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). The normal hearing range is 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.

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.

Implantable aids:

Cochlear implants: These are devices designed to stimulate the cells of the auditory spiral ganglion to provide a sense of hearing to those with neurosensory hearing loss. The duration and severity of the deafness, the progression of the hearing loss and educational setting help predict success (or otherwise) in this kind of surgery but despite best assessments, there will still be a strong unpredictable element in outcome

performance. 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): 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 cosmetics (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 ranges from hearing loops to vibrating pagers, visual trigger units for different situations (baby monitors, doorbells, fire alarms) and specialist alarm clocks and telephones.
The deafness may be classified as conductive (where there is a failure of the sound waves to reach the inner ear through the normal air conduction channels of the outer and middle ear) or sensorineural (where there are abnormalities or there is damage to the sensory cells ± nerve fibres of the inner ear).