Presbyacusis is associated with loss of the hair cells of the cochlea and secondary degeneration of the neurons of the cochlear nucleus.
Although it has been postulated that failure of cochlear microcirculation is a factor in the development of presbyacusis, no direct link with hyperlipidaemia or diabetes has yet been established.
Due to the involvement of higher frequencies in this type of hearing loss, the first clinical evidence of this condition is the poor perception of consonants, which are high-frequency sounds. Thus, patients often tend to mishear certain consonants, especially under conditions of slightly raised background noise. To add to this ‘distorsion’ of sound is the phenomenon of recruitment, where the patient perceives a relatively large increase in the loudness of a particular sound. A slightly raised voice may be perceived as uncomfortable by the patient, while a softer word may not be understood properly.
At the present time the only way one may help patients with such a sensorineural disability is to prescribe a hearing aid although middle ear hearing aids are being evaluated Hearing aids are indicated in persons with a hearing disability and who are motivated enough to try one. Since most people develop a bilateral symmetrical type of loss it makes good sense to have binaural hearing aids, where speech discrimination and sound localisation are improved.
Most hearing aids available are of the air-conducting type, ie they direct sound through the ear canal and onto the tympanic membrane as happens normally. The commonest types of air-conducting hearing aids are ‘behind the ear’ and ‘in the ear’ varieties and depend on an intact ear canal and tympanic membrane. In the ear instruments are suitable for persons with hearing losses up to 60dB, while the more powerful behind the ear aids are required for more profound losses. Where these conditions cannot be satisfied bone conducting hearing aids are considered. Bone conducting hearing aids direct sound through bone to the cochlea, and depend on some form of direct contact to bone, such as the ‘spectacle type’ where the spectacle frame presses onto the mastoid bone. Bone anchored hearing aids latch onto a titanium implant that has been screwed into the skull.
A prerequisite for hearing aid fitting is to obtain a good audiogram. Decisions of which ear to fit, whether to fit the better ear or worse ear may be taken on the basis of the audiogram. Other factors have to be taken into consideration during the consultation. Wax has to be cleared out before taking an impression, or mould of the ear canal. Eczema of ear canal skin is another problem as wearing an aid may encourage chronic otitis externa, especially with diabetic patients. Patients with chronic otitis media and perforations of the tympanic membrane may not have an aid fitted until their perforations are closed as an aid encourages further discharge.
Early (analogue) hearing aids served as a sound amplifier as this simple diagram illustrates:
Analogue hearing aids are simple and cheap. However, their output is non-linear and cannot be accurately adjusted to fit an individual’s hearing loss pattern. Background noises therefore tend to be amplified at the cost of speech. Other drawbacks include the tendency to acoustic feedback. ‘Feedback’ is the annoying whistling noise resulting from the microphone picking up sound leaking from around the hearing aid mould. Hearing aid users have to continuously vary the output of their instrument (the ‘volume control’) to adjust to external conditions.
Since the end of 1996 there has been a leap in hearing aid technology pioneered by Danish hearing aid manufacturers. Digital hearing aids have been compared to Compact Disc technology in the place of vinyl discs.
Digital hearing aids have the capacity to change sound waves (inherently analogue signals) to digital signals. Digital Signal Processing (DSP) is very precise and is a mathematical manipulation of sound data. Advanced alogrithms such as statistical analysis may then be carried out by ‘chip’ in the hearing aid. This means incoming sound can be sampled and the hearing aid would react accordingly. Background noise may be filtered out and one may also vary which frequencies are to be amplified and to what degree. This feature helps most patients, where loss of hearing is worse at some frequencies and better at others. Amplification can be adjusted according to the patient’s audiogram. Some hearing aids also have the facility to carry out an audiogram with the aid in place, so giving real data feedback to the professional and helping accurate adjustment.
Digital circuits are largely immune to external interference (eg mobile phones) and are energy efficient, giving increased battery life for the hearing aid user. The current drawback of digital hearing aids is cost but it is hoped that these will be brought down in the future.
© Mr Adrian M Agius
