Hearing loss refers to a reduced ability to hear sounds. Healthcare professionals use diagnostic tools like the audiogram to understand specific types of hearing loss. An audiogram visually represents an individual’s hearing abilities across different pitches and loudness levels. This tool helps distinguish various types of hearing loss, including conductive hearing loss.
Understanding Conductive Hearing Loss
Conductive hearing loss occurs when sound waves are prevented from traveling efficiently through the outer or middle ear to reach the inner ear. The inner ear, which contains the cochlea responsible for converting sound into nerve signals, and the auditory nerve remain unaffected and function normally in cases of purely conductive hearing loss. In contrast, sensorineural hearing loss involves issues with the inner ear itself or the auditory nerve.
When sound cannot be effectively conducted, softer sounds may be difficult to hear, and louder sounds might sound muffled. This condition is prevalent in young children. Individuals with conductive hearing loss might perceive their own voice as louder.
The Audiogram Explained
An audiogram is a graph illustrating the results of a hearing test, providing a comprehensive picture of an individual’s hearing thresholds. The horizontal axis represents frequency, or pitch, measured in Hertz (Hz). Lower pitches are on the left side, extending to higher pitches on the right, ranging from 125 Hz to 8000 Hz.
The vertical axis indicates intensity, or loudness, measured in decibels (dB HL). Softer sounds are at the top of the chart, with loudness increasing as you move down, from -10 dB to 120 dB. During testing, different symbols mark the softest sounds a person can hear: ‘O’ or a square for air conduction in the right ear, and ‘X’ for the left ear. Bone conduction thresholds, which bypass the outer and middle ear, are marked with ‘<' for the right ear and '>‘ for the left ear. Normal hearing thresholds are below 20 dB HL across all frequencies.
Identifying Conductive Hearing Loss on an Audiogram
The distinguishing characteristic of conductive hearing loss on an audiogram is the presence of an “air-bone gap.” An air-bone gap refers to a significant difference between the air conduction and bone conduction thresholds for a specific frequency. This difference is 10 dB or more.
On the audiogram, air conduction thresholds, represented by ‘O’ or ‘X’ symbols, will show elevated levels, indicating a hearing loss. This means sounds need to be louder for the individual to hear them through the ear canal and middle ear system. Simultaneously, the bone conduction thresholds, marked by ‘<' or '>‘ symbols, will remain within or near the normal hearing range, below 20 dB HL. The normal bone conduction results confirm the inner ear is functioning adequately, while elevated air conduction results point to a problem in the outer or middle ear obstructing sound transmission.
Common Causes of Conductive Hearing Loss
Conductive hearing loss arises from various conditions that impede sound transmission through the outer or middle ear. Common causes include earwax impaction, where cerumen completely blocks the ear canal. Foreign objects lodged in the ear canal can also create an obstruction, particularly common in children.
Middle ear infections, known as otitis media, are a common factor, especially in children, leading to fluid accumulation behind the eardrum. Poor Eustachian tube function, which connects the middle ear to the nose, can prevent this fluid from draining properly. A perforated eardrum, or a hole in the tympanic membrane, directly interferes with sound vibration. Issues with the ossicles, the three tiny bones in the middle ear (malleus, incus, and stapes), such as disarticulation or abnormal bone growth like otosclerosis, can also disrupt sound conduction.