Congenital hearing impairment requiring amplification has serious and lasting effects on language development for many children. It has traditionally been diagnosed late.¹ With equipment suitable for newborn screening now feasible and available, many countries and states are embracing the philosophy and practice of universal newborn hearing screening.^2–,⁴ Two influential recent studies^5,6 build on earlier studies^7–,⁹ in suggesting that very early diagnosis and entry to services improve language outcomes, independent of severity of hearing impairment,^5,6 intelligence,^5,6 and socioeconomic status.⁵

However, the published research has important flaws. Most importantly, rigorous epidemiological approaches are needed to minimise selection bias and maximise generalisability. While recruitment within one or two early intervention services offers the opportunity to study within-group outcomes by age of entry, it adequately characterises neither the whole population of children with congenital hearing impairment, nor those who left the service prior to outcome data collection. Secondly, the most persuasive studies^5,6 assessed outcomes before children completed their preschool years. Research with profoundly deaf children receiving cochlear implants has established that catch-up can occur at later ages.¹⁰ Assessment of longer term outcomes is therefore crucial.

Additional criticisms include the use of parent reported language measures which, while well respected, may be open to serious bias in unblinded studies of the impact of age of diagnosis on outcome.⁵ Language measures in some studies canvass only limited domains, such as receptive vocabulary⁶ or intelligibility.⁷ Not all studies have adjusted for child intelligence,¹¹ despite known correlations of 0.70–0.75 between global cognition and general language abilities in normally hearing children.^12,13 Those that included cognitive measures⁵ did so at an age when IQ is difficult to measure accurately and thus lacks longitudinal stability. Finally, although the foundations of language are laid down in early childhood, language development continues throughout the childhood years; thus it seems important to study outcomes for older, as well as younger, children.

The Children with Hearing Impairment in Victoria Outcome Study (CHIVOS) aimed to address many of these issues. The study targeted an entire state’s population of 7–8 year old hearing impaired children. Access to amplification and early intervention services was available to all. Accurate data on severity of hearing loss and ages of diagnosis and amplification were collected prospectively. Objective measures of cognition and a broad range of language skills were assessed by researchers blind to hearing status and age of diagnosis.

This paper reports on the relation between severity of hearing loss at diagnosis, age at diagnosis, and language outcomes for this cohort. In line with the current literature, we hypothesised that children whose hearing impairment was diagnosed early would have language outcomes superior to children who were diagnosed later, irrespective of severity of hearing impairment or intelligence.

METHODS

RESULTS

A total of 132 eligible children and families were identified (fig 1⇓). The 89 respondents did not differ from the 43 non-respondents in mean age at diagnosis, mean age at which hearing aids were first fitted, mean severity at diagnosis, gender, or mean Education and Occupation Index score.

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Figure 1

 Participant flowchart.

After completion of the study, we realised that a further 45 potentially eligible children had not been identified due to a 1997 administrative change in the way Australian Hearing provided data to the hearing screening programme. These children were similar to those approached on all available indices (age at diagnosis, age hearing aids first fitted, severity at diagnosis).

Table 1⇓ shows demographic and audiological characteristics of the sample. On average, 6.1 years had elapsed since diagnosis (SD 1.1, range 3.5–8.1). Eleven children had been diagnosed by 6 months, of whom one had mild, three moderate, four severe, and three a profound loss; 29 had been diagnosed by 12 months, of whom two had mild, eleven moderate, six severe, and nine a profound loss. Mean severity was 65 dB HL at diagnosis (SD 25, range 30–120) and 57 dB HL at outcome (SD 22, range 23–105). On average, children’s acuity remained stable from diagnosis to outcome (mean change = 0.06 dB HL, SD 14, range −27 to +50 dB HL). Fourteen children showed evidence of progression 10 dB HL or greater, with a median change of 17 dB HL (range 12–50), and a further 14 children showed an improvement in hearing levels of 10 dB HL or greater (median 15, range 12–27). Eighty eight per cent of the sample had attended an early intervention service. Forty six per cent were currently attending a school for the deaf or a school with a deaf facility; 93% of those attending a school without a deaf facility had a visiting teacher or teacher’s aide.

Diagnosis (mean age 1.8 years) was followed closely by hearing aid fitting (mean age 1.9 years; Pearson’s r = 0.99, p < 0.001), and entry to early intervention service (mean age 2.0 years; Pearson’s r = 0.76, p < 0.001). Age at diagnosis was negatively correlated with severity at diagnosis (Pearson’s r = −0.39, p < 0.001). At outcome, mean non-verbal IQ was 104.6 (SD 16.7); mean total CELF score was 76.7 (SD 21.4); and mean PPVT score was 78.1 (SD 18.1).

Table 2⇓ shows bivariate correlations between outcome measures and potential predictors/confounders. Severity at diagnosis, severity at outcome, and non-verbal IQ were significantly correlated with almost all speech and language measures, but not reading score. In contrast, there were no significant correlations between age at diagnosis or hearing aid fitting and any of the outcome measures. Family functioning did not correlate with any outcome measure. Socioeconomic indicators were only weakly related to outcome measures, the strongest correlation being with the reading score.

Seven regression equations were calculated using the enter method (table 3⇓). Age at diagnosis did not contribute significantly to variance on the language, speech, or reading measures, with the exception the PPVT which approached significance (R² change = 0.07, p = 0.05). Severity of hearing impairment contributed significantly to the variance in scores on all the language and speech measures, but not the RPT. More severe hearing impairment very strongly predicted lower scores on all language measures, independently accounting for 12–29% of outcome variance. Interaction effects between severity of hearing impairment and age at diagnosis on outcome were sought but not found. Our cohort had a slightly higher proportion of mild losses (21%) than Moeller’s (8%) or Yoshinaga-Itano’s (13%) studies. In case these children unduly influenced our results, we re-ran the regression analyses including only the children with losses >40 dB HL. The findings were unchanged.

In the GLMs, estimated mean language and reading scores did not vary significantly by age of diagnosis, adjusted for severity of hearing impairment and non-verbal IQ (table 4⇓), though we note a tendency for those diagnosed by 12 months to do better than those diagnosed between 1 and 3 years. In contrast, mean scores fell sharply with increasing severity of hearing impairment, adjusted for age at diagnosis and non-verbal IQ (table 5⇓).

Figure 2⇓ presents scatterplots, stratified by severity, of age at diagnosis by outcome for the CELF. No discernable trends are evident in any of the severity groupings.

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Figure 2

 Scatterplots showing CELF score against age at diagnosis, stratified by severity of hearing impairment.

DISCUSSION

Language outcomes for this cohort of 7–8 year olds with congenital hearing impairment from Victoria were very poor—more than 25 points lower than expected from their IQ scores. In contrast to other studies, we found that these outcomes were strongly and linearly related to the severity of impairment but not to age at diagnosis. How do we interpret these findings?

We have confidence in the quality of our data. The study minimised potential bias through its size, sample selection methods (population based), measurement of predictors (objective and prospective), and measurement of outcomes (objective, standardised, and blinded with respect to the predictors of interest). Likely confounders were considered and where necessary adjustment occurred. Diagnosis was followed rapidly by amplification and early intervention, which was accessed by all but the mildest and latest diagnosed in the cohort. Non-respondents did not differ from respondents on key predictors or sociodemographic indicators.

The main differences between this and previous studies^5,6 is that our subjects were older and our study sampled from a geographical, rather than a service based, population. It may be that by age 7–8 years, severity of impairment has prevailed over age at diagnosis—the longer a severe disability is present, the greater its relative impact on outcomes. A population sample may give a different picture to studies conducted entirely within one or two early intervention programmes. Although almost all our children received intervention as soon as the diagnosis was confirmed, this was variable in approach (and presumably intensity) since parents were free to choose which of the available programmes they wished to attend. This is probably similar to the scenarios in most countries, in which a variety of approaches are usually available.

It may be that there were simply too few children in this cohort diagnosed younger than 6 months of age. However, examination of the raw data did not indicate that the 11 children diagnosed before 6 months showed a trend to do better, and in fact those with the best outcomes were diagnosed after 6 months. The most direct comparison that can be made is with Moeller’s study of children with similar numbers, spreads of age at diagnosis and severity, and outcome measures.⁶ Using a comparable age categorisation, Moeller found a stepwise relation between language outcomes and age at diagnosis, but not IQ or severity; we found the opposite. Nonetheless, the small numbers of cases diagnosed prior to 6 months leaves wide confidence intervals around the outcome figures and therefore leaves the question of possible benefit of very early intervention (made feasible with universal neonatal screening) open.

Other limitations include lack of a parent-child interaction or family participation measure, which is difficult to assess by this age and in the setting of a non-service based research outcome study. Parents play a vital role in optimising the development of children’s language,²⁴ regardless of hearing status. We doubt addition of a good parent-child interaction measure would have negated the findings, though they may have added to the variance explained. Our family functioning measure did not add to the explanatory power of our models, even though we might expect this to be related to both positive parent-child interaction and family participation. Because we excluded children with intellectual disability, we cannot generalise these findings to such children. While it could be possible that some children who had poorer outcomes were excluded as having “intellectual disability”, we doubt that this is the case since the mean non-verbal IQ (104.6) was equivalent to normative populations, and we found no evidence (such as downward skew) to suggest that performance IQ was altered by the hearing loss itself.

Like other studies, we could not be completely sure that all hearing impairments were congenital. However, the hearing screening programme prospectively recorded whether hearing impairments were believed to be acquired or congenital, and acquired losses were excluded from our sampling frame. Minimal overall progression was measured from diagnosis to outcome, and the prevalence of moderate or greater bilateral hearing loss in our population at age 4.5 years approximates that usually found in universal newborn hearing screening programmes.²⁵ Thus we do not think that inadvertent inclusion of acquired or progressive losses has influenced these findings greatly.

In 2001, the US Preventive Services Task Force evidence based systematic review of newborn hearing screening noted important design weaknesses in all published studies examining age of diagnosis and language outcomes.²⁶ In addressing many of these weaknesses, this study did not find the expected relations between earlier diagnosis and better outcomes, even when amplification and prompt intervention were readily available. The strongest evidence continues to indicate that the best outcomes follow very early diagnosis of congenital hearing loss (before 6 months of age). This study, which contained only 11 such children, should not change that conclusion but does sound a note of warning. Countries considering implementing newborn hearing screening should be aware that excellent infant, preschool, and primary school services are also essential if early diagnosis is to reap the desired dividends. They should also systematically monitor outcomes of any new programme well into the primary school years, and not assume that early diagnosis alone translates into long term benefit. Outcome data such as these should provide a baseline against which to measure our successes, an impetus to rigorously study the effectiveness of different approaches, and a mechanism to make long term decisions about the future of newborn hearing screening.