Understanding the Audiogram
An audiogram serves as a visual representation of a person’s hearing ability. It is a graph audiologists use to plot hearing thresholds, which indicate the softest sounds an individual can hear at various pitches. This diagnostic tool helps identify the presence, type, and severity of hearing loss. By interpreting the patterns on an audiogram, hearing professionals can pinpoint problems within the auditory system.
The audiogram features two primary axes. The horizontal X-axis represents sound frequency, measured in Hertz (Hz), ranging from low pitches (like a tuba) to high pitches (like a flute). The vertical Y-axis indicates sound intensity or loudness, measured in decibels (dB), with softer sounds at the top and louder sounds at the bottom. Sounds at 0-20 dB are considered within the normal hearing range.
Different symbols are plotted on the audiogram to represent hearing thresholds for each ear. Circles (O) denote air conduction thresholds for the right ear, while X symbols represent the left ear. Air conduction tests assess the entire auditory pathway, from the outer ear through the middle ear to the inner ear. Bone conduction thresholds, marked with symbols like `<` for the right ear and `>` for the left ear, measure how sound bypasses the outer and middle ear, directly stimulating the inner ear through skull vibrations.
A normal audiogram shows all symbols clustered at the top of the graph, between 0 and 20 dB across all tested frequencies. This indicates a person can hear soft sounds at different pitches. Any symbols falling below this range suggest some degree of hearing difficulty, requiring louder sounds to be perceived. The specific configuration of these symbols helps differentiate between various forms of hearing loss.
What is Conductive Hearing Loss?
Conductive hearing loss occurs when sound waves are unable to travel through the outer ear canal, the eardrum, or the tiny bones of the middle ear. Sound is blocked or attenuated before it can reach the inner ear, where it is converted into electrical signals for the brain. The inner ear itself, including the cochlea and auditory nerve, functions normally in cases of purely conductive hearing loss.
The normal pathway of sound begins when waves enter the outer ear, travel down the ear canal, and cause the eardrum to vibrate. These vibrations transfer to three small bones in the middle ear—the malleus, incus, and stapes—which amplify and transmit sound to the inner ear. In conductive hearing loss, an obstruction or dysfunction in the outer or middle ear prevents this transmission.
This type of hearing loss differs from sensorineural hearing loss, which involves damage to the inner ear (cochlea) or the auditory nerve. Sensorineural hearing loss results in permanent damage and distortion of sound. Conversely, conductive hearing loss means the inner ear is intact and capable of processing sound if it reaches it directly, such as through bone conduction. Mixed hearing loss is a combination of both conductive and sensorineural components.
Visualizing Conductive Hearing Loss on an Audiogram
The distinguishing characteristic of conductive hearing loss on an audiogram is the “air-bone gap.” This gap represents a difference between air conduction thresholds and bone conduction thresholds at the same frequencies. It occurs because sound transmission through the outer and middle ear (air conduction) is impaired, while the inner ear’s ability to hear sound delivered directly through bone (bone conduction) remains intact.
On the audiogram, air conduction thresholds (O for the right ear, X for the left) appear below the normal hearing range, indicating louder sounds are required for perception. For example, air conduction symbols might cluster around 40-60 dB, suggesting moderate hearing loss. In contrast, bone conduction thresholds (`<` for the right ear, `>` for the left) fall within the normal hearing range, above 20 dB. Normal bone conduction thresholds confirm the inner ear functions correctly.
The visual representation of conductive hearing loss shows air conduction symbols plotted much lower than bone conduction symbols. This creates a vertical separation between the two sets of symbols across frequencies. For instance, Xs and Os might plot in the moderate hearing loss range, while corresponding `<` and `>` symbols for the same ear remain in the normal or near-normal range above 20 dB. This disparity is the hallmark sign.
The size of this air-bone gap reflects the degree of the conductive problem. A larger gap indicates a greater blockage or impediment to sound transmission through the outer and middle ear. For example, if air conduction thresholds are at 50 dB and bone conduction thresholds are at 10 dB, there is a 40 dB air-bone gap. The overall severity of the hearing loss is determined by the level of the air conduction thresholds. Even with normal bone conduction, if air conduction thresholds are in the severe range (e.g., 70 dB), the person experiences severe conductive hearing loss.
Common Causes and Significance
Conductive hearing loss can stem from various issues affecting the outer or middle ear. Common causes include blockages in the ear canal, such as earwax accumulation or a foreign object. Middle ear problems are also common, including infections (otitis media) that cause fluid buildup behind the eardrum, or a perforated eardrum. Other conditions involve issues with the middle ear bones, such as otosclerosis, where the stapes bone becomes fixed, or ossicular chain discontinuity, where middle ear bones become disconnected.
This type of hearing loss is temporary or treatable because the problem lies with sound transmission rather than permanent inner ear damage. Removing earwax, treating an infection with medication, or surgical repair of a perforated eardrum or ossicular chain can restore hearing. The intact inner ear means that once the conductive barrier is resolved, sound can travel to the cochlea.