all test electrodes in children with
normal-size CNs. In contrast, the eCAP could not be recorded at any
electrode site in 4 children with CND. For all other children with CND,
the percentage of electrodes with measurable eCAPs decreased as the
stimulating site moved in a basal-to-apical direction. For children with
CND, the stimulating site had a significant effect on the slope of the I/O
functions and the relative refractory period but showed no significant
effect on eCAP threshold and the maximum eCAP amplitude. Children
with CND had significantly higher eCAP thresholds, smaller maximum
eCAP amplitudes, flatter slopes of I/O functions, and longer absolute
refractory periods than children with normal-size CNs. There was no significant
difference in the relative refractory period measured in these two
subject groups.
Conclusions: In children with CND, the functional status of the CN varied
along the length of the cochlea. Compared with children with normalsize
CNs, children with CND showed reduced CN responsiveness to electrical
stimuli. The prolonged CN absolute refractory period in children
with CND might account for, at least partially, the observed benefit of
using relatively slow pulse rate in these patients.