Your Audiology Tutorial: "Noise Notch"

 

The above audiogram displays what is commonly described by audiologists and ENTs as a "noise notch".  What you are looking at, to refamiliarize, is a graph with frequency (pitch) on the horizontal axis and intensity (sound level) in decibels on the vertical.  Circles are right ear, Xs are left.  Hearing sensitivity within normal limits is considered to be at 25 dB or softer (the upper part of audiogram). 

You'll note that this patient's hearing drops off at 3-4 k Hz, then improves at 6 - 8 k Hz.   We learned in grad school that the high frequencies degrade as you get older.  "Hair cells" - sensory cells that convert sound to electrical signals that the brain's auditory region can interpret - line the cochlea (hearing organ) and those at the base, coded for the highest pitches, break down first.  Due to age, primarily.  Ototoxic medications are another trigger.

Noise exposure, usually the chronic sort over time, can cause this degradation at any age.  But why do we often see the above pattern? Why only at 3-4 k?

There are some theories:

- Vascular insufficiency (inhibition of blood flow to cochlea) to that particular region.

- Torque from the "traveling wave" (a rotation across the membrane in response to sounds) in the base.

- The particular angle of the hair cells.

The notch is often seen in patients who've spent significant amounts of time in noisy environments.  Military veterans, law enforcement, musicians, and construction workers may present with such an audiometric configuration.  But it is important to consider that the notching may be due to head trauma, viral infections, and perilymph fistula (sensory fluid leak between thin membranes in the bony labyrinth and the middle ear space).  So wholesale designation of this sloping hearing loss as "noise notch" may not be entirely accurate.