Types of hearing loss
Conductive hearing loss
Conductive hearing loss occurs when the outer or middle ear is dysplastic or does not work properly. Consequently, sound waves cannot be efficiently conducted to the inner ear. In case of a temporary dysfunction, it is often possible to correct conductive hearing loss with surgery and/or treatment with medication.
Common causes of conductive hearing loss include:
- Injury of the outer ear itself
- Blockage of the ear canal due to ear wax (cerumen) or other small objects like food, beads or insects
- Infections of the outer or middle ear
- Perforation of the tympanic membrane
- Congenital deformities (e.g. Down syndrome, Franceschetti syndrome, Treacher Collins syndrome or Achondroplasia (dwarfism)
The audiogram below demonstrates conductive hearing loss. In this example, the inner ear works properly but something inhibits sound from getting through the outer/middle ear to the inner ear.
The hearing loss displayed above ranges from 30 to 50 dB HL in the right ear and between 35 and 45 dB in the left ear. This individual would only be able to perceive fragments of normal speech, which has a loudness level of about 65 dB. A normal hearing person has a dynamic range of up to 65 dB between listening threshold (at 0 dB) and speech level (at 65 dB). The person with the described hearing loss only has a dynamic range of about 20 dB. This difference of about 45 dB must be amplified to enable the person to perceive speech and sounds appropriately.
Depending on the general diagnosis, the necessary amplification is usually provided by:
- Conventional hearing instruments (sometimes not possible due to chronic inflammation, effusion and/or dysplasia of the outer ear/ear canal)
- Bone conduction hearing instruments: Behind-the-ear shell with a vibrating transducer, placed on the skull using a headband or retainer
- Bone anchored hearing aid (BAHA): A vibrating hearing solution which is attached to the skull using a screw which is implanted behind the ear
If the inner ear is unimpaired, bone conduction hearing solutions do not deliver any amplification. Their task is only to make sounds audible by an adequate vibration of the skull, which transfers the sound information directly to the inner ear. Starting from there, the hearing process continues normally.
Sensorineural hearing loss
Sensorineural hearing loss is the most common type of hearing loss. It has its origin in the inner ear or along the auditory nerve. Most commonly, the damage occurs in the inner ear (cochlea). In this instance, the hair cells in the cochlea are damaged and cannot transmit neuro-electrical impulses to the brain. Sensorineural hearing loss can be congenital (present at birth) or acquired after birth.
Common congenital causes include:
- Hereditary factors
- Viral infections
- Birth trauma such as anoxia
Acquired causes include:
- Reactions to ototoxic drugs (damaging to the hearing system)
- Head injury
- Noise exposure
- Ear infections
- Other diseases
The audiogram displays a sloping hearing loss from 20 dB HL in the low frequencies up to 75 dB HL in the higher frequencies and (nearly) equal in both ears. Such a hearing loss can usually be amplified sufficiently by using conventional hearing instruments.
If sensorineural hearing loss in speech relevant frequencies exceeds values of about 80 to 90 dB HL, it might become difficult to make speech adequately audible. The reason here is that even though the person suffers from profound hearing loss, he/she nevertheless regards extremely loud sounds just as uncomfortable as a normal hearing person would. The challenge here is to fit the complete speech spectrum into the individual’s residual dynamic hearing range. If this range is reduced to 10 – 20 dB, conventional hearing instruments cannot provide sufficient amplification anymore.
In these cases a cochlear implant (CI) might be the solution. A cochlear implant processes sounds and transforms them directly into electric impulses. The cochlear implant replaces the functionality of the inner ear. The electric impulses directly stimulate the auditory nerve. The auditory process in the brain is thus triggered. However, for the brain to interpret sounds correctly, patients need to have either adequate former listening experiences, or should be treated with a CI within the first four years of life.
Even under ideal conditions, patients provided with a cochlear implant need a lot of auditory training and continuous support. Even then, there is no guarantee that a CI works to every patient’s satisfaction
Mixed hearing loss
If air conduction and bone conduction measurement values exceed 20 dB HL, and the difference between the two is more than 15 dB, this is called a mixed hearing loss. Such hearing loss is treated according to the severity, the given anatomic structures, and other possible influence factors. Therefore, all of the hearing solutions mentioned previously might apply here:
- Conventional hearing instruments
- Bone conduction hearing instruments
- Bone-anchored hearing instruments (BAHA)
- Cochlear implants (CI)
Selecting the right method for treatment of a mixed hearing loss depends on the degree of the conductive and sensorineural components of the given hearing loss.