An audiogram is a graph of behavioral responses to tonal sounds. The marks on the graph signify the softest level at which a tone is responded to by a patient, at each of a set of conventionally measured frequencies across the speech spectrum of sound.
Below is an audiogram chart:
The further down on the audiogram graph the marks are, the poorer the hearing is, because the louder the sound had to be before the person responded to it. Each colored section of the graph represents a different conventionally accepted degree of hearing loss.
As you move from left to right on the graph, you are moving from bass (low rumble sounds) to treble (tinkly sharp sounds), like on a piano.
Below, is a typical audiogram of a person with presbycusis (age-related hearing loss), and also a common hearing loss among the noise-exposed:
The blue X represents the thresholds (softest sound responded to) for the left ear, and the red O represents the thresholds for the right ear. As you can see, there can be different degrees of hearing loss at different frequencies. It is most common to see poorer hearing in the high frequencies (treble) than in the low frequencies (bass).
From that audiogram we don’t know whether that is a conductive hearing loss, sensorineural hearing loss, or mixed. That is because we only see the thresholds found through earphones, the “air conduction” thresholds. This measures the hearing ability of the inner and middle and outer ears, together. This is mostly likely a sensorineural hearing loss because of the configuration (conductive loss is usually in the lower frequency, or the left side of the graph), but to be sure, we need more information.
We need bone conduction thresholds. Below, is a graph of a conductive hearing loss:
The brackets ] for left ear and [ for right ear indicate the “bone conduction” thresholds (sometimes these brackets are > and <) . Rather than hearing through headphones, a vibrating device on the mastoid bone (behind the ear) sends sound through the bone of the skull directly to the cochlea (inner ear, where the hearing “nerve endings” are), so if there is any blockage at the ear canal or behind the eardrum, this test bypasses it. If there is a gap between the bone conduction thresholds and the air conduction thresholds, we have an air-bone gap, and a need for medical consult, and likely an otolaryngology consult to identify the problem with the outer or middle ear, and determine whether there is a medical remedy. Outer ear blockages (earwax or foreign object or malformation) are generally ruled out by the audiologist during otoscopy (looking in the ear with an otoscope).
In a sensorineural loss, these brackets will be at or very close to the X‘s and O‘s, so the air conduction thresholds and bone conduction thresholds are almost the same, and there is no air-bone gap. The hearing loss is purely sensorineural.
HEARING LOSS AND THE EAR
So, what does this mean in the ear? The diagram below should help to explain that for you:
A conductive loss is due to a malformation or blockage in the outer or middle ear. It can be as simple as cerumen (earwax) built up in the ear canal, it could be a hole or damage to the eardrum (the membrane between the ear canal and middle ear), fluid in the normally air-filled middle ear (area behind the eardrum), or in a more serious situation, infection or a tumor in the middle ear space. A conductive loss can often be treated with medicine or surgery. Sometimes it can’t.
Most sensorineural loss is due to damage or deterioration in the inner ear, the section that looks like a snail shell. In that snail-shaped cave in your skull, are the sensory cells for sound. These cells transfer the sound collected by the ear into nerve firings, which are then transmitted to the brain. Very rarely, is a sensorineural hearing loss treatable with medicine or surgery. Almost all sensorineural hearing losses are permanent. The only treatment plan for most sensorineural hearing loss is hearing aids.
Some sensorineural loss is further along the hearing nerve pathways. Audiologists look for indications of this when evaluating hearing, and if there are signs and symptoms of damage or deterioration further into the nerve pathways, the patient will be referred for medical consultation and further testing.
Joely E. Viveiros, M.Sc., Aud (C)
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