The ability to hear sound is an important sense for humans. Hearing allows us to communicate with each other through speech and language. It also alerts us to important sounds in our environment that provide information or warn of potential danger.
Hearing relies on a series of complex structures within the ear that help turn sound waves into meaningful nerve impulses that the brain can interpret. The main structures involved in hearing are the outer ear, middle ear, inner ear and auditory nerve pathways to the brain.
The Role of the Middle Ear
The middle ear contains three tiny bones called ossicles – the malleus, incus and stapes. These bones form a chain that reaches from the eardrum to the inner ear. When sound waves enter the ear canal and cause the eardrum to vibrate, the ossicles transmit and amplify these vibrations to the inner ear.
The middle ear acts as an impedance matcher. It overcomes the large difference in acoustic impedance between air in the ear canal and fluid in the inner ear. This allows efficient transfer of sound energy from air vibrations to fluid vibrations and activation of the inner ear’s sensory cells.
Without the middle ear, the ossicles and eardrum, most sound waves would simply be reflected away from the inner ear. Only very loud, low frequency sounds would be transmitted. The middle ear therefore plays a vital role in hearing for most frequencies and intensities of sound.
Hearing Loss from Middle Ear Problems
Damage or defects affecting the middle ear structures can significantly reduce hearing ability. Common middle ear problems that impair hearing include:
- Eardrum perforation – a hole or rupture in the eardrum
- Otitis media – a middle ear infection
- Otosclerosis – abnormal bone growth around the stapes bone
- Cholesteatoma – skin cyst in the middle ear
- Ear trauma or injury
These conditions can prevent proper vibration of the ossicles and transfer of sound to the inner ear. Conductive hearing loss occurs, in which sounds become muffled or muted.
Hearing Aids and Surgery for Middle Ear Problems
Many types of middle ear hearing loss can be treated with hearing aids or surgery. Examples include:
- Acoustic hearing aids – These funnel amplified sound directly into the ear canal and eardrum, bypassing the middle ear.
- Bone conducting hearing aids – These transmit vibrations through the skull bone directly to the inner ear, bypassing the middle ear.
- Stapedectomy – Surgery to replace or repair damaged or fixed stapes bones in otosclerosis.
- Tympanoplasty – Surgery to repair eardrums perforations.
- Tympanostomy tubes – Small tubes inserted in the eardrum to ventilate the middle ear and prevent recurring infections.
With treatment, many people with middle ear problems can regain significant hearing. However, devices that bypass the middle ear are usually only partially effective compared to normal acoustic hearing through an intact middle ear system.
Hearing through Bone Conduction
Bone conduction pathways allow us to hear sound vibrations transmitted through the bones of the skull, bypassing the outer and middle ear. The cochlea of the inner ear has two different input pathways:
- Air conduction – This is the normal route for hearing through the ear canal and ossicles.
- Bone conduction – Sound vibrations are transmitted from the bones of the skull directly to the cochlea.
Bone conduction pathways mean that sound information can still reach the cochlea even when there is damage to the external ear or middle ear. However, bone conduction hearing relies on higher intensities of sound and transmission is less efficient than through air conduction.
How Bone Conduction Works
There are two main mechanisms transmit vibrations through the bones of the skull to the inner ear:
- The inertial mechanism – Vibrations cause movement of the bone and attached perilymph fluid inside the cochlea, activating hair cells.
- The compression mechanism – Vibrations generate pressure waves within the cochlear fluid, activating hair cells.
Both these mechanisms allow hearing through bone conduction when the external and middle ear are bypassed. However, the quality of sound perception is often reduced compared to normal hearing through the air conduction pathway.
residual Hearing in Conductive Loss
When there is hearing loss due to middle ear or external ear problems, there is often some residual hearing through bone conduction. The amount of residual hearing depends on factors like:
- The type and severity of damage or defect to structures like the eardrum, ossicles or ear canal.
- How much acoustic energy is transmitted through the skull bones.
- Whether there is any cochlea damage in the inner ear.
People with chronic middle ear infections or otosclerosis commonly have very poor air conduction hearing, but better preserved bone conduction hearing. In conditions like otosclerosis where the stapes bone is fixed, loud sounds may still be transmitted to the cochlea through skull vibrations.
In cases of minor external or middle ear problems, bone conduction pathways may allow hearing to be fairly close to normal thresholds. With more severe defects, residual hearing through bone conduction is significantly impaired.
Limits of Bone Conduction Hearing
Although bone conduction provides a second pathway for sound to reach the inner ear, there are some disadvantages compared with normal air conduction hearing:
- Higher thresholds – More intense or loud sounds are needed before they can be transmitted through the bone.
- Poor sound quality – High frequency sounds in particular are poorly transmitted.
- Limited amplification – There are limits to how much sound can be amplified and boosted through the skull bone.
- Less spatial perception – Ability to locate direction of sounds is reduced.
Therefore, hearing purely through bone conduction results in hearing loss for many sounds, and a lack of clarity compared to normal hearing. However, bone conduction can provide a useful low-fidelity backup route for accessing sound when air conduction pathways are disrupted.
Assessing Bone Conduction Hearing
Audiologists can assess bone conduction hearing thresholds using a special type of headphone called a bone conductor oscillator. This has a vibrating pad that is placed against the mastoid bone behind the ear to transmit sound vibrations directly into the skull.
By comparing air conduction and bone conduction thresholds for a range of sound frequencies, the level of conductive hearing loss versus sensorineural hearing loss can be determined. This helps diagnose conditions like otosclerosis, chronic ear infections or ear canal obstructions that interfere with middle ear function.
When there is a mismatch between the air conduction and bone conduction thresholds, this indicates a conductive hearing loss component that may be medically or surgically treatable.
Bone Conduction Testing Limitations
Although bone conduction testing is useful, it has some limitations:
- Results can vary depending on exactly how the bone oscillator is positioned and coupled to the skull bone.
- Thresholds measured can sometimes overestimate the true bone conduction ability.
- Does not give information about hearing ability when both air and bone conduction pathways are disrupted.
Cochlear Implants for Sensorineural Loss
Cochlear implants are small electronic devices surgically implanted into the cochlea. They stimulate the cochlea’s auditory nerve fibers directly using electrical signals.
Cochlear implants can provide a level of hearing sensation and speech understanding to people with severe inner ear damage who don’t benefit enough from traditional hearing aids.
The external sound processor captures and processes sound into electrical signals, which are transmitted to the implanted electrodes in the cochlea. Targeted stimulation of auditory neurons allows hearing of environmental sounds and speech.
Cochlear implants bypass both the external ear canal and middle ear structures. They demonstrate that a form of useful hearing can be achieved through electrical stimulation of the cochlea’s nerve cells alone, without any mechanical conduction through the ossicles or even a functional cochlea.
Limitations of Cochlear Implants
While cochlear implants can provide hearing ability to the profoundly deaf, there are some limitations:
- Hearing through an implant does not sound exactly the same as normal acoustic hearing.
- Speech understanding is often slower and requires more concentration.
- Music perception is usually poor due to limited pitch perception.
- Implants may not work for people whose auditory nerves are severely damaged.
Overall, cochlear implants provide a partial substitute for normal hearing. But they demonstrate stimulation of the auditory pathway is possible without relying on middle ear or even inner ear structures.
Vestibular Schwannoma Tumors
Vestibular schwannoma tumors are benign growths that arise from the balance nerves supplying the inner ear. As they slowly enlarge, they can compress structures of the middle and inner ear.
Larger tumors can eventually cause:
- Gradual conductive hearing loss by impinging on the ossicles.
- Sensory hearing loss by damaging the cochlear nerves.
- Facial paralysis or imbalance when vestibular nerve fibers are affected.
Smaller tumors may be monitored with regular MRI scans. Larger tumors causing significant symptoms may require surgical removal.
After tumor removal surgery, patients are often left with permanent conductive hearing loss and vestibular problems. However, bone conduction hearing aids can provide amplification by bypassing non-functional middle ears.
Atresia of the Ear Canal or Eardrum
Some babies are born with absent or closed off ear canals, or an absent eardrum. This condition is called atresia or microtia.
Since the ear canal is blocked, this results in substantial conductive hearing loss. However, bone conduction hearing is usually present. Bone conducting hearing aids that attach to glasses or headbands can amplify these bone conduction pathways and provide functional hearing.
Surgery may also be able to create or open an ear canal in some cases, allowing improved hearing through a reconstructed middle ear system.
Swimmer’s Ear Otitis Externa
Swimmer’s ear is an infection of the outer ear canal caused by trapped water. It causes inflammation, pain, and discharge.
As well as being painful, swimmer’s ear can cause temporary conductive hearing loss by blocking and swelling the ear canal. Bone conduction allows some residual hearing ability until the infection clears.
Treating the infection with prescription ear drops helps reduce swelling in the canal and improve hearing. Preventing water trapping in the ears helps avoid repeat infections.
Ruptured Eardrum
A ruptured or perforated eardrum is a hole or tear in the thin eardrum membrane. This can be caused by:
- Middle ear infection
- Pressure injury from loud blasts or explosions
- Foreign objects pushed into the ear
- Slaps or blows to the ear
As well as pain, a ruptured eardrum causes conductive hearing loss by disrupting the chain of ossicles. Drainage and antibiotics treat infection, while the rupture often heals spontaneously after a few weeks.
Significant residual hearing is often still present via bone conduction which allows time for the rupture to repair.
Conclusion
The middle ear is vital for transmitting and amplifying sounds efficiently to the inner ear through the air conduction pathway. When middle ear structures are disrupted by damage, defects or disease, significant conductive hearing loss occurs.
However, bone conduction pathways allow someresidual hearing by bypassing the middle ear and transmitting sounds via the skull bone directly. Although bone conduction hearing has limitations compared to normal hearing, it provides a backup route for sound information to reach the cochlea.
Devices like bone conducting hearing aids and cochlear implants that stimulate the cochlea via alternative pathways demonstrate that some level of hearing is possible without a functional middle ear system.
