How we hear

Ears and hearing

Most baby books do not include much information about how the ear works, about hearing tests or about the language used to measure, explain and record hearing levels. That important component of your child’s overall development is, however, likely to have become much more relevant to you if you have been told that your baby is deaf.

Often, the information that you would like to understand better has been given at a time when your head was full of questions, you may have been in shock, or you were diverted by the needs of your very new baby.

We will now give you an overview of how we hear, what the different parts of the ear are called, what they should do when fully-functional, where problems can arise, and how you and your support team can begin to help meet these challenges. You can refer back to this information whenever you need it!

The process of hearing

When all parts of the ear are functioning fully, every sound we hear and every word that is spoken to us, follows this pathway to our brains and our understanding:

• A sound source vibrates and this in turn causes the air around it to move.
• These movements (or sound waves) travel down our ear canal and reach our eardrum, making it vibrate.
• The vibrating eardrum sets a chain of three little bones (the ossicles) in motion.
• As the last of these little bone moves, it causes fluid in the inner ear to flow back and forth, bending tiny hair cells in the cochlea – a spiral structure in the inner ear.
• As the tiny hair cells bend they trigger an electrical impulse that stimulates the auditory nerve to send a message.
• The auditory nerve takes the message to our brain so that the brain can interpret it into something meaningful. For example, we hear the sound of our baby cry and our brain interprets it so we respond and move to meet our baby’s needs

When any one element – or combination of elements – in this process does not work fully, it can cause deafness. It helps to break the process down into smaller parts so that we may identify where the issue occurs and the effect it has.

Parts of the ear

Different parts of the ear work in different ways and difficulties with particular parts of the ear can result in different types of deafness – and different approaches to managing this deafness.

A consultant audiologist describes the parts of the ear

In this section we will look at each part of the ear more detail, so we can explain where problems may arise and identify positive ways of approaching them.

Before getting started, you might like to watch this video of Consultant Audiological Scientist Huw Thomas describing the different parts of the ear, how they help us hear, and how they can cause deafness.

We will now look at the main parts of the ear in turn, focusing on their role in the hearing process and issues that can effect their function:

• The outer ear
• The middle ear
• The inner ear
• The auditory nerve

The outer ear

The first part of the ear is the outer ear – consisting of the pinna and the ear canal which ends at the eardrum.

Pinna and ear canal

The pinna (the visible part of your ear) and the ear canal work together to funnel sound vibrations to the eardrum (also known as the tympanic membrane). Sometimes these structures are not formed well, leaving the child with a misshapen or missing pinna or an ear canal that may be very narrow or even blocked.

In the short term, assistance may take the form of a bone conduction hearing aid. In the longer term, surgery can be an option to open up the ear canal or rebuild the pinna to improve hearing.

Ear wax

Much more typical, for all young children, is temporary deafness caused by a build-up of wax blocking the ear canal. A little wax in the ear canal is natural and is helpful as it surrounds and helps reject tiny bits of dust and debris that can get into the ear canal. However, if too much wax builds up this can cause a blockage which prevents sound vibrations from travelling down the ear canal to the eardrum.

A doctor or audiologist can check the ear canal for blockages using an otoscope (or auriscope), which is a small torch with a magnifying lens. Massaging a little olive oil in the ear canal or using prescribed eardrops may be sufficient to soften the wax.

The middle ear

The middle ear is the next part of the ear – consisting of the ossicles in the middle ear cavity.

The ossicles

The middle ear is a cavity of air inside the head containing a chain of three small bones (known as the ossicles). Due to their shapes, these little bones are called the hammer (or malleus), anvil (or incus) and stirrup (or stapes). Sound waves travel down the ear canal and make the eardrum vibrate, this in turn causes each of the small bones to vibrate, transmitting the sound pattern to the inner ear.

Deafness may arise when these little bones are not formed well and cannot knock against one another, or when there is no functioning eardrum. Again, in the short term, hearing can be assisted with either a bone conduction or a behind the ear hearing aid. In the longer term, surgery can be an option to overcome these challenges.

Glue ear

Much more prevalent, for all young children, is the middle ear cavity becoming congested with ‘glue ear’ (the medical term for this middle ear inflammation is otitis media).

The eustachian tube helps to regulate airflow in the middle ear, keeping the cavity clear and dry so that the eardrum and little bones (ossicles) may move easily. The eustachian tube tends to be very narrow in babies and it is positioned at a very shallow angle. This means that any congestion may not be able to drain away easily and may then block the eustachian tube.

If the eustachian tube becomes blocked, the air pressure in the middle ear becomes negative. After a while, this acts like a vacuum and draws in fluid from the surrounding cells. The middle ear then fills up with fluid – or ‘glue’ – and this restricts the movement of the little bones (ossicles), causing temporary deafness known as ‘glue ear’.
‘Glue ear’ is particularly common during the winter months as many children suffer from winter colds and the cold weather prevents the ear from drying out. This type of seasonal glue ear can often get better on its own as young children recover from winter colds and the weather becomes warmer and dries out the ear. Alternatively, medication such as antibiotics may be prescribed. It is not unusual for newborn babies to retain fluid well into their first months.

A consultant audiologist discusses the causes of glue ear and how it is treated

In this video, Consultant Audiological Scientist Huw Thomas discusses what causes glue ear and how it is treated.

Treating glue ear

If glue ear remains a problem the young baby or child may be fitted with grommets. These are small pressure-equalizing tubes inserted in the eardrum which help regulate airflow through the middle ear to the eustachian tube. Grommets are fitted by surgery: the surgeon first clears any middle ear congestion and then inserts the grommets in a tiny hole in the eardrum.

As the child grows the eustachian tubes become wider and the angle steeper, so fluid is less likely to ‘glue up’ the middle ear as the child nears junior school age.

Testing for glue ear

Doctors and audiologists can check for fluid and congestion in the middle ear using a type of hearing test called tympanometry. They can also compare results of testing through air conduction and bone conduction to find out if there is a problem with the way the middle ear is formed.

Conductive deafness

Difficulties in hearing caused by the outer or middle ear are referred to as conductive deafness. The sound vibration cannot be ‘conducted’ through to the sensory part of the ear – the inner ear.

The inner ear

The third part of the ear is the inner ear – this part of the ear includes the vestibular system and the cochlea.

The vestibular system

The vestibular system (also known as the semi-circular canals) is not part of the hearing process. Its function is to provide the brain with information about movement and spatial orientation to help with balance.

The cochlea

The cochlea looks a little like a snail shell and is the size of a garden pea. The cochlea receives sound vibrations transmitted by the stirrup (stapes). Within the cochlea, thousands of tiny hair cells respond to different pitches of sound.

Hair cells deep in the more protected internal curves of the cochlea respond to low sounds, with higher pitches stimulating the hair cells near the opening of the cochlea. As each hair cell is stimulated it generates a small electrical signal that travels up the auditory nerve to the brain.

Problems with the hair cells in the cochlea can cause deafness. They may not be formed at all, may be reduced in number, may not have formed correctly or they may have become damaged. There are many reasons why this may happen and often the cause is unknown, but some possible factors include the family’s genetic make up; complications or infections during pregnancy; medications, or lack of oxygen during a difficult birth.

Sensory deafness

Where hair cells are damaged or absent, complete sound messages will not be sent to the brain, causing what is known as sensory deafness. It is rare for the entire cochlea to be damaged. Any remaining hearing is referred to as ‘residual hearing.’

Sensory deafness, where the cochlea is impaired, is helped through the use of behind-the-ear (post aural) hearing aids or cochlear implants, depending on the severity of the impairment. Longer term, technical help using FM or infrared sound systems may also be added on. More details can be found in our special interest paper All About Hearing Aids.

Hearing aids and cochlear implants do not completely restore the sense of hearing, but when they are combined with informed, early intervention and education in a good acoustic environment, most severely and profoundly deaf children can use them to learn to listen and talk.

Testing for sensory deafness

Otoacoustic Emission (OAE) testing is an effective way to identifying problems in the cochlea. Our guidance paper on Hearing tests and your child provides further information on this type of testing.

The auditory nerve

The final part of the ear is the auditory nerve.

The tiny electrical signals produced by the hair cells in the cochlea combine to travel along the auditory nerve, through various levels of the cortex, to the parts of the brain which process sound.

At any one of a number of junctions along this journey along the auditory nerve, the signal may not transfer efficiently and the sound message may become distorted or out of synchrony.

Auditory Neuropathy Spectrum Disorder

Hearing issues in this area are known as Auditory Neuropathy Spectrum Disorder (ANSD), but have also been called Auditory Dysynchrony.

As ANSD is an umbrella term for a range of challenges, children with these hearing difficulties can present with a wide variation of responses to sounds that affect listening and spoken communication development.

Hearing aids or cochlear implants can often help with Auditory Neuropathy Spectrum Disorder. In rare cases where these are not appropriate, auditory brainstem implants can be a possibility. Our special interest paper All About Hearing Aids provides more information on auditory brainstem implants.

Testing for Auditory Neuropathy Spectrum Disorder

Auditory Brainstem Response (ABR) Testing is used to help identify impairments of the auditory nerve.

Types of deafness

As we have just seen, there are the following terms for deafness, defined by the cause of the deafness.

• Conductive deafness – this is the result of something in the outer or middle ear that prevents sound vibrations from being conducted to the inner ear.
• Sensorineural deafness – this is a generic term for an impairment of the sensory or neural pathway:
  • Sensory deafness – this is the result of an impairment of the inner ear that prevents the cochlea from sensing sound or turning it into an electrical signal.
  • Auditory Neuropathy Spectrum Disorder – this is the result of the electrical signals failing to be transferred or interpreted effectively by the brain.
• Mixed deafness – this is the result of a combination of both sensory deafness and conductive deafness.

A consultant audiologist describes the different types of deafness and their treatment

In this video, Consultant Audiological Scientist Huw Thomas describes the main types of deafness.