Hearing is one of the major senses that we possess other than taste, smell, vision, and touch. These senses of ours play an important role in judging our surroundings and communicating with others. This sense of ours has helped us to get on top of the food chain, to communicate fear, react and adapt to our surroundings.
Our ears perceive vibrations and decode them as various sounds. The function of the ear is to gather these vibrations, encode them and send the nerve impulses to the brain for further processing. To understand the mechanism of hearing we will have to take a look at the structural and functional working of the human ear.
The anatomy and physiology of the ear
The ear is the main organ responsible for hearing and balance. Its anatomy is unique and important to understand the mechanism of hearing.
Here are the parts of the ear,
- The external ear or outer ear consists of:
-Auricle or Pinna (This is the outer part of the ear that is visible to the human eye)
-External auditory canal or tube (This tube connects the outside of the ear to the middle part of the ear)
- The tympanic membrane or the Eardrum
The tympanic membrane separates the external ear from the internal ear and passes the sound vibrations forward.
- The middle ear or the Tympanic cavity consists of:
-Ossicles (these are a set of 3 small bones that work to transmit sound.)
These 3 bones are
-Malleus
-Incus and
-Stapes
- Eustachian canal
The Eustachian canal is a connecting canal between the middle ear and the nasal passage. This canal is lined with a mucous membrane just like our nose and throat. It regulates the pressure in the middle ear and equalizes it. This helps in the better transfer of sound waves.
- The inner ear consists of 3 important structures, they are:
-Cochlea (rich in nerves that aid in hearing)
-Vestibule (contains receptors for hearing and balance)
-Semicircular canals (contain receptors for balance)
These were the main components of the hearing and balance system in the ear, now let’s see how they work and help us hear.
Understanding the hearing process
The Pinna which is a part of the outer ear is made up of cartilage covered in the skin. It protrudes from both sides of the skull and performs the function of gathering sound from the surroundings. This sound is transmitted via the external auditory canal or tube to the middle ear.
The ear canal is slightly bent towards the outer cartilaginous portion. This bent structure is a part of a protective mechanism of the ear that prevents any foreign objects or entities from entering the ear canal and thus the tympanic membrane. The ear canal is the place where the ear wax (made from oil, dead cells, and debris) is present, it is also a protective mechanism against foreign entities.
The tympanic membranes separate the ear canal from the middle ear. It is an extremely thin membrane (1/10th of a millimeter thick) The tympanic membrane is lined with epithelial cells and it functions similar to a loudspeaker. The sound waves that pass through the ear canal hit the tympanic membrane. This causes the tympanic membrane to vibrate and these vibrations are further transferred to the middle eat where the Ossicles are situated.
The Ossicles comprise the Malleus, Incus, and Stapes or they are also known as the hammer, the anvil, and the stirrup. The sound vibrations from the tympanic membrane are received by the Malleus, Incus, and Stape. The 3 tiny bones amplify this sound and send it further to the inner ear. The tympanic membrane is usually called the eardrum but in actuality, the tympanic membrane acts as the drum skin and the middle ear acts as the eardrum.
Now the sound waves are received by the fluid-filled hearing organ cochlea in the inner ear. This is a shell-like organ made up of membranes and surrounded by a fluid called the perilymph. The cochlea has thousands of hair cells that help us hear. The cochlea is the organ that keeps the body balanced against gravity. This organ converts sound waves into electrical impulses that can be transferred to the brain.
The brain on receiving these electrical impulses perceives the sound as we hear it.
Hearing Loss and its types
The inability to hear with one or both ears is known as Hearing loss. It can be a result of old age or a traumatic incident. The loss of hearing is usually gradual. People between the age of 60 to 70 are often likely to experience a gradual loss of hearing.
According to experts, hearing loss can be of 3 types. They are as follows:
Conductive hearing loss
This condition is a result of something that stops the sound waves from getting through the outer or inner ear. Conductive hearing loss may lead to the hearing loss of low-intensity soft sounds or muffled noises. This type of hearing loss can be treated with medication and surgery. Conductive hearing loss can occur due to allergies, infection, or buildup of ear wax.
Sensorineural hearing loss
One suffers from Sensorineural hearing loss when there is damage to the inner ear or the nerves in the brain that help in hearing. This type of hearing loss usually occurs due to trauma, birth defects, aging, loud noise, or some diseased state. SHL is usually permanent.
Mixed hearing loss
This type of hearing loss is a combination of conductive and sensorineural hearing loss. It is a condition that occurs when both these ailments occur at the same time.
Tests to detect hearing loss
- Bone conduction test
A bone conduction test is performed to detect if the inner ear is functioning properly or to detect if there is a problem with the outer ear or middle ear of a person.
To perform a bone conduction test, A device or a headband is placed on the patient’s forehead or behind the ear. This device sends vibrations through the skull and is perceived by the cochlea in the inner ear. If the patient can hear the sound, then it can be concluded that there is no defect in the functioning of the cochlea or the inner ear.
- Absolute bone conduction test
The absolute bone conduction test is a modified Schwabach test.
To perform this test the clinician will close the ear by gently pressing the tragus against the ear canal. This stops the ambient noise from the surroundings to interfere with the results. This is the unique step in the ABC test. Now The clinician will place a vibrating tuning fork on the patient’s mastoid, when the patient stops hearing the sound they will signal the clinician. The clinician then transfers the same tuning fork to their mastoid to check if they can still hear the sound. The interpretations are as followed
| Test outcomes | Result | Interpretation |
| Clinician listens longer than patient | ABC reduced | Sensorineural hearing loss |
| Clinician listens equally to that of patient | ABC normal | Normal |
| The patient listens longer than the clinician | ABC lengthened | Conductive hearing loss |
- Rinne test
It is used to detect hearing loss in an ear by comparing the time required for bone conduction versus air conduction.
To perform the Rinne test, the doctor will place a vibrating tuning fork on the patient’s mastoid bone. When the patients stop hearing the sound via bone conduction they signal the doctor. The doctor records this time.
Now the doctor places a vibrating tuning fork near the patient’s eat canal. The patient is told to signal when they stop hearing the sound. Once the patient does so, the doctor records the time it took for air conduction of the sound.
In normal hearing, the time taken for air conduction is double that of bone conduction
In conductive hearing loss bone conduction is heard longer than the air conduction sound.
In the sensorineural hearing loss, the air conduction is heard longer than bone conduction.
- False Rinne test
A patient with profound sensorineural deafness may give out a false negative Rinne test. When a patient has completely lost hearing in one ear and a vibrating tuning fork is placed on the mastoid of that ear, the patient does not hear anything in that ear. However, these vibrations travel through bone conduction to the inner ear of the opposite ear, and the patient hears a sound. This can confuse the patient as they are unable to identify in which ear they can hear the sound.
To perform a confirmatory test, placing a vibrating tuning fork near the ear canal of the nonfunctional ear while closing the entrance of the functional ear can confirm that the patient has lost all hearing in one ear.
- Schwabach test
A Schwabach test is performed to check the patient’s bone physical phenomenon called conductivity in comparison with the hearing of a healthy person (usually the examiner).
To perform this test, A vibrating tuning fork is placed on the patient’s mastoid bone and they are told to signal immediately when they stop hearing the sound. When the patient signals this, the examiner places the same tuning fork on his own mastoid and checks if he can still hear any sound.
If the examiner was able to hear the sound even after the patient stopped hearing it, the conclusion implies a sensorineural loss.
If the patient hears the sound for longer than the normal duration the conclusion is that they suffer from conductive hearing loss.
These tests are great ways to find out hearing abnormalities and can help an individual to seek early treatment for the same.
References:
- Types of Hearing Loss at https://www.cdc.gov/ncbddd/hearingloss/types.html accessed on 27th January 2022
- Anatomy and Physiology of the Ear – Health Encyclopedia – University of Rochester Medical Center at https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=90&ContentID=P02025 accessed on 27th January 2022
- Rinne & Weber Tests | Definition & Patient Education at https://www.healthline.com/health/rinne-and-weber-tests#results accessed on 27th January 2022
- What is a Bone Conduction Test? at https://hearillinois.com/hearing-health-blog/what-is-a-bone-conduction-test accessed on 27th January 2022
FAQs
A bone conduction test is performed to detect if the inner ear is functioning properly or to detect if there is a problem with the outer ear or middle ear of a person.
To perform this test the clinician will close the ear by gently pressing the tragus against the ear canal. This stops the ambient noise from the surroundings to interfere with the results. This is the unique step in the ABC test. Now The clinician will place a vibrating tuning fork on the patient’s mastoid, when the patient stops hearing the sound they will signal the clinician. The clinician then transfers the same tuning fork to their mastoid to check if they can still hear the sound.
It is used to detect hearing loss in an ear by comparing the time required for bone conduction versus air conduction.
A patient with profound sensorineural deafness may give out a false negative Rinne test. When a patient has completely lost hearing in one ear and a vibrating tuning fork is placed on the mastoid of that ear, the patient does not hear anything in that ear. However, these vibrations travel through bone conduction to the inner ear of the opposite ear, and the patient hears a sound. This can confuse the patient as they are unable to identify in which ear they can hear the sound.
A Schwabach test is performed to check the patient’s bone physical phenomenon called conductivity in comparison with the hearing of a healthy person.