<s>[INST] context: Carharts notch

Bone conduction is normal in otosclerosis.
In some cases there is a dip in bone conduction curve which is maximum at 2000 Hz / 2 KHZ called as Carharts notch.
Carharts notch is seen only in bone conduction curve.
It disappears after successful stapedectomy/stapedotomy.
The reason why it disappears after successful surgery is that when the skull is vibrated by bone—conduction sound, the sound is detected by the cochlea via 3 routes:

– Route (a)—is by direct vibration within the skull.
– Route (b)—is by vibration of the ossicular chain which is suspended within the skull.
– Route (c)—is by vibrations emanating into the external auditory canal as sound and being heard by the normal air-conduction route.

In a conduction type of hearing loss (as in otosclerosis) the latter two routes are deficient but regained by successful reconstruction surgery. Hence bone conduction thresholds improve following surgery.

ALSO KNOW

Dip in noise induced hearing loss is seen at 4 KHZ.
In noise induced hearing loss—Dip is seen in both air and bone conduction curves.
Trough shaped audiogram is seen in congenital SNHL.
Flat audiogram with moderate to severes SNHL is characteristic of presbycusis. [/INST] question: Carhart's notch in audiometery is seen in - | options: A: Ossicular discontinuity B: Haemotympanum C: Otomycosis D: Otosclerosis | answer: D </s>