The normal hearing threshold is -10 to 25 decibels. Hearing Impairment (HI) is defined when an ear can only receive sound of 26 decibels or more. HI can be divided into five levels of severity: mild, moderate, moderately severe, severe and profound. HI is considered ‘significant’ when the degree of impairment is moderate grade or worse in the better ear.
The nature of HI can be conductive, sensorineural or mixed. Conductive HI is caused by problems in the conduction of sounds in the outer and/or middle ear, such as earwax occlusion and middle ear effusion. Sensorineural HI involves impairment found in the inner ear and/or auditory nerves, whereas mixed HI includes both conductive and sensorineural components. Management plans for HI vary from person to person, depending on the severity and nature of the problem.
The impact of HI follows the degree of impairment. Children with mild HI demonstrate difficulty perceiving faint sound and understanding soft-spoken speech. Children with moderate HI demonstrate understanding of speech at 3-5 feet distance but they have difficulty in perceiving conversation in a noisy environment, while those with severe HI can only understand speech at 1 foot from the ear with poor speech sound discrimination. Children with profound HI generally show lack of response to environmental sound and they cannot rely on audition as primary modality of communication.
HI will affect the different stages of language development. Language development of these children will depend on the severity and the onset time of the impairment. The following descriptions are mainly describing the language development of children whose HI is at a significant level with the onset at a pre-verbal stage:
HI itself does not hinder cognitive development but communication and language development will be affected. The lack of a solid first language (sign or spoken) and of interactive learning experience will affect acquisition of information and knowledge. These children may as a result fail to develop their full potentials.
Many studies have shown that children with HI are at risk of difficulties in literacy development. In addition to the delay in the growth of vocabulary, grammar and sentence structures, children with severe HI do not have solid knowledge of the sounds of words, and hence unable to “read out the words” in their brains. Reading and comprehension will therefore be more difficult.
Because of their difficulties, children with HI may limit their communication and interactions with others, thus hampering the relationship with their caregivers and friends. Older children often have emotional or behavioral problems due to ineffective communication with peers and low self-esteem.
Motor coordination problems are noted in children with HI. There may be easy loss of balance, and frequent falls, hindering their functioning in the daily life. The problem is often more obvious in a dark environment.
International data shows that the prevalence of congenital HI varies from 1-2 per 1000 newborn infants. In Hong Kong, for every 1000 children under the age of 15 years, 1.3 was registered with significant HI in the Central Registry for Rehabilitation in 2014. In Child Assessment Service (CAS) of Department of Health (DH), there are around 70-80 children diagnosed with significant HI every year.
Genetic causes account for roughly 50-60% of children with HI but most cases are autosomal recessive. These include gene mutations, chromosomal abnormalities and genetic syndromes. Studies found that many genes are related to HI, with some causing mild HI and others significant HI. Common genetic mutation testing is provided by the Clinical Genetic Service of DH.
Meanwhile, many non-genetic factors can also lead to HI. Sensorineural HI may result from multiple causes, including intrauterine infections or certain drugs taken by the pregnant mother, adverse events during delivery, brain and ear infections, as well as trauma or tumors affecting related structures during infancy and childhood. Conductive hearing loss may result from infection or obstruction of the ear canal and middle ear such as impacted ear wax and secretory otitis media.
Some developmental conditions commonly seen in children may be confused with HI. These include autism spectrum disorder, severe language delay, developmental delay or intellectual disability and severe behavioral disorders.
Depending largely on the cause of the HI, there may also be developmental delay, intellectual disability, emotional and behavioral problem, cerebral palsy or visual impairment in respective children with HI. HI may also be part of the features of congenital syndromes in which hearing is affected.
Residual hearing should be optimized through early amplification, provisions of favorable acoustic environment and appropriate training. Effective comprehensive habilitation programs will be able to enhance these children’s auditory awareness and speech discrimination ability, foster the development of speech and language, and facilitate their intellectual and social development.
The language development of children is almost always being affected by their HI to different degrees. Therefore, these children need to receive training on their speech, language and communication. The earlier the commencement of training, the better will be the outcome. However, the ultimate progress will also depend on other factors including the degree of hearing impairment, how long it took for hearing aids or cochlear implants to be effectively fitted, the learning abilities of the child, motivation of the parents, and so on.
Some children with HI may not attain a satisfactory level of verbal language development even though they have been fitted with hearing aids or cochlear implant. Therefore, in these cases, the therapists might need to consider other modes of communication besides the verbal one. In Hong Kong, most of the aural rehabilitation programs use total communication as the basic concept. Recently, the concept of sign-oral language bilingualism has also been put into the rehabilitation programs. Auditory-verbal therapy and the cued-speech method are also widely used in western countries. Below are some descriptions of the aforesaid rehabilitation methods:
Developing a fluent first language at an early age will facilitate the child’s development of cognition and written language. When reading, children need to use existing knowledge and a "top down" approach to understand the content. At the same time, children may need to use the "bottom up" approach through reading of words and sentences to understand the article and learn new concepts. In addition, we can enhance children's reading motivation through interesting learning activities, reading different kinds of books, and use of multi-sensory methods to help word learning, promote grammar learning and enrich their background knowledge.
Treatment of the underlying cause is possible in some conditions (e.g. earwax removal, antibiotics for otitis media, grommet insertion for middle ear effusion). Genetic counselling or related medical treatment can be provided for genetic and syndromal conditions.
With overwhelming research evidence demonstrating the benefit of early aid fitting on language development, children with HI should be fitted with hearing aids as early as possible. Binaural use of hearing aid is encouraged for more effective receiving of sound signals and sound localization. However, even with advanced hearing aids, hearing cannot be fully restored. Children must receive appropriate auditory training in order to enhance the hearing and language development. In noisy environments, wireless frequency modulation (FM) systems or other assistive listening devices may be used together with hearing aids to reduce the impact of noise.
For children with severe to profound hearing loss who show limited or no benefit from hearing aids, cochlear implantation (CI) may be considered. A cochlear implant is an electronic device that is implanted surgically, allowing the recipient to receive auditory information by electrical stimulation of the cochlear portion of the ear. Another audiological intervention, auditory brainstem implant (ABI), is used to treat total deafness in both ears which cannot be improved by hearing aids or cochlear implants. The external receivers of CI and ABI are similar, but in ABI the internal device is connected to the brainstem and directly stimulates the cochlear nucleus. Both CI and ABI require intensive auditory, speech and language rehabilitation and training after implantation.
Professional advice, counseling and psychological support must be provided to parents of children with HI. Their compliance with the child’s use of hearing aids, provision of adequate language stimuli and participation in parent support groups should be encouraged. Advice on school accommodations such as classroom seating arrangement and attendance at various medical reviews should be given.
Family Health Service from DH and neonatal units from the Hospital Authority(HA) provide universal newborn hearing screening programs using the Distortion Product Otoacoustic Emission or Automated Auditory Brainstem Response. Student Health Service (SHS) also provides hearing screening for Primary 1 to Form 7 students who join their voluntary annual health check program.
Ear, Nose and Throat (ENT) Departments of HA, and Audiological services of CAS and SHS of DH provide diagnostic assessment for children with HI.
HA offers medical and surgical interventions for children with HI. Since 1995, three cochlear implantation centres have been established including at Queen Mary Hospital, Prince of Wales Hospital and Queen Elizabeth Hospital. In current practice, the age for paediatric cochlear implantation is around 1 year old. Hearing aids and assistive listening devices can be obtained through the Education Bureau (EDB) or the private sector. Auditory and speech training is available in speech therapy clinics under HA, EDB, non-governmental organizations and the private sector.
In Hong Kong, there are early educational training centres and two special child care centres to provide pre-school habilitation training for children with HI and their parents. They provide intensive auditory and speech training, and children are encouraged to practice their speech and communication skills into daily experience through various activities. Integrated programmes in child care centres as well as special child care centres for children with disabilities are also provided with additional resources to help children with HI.
Lutheran School for the Deaf is the only special school for hearing-impaired school-aged children in Hong Kong today. Mainstream curriculum is implemented in this school with special educational support from teachers. Total communication method is applied with emphasis on a balanced development in spoken, sign and written language. There are also continual auditory and speech training at the school.
In recent years, there has been a shift in enrolment from special schools to mainstream schools for children with HI. Mainstream schools provide varying degrees of special education support to students with HI. These include outreach services by special school or through special funding assigned to these mainstream schools.
The use of sign language together with oral language is believed to enable the children with severe HI to be better able to attain their full cognitive, linguistic and social potentials.
Since 2006 the Centre for Sign Linguistics and Deaf Studies in the Chinese University of Hong Kong has offered “Sign Bilingualism and Co-enrollment in Deaf Education Programme” which promotes simultaneous learning sign language and oral language. Besides “Sign language classes for babies”, the Centre provides “Sign bilingual Chinese reading classes” and “Parents signing classes”. The Centre partners with kindergartens, primary and secondary schools to deliver this educational programme in the school setting.
Parent resource centres, support groups and parent training programmes are organized by a wide variety of institutions including government and non-governmental organisations, including The Hong Kong Society for the Deaf, HA cochlear implant centres, special schools and CAS of DH.
Prognosis is dependent on the degree of HI, age of onset and diagnosis, age at which amplification is introduced, developmental characteristics of the child, psychosocial factors and educational experience. Early diagnosis and timely intervention are key factors for favorable outcome.