An audiogram is the primary tool for objectively assessing a patient’s hearing ability. This standardized chart visually displays the softest sounds a person can hear, known as hearing thresholds, across different pitches. By analyzing specific patterns, specialists determine the severity and type of hearing impairment, categorized as conductive, sensorineural, or mixed hearing loss. The results are especially informative for diagnosing conductive hearing loss, which involves a mechanical issue in the outer or middle ear.
Understanding Conductive Hearing Loss
Conductive hearing loss (CHL) results from a problem that physically obstructs or reduces the transmission of sound waves from the external environment to the inner ear. Sound energy is gathered by the outer ear but encounters a roadblock within the ear canal, eardrum, or middle ear space. The problem is essentially mechanical, preventing sound vibrations from reaching the cochlea, the organ of hearing.
The distinction in CHL is that the inner ear, including the sensory cells and the auditory nerve, is functioning within normal limits. Sound is not being processed correctly because it is not physically delivered to the cochlea with enough intensity. This localized malfunction creates the unique pattern on the audiogram.
Decoding the Audiogram Chart
The audiogram is a graph used to plot hearing sensitivity, with two axes representing different properties of sound. The horizontal axis (X-axis) shows the frequency or pitch, measured in Hertz (Hz). The vertical axis (Y-axis) represents the intensity or loudness, measured in decibels (dB), where softer sounds are near the top and louder sounds are toward the bottom.
Two separate measurements are plotted: Air Conduction (AC) and Bone Conduction (BC). Air conduction testing uses headphones to measure how well sound travels through the entire auditory pathway, from the outer ear to the inner ear. Results are marked by symbols, typically an ‘O’ for the right ear and an ‘X’ for the left ear.
Bone conduction testing uses a small vibrator placed directly on the mastoid bone behind the ear. This bypasses the outer and middle ear entirely, sending vibrations straight to the cochlea, effectively testing the inner ear’s function directly. Bone conduction thresholds are plotted using symbols like angle brackets, such as ‘<' or '>‘.
The Significance of the Air-Bone Gap
The definitive sign of conductive hearing loss on an audiogram is the presence of an Air-Bone Gap (ABG). This gap is the space between the plotted air conduction thresholds and the bone conduction thresholds for the same ear and frequency. An ABG is typically defined as a difference of 10 to 15 dB or more between the two scores.
In pure conductive hearing loss, the bone conduction scores will be within the normal hearing range (0 to 20 dB HL). This indicates that the inner ear is functioning correctly once the problem area is bypassed. However, the air conduction scores will be lower on the chart, falling into the range of hearing loss.
This characteristic pattern visually confirms that sound transmission is compromised only in the outer or middle ear. If bone conduction thresholds were also significantly depressed, the condition would be classified as a mixed hearing loss, indicating damage in both the conductive and sensorineural parts of the system. The air-bone gap serves as the precise visual marker that isolates the hearing problem to the mechanical structures of the ear.
Common Sources and Outcomes
The conditions leading to this specific audiometric pattern all create a physical barrier to sound transmission. Frequent causes include earwax impaction (cerumen buildup), which blocks the ear canal. Middle ear infections (otitis media) can also cause CHL by filling the middle ear space with fluid, dampening the vibration of the eardrum and the ossicles.
Structural issues like a perforation in the eardrum, or problems with the ossicles, such as abnormal bone growth called otosclerosis, also result in a CHL pattern. These conditions create the physical impedance that the air conduction test registers as a loss, while the bone conduction test bypasses the impedance to show normal inner ear function. Conductive hearing loss often carries a favorable prognosis; because the inner ear is usually undamaged, many cases are medically or surgically treatable, and the hearing loss can be reversed or significantly improved.