By Dr. Maddie Swannack and Dr. Sachin Sudhakaran
Next Lesson - Ear Pathology
Core
The ear is the human organ of hearing and balance. The anatomy of the ear can be organised into the external, middle and inner ear. This article aims to outline the key anatomical features.
This article will cover the basic structure and function of the ear. See our article on pathologies of the ear here.
This image shows the basic anatomy of the ear. Note the three segments of the ear.
Diagram - General overview of the anatomy of the ear
Creative commons source by OpenStax [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The external ear can be divided into the pinna and external acoustic meatus.
The pinna is a structure found on the side of the head that acts to direct sound waves toward the external acoustic meatus. The majority of the pinna is cartilaginous (elastic cartilage). Helpful structures to remember are the tragus (the small flap in front of the ear canal), helix (the curved outer rim), concha (the bowl-shaped hollow leading into the ear canal) and lobule (the soft earlobe, which has no cartilage). These landmarks help you describe where on the ear a lump, wound or infection is. The pinna has a number of parts as demonstrated in the image below:
Diagram - Anatomy of the pinna
Creative commons source by Anatomist90 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The external acoustic meatus (EAM) (ear canal) is an S-shaped tube which extends from the pinna to the external surface of the tympanic membrane. The walls of the outer 1/3 of the canal are formed by cartilage whilst the walls of the remaining inner 2/3 are made up of the temporal bone.
The sigmoid shape is clinically relevant as it can have implications on otoscopy (looking into the ear with an otoscope). If the tympanic membrane is not immediately visible, in adults gently pull the pinna in a posterior and superior direction (backwards and upwards). This straightens the meatus and allows the ear drum to be visualised. In young children, the pinna is pulled posteriorly and inferiorly (backwards and downwards).
The EAM is lined with keratinised, stratified squamous epithelium (a tough, layered lining of flat cells that contain the protein keratin) like most skin. The canal is usually self-cleaning with shed epithelium moving out through the canal. This shedding of the epithelium helps to remove ear wax, which exists to clean and lubricate the ear. Ear wax is made up of desquamated (shed) epithelial cells mixed with secretions from sebaceous (oil-producing) and ceruminous (wax-producing) glands. The wax has a low pH to inhibit bacterial growth and is hydrophobic (water-repelling) to try to reduce water in the outer ear.
The tympanic membrane (TM) is found at the medial end of the external acoustic meatus. It is usually translucent and on otoscopy, some structures of the middle ear can be observed through it.
The cone of light (a reflection from the otoscope) is often seen on a normal tympanic membrane, although its absence does not necessarily indicate pathology.
The tympanic membrane is key to hearing. It converts sounds (vibrations in the air) into vibrations of the membrane which can then be transmitted along the ossicles.
Diagram - The tympanic membrane of the right ear. Note the cone of light, and the lateral process and handle of the malleus (the hammer-shaped bone of the middle ear)
Creative commons source by EMAD KAYYAM [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Image - The tympanic membrane of the left ear on otoscopy. This is the normal colour for the tympanic membrane, and there is no fluid level visible (which would indicate an abnormal collection of air or fluid in the middle ear)
Creative commons source by Michael Hawke MD [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The middle ear is an air-filled cavity which lies within the temporal bone, extending from the tympanic membrane (on its lateral side) to the inner ear (on its medial side).
The facial nerve runs in a narrow bony channel (the facial canal) along the medial wall of the middle ear, next to the cavity. As a result, pathology in the cavity can cause symptoms relating to the facial nerve.
The middle ear contains three ossicles (tiny bones): the malleus (hammer), the incus (anvil) and the stapes (stirrup). These relay vibrations from the TM to the oval window, the membrane-covered opening into the inner ear.
The strength of vibration can be dampened by two small muscles in the middle ear, the tensor tympani and the stapedius. They contract in response to loud sounds, which helps to protect the inner ear.
The order of the ossicles, starting from the tympanic membrane, is Malleus, Incus, Stapes: MIS. The Malleus sits at the tympanic Membrane (two Ms together). Alternatively, picture a blacksmith using a hammer on an anvil to make a stirrup.
Eustachian Tube (Pharyngotympanic Tube)
The Eustachian tube is a communication from the middle ear to the nasopharynx which allows for equalisation of pressure within the middle ear. This is required as the middle ear is lined with a mucous membrane which continuously resorbs air. This results in a negative pressure inside the middle ear compared with the atmosphere, shown by the concave tympanic membrane. The Eustachian tube facilitates a relative equalisation of this pressure gradient (stops the gradient getting too steep).
However, the Eustachian tube can act as a passage for infections of the nasopharynx to pass into the middle ear and cause infection there. This is a more common occurrence in children as their Eustachian tubes are shorter and have a flatter course within the skull. This means that mild infections like colds are more likely to cause middle ear infections in children. As the skull increases in size, the Eustachian tube takes on a steeper angle and lengthens. Infections of the nasopharynx are therefore less likely to pass into the middle ear in adults.
Diagram - The anatomy of the middle ear
Creative commons source by BruceBlaus [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The inner ear contains two key organs: the vestibular apparatus (the balance organ) and cochlea (the snail-shaped hearing organ).
Vibrations passing through the ossicles in the middle ear result in movement of the oval window (the membrane-covered opening between the middle ear and the vestibule, the central part of the inner ear that connects to the cochlea) causing movement of the fluid within the cochlea. This movement of fluid causes movement of stereocilia (tiny hair-like projections) on the hair cells of the organ of Corti, the hearing receptor inside the cochlear duct (the fluid-filled tube within the cochlea). Movement of the stereocilia generates receptor potentials (small electrical changes) in the hair cells, which then lead to action potentials in the afferent fibres (nerve fibres carrying signals towards the brain) of the cochlear portion of cranial nerve VIII (vestibulocochlear nerve). The cochlea also has a structure called the round window. This membrane-covered opening bulges outwards when the oval window is pushed inwards, which lets the fluid within the cochlea move.
The vestibular apparatus uses movement of fluid within three semi-circular canals (anterior, lateral and posterior) to help maintain a sense of balance. Movement of the head leads to movement of stereocilia within these structures, leading to action potentials to propagate through the vestibular portion of cranial nerve VIII.
Diagram - The anatomy of the inner ear
Creative commons source by BruceBlaus [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The process of hearing is quite simple:
- The pinna and the external acoustic meatus focus and funnel sound onto the tympanic membrane, which vibrates when sound hits it.
- The vibration of the tympanic membrane vibrates the ossicles.
- The vibration of the stapes at the oval window causes movement in the cochlear fluid.
- This movement of the fluid stimulates the stereocilia in the cochlea.
- Stereocilia then trigger action potentials in the vestibulocochlear nerve.
- Impulses travel to the primary auditory cortex (in the temporal lobe), which interprets the input.
There are two main types of hearing loss, depending on which part of the ear is affected. Some people have both at once, which is called mixed hearing loss.
The first is conductive hearing loss. This is pathology of the outer or middle ear and causes problems with the transmission of sound wave vibrations to the cochlea. Air conduction (sound travelling through the ear canal and middle ear) will be impaired as sound is not transmitted. However, as the cochlea is preserved, bone conduction (sound travelling through the bones of the skull) is typically relatively preserved compared with air conduction. Bone-conducted sound may even seem louder in the affected ear. One explanation is reduced masking (less background noise reaches the affected ear to drown out the sound), and other mechanisms have also been proposed.
The second is sensorineural hearing loss. This is pathology of the inner ear or the vestibulocochlear nerve. This is due to defects with the transmission of the impulses from the stereocilia. Both air and bone conduction of sound will be reduced as the cochlea or nerve are damaged so sound cannot be transmitted appropriately to the brain.
Hearing tests with tuning forks can help to differentiate between these two types of hearing loss.
Examination of Hearing in General Practice
If a patient presents with concerns about their hearing, there are a number of things which can be done to find the cause of the hearing loss and decide on treatment:
History
It is important to take a full history of the hearing loss. Focus on how it developed (in particular, whether it came on suddenly), the exact nature of the patient’s concerns, and any other ear-related symptoms, such as vertigo (a spinning sensation), discharge from the ear or tinnitus (ringing or buzzing in the ear).
It is also important to establish whether the patient thinks the hearing problems are bilateral or unilateral, as this can help differentiate the causes of the deficit.
As always, ensure you establish a wider history including medication history, past medical history, any occupational exposure to loud noises, or a family history of hearing issues.
Visual Examination
Examination of the external ear and otoscopy to visualise the tympanic membrane should be attempted.
Gross Hearing Assessment
A crude test of hearing should be performed on every patient who presents with concerns. One example is to whisper a number into the patient’s left ear while masking the right ear (for example by crumpling paper next to their right ear), and then ask the patient to repeat the number. Repeating this on both sides gives an indication of whether the hearing is better on one side, or whether both are impaired equally.
Tuning Fork Tests
There are two tests performed as an assessment of hearing using tuning forks.
A tuning fork is an instrument that produces vibrations at a specific frequency when struck, creating a noise. It can be used in two tests to assess hearing.
To perform these tests, a tuning fork of 512Hz should be used.
To conduct Weber’s test, the tuning fork is struck and is placed in the centre of the forehead. The patient is asked if it sounds louder in either ear.
- A normal result is that the sound is equally loud in both ears.
- An abnormal result is that the sound localises to one ear.
If the hearing loss is sensorineural, the louder ear will be the normal ear. This is because the damaged cochlea or nerve picks up the sound less well than the healthy one.
If the hearing loss is conductive, the louder ear will be the pathological ear. This is because bone conduction is relatively preserved and less background noise reaches the affected ear to mask the sound, so it seems louder there.
Weber’s test is used in conjunction with Rinne’s test to help decide whether hearing loss is sensorineural or conductive.
The second test is called Rinne’s test and is a conduction test.
The tuning fork is struck and placed on the mastoid process (the bony area just behind the ear). The patient is asked to indicate when they can no longer hear the sound of the fork. At this point, the tuning fork is moved to in front of the pinna, and the patient asked if they can hear it now.
A normal result is that the patient can hear the noise of the fork through the air after they have ceased to hear it through the bone, so air conduction > bone conduction. This is a positive Rinne’s test and is normal.
A patient with conductive hearing loss will have bone conduction > air conduction. This is because their ear canal or middle ear is not transmitting vibrations to the cochlea, whilst the mastoid bone still can as the cochlea is undamaged.
A patient with sensorineural hearing loss will usually have a Rinne’s positive result. Both air and bone conduction are reduced to a similar degree, so air conduction > bone conduction still holds. The exception is severe one-sided or total sensorineural hearing loss, where the test can give a false negative (bone > air) because sound crosses the skull to the better ear.
This is why the two tests are combined: neither should be interpreted in isolation.
Image - The different positions of the tuning forks in Weber’s and Rinne’s tests
SimpleMed original by Dr. Maddie Swannack
Table - The results of Weber's and Rinne's Tests
SimpleMed original by Dr. Maddie Swannack
It can be easy to confuse these tests and their results without context. The best way to help understand them is through examples.
Patient 1
Patient 1 has presented to the GP with reduced hearing in his right ear after a head cold. After taking a history the GP examines the patient and conducts Rinne’s and Weber’s tests. A gross hearing test confirms that Patient 1’s hearing is reduced in his right ear.
Patient 1 says that Weber’s test is louder in his right ear. This means that either he has sensorineural hearing loss on the left side, or conductive hearing loss on the right side.
Rinne’s test on the right side is negative (bone > air). This means that the right ear has conductive hearing loss. This makes sense in the context of Weber’s test: in conductive hearing loss, bone-conducted sound seems louder in the affected ear, making Weber’s louder in his right ear.
After combining Weber’s and Rinne’s tests, the findings point to right ear conductive hearing loss, with the left ear most likely normal. A formal hearing test would confirm this.
Patient 2
Patient 2 has presented to the GP after noticing reduced hearing in her left ear after a previously treated and resolved head injury. After taking a history the GP examines the patient and conducts Rinne’s and Weber’s tests. A gross hearing test confirms that Patient 2’s hearing is reduced in her left ear.
Patient 2 says that Weber’s test is louder in her right ear. This means that either she has sensorineural hearing loss on the left side, or conductive hearing loss on the right side.
Rinne’s test on the right side is positive (air > bone). This makes right-sided conductive loss unlikely.
Rinne’s test on the left side is positive (air > bone). This means that the left ear is either normal or has sensorineural hearing loss. When combined with the results of Weber’s test and the gross hearing test, this points to sensorineural hearing loss in the left ear.
In summary: conductive hearing loss is unlikely given the positive Rinne’s results on both sides, and Weber’s points to sensorineural hearing loss on the left side. Because her hearing loss is one-sided and followed a head injury, she needs prompt referral to ENT (ear, nose and throat) for a formal hearing test and further assessment.
These tests are quick bedside aids that help to tell conductive from sensorineural hearing loss. On their own they cannot rule hearing loss in or out, and they do not replace a formal hearing test (audiometry). A referral for a formal hearing assessment should be made if any abnormalities are found on either Rinne’s or Weber’s tests.
Some presentations need faster action. NICE advises referring immediately (to be seen within 24 hours) to an ENT (ear, nose and throat) service or an emergency department if hearing loss developed suddenly (over three days or less) within the past 30 days and is not explained by an outer or middle ear cause. NICE Clinical Knowledge Summaries also advise immediate referral for hearing loss that follows a head or neck injury. Hearing loss that mainly affects one ear should also be considered for referral for further investigation.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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