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The human ear and its central connections are well developed and help us to hear a range of sounds in complex listening conditions, have a sense of direction and movement of sounds and also help to maintain our balance. In some ways it is probably the most developed sensory organ we have. The outer hair cell system within the inner ear help us to hear a vast range of sounds from very quiet to very loud by dampening down the loud and amplifying the quiet sounds. The whole auditory system is capable of “filtering out” unwanted background noise. When this system fails, not surprisingly, the first symptom we experience is difficulty in hearing speech when there is background noise. Similarly if the balance organ within the inner ear is not functioning well we experience dizziness or vertigo and a sense of motion, and in a child, this may manifest as delayed physical milestones e.g. delay in walking. Hearing is extremely important for the development of spoken language. Those with a pre-lingual (prior to development of speech) profound hearing loss will not have access to speech sounds and hence will not develop oral language, while those with a severe pre-lingual hearing loss will not develop normal speech. Moderate hearing loss may affect speech and (oral) language development and will have a considerable effect on mainstream education of children. Mild hearing losses also may affect the ability to learn depending on a number of other factors. Permanent hearing impairment The incidence of significant permanent congenital hearing impairment (PCHI) is about 1 in 1,000 live births in most developed countries although this may be 3-4 times higher in certain communities or parts of the UK. The incidence almost doubles by ten years of age because of acquired hearing loss from meningitis (see entry), mumps, measles, trauma and other causes. The most common reason for PCHI is loss of hair cells in the inner ear but it is sometimes due to malformation of the middle ear ossicles (small bones that transmit vibrations of the ear drum) or the ear canal (tube from the outer to the inner ear). If the hearing loss is due to a problem within the inner ear (sensory), or occasionally due to abnormalities in the hearing nerve (neural), the term sensorineural hearing loss is used. Hearing loss due to a problem within the middle ear or the outer ear is called a conductive hearing loss while a combination of sensorineural and conductive hearing loss is called a mixed loss. Glue Ear In addition to PCHI and postnatally acquired permanent hearing loss, almost 40 – 60% of children develop 'glue ear' (synonyms: otitis media with effusion, fluid in the middle ear, middle ear effusion) during the first 6 years of life due to dysfunction of the Eustachian tube (tube that links path of the throat to the ear) with the incidence reducing as the child gets older. Otitis media with effusion (OME) is usually a temporary condition with mucus-like fluid accumulated within the middle ear (behind the ear drum) becoming thick with time (hence the name 'glue ear'). Glue ear during the first few critical developmental years of life, may lead to some speech and language delay and sometimes auditory processing difficulties. Persistent glue ear, with a significant hearing loss or other associated problems such as frequent ear infections, is normally treated by draining the middle ear fluid and inserting a tube called a 'grommet' to ventilate the middle ear. Hearing aids are also recommended as an alternative or while waiting for grommets, or when surgery is not possible. The hearing loss with glue ear is never severe or profound unless there is an additional permanent sensorineural hearing loss. Hence 'glue ear' alone is unlikely to cause severe speech and language delay. Auditory Neuropathy/ Auditory Dys-syncrony A form of deafness that has attracted interest of audiologists and clinicians more recently is Auditory Neuropathy/ Dys-synchrony. This is a condition where there is often a mis-match between objective tests of hearing and the behavioural tests. This group belongs to the “neural” component of snesorineural deafness and is estimated to be up to 10% of all SNHL. The dys-synchronous firing of the hearing nerve fibres can be due to a number of reasons including dysfunction of the (1) inner hair cells, (2) synapse (the junction between the IHC and the hearing nerve), (3) hearing nerve itself, (4) VIIIth nerve nucleus in the brainstem and (5) brainstem pathways themselves. The pathology / aetiology may vary from genetic (otoferlin mutation that leads to absent or malfunctioning IHC) and, jaundice and hypoxia at birth, to delayed maturation of the hearing pathway. Diagnosis of this condition entails specific testing and multidisciplinary assessments with management requiring an experienced team. Some of the aetiological investigations need to be carried out within the first few months of life and others repeated later to maximise the potential for arriving at a diagnosis. Amongst the causes of congenital hearing loss are: maternal infections such as Rubella which is now very rare because of immunisation; cytomegalovirus which is probably the commonest infective aetiology that can cause a progressive hearing loss; prematurity; severe lack of oxygen to the fetus; severe jaundice; some drugs that are harmful to inner ears; and genetic inheritance. How is it diagnosed? NHS Newborn hearing screening program was launched to cover the whole of England in April 2005 and this gives an opportunity for all parents to have their newborn baby’s hearing screened at birth, leading to early diagnosis of hearing loss and any associated medical conditions, giving a chance to make an informed choice for their child, intervene early leading to better hearing, speech and educational outcome. How is it treated? Interventions include introducing a communication medium of parental choice, hearing aid fitting and cochlear implants when hearing aids do not help. Those children with absent ear canals or significant middle ear abnormalities may be helped with specialised devices such as bone conduction or bone anchored hearing aids. Once confirmed it is also important to find the reason for the hearing loss for the following reasons:
Inheritance patterns About 20% of PCHI is due to an autosomal dominant (AD) inheritance, where the child needs only one copy of the dominant gene to be affected. Often there is deafness on one side of the family with each baby carrying approximately 50% chance of having the dominant gene responsible for deafness. Examples of dominant syndromic deafness include Treacher Collins syndrome, Branchio Otorenal (BOR) syndrome and Waardenburg syndrome. Often in AD syndromic hearing loss the degree of symptoms and signs may vary from member to member because of variability of penetrance and expressivity of the gene responsible. Other forms of inheritance include X-linked and mitochondrial. Inheritance of X-linked deafness e.g. Alport syndrome (hearing loss and kidney involvement) is from affected father to daughters (because the father passes on the Y chromosome to his sons) and from affected mother to approximately 50% of all children irrespective of the sex. As mitochondria are in the cytoplasm, and it is only the ovum that contains cytoplasm (sperm has only the nucleus and no cytoplasm) the inheritance of mitochondrial deafness is from mother to children. Often in mitochondrial deafness there is more than one system/ organ involved. A1555G is one such mitochondrial mutation that makes individuals more susceptible to ototoxicity from aminoglycoside drugs such as gentamicin. It must be remembered that more than one gene could be responsible for a given syndrome and some syndromes may show more than one pattern of inheritance. Your geneticist should be able to tell you more about the small number of genes that can be tested for, but as new mutations are identified it is expected that this number will increase. Prenatal diagnosis Medical text written January 2003 by Dr Tony Sirimanna. Last updated December 2007 by Dr Tony Sirimanna, Consultant Audiological Physician, Great Ormond Street Hospital for NHS Trust, London, UK
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