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Assessment of Vocabulary Development in Children After Cochlear Implantation
Hamdy El-Hakim, FRCS (ORL);
Josée Levasseur, MSc S;
Blake C. Papsin, MD, MSc, FRCSC;
Jaswinder Panesar, FRCS;
Richard J. Mount;
Derek Stevens;
Robert V. Harrison, PhD, DSc
Arch Otolaryngol Head Neck Surg. 2001;127:1053-1059.
ABSTRACT
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Objectives To assess vocabulary development in children following cochlear implantation
and to evaluate the effect of age at implantation on performance.
Design Retrospective study (mean follow-up, 3 years).
Setting Tertiary center.
Patients Children with prelingual deafness provided with a cochlear implant between
1988 and 1999, who serially performed the Peabody Picture Vocabulary Test–Revised
(60 patients) and the Expressive One-Word Picture Vocabulary Test–Revised
(52 patients). The children were subgrouped into those receiving implants
at younger than 5 years and at 5 years or older.
Outcome Measures Age-equivalent vocabulary test score and gap index (chronological age
minus the age-equivalent score, divided by the chronological age at the time
of testing) were calculated. For each test, the following were performed:
calculation of rate of change for age-equivalent score; comparison of earliest
and latest gap indices means (the cohort and intergroup and intragroup comparison);
and multiple regression analysis demonstrating the effect of age at implantation,
sex, communication mode, etiology of deafness, and residual hearing on the
rate of vocabulary development.
Results Expressive and receptive vocabulary development rates were 0.93 and
0.71 (age-equivalent scores per year), respectively. Subgrouped by age at
implantation, the children's rates (for both vocabularies) were not statistically
different (Peabody Picture Vocabulary Test–Revised, P = .90; Expressive One-Word Picture Vocabulary Test–Revised, P = .23). The global latest gap indices were significantly
less than the earliest (Peabody Picture Vocabulary Test–Revised, P = .048; Expressive One-Word Picture Vocabulary Test–Revised, P<.001), indicating an improvement in age-appropriate
vocabulary development over time. The age subgroups demonstrated similar results,
except for the younger group's receptive gap index. On multiple regression
analysis, the significant predictive variables were residual hearing (Expressive
One-Word Picture Vocabulary Test–Revised) and male sex and oral communication
mode (Peabody Picture Vocabulary Test–Revised).
Conclusions Children with cochlear implants developed their vocabularies at rates
that were sufficient to prevent an increase in their gap indices as related
to ideal scores at testing. A late age at implantation does not singularly
preclude beneficial development of vocabulary.
INTRODUCTION
DURING THE past decade, investigations have attempted to assess the
effect of cochlear implantation on oral communication in children with severe
to profound deafness.1-8
These studies have generally demonstrated that, in children with prelingual
deafness, the age-equivalent scores (on norm-referenced tests of expressive
and receptive language or vocabulary) increased significantly over time. However,
the subjects maintained a considerable linguistic delay after cochlear implantation.
To our knowledge, this delay, or gap between the children's performance and
the ideal performance for their chronological age, has not been quantified
in any investigation. In some studies,5-6
inferences have been made about how the rates of language development in children
with implants compare with those of children without hearing abnormalities,
or children with deafness without implants. These investigations did not include
similarly assessed and concurrent control subjects, and some results were
drawn from cross-sectional data. Also, the type of habilitation of these children
after implantation may not have been necessarily equivalent to that of the
other groups. Moreover, the follow-up time in the reported studies is generally
short, not allowing for the fact that language growth in children may fluctuate
over time.9
Dawson2 and Robbins5-6
and their colleagues tried to identify factors that may significantly affect
language development as an outcome measure for cochlear implantation, but
found none of the factors studied to be significant. Recently, Nikolopoulos
et al10 noted the absence of robust statistical
evidence supporting claims that age at implantation is a significant predictor
of speech perception and intelligibility after pediatric cochlear implantation.
After searching the literature, we similarly found no evidence in relation
to oral language development outcomes. Therefore, we aimed to address the
effect of age at implantation as a predictive factor of vocabulary development.
We concur with Robbins and colleagues6
that a comprehensive analysis of serially measured age-equivalent scores should
include measures of the performance of the children over time and the language
skills achieved by the end of the follow-up period. In addition, the method
should relate the actual performance to that ideally expected, so that one
can compare the relative performance of individuals at different time points.
We retrospectively reviewed our database of patients with prelingual
deafness who received implants over a 12-year period in The Hospital for Sick
Children, Toronto, Ontario,11 and documented
the rates of acquisition of vocabulary (receptive and expressive). We used
a novel method to judge the change in vocabulary age gap from the earliest
to the latest assessment. Furthermore, we evaluated the effect of age at implantation
on these outcomes and explored the predictive value of residual hearing, communication
mode, sex, and etiology of deafness on rates of vocabulary acquisition.
MATERIALS AND METHODS
PATIENTS
The database of the Cochlear Implant Program at The Hospital for Sick
Children contains information on 133 children who underwent implantation between
1988 and 1999. Only children with prelingual deafness who performed the 2
vocabulary tests detailed in the next subsection, "Language Tests," were considered
for this retrospective study. Of these, the patients whose performance could
be quantified by the tests were included. Because the scores before implantation
were not available for all patients and represented variable time points before
implantation, none of these scores were included in the analysis. Consequently,
only patients with scores on at least 2 occasions after implantation were
included (Peabody Picture Vocabulary Test–Revised [PPVT], 60 patients;
Expressive One-Word Picture Vocabulary Test–Revised [EOWPVT], 52 patients).
LANGUAGE TESTS
The PPVT12 is an individually administered,
norm-referenced test to estimate receptive (hearing) vocabulary for standard
American English. The test format contains 5 training items, followed by 175
test items arranged in order of increasing difficulty. Each item has 4 simple
black-and-white illustrations arranged in a multiple-choice format. The subject's
task is to select the picture considered to illustrate the best meaning of
a stimulus word presented orally by the examiner. The raw scores are converted
to age-referenced norms (age-equivalent scores). This test was designed for
subjects aged 2 to 40 years who can see and hear reasonably well and understand
standard English to some degree.
The EOWPVT13 is an individually administered,
norm-referenced test to estimate a child's expressive vocabulary in standard
scores. It is composed of 143 items, and the child is required to perform
a naming task. Again, the raw scores are converted to age-referenced norms
(age-equivalent scores). This test was designed for children aged 2 to 12
years (maximum score achievable at 11.9 years equivalent age).
In both tests, the stimulus was only presented orally (the stimulus
word was not presented with signed support for children whose primary mode
of communication was signing). For the expressive vocabulary test, only spoken
answers were considered to determine the child's score. During testing, all
children were using amplification (cochlear implants with or without hearing
aids).
OUTCOME MEASURES
An age-equivalent score was calculated for each patient after every
assessment. For the purpose of this investigation, an additional value was
derived, the gap index. This was calculated by subtracting the age-equivalent
score from the chronological age of the child and then dividing by the chronological
age (at the time of the test). This index provides a measure of the linguistic
gap in relation to age at the time of testing (or to ideal score at the time
of testing). If language develops favorably, the gap index should approach
zero.
The other variables we documented were age at implantation, duration
of follow-up (in years), sex, communication mode, etiology of deafness (if
known), and residual hearing (mean of the preoperative unaided threshold for
the ear receiving the implant at 250, 500, 2000, and 4000 Hz) expressed as
a percentage of maximum threshold (120 dB).
IMPLANTS USED AND HABILITATION
A multichannel cochlear implant (Nucleus; Cochlear Ltd, Lane Cove, Australia)
was used in all children. All implants were programmed using SPEAK code strategy
from 1994 onward, before which the MPEAK strategy was used. The children had
various types of habilitation programs and educational placements before and
after implantation. Some of the children had changes in educational placement
over the years.
ANALYSIS
For each vocabulary test, we:
- Determined the rates of change of the age-equivalent
scores (regression coefficients for scores of individual patients over time)
for all patients (mean, SD, and 95% confidence interval [CI]).
- Compared the mean rates of change of the age-equivalent scores
of older and younger children by age at implantation (2-tailed t test with unequal variance).
- Compared the means of the earliest and latest available gap indices
of the whole population (2-tailed paired t test).
- Performed multiple regression analysis (analysis of covariance)
using the rate of change of the age-equivalent scores as a dependent variable,
and age at implantation, mode of communication, percentage of residual hearing
(in the ear receiving the implant), sex, and etiology of deafness as independent
variables.
RESULTS
As part of the Cochlear Implant Program protocol,11
all patients undertake vocabulary tests preoperatively for baseline assessment,
then postoperatively every 6 months for the first 2 years and once yearly
thereafter. Therefore, at a given time after implantation, every patient should
have performed both language tests an equal number of times. As this was not
achieved because of several constraints (mainly missed appointments and relocated
families), 2 patient populations are described.
Satisfying the inclusion criteria were 60 children (28 boys and 32 girls)
for the PPVT and 52 children (27 boys and 25 girls) for the EOWPVT (Table 1). Thirteen patients had postlingual
deafness and did not qualify for inclusion in the study. Of 120 patients with
prelingual deafness, 60 were excluded from the PPVT group and 68 were excluded
from the EOWPVT group. Exclusion criteria included patients who did not possess
2 or more assessment scores after implantation, either because of their not
being tested or because the obtained scores fell outside the sensitive range
of the language tests.
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Table 1. Characteristics of Prelingually Deaf Patients Included in
the Study*
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The mean age at implantation was 5.1 years for the PPVT group and 5.3
years for the EOWPVT group (range, 1.9-11.6 years for both groups). Accordingly,
on subgrouping the populations by age at implantation, we chose a cutoff point
of 5 years. These subgroups (<5 years and  5 years) are referred to
as "younger" and "older" groups.
The subgroups were not different statistically in follow-up duration
(PPVT, P = .40; EOWPVT, P
= .63) or in proportions of sex (PPVT, P = .51; EOWPVT, P = .28), communication mode (PPVT, P = .72; EOWPVT, P = .48), or etiology of
deafness (PPVT, P = .42; EOWPVT, P = .31). There was a significant difference between the subgroups
in mean residual hearing in the EOWPVT group (P =
.03), but not in the PPVT group (P = .06). These
results are given in Table 2.
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Table 2. Comparison of Characteristics of Subgroups by Age of Implantation
in the Studied Populations*
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All children used their implant devices consistently. They had been
followed up for a mean of 3.5 years (range, 12 months to 9 years for both
groups). Most were oral communicators (85% of both groups), with only 1 child
using American Sign Language. The remainder used total communication (Table 1).
Figure 1A is a plot of the
mean PPVT age-equivalent scores against time after implantation for the whole
group that performed the test and for the younger and older subgroups. Four
observations can be made. First, there is a consistent rise in the age-equivalent
scores of both tests over time. Second, the rise is uneven between consecutive
time points, indicating a fluctuating rate of growth. Third, the older group
of patients has scores that were higher than those of the younger ones at
any time point. Finally, the growth pattern of the scores of the younger and
older groups do not appear different. Figure
1B is a plot of the EOWPVT age-equivalent scores against time after
implantation. It has been constructed in a similar fashion to Figure 1A, and the same observations can be made on it, the major
difference being the steeper growth curve of the mean EOWPVT scores.
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Figure 1. A, Mean age-equivalent scores
of the Peabody Picture Vocabulary Test–Revised. B, Mean age-equivalent
scores of the Expressive One-Word Picture Vocabulary Test–Revised over
time after implantation. After 7 years, the number of patients followed up
decreases significantly for both tests.
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However, because the growth curves were constructed using mean scores
at separate time points, with variable numbers of patients at each point,
caution is advised on interpretation, as many of the details of the data are
lost in this representation.14 The results
of the method of summary statistics we used for the definitive analysis follow.
RATE OF CHANGE OF THE AGE-EQUIVALENT SCORES
Figure 2 illustrates the mean
rates of change in the age-equivalent score per year in the 2 vocabulary tests
for the whole group and for the older and younger subgroups. Although the
mean PPVT rates are not notably different, the older group has a greater mean
EOWPVT rate than the younger group; however, this difference is not significant
(P>.05). The details of the results are given in Table 3. The mean PPVT rate for the whole
group was 0.71 age-equivalent score per year (SD, 0.50; 95% CI, 0.20), while
the EOWPVT rate was 0.93 age-equivalent score per year (SD, 0.66; 95% CI,
0.99). For the PPVT, the older group's mean rate was less than that of the
younger group (0.69 vs 0.72), but the difference was not significant (P = .90). For the EOWPVT, the older group's rate was higher
(1.06 vs 0.83), and again this difference was not significant (P = .23).
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Figure 2. Rates of change in Peabody Picture
Vocabulary Test–Revised and Expressive One-Word Picture Vocabulary Test–Revised
in all groups.
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Table 3. Rates of PPVT and EOWPVT Age-Equivalent Scores*
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THE GAP INDEX
Figure 3A shows histograms
representing means of the earliest and latest PPVT gap indices for the whole
group and for the older and younger subgroups. There is a significant decrease
in the older group's gap index (P<.01). Figure 3B shows the mean earliest and latest
gap indices of the EOWPVT. There is a similar and statistically significant
decrease in gap index, over time, for each of the 3 groups.
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Figure 3. A, Comparison of 52 early and
late Peabody Picture Vocabulary Test–Revised gap indices (whole group, P = .05; younger group, P = .57;
older group, P<.01). B, Comparison of 60 early
and late Expressive One-Word Picture Vocabulary Test–Revised gap indices
(whole group, P<.001; younger group, P<.01; older group, P<.001).
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The results comparing the earliest and latest gap indices of the PPVT
and EOWPVT are given in Table 4
and Table 5. The gap index value
is expressed as a fraction of the ideal score for age at the time of testing.
A significant difference was demonstrated between the means of the earliest
and latest available gap indices of the whole population. The PPVT gap indices
changed from 0.53 to 0.49 (P = .05; 95% CI, -0.0002
to 0.08) and those of the EOWPVT from 0.45 to 0.38 (P<.001;
95% CI, 0.04-0.10).
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Table 4. Intragroup Comparison of Early and Late Gap Indices*
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Table 5. Intergroup Comparison of Early and Late Gap Indices*
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Within the younger and older groups, comparisons of earliest and latest
available gap indices demonstrated significance. The PPVT gap indices of the
older group changed significantly from 0.62 to 0.55 (P<.01;
95% CI, 0.02-0.12), while the younger group had a nonsignificant change from
0.47 to 0.46 (P = .57). For the EOWPVT, the gap indices
of the older group also changed significantly from 0.47 to 0.38 (P = .001; 95% CI, 0.04-0.14), while for the younger group gap indices
changed significantly from 0.43 to 0.37 (P<.01;
95% CI, 0.02-0.10).
An intergroup comparison of the younger and older groups' earliest available
and latest available indices demonstrated that, while the younger patients
had significantly lower gap indices for receptive vocabulary, the 2 groups
showed no significant difference with respect to expressive vocabulary (earliest
index, P = .45; latest index, P = .83). The details of these results are found in Table 5.
MULTIPLE REGRESSION ANALYSIS
For the PPVT, multiple regression analysis using a backward stepwise
model showed male sex and oral communication mode to be significant factors
(P = .04 and .03, respectively). For the EOWPVT,
running a best subset model demonstrated residual hearing as the only significant
predictive factor (P = .03). Table 6 contains the results of the multiple regression analysis
for the 2 test groups.
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Table 6. Significant Factors on Multiple Regression Analysis (Analysis
of Covariance) of Age-Equivalent Scores of Both Tests*
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COMMENT
Presentation of the results in the form of a gap index and a rate is
necessary to provide as extensive an evaluation as possible of the evolving
vocabulary skills of these children. The rate (calculated as the coefficient
of regression of the age-equivalent scores over time) represents performance
over time and takes into consideration every score for the individual patient.
At the same time, the gap index allows an evaluation of the end product and
compares it with the state at a starting point. It is our view that neither
is exclusive to the other, especially given that the rate of language acquisition
demonstrates considerable intersubject and intrasubject variation.
In our study, rates of vocabulary development of children with cochlear
implants demonstrated considerable individual variation, as evidenced by the
wide SDs (Figure 2 and Table 3). This is in agreement with previous
reports.4, 6 In the absence of
a concurrent control group, our findings cannot support those of other reports5-6 that children with cochlear implants
equate or supersede rates of vocabulary acquisition of their counterparts
without hearing abnormalities.
On analyzing the subgroups by age at implantation, there was no demonstrable
difference in vocabulary growth rates. The older group tended to acquire expressive
vocabulary faster, but this may be partly explained by their significantly
higher residual hearing, especially as it was the only significant predictor
for the EOWPVT on multiple regression analysis. Comparison of the corresponding
gap indices (Table 5) showed that
the receptive vocabulary indices of younger patients were significantly better
than those of the older children, whereas the expressive indices were similar
for the 2 groups. Although this supports the notion that earlier implantation
may reduce the receptive vocabulary loss caused by the duration of auditory
deprivation, as others have suggested,4 expressive
vocabulary may not be similarly affected. In addition, the younger children
did not improve their receptive indices to any demonstrable degree, whereas
the children who underwent implantation after a longer period of auditory
deprivation demonstrated benefit over time.
There was a reduction in the gap index as a proportion of the ideal
score for age at testing. This can be illustrated by a hypothetical example
(Figure 4) of a child aged 3 years
at implantation and followed up for 3 years. According to mean gap
indices, the PPVT gap would change from 1.65 years at the 6-month assessment
after implantation to 2.99 years after 3 years of follow-up. On the
other hand, the child's EOWPVT gap of 1.51 years would change to 2.41 years.
Although the rates of vocabulary growth prevented a perpetual increase in
the initial delays and led to their decrease in the expressive test, the gaps
amounted to 46% of the age-appropriate PPVT scores for age and 37% of the
EOWPVT scores.
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Figure 4. Hypothetical example of a child
with prelingual deafness who underwent implantation at the age of 3 years.
The example assumes 6-month and 3-year assessments after implantation
using the Peabody Picture Vocabulary Test–Revised and the Expressive
One-Word Picture Vocabulary Test–Revised. The calculation of the expected
language gaps and age-equivalent scores is explained.
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The only significant predictive variables were residual hearing for
the EOWPVT and male sex and oral communication mode for the PPVT. Although
the sex effect has not been found previously in investigations on children
undergoing implantation for deafness, another author15
found a similar result in a population without deafness with respect to the
same test. Meanwhile, children with oral communication did better, probably
because our testers administered the tests only in the oral mode, irrespective
of the communication mode of the child. This is slightly different from the
findings of Miyamoto et al,4 most likely because
they administered their tests in the preferred mode of the child. Age at implantation
was not a significant predictive factor, irrespective of prior expectations.
Despite the retrospective design, there are several strengths of our
investigation. It is one of the larger studies of communication in children
undergoing implantation for prelingual deafness, and its follow-up has extended
longer than that of most others. Regarding the tools of the investigation,
the tests were administered only in the oral mode, unlike those of Dawson,2 Miyamoto,4 Robbins,6 and Bollard7 and their
colleagues and most other investigations. Using only the oral mode eliminated
all nonauditory sensory inputs from the results as far as feasible, allowing
more credible extrapolation of our results. Our elimination of the data before
implantation is well-founded, because it was only available for some patients
and hard to accurately designate on a time scale. This limits our conclusions
to the course of events after implantation.
As an effectiveness study, we introduce the use of the gap index as
a potential outcome measure that simplifies the analysis of serial measurements
of scores that grow over time. It allows comparison of different age groups
and assessment of end stage in relation to the initial one.
Further research should be directed at conducting prospective longitudinal
clinical trials that compare concurrent groups of children with profound deafness:
those who have been fitted with hearing aids, those who received cochlear
implants, and those who received neither. The credibility of such trials would
be enhanced if the outcome measures used are reflective of real-life communication
and the extent of inclusion of these children in mainstream activities.
CONCLUSIONS
The evaluation of language development of children with deafness after
cochlear implantation should include, in addition to the rate of such development
over time, an expression of the language gap at the latest point in the follow-up.
Age at implantation, on its own, cannot preclude a beneficial outcome but
may suggest a different pattern of development.
AUTHOR INFORMATION
Accepted for publication February 7, 2001.
Presented at the American Society of Pediatric Otolaryngology meeting,
Orlando, Fla, May 18, 2000.
Corresponding author and reprints: Hamdy El-Hakim, FRCS (ORL), Ear,
Nose, and Throat Department, Ward 45, Aberdeen Royal Infirmary, Foresterhill,
Aberdeen AB9 2ZB, Scotland (e-mail: helhakim{at}aol.com).
From the Departments of Otolaryngology (Drs El-Hakim, Papsin, Panesar,
and Harrison and Mr Mount) and Epidemiology (Mr Stevens), and the Cochlear
Implant Laboratory (Ms Levasseur), The Hospital for Sick Children, Toronto,
Ontario. Dr El-Hakim is now with the Ear, Nose, and Throat Department, Aberdeen
Royal Infirmary, Scotland.
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