By Dr Ananya Mandal, MD
Hearing loss of hearing impairment may be of two major types - conductive hearing loss and sensorineural hearing loss. A third type is a mixed type that has underlying symptoms of both these types of deafness of hearing loss. 1-6
Normal ear anatomy
The normal ear consists of a narrow canal that lets in the sound waves. This is called the external ear or the ear canal. These waves enter the ear canal and strike the ear drum.
The ear drum (called the tympanic membrane) is a membrane that vibrates as the sound waves hit it. These vibrations are passed to the three small bones (ossicles) inside the middle ear. These are called malleus, incus and stapes bones.
The ossicles move to amplify the vibrations and pass them on to the inner ear. The inner ear contains a shell shaped organ called the cochlea. Within the cochlea are tiny hair cells all along the inner walls. These move in response to the vibrations and send a signal through the auditory nerve to the brain.
Decibels hearing loss
The normal hearing range is 0-20 decibels (dB). Around 30 dB are for whispers, 50 dB for average home noises and 60 dB for conversational speech. Sounds like jet engine noises are over 140 dB and are painful.
Hearing loss is measured in decibels hearing loss (dB HL).
- 25-39 dB HL means mild hearing loss (cannot hear whispers)
- 40-69 dB HL means moderate (cannot hear conversational speech)
- 70-94 dB HL is severe (cannot hear shouting)
- more than 95 dB HL is profound (cannot hear sounds that would be painful for a hearing person)
Types of hearing loss
Types of hearing loss include conductive hearing loss, sensorineural hearing loss and mixed type.
Conductive hearing loss
In this the sound waves are unable to pass from the external ear into the inner ear resulting in a hearing loss. The most common reasons are due to:
- blockage of the ear canal by ear wax
- perforation of the ear drum
- build-up of fluid due to an ear infection called glue ear
Sensorineural hearing loss
This occurs where the auditory nerve and other nerves that carry the information from the sounds heard to the brain are damaged due to age or injury.
Hearing loss due to aging is called presbyacusis. After the age of 30 to 40, many people start to lose their hearing in tiny amounts. This increases with age and by 80 many people may have significant hearing impairment.
Presbuacusis occurs when the sensitive hair cells inside the cochlea gradually become damaged or die. The initial symptoms include loss of high-frequency sounds, such as female or children’s voices and difficulty in hearing consonants, making hearing and understanding speech difficult.
Ear injury is another common cause of hearing loss. This occurs due to damage caused by loud noises. The inner structures due to constant exposure to noise become damaged. Exposure to noise causes the hair cells inside the cochlea to be inflamed.
Some drugs may also cause damage to the nerves of the ears leading to sensorineural hearing loss. These include notable antibiotics like aminoglycosides (Gentamicin, Amikacin etc.)
Mixed type of hearing loss
When people get both types together, the condition is termed mixed type of hearing loss.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)