Congenital Deafness

Congenital deafness refers to hearing loss which is believed to have been present since birth.¹ This is distinct from progressive impairment which is a problem noticed at birth but which worsens with time. Late onset impairment is when the hearing loss manifests itself postnatally but with no identifiable exogenous cause.

Hearing loss is measured in decibels hearing loss (db HL). Normal hearing can detect sounds at 0-20dB. To be diagnosed with congenital deafness, the patient must have bilateral hearing impairment of at least 40dB HL in the better ear - that is, not be able to hear sounds of less than 40dB.²

• Overall, 1.64 per 1000 live births (which corresponds to about 1000 new cases per year)
• Bilateral hearing loss: 1.00 per 1000 live births
• Unilateral hearing loss: 0.64 per 1000 live births

Genetic causes

• Non syndromic:
  • Deafness occurs in isolation with no associated findings
  • Accounts for about 80% of genetic hearing loss
  • Highly heterogenous group of conditions but mutation of the gene encoding for the connexin-26 molecule is most commonly found (49% of cases)
  • Where there is an autosomal dominant inheritance, a positive family history will have been noted
  • Autosomal recessive abnormalities account for about 75% of all congenital deafness
• Syndromic:
  There are over 100 syndromes, mostly associated with sensorineural deafness. They include:
  • Alport syndrome - progressive sensorineural deafness
  • Branchio-Oto-Renal syndrome - there may be associated Mondini malformations (see below)
  • X-linked Charcot Marie Tooth - sensorineural deafness (not all patients affected)
  • Goldenhar syndrome - conductive deafness
  • Jervell-Lange-Nielsen syndrome - severe, profound sensorineural deafness
  • Mohr-Tranebjaerg syndrome - sensorineural deafness (there is an element of mitochondrial DNA abnormality in these patients)
  • Norrie disease - progressive sensorineural loss (not all patients affected)
  • Pendred syndrome - progressive sensorineural deafness
  • Stickler syndrome - conductive hearing loss
  • Treacher Collins' syndrome - conductive hearing loss
  • Waardenburg syndrome - sensorineural deafness
  • Usher syndrome - sensorineural deafness
• Mitochondrial disorders⁶ - mutations of mitochondrial DNA commonly cause progressive hearing loss of the sensorineural type, often through damage to the cochlea.

Non-genetic causes

• Mondini dysplasia (malformation) - the two and a half turn inner cochlear cavity is replaced by a single open sac-like cavity due to an arrested development in utero. Other associated malformations give rise to a predisposition for perilymphatic fistulae. Mondini-type deformities are also seen in CHARGE syndrome (Coloboma, Heart disease, Choanal Atresia, Retarded development, Gonadal aplasia and Ear abnormalities).
• Enlarged Vestibular Aqueduct syndrome - the vestibular system consists of a series of semicircular canals along with the utricle and the saccule. In this syndrome, the diameter of this system is increased (as measured on CT and high-resolution MRI) giving rise to fluctuating sensorineural hearing loss.
• Other malformations which give rise to conductive hearing loss: cleft palate, ossicular malformation, ossicular fixation, external auditory canal atresia, congenital cholesteatoma.
• Teratogenic such as gentamicin and thalidomide.
• Infectious causes - notably viral: Toxoplasmosis, Other (HIV, syphilis), Rubella, Cytomegalovirus, Herpes (TORCH).

Factors raising suspicion⁵

• Family history of congenital deafness
• Pre-natal infection: TORCH - see above
• Premature delivery and low birth weight
• Difficult delivery and fetal distress at time of delivery
• Parental suspicion
• Head malformation
• Jaundice (causing retrocochlear deafness)
• Administration of ototoxic drugs
• Infection such as bacterial meningitis

This is a nationally run screening programme that was started in 2001and which is currently fully implemented in England (and equivalent in Scotland). It is carried out by technicians or health visitors and involves two tests:

• Automated Otoacoustic Emissions (AOAE): an earpiece placed within the baby's external auditory meatus sends an acoustic signal which is picked up by the cochlear hair cells (on the organ of Corti). Activation of these produces movement and sound which in turn is detected by a microphone on the ear piece. It is decoded and transformed into a reading. If this reading is not clear, a second test may be done before moving onto the AABR.
• Automated Auditory Brainstem Response (AABR): earmuffs are placed over the baby's ears and sensors on the head and neck. A stimulus is sent and the response of the cochlear cells as well as of the auditory nerve is assessed via analysis of the EEG signal emitted. This is a more complicated and time-consuming test. Failure to respond prompts an audiology referral.

All babies are tested, some earlier than others:

• Normal, healthy babies - between 10 and 24 days of age in hospital based screening programmes and 5 weeks of age in community based programmes. If a repeat AOAE needs to be done, this should be done within one week of the first test.
• Babies who have been in the neonatal intensive care unit or the special baby care unit - by 44 weeks of gestational age. Only one attempt at AOAE is made; if there is no response, an AABR test will ensue.
• Babies having received gentamicin - as for normal babies unless the gentamicin dose is known to have exceeded normal therapeutic levels.

These are largely the syndromes identified above as well as a very large number of additional syndromes.

• Parental or carer education and support - it is vital that the parents or carers of the child receive adequate support (see numerous organisation sites listed below). This is a problem that will affect all the family (they will have to learn how to sign) as well as the child for the rest of their life.
• Registration of patient as deaf - local authorities are required to maintain classified registers of people who are deaf (as well as those suffering from a number of other disabilities).⁷ Form SSDA910 is kept by the social services department and needs to be completed in order for the patient to be registered and should enable the activation of support services.
• Hearing aids:
  • Externally worn aids - these are devices that increase the volume of the sound reaching the ear (effectively, amplifiers). They sit either behind the ear (although these devices are not powerful enough for patients with severe impairment) or just inside. Hearing aids that are placed right inside the external auditory meatus are available for patients with mild hearing loss. Bone conduction hearing aids for patients with conductive hearing loss are available in the form of headbands.
  • Analogue versus digital aids - digital aids often (but not always) improve the quality of sound. Aids available on the NHS usually have a digital component to them.
  • Implantable aids:
    • Cochlear implants⁸ - these are surgically placed devices designed to convert sound into an electrical signal. They stimulate the cells of the auditory spiral ganglion to provide a sense of hearing to those with neurosensory hearing loss. Although their performance is improving dramatically over recent years due to technological advances, there is still great variability in individual responses to the implant which cannot be fully mitigated despite best assessments. The duration and severity of the deafness, the progression of the hearing loss and educational setting can help determine success (or otherwise). Children should be at least of an age where they can wear a hearing aid prior to consideration for surgery as all implants use an external processor. Surgery carries the risk of poor auditory outcome, cerebrospinal fluid leakage and meningitis.
    • Bone anchored hearing aids (BAHA)⁹ - this type of surgery is reserved for patients with conductive and mixed hearing loss. It involved the fixing of a titanium implant just behind the ear, to which is connected an external abutment and a sound processor. Thus, it allows sound to be conducted through the bone rather than through the middle ear ('direct bone conduction'). The advantages are a better quality of sound and improved cosmesis (as they are less bulky than the headband system) but as with cochlear implant surgery, there is a risk of unrealistic patient expectation about outcome. There is also a risk of soft tissue reactions and loss of the fixture from its position in the skull.
  • Other equipment - there are a variety of other products on the market to assist with hearing. These range from hearing loops to vibrating pagers, visual trigger units for different situations (baby monitors, doorbells, fire alarms) and specialist alarm clocks and telephones.

There are huge personal, educational and social ramifications for the child born with hearing impairment.^1,10 A holistic approach to managing these patients and their families is necessary. They may encounter problems stemming from lack of understanding by people around them and therefore educating the family is essential in order to help them pave the way for their child to live a fulfilled life.

This depends on the underlying cause but essentially, the degree of hearing impairment either stays at the same level or gets worse. It never improves.