Case 6

Age: 65

Gender: F

History: The patient was sent for audiologic evaluation by her ENT physician who requested an audiogram and noted that he suspected otosclerosis.

  • What is otosclerosis, which part of the middle ear is affected?

There are different ways in which bone conducted sound creates movement of cochlear fluids.

  1. Compression / distortion.  The cochlea’s shell shape is distorted as the skull vibrates back and forth, creating movement of the cochlear fluids.
  2. Bone conduction by air conduction.  The bony sides of the external auditory meatus are vibrating, and this creates an air-conducted sound, which travels both outward and towards the tympanic membrane and into the middle ear.
  3. Ossicular lag.  The shell of the cochlea is set into vibration by the bone vibrator; however, at some frequencies the stapes footplate, which is held in place with ligaments, does not move at the same time. The stapes footplate lags behind the vibrating cochlear shell.  This has the same effect as if the stapes footplate were vibrating from an air conducted sound. The vibration creates more movement of the cochlear fluids.

You may wish to review the mechanisms of bone conduction.

  • Which of the normal mechanisms for bone-conduction hearing would be reduced or eliminated by otosclerosis?