Monday, October 19, 2015

Study group discussion: What causes Carharts notch?

In otosclerosis, why carharts notch at 2000 Hz in PTA?

Carhart attributed this phenomenon to "mechanical factors associated with stapedial fixation."

But why the greatest dip at 2 kHz?

The ossicular chain has two basic modes of vibration. The first mode, with a peak around 1200 Hz, is the primary mode for AC stimulation. This mode is associated with a "hinging" motion of the ossicles caused by AC stimulation of the tympanic membrane at the umbo. The second mode, with a peak around 1700 Hz, is described as a "pivoting" motion of the malleus/incus, with an axis of rotation somewhat orthogonal to the axis of rotation associated with the "hinging" motion. The second mode is less robust than the primary mode for AC stimulation, but it is the dominant mode when excited by BC stimulation. A decreased mobility of the ossicular chain at 1700 Hz due to otosclerosis also affects the surrounding frequencies, but is seen most prominently as a BC loss at 2000 Hz in audiometric testing.

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