Hearing assessments map a person’s ability to perceive sound and provide detailed information about how sound travels through the ear structures and which components may be impaired. Identifying the specific type and location of any hearing loss is important for determining appropriate intervention strategies. The diagnostic process relies on comparing different ways sound stimulates the inner ear.
Air Conduction Versus Bone Conduction
Sound naturally travels through the ear canal, vibrates the eardrum (tympanic membrane), and moves the three tiny bones (ossicles) in the middle ear before reaching the fluid-filled cochlea. This measurement reflects the function of the entire hearing mechanism, encompassing the outer ear, middle ear, and the sound processing structures of the inner ear. Air conduction testing measures hearing thresholds, which is the softest level, measured in decibels (dB), at which a person can detect a pure tone. These tests are typically conducted using headphones or insert earphones in a sound-treated booth.
Bone conduction is a specific technique used to directly stimulate the inner ear, completely bypassing the outer and middle ear structures. A small vibrator is placed directly on the mastoid bone behind the ear. This device causes the skull to vibrate, which then transmits the sound energy directly to the fluid within the cochlea. By isolating the inner ear, bone conduction testing reveals the cochlea’s functional capacity.
Defining the Air-Bone Gap
The air-bone gap (ABG) is a specific diagnostic term that represents the numerical difference between the air conduction threshold and the bone conduction threshold. It is calculated by subtracting the bone conduction threshold from the air conduction threshold at a specific test frequency. This difference is measured in decibels (dB), the standard unit used to quantify sound intensity.
The presence of a difference suggests that sound transmission is being impeded in the outer or middle ear structures. An air-bone gap is only considered clinically relevant when the difference between the two thresholds is 10 dB or greater. A gap smaller than 10 dB is usually considered insignificant.
This comparison is visually represented on an audiogram, which plots hearing thresholds across various frequencies. Air conduction thresholds are typically marked using symbols, such as an ‘X’ for the left ear and an ‘O’ for the right ear. Bone conduction thresholds are plotted using different symbols, such as brackets. The spacing between the two sets of symbols on the audiogram highlights the presence and magnitude of the air-bone gap.
Clinical Significance of the Gap
The existence of a measurable air-bone gap isolates the problem to the conductive parts of the hearing system. A gap means that sound is being blocked, dampened, or attenuated as it passes through the outer ear canal or the middle ear space. The bone conduction results show that the inner ear itself is functioning normally or near-normally.
This pattern of results confirms a diagnosis of conductive hearing loss. The degree of the air-bone gap directly correlates with the severity of the obstruction in the outer or middle ear pathways. For instance, a 30 dB gap means that 30 dB of sound intensity is lost before reaching the cochlea.
If a person experiences hearing loss, but the air conduction and bone conduction thresholds are nearly identical (less than a 10 dB difference), this indicates a sensorineural hearing loss. In this case, sound transmits correctly through the outer and middle ear, but the hearing loss originates within the cochlea or the auditory nerve.
A third possibility is a mixed hearing loss, which occurs when both pathways show significant problems. A mixed loss is characterized by poor bone conduction results, showing an inner ear problem, plus a significant air-bone gap, indicating a simultaneous issue in the outer or middle ear. The air-bone gap component represents the portion of the hearing loss due to the conductive problem.
Conditions That Create the Gap
The presence of an air-bone gap directs the clinician toward investigating specific anatomical issues within the outer and middle ear. One common cause is otitis media, which involves the accumulation of fluid in the middle ear space. This fluid dampens the vibration of the eardrum and the ossicles, creating a measurable gap.
Other structural issues can also lead to this finding, such as a perforation or hole in the eardrum (tympanic membrane perforation). Blockages in the ear canal, such as excessive earwax (cerumen impaction), prevent sound from reaching the eardrum effectively.
More complex causes involve the ossicular chain, where the bones may become stiff (otosclerosis) or physically disrupted due to trauma or chronic infection. Identifying the precise condition causing the air-bone gap guides the audiologist or physician to recommend the most appropriate medical or surgical intervention.