Thus, the air-bone gap is crucial information used to determine whether a hearing loss is conductive, sensorineural, or mixed based on the measured values.

Air-bone gaps in audiometric hearing assessments associated with conductive and mixed hearing losses can have a variety of causes.

Causes of conductive hearing loss include:

• Ear infections
• Otitis Media with Effusion (fluid in the middle ear)
• Tympanic membrane perforation (hole in the eardrum)
• Abnormalities of the ossicles (middle ear bones), including:

- Malformations of the ossicles

- Ossicular discontinuity (a break in the middle ear bones)

- Ossicular fixation (fusion of the middle ear bones)

• Wax occlusion - this resolves once the wax is removed
• Exostoses - bony growths in the ear canal - these can cause conductive hearing loss when they cause occlusion of the ear canal.

Mixed hearing losses, which are those with conductive hearing loss in addition to sensorineural hearing loss, can be caused by:

• Inner ear disorders such as endolymphatic hydrops (Meniere’s disease)
• Metabolic bone disorders such as Paget’s disease
• Trauma to the head that causes damage to both the middle ear and inner ear structures.
• Otosclerosis - while this predominantly causes conductive hearing loss, it can also cause mixed hearing loss.
• Hearing loss with multiple causes that affect both the middle ear and the inner ear.

Multiple diagnostic tests are used together to identify the existence and likely cause of an air-bone gap in a person’s hearing. These include otoscopy, pure tone testing, immittance testing, and the Weber test.

The pure tone testing is crucial in identifying air-bone gaps. An audiogram graphically represents a person’s hearing sensitivity (how loud a sound needs to be for it to be heard) at different frequencies (pitches).

Pure tone audiometry determines the nature and severity of a patient’s hearing loss.

Air conduction thresholds are obtained through headphones or insert phones, while bone conduction testing transmits sound directly to the inner ear by the skull bones, bypassing the outer and middle ear.

An air-bone gap is identified, and its magnitude is assessed by subtracting the bone conduction thresholds from the air conduction thresholds at a specific frequency.

There can be some natural variability between air and bone conduction thresholds. To allow for this, only a difference between the air and bone conduction thresholds of more than 10dB is considered a clinically significant air-bone gap consistent with some degree of conductive hearing loss.

Otoscopy is vital in identifying an air-bone gap, as specific pathologies that can cause an air-bone gap can be seen by otoscopy. For instance, a perforated tympanic membrane can be identified through otoscopy, which is then used in conjunction with other results to determine appropriate management of the air-bone gap.

Otoscopy can also be used to identify potential causes of an air-bone gap that can be easily rectified, such as wax in the ear canal. By identifying this prior to audiometry, it is possible to remove the wax and get an accurate assessment of the patient’s underlying hearing loss.

Tympanometry is a key component of immittance testing in identifying an air-bone gap and its likely origin.

Tympanometry assesses the health and function of the middle ear. It can be used to identify causes of an air-bone gap, such as a perforated eardrum, fluid behind the ear drum, or problems with the ossicles. This information is used to support the findings on other tests, such as otoscopy and the pure tone audiogram and determine appropriate next steps for the patient.

Abnormal or absent acoustic reflexes also play a role in identifying an air-bone gap and supporting other test results.

The Weber test uses a tuning fork or bone conductor to differentiate between sensorineural and conductive hearing losses. It supports other test results, such as pure-tone audiometry.

Air-bone gaps that are <10dB are not considered clinically significant. However, air-bone gaps greater than 10dB indicate a conductive component to hearing loss and can impact a person’s overall hearing ability.

Air-bone gap = air conduction threshold - bone conduction threshold at a specific frequency.

If the figure derived from the above calculation is greater than 10dB, there is a conductive component to the hearing loss. If the bone conduction results are in the normal range and the air conduction results are elevated outside the normal range, this is considered conductive hearing loss. If the air and bone conduction results are elevated with an air-bone gap greater than 10dB, this is regarded as a mixed hearing loss.

A False Air-Bone Gap
A false air-bone gap in pure-tone audiometry occurs when test results indicate a difference between air and bone conduction thresholds, even though there is no actual conductive hearing loss. This misleading result can be due to various factors in the testing process or patient-specific characteristics.

Your hearing care professional is trained to detect false air-bone gaps and will use their expertise to determine the validity of the air-bone gap based on the various tests they do.

One common cause of a false air-bone gap is the "occlusion effect." This effect occurs when headphones or ear inserts are used in addition to the bone conductor during bone conduction testing to trap sound within the ear canal, amplifying low-frequency sounds and potentially leading to inaccurate results, especially in lower frequencies.

Another possible cause is improper placement of the bone oscillator, which may fail to stimulate the inner ear effectively. If the bone conductor is not positioned securely or accurately, bone conduction thresholds may appear artificially high, creating an apparent gap. Additionally, individuals with specific skull shapes or densities may show slight discrepancies between air and bone conduction results.

Hearing care professionals may repeat tests, use masking procedures, or perform additional procedures like tympanometry, the Weber test, and other cross-checks to rule out a false air-bone gap. Careful interpretation and accurate equipment setup are essential to ensure reliable pure-tone audiometry results.

Identifying the presence of an air-bone gap and its likely cause is important as it affects the patient's management options.

The treatment of an air-bone gap will depend on its likely cause. Options include treating the air-bone gap to reduce it, using hearing devices to compensate for the hearing loss caused by the air-bone gap, or a combination of these things.

It may be possible to treat an air-bone gap surgically. Options for surgical intervention are dependent on the cause of the air-bone gap but include:

• Draining the fluid from behind the ear and inserting ventilation tubes (grommets)
• Repairing the ossicular chain
• Repair of the eardrum - tympanoplasty
• Removal of exostoses in the ear canal

It is important to note that children can be particularly prone to ear infections and chronic fluid behind their eardrums due to the anatomical differences associated with smaller heads than adults. This can cause significant conductive hearing loss. While the fluid behind the eardrum will self-resolve for some children, persistent fluid can cause delays in speech development, poor social skills, and poor educational outcomes. This is why it is common to hear about children having ventilation tubes (grommets) put in their ears. As children grow up, they usually stop having these issues.

Options to compensate for the hearing loss include:

• Traditional hearing aids
• Bone-anchored hearing aids
• Cochlear implants

Hearing aids are a common way to treat hearing loss. They can be used for mixed or conductive hearing losses where surgical or medical interventions are not appropriate or the patient has not chosen to pursue these options.

There are a range of hearing aids available in different styles and technology levels. If a hearing aid is the best option, your hearing care professional will discuss all of the options with you to help you decide which hearing aid options are suitable for you. 

Bone Anchored Hearing Aids
Bone-conduction hearing aids may be recommended when traditional air-conduction hearing aids are not an option. This can include cases where there is an ongoing infection in the ear or an absent or obstructed ear canal.

A bone anchored hearing aid works according to the same principles as bone conduction in pure-tone audiometry. It vibrates the skull to simulate the inner ear directly, bypassing the middle and outer ear components. There are a variety of bone-anchored hearing aids available, and your hearing care professional will help you choose the most suitable one.

Cochlear Implants
Cochlear implants may also be used for people with an air-bone gap. However, this is only the case when there is a severe sensorineural component to a mixed hearing loss.

Prompt Treatment and Monitoring of an Air-Bone Gap
Prompt treatment of ear conditions such as ear infections can help minimize air-bone gaps that cause hearing loss. This is particularly important for children prone to getting fluid behind their eardrums. Children with a history of ear infections or fluid behind their eardrums may need to have their hearing regularly monitored to ensure there is no hearing loss that may affect speech, social, or educational development. 

Correct Nose-Blowing
The correct technique for blowing your nose can also minimise the risk of fluid building up behind the eardrum. Children often need to be encouraged to use correct nose-blowing methods to help reduce the likelihood of their eustachian tubes becoming blocked, which leads to fluid building up behind their eardrums

Wax/Debris in the Ear Canal
Wax or a build-up of other debris in the ear canal, such as dried skin, causing conductive hearing loss can often be treated by removing the wax and other debris from the ear canal by a hearing care professional. It is essential to avoid trying and removing these debris yourself using Q-tips (cotton buds). This can result in the wax/debris becoming more impacted, being pushed down the ear canal, and causing damage to the ear, such as damaged skin in the ear canal or a perforated tympanic membrane. You should see a professional to manage wax/debris in the ear canal.

See your Hearing Care Professional for a Hearing Assessment
If you suspect that your hearing may be affected by an air-bone gap, whether you have conductive or mixed hearing loss, the best thing to do is have a hearing assessment by your hearing care professional. This way, an appropriate diagnosis can be made, and you can learn about your treatment options.