Hearing is an integral part of human communication. Most daily communication is aural and any individual with hearing loss risks social isolation. Genetic hearing loss is considerably more common than most people realise.
How many people are affected by hearing loss and deafness?
Hearing loss and deafness are the most common sensory deficits and result in difficulty communicating by spoken language.
One in a thousand children is born deaf or will become severely or profoundly deaf before speech is acquired. Another two to four per thousand will become deaf or hearing impaired before adulthood. In New Zealand over half of childhood deafness is attributable to genetic causes. By the seventh decade, nearly two thirds of the population has a significant hearing loss (> 25 dB) much of which is also genetically determined.
Genetic hearing loss in children
Genetic deafness is categorised as non-syndromic or syndromic. Syndromic deafness is diagnosed when other anomalies occur in addition to those of the ear. Most commonly, genetic hearing loss is sensori-neural.
Approximately 70-80 percent of genetic hearing loss is recessive, 20 percent is dominant, and the rest is sex linked or mitochondrial. The majority of recessive deafness is non syndromic. The gene responsible for approximately half of congenital non-syndromic deafness in New Zealand codes for a gap junction protein, "Connexin 26." Testing is available in New Zealand for specific mutations in this gene. Many other genes which cause hearing loss have now been described, and mutational testing for several is available commercially. Often blood or DNA is sent to overseas laboratories for this.
Auditory Neuropathy is a condition which is now being diagnosed more frequently, probably because it is being recognised.
It is a disorder where poor auditory signalling is passed from the inner ear through the auditory nerve to the brainstem. The exact site of abnormality is unknown, although the junctions between the sensory hair cells and the afferent nerves are suspected. Usually there is no true "neuropathy" as such therefore the term is a misnomer. It is best thought of as "auditory dys-synchrony". A genetic aetiology of this condition was first described in 2004 although, in some cases, it has also been associated with premature birth and jaundice. Some children with "auditory neuropathy" can do well with hearing aids, but many require cochlear implantation.
A potentially overlooked cause of genetic deafness is "mitochondrial". The products of the mitochondrial genes are used in metabolism, and the high metabolic demands of the inner ear reflect its dependency on normally functioning mitochondria.
The association between diabetes mellitus and sensori-neural hearing loss has been traced to the mitochondrial mutation A3243G. The sensori-neural hearing loss does not manifest until the patient develops diabetes. Aminoglycoside ototoxicity is due to a specific mitochondrial mutation in some patients. They are susceptible to aminoglycosides at low doses. Testing is available for this mutation.
Assessment of early childhood deafness
Unfortunately, New Zealand has a poor record in the early detection of childhood deafness. A neonatal screening programme would help correct this. Such screening programmes are now widespread across the developed world.
No child is too young to test. If there is any doubt about a child's ability to hear they should be referred to an audiologist for testing. When hearing loss is not detected and treated by six months of age language delays will be permanent.
Dilworth Hearing is fully equipped to test children of all ages, including newborns. Such testing in babies may involve Oto-acoustic Emission Testing and/or Auditory Brainstem Response (ABR) testing. Dilworth Hearing is the only private audiology facility with ABR facilities in New Zealand. ABR is performed when the infant is asleep. Some older children may require general anaesthesia to enable this.
Children, from six to nine months, are usually able to be tested using behavioural techniques. These involve visual reinforcement testing where children are rewarded (with images of toys or puppets) for turning to sounds of different frequencies and intensities played through loudspeakers. Children from 30-36 months are able to wear testing ear phones and to sit and play at a table. They may complete pieces of a puzzle or put pegs into a board when they hear testing signals.
What are the options for referring children with deafness or hearing loss?
Because most inherited childhood deafness is recessive, the parents are most likely to hear normally, and are most likely to wish their children to hear and to learn to speak. They are usually referred by the audiologist to an oral habilitation programme, such as Hearing House and also to an advisor on deaf children. When parents prefer their children to communicate with New Zealand Sign Language, they will usually attend one of the two schools for the deaf (and preschools): van Asch in Christchurch or Kelston in Auckland.
Children with suspected hearing loss may be referred to Dilworth Audiology or to a public hospital audiology department.
If hearing loss is diagnosed, hearing aids may be fitted privately or in the public system. Hearing aids are fully funded in New Zealand for children aged up to 18 years (or to 21 years for those in full-time study). A fitting fee only is charged for children who have a hearing aid fitted privately.
If hearing loss is confirmed the audiologist refers the child to an ENT specialist for further assessment. The ENT specialist is likely to also refer for opthalmologic evaluation (as up to third of congenitally deaf children have visual problems) and also for genetic assessment and counselling.
The child is also referred to an habilitation service (eg. Hearing House), and to an Advisor on Deaf Children from the Ministry of Education.
Dilworth Hearing is uniquely placed to diagnose and manage hearing loss in children because it is committed to providing a comprehensive and complete audiology service, has specialised and highly trained staff and because of its long term close association with ENT specialists.
