Introduction to the Longitudinal Outcomes of Children with Hearing Impairment (LOCHI) study: background, design, sample characteristics

Unique research environment in Australia

Table 2 shows the commencement year, screening protocol, and relative proportion of Australian births (as at 2002) for each of the state- and territory-wide UNHS programs in Australia. Notably, the three most populous states—New South Wales (NSW), Victoria (VIC), and Queensland (QLD)—each commenced programs within a 3-year period from 2002 to the 2005. In New South Wales the program was implemented across all regions of the state simultaneously whereas in Queensland and Victoria the roll-out of the program was a staged process with different regions of each state commencing screening progressively over periods of 2 years and 6 years respectively. Notably, however, the programs that were implemented in all three states used the same screening protocols and screening techniques (i.e., Automated Auditory Brainstem Response—AABR—testing).

Regardless of the state or territory jurisdiction in which they are born or the age at which their hearing loss is diagnosed, Australian children with childhood hearing loss are referred to a single government-funded organization (i.e., Australian Hearing) for the delivery of hearing services. Those services are provided free of charge according to nationally consistent protocols. This means that all newborns who are identified with hearing loss through UNHS receive a common standard of care in regard to post-diagnostic audiological assessment and the provision of amplification. Taken together, this common standard of care, the employment of common protocols for screening across the three largest states, and the differential rates and patterns of program implementation across those states provided the opportunity to undertake a population-based study investigating the effects of earlier versus later identification of permanent childhood hearing impairment. By seeking to recruit all children with hearing loss who were born in the three states across a five year period from 2002 to 2007, the differential implementation of UNHS programs provided a sample of children that was naturally divided according to whether their impairment was identified early (through UNHS) or later. Further, because of the similarity of the three state-wide screening programs and the consistency of the other services available to all children in those states, such a study could effectively control for a wide range of other important factors.

Predictors of outcomes

The US Preventative Services Task Force questioned whether there were factors other than age of intervention that might contribute to language development (Thompson et al., 2001). This question was addressed in the LOCHI study by measuring a range of demographic characteristics of the children while they were occurring, rather than being gathered retrospectively. A range of predictor variables that have been implicated in previous studies to affect outcomes were measured. These included child-related factors (gender, birthweight, age of diagnosis, age of hearing aid fitting, age of cochlear implantation, degree of hearing loss, presence of additional disabilities, presence of auditory neuropathy), family-related variables (maternal education, socio-economic status, language used at home, communication mode at home, involvement in intervention), and other education-related variables (communication mode, changes in communication mode over time, hours of service). The relative importance of these variables has not been quantified within a single cohort before, as studies did not have sample sizes that were sufficient for investigations of all variables with adequate statistical power.

In addition to age of hearing aid fitting, we hypothesized that changes in real-ear aided gain as a result of changes in hearing thresholds over time, hearing aid prescription used in fitting, and frequency lowering processing in hearing aids would affect outcomes. Therefore, we incorporated two randomised controlled trials in the LOCHI study. The first examined the effect of choice of prescription. Two prescriptions, the National Acoustic Laboratories (NAL) and the Desired Sensation Level (DSL) prescriptions are widely used internationally for fitting hearing aids to children. The prescriptions differ (sometimes markedly) in real-ear gain at different frequencies (Byrne et al., 2001; Ching et al., 2010), and neither has been validated for children. Although there is some evidence to show that the choice of prescription has little consequence in speech understanding and listening comfort for school-aged children with moderate degrees of hearing loss (Ching et al., 2010; Scollie et al., 2010), the differences may be important for development of speech and language skills in the short and longer term when amplification is provided during the first few months of life, especially when the hearing loss is severe. In this trial, we randomly assigned newly identified children to one of the two prescriptions for fitting, and assessed the impact of prescription on outcomes after allowing for a range of predictor variables.

The second randomized trial within the LOCHI study investigated the effect of nonlinear frequency compression in hearing aids on children’s speech and language development. This sub-study was necessitated by the wide availability of frequency-lowering technology in commercial hearing aids for children despite a total lack of evidence on the impact of the processing on language development of young children (McCreery et al, 2012). We randomly assigned children to being fitted with hearing aids that provided either conventional processing or nonlinear frequency compression, and examined whether their outcomes were significantly affected by hearing-aid processing.

There is also a proportion of children for whom management of hearing loss may require cochlear implantation. In previous population studies, only about 2or 3% of participants had cochlear implants although about 30% of the cohorts had a profound degree of hearing loss. Early diagnosis of hearing loss would be expected to not only enable early hearing aid fitting, but also early cochlear implantation for those with profound loss.

Outcomes measures

The test battery was carefully selected to assess the skills that rely on auditory input to develop. These included auditory functional ability, speech production, receptive and expressive language, and psychosocial development at 3 years of age. The measures included tests directly administered to children, and reports solicited from parents or primary caregivers. At later measurement points, nonverbal cognitive ability, literacy, educational attainment, mental health, and quality of life will also be evaluated. Direct evaluations included the Pre-school Language Scale version 4 (PLS-4; Zimmerman et al., 2002) for assessing receptive and expressive language, the Peabody Picture Vocabulary Test, fourth edition (PPVT-4; Dunn & Dunn, 2007) for assessing receptive vocabulary, and the Diagnostic Evaluation of Articulation and Phonology (DEAP; Dodd et al., 2002) for assessing speech production. In addition, parents were requested to complete the Child Development Inventory (CDI; Ireton, 2005) on their child’s language and psychosocial skills, and the Parents’ Evaluation of Aural/oral Performance of Children (PEACH; Ching & Hill, 2007) on their child’s functional performance in real life. Early education teachers were requested to complete the Teachers’ Evaluation of Aural/oral performance of children (TEACH; Ching et al., 2008) to assess the child’s functional performance in educational settings. Although other cross-sectional or longitudinal studies have reported on language and speech production and perception skills of children using hearing aids or cochlear implants (e.g., Blamey et al., 2001; Geers et al., 2003), none included the entire range of hearing loss, and hence users of hearing aids and cochlear implants, who received very early intervention in a large cohort to allow investigations of the effect of timing of intervention on development in multiple domains while controlling for a range of predictor variables.

Rationale for the study

As the research arm of Australian Hearing, the National Acoustic Laboratories was well placed to take advantage of the national service provision network to sample from across an entire population, enroll sufficiently large numbers of early or later identified children who accessed hearing services, and include children with hearing loss ranging from mild to profound degrees, who received hearing aids and/or cochlear implant interventions. The number of predictors measured that potentially could affect outcomes, and the range of outcomes measured at several intervals for the large sample size provided data that have unprecedented potential for examining the relationships between predictors and outcomes.

The LOCHI study is important because (a) the efficacy of UNHS is not established and, without evidence of effectiveness, the costliness of UNHS programs may threaten their sustainability; and (b) The factors influencing language, literacy, psychosocial and quality of life outcomes and their developmental trajectories is unclear, and without evidence on significant factors on which to base post-diagnostic intervention, the potential benefits of early detection may not be fully realised.

This paper describes the design of the study and documents the demographic characteristics of the LOCHI cohort. The information can provide clinicians and researchers with insight regarding the population being studied. To adequately determine the effect of early intervention on outcomes, it is important to control for as many of the predictors as possible. Failure to control for any of these intervening variables may obscure the underlying causes of good or poor performance. Therefore, not only do the demographic and educational characteristics of the LOCHI cohort have inherent descriptive value, they will be even more valuable as predictors of measured outcomes.

Design

This is a prospective cohort study. Outcomes and potential predictors affecting speech, language, psychosocial, and functional performance of children were measured with normed, well validated instruments. Additionally, randomized controlled trials of hearing-aid prescription and of nonlinear frequency compression were conducted.

Sample size was determined according to two principles. First, because the primary analysis tool was multiple non-linear regression, we needed to ensure an adequate ratio of data points to unknown coefficients. With 15 raw predictors, and if necessary a larger number to allow for non-linear transformation of some predictor variables, and potential interactions between the variables, our target sample size was 400. The second principle was the anticipated need to perform analyses of variance. The target sample size was sufficient to detect a difference of 0.4 population standard deviation units on quantitative measures, with 80% power at an error rate of 5%. Because the outcomes data would be corrected for the effects of all the other predictor variables, the within-group standard deviation that applies for this power analysis is likely to be considerably smaller than the raw standard deviation that would otherwise apply. The target total of 400 children therefore provided a design that was very sensitive to the effect of early intervention.

Results

Of the 451 children (247 boys) enrolled in the study, 239 children were first fitted with hearing aids before 6 months of age. There were 86 (19%) children with mild hearing loss (average between 0.5 and 4 kHz in the better ear; 4FA ≤ 40 dB HL), 149 (33%) with moderate hearing loss (4FA: 41–60 dB HL), 71 (18%) with severe hearing loss (4FA: 61 – 80 dB HL), and 145 (32%) with profound hearing loss (4FA ≥81 dB HL). Figure 1 shows the distribution of age at first fitting of hearing aids for different degrees of hearing loss.

At 3 years of age, 134 children were using cochlear implants, with 46 of them having received an implant before 12 months of age. The remainder, except for six children, were using hearing aids. Parents reported that 85% of the children were using their hearing devices for more than 75% of their waking hours.

The cohort included 107 (24%) children who were reported to have disabilities in addition to hearing loss. There were 44 children diagnosed with auditory neuropathy spectrum disorder.

Based on parent reports, 302 children were using an aural/oral mode of communication at home, 3 were using manual communication, and the remainder were using a combination of oral and manual methods at home. Parents’ reports on communication mode in educational intervention showed that 301 (67%) used an aural/oral mode of communication; the remainder used a combined oral and manual mode. Between first enrolment in educational intervention and 3 years of age, parents of 50 children reported a change in communication mode in early education.

Summary

This article has described the design of the study and the characteristics of the cohort whose performance were analysed in the subsequent articles in this supplement and in Ching et al., 2013. In addressing challenges for clinicians posed by implementing early intervention, the articles will provide an increased understanding of family perspectives on early intervention, the relative effectiveness of hearing aid prescription and frequency lowering technology for supporting children’s speech and language development, and the impact of auditory neuropathy on outcomes. An evidence-based approach to addressing these questions is critical because clinical decisions on management strategies potentially have lifelong impact on children’s outcomes.