Editor's note: Dr. Emanuel provided an update of this Ask the Expert that included new research findings into this phenonmenon. Please read the latest information here: http://www.audiologyonline.com/askexpert/display_question.asp?question_id=636
Why does the Carhart notch appear at 2000Hz in otosclerotic patients and at no other frequency?
An audiometric finding characteristic of otosclerosis is an increase in bone conduction threshold with a peak at 2,000 Hz known as Carhart's notch (Carhart, 1950). Although the notch occurs at 2,000 Hz, a reduction in bone conduction sensitivity is seen from 500 to 4,000 Hz which is, on average, 5 dB at 500 Hz, 10 dB at 1000 Hz, 15 dB at 2000 Hz, and 5 dB at 4,000 Hz (Carhart, 1971). Carhart attributed this phenomenon to "mechanical factors associated with stapedial fixation." The Carhart notch is not a true indication of "cochlear reserve" and this apparent bone conduction loss may be corrected by surgical intervention (Tonndorf, 1971). Carhart (1971) and Tonndorf (1971) provided a review of several theories to explain this phenomenon. Further, Tonndorf (1971) provided a summary of the results of his studies of stapes fixation in various mammals (cats, dogs, guinea pigs, rats, and humans). Tonndorf (1971) explained that the middle ear contribution to the total bone conduction response consists of an ossicular inertial component and a middle ear cavity effect. He found the magnitude of the Carhart notch depended on the extent the middle ear contributed to the total bone conduction response in each of the species tested. Further, he explained the frequency of the notch varied depending on the resonant frequency of the ossicular chain for bone-conducted signals. The resonant frequency of the human ossicular chain was at a relatively high frequency compared with other species. Among the species he studied, this frequency was lowest in cats and highest in rats. Based on the work of Tonnndorf, it appears the Carhart notch peaks at 2,000 Hz due to the loss of the middle ear component close to the resonance point of the ossicular chain.
Carhart, R. (1950). Clinical application of bone conduction audiometry. Archives of Otolaryngology, 51, 798-808.
Carhart, R. (1971). Effects of stapes fixation on bone-conduction response. In I.M. Ventry, J.B. Chailkin, & R.F. Dixon (Eds.), Hearing measurement: A book of readings (pp. 116-129). New York, NY: Appleton-Century-Crofts.
Tonndorf, J. (1971). Animal experiments in bone conduction: Clinical conclusions. In I.M. Ventry, J.B. Chaiklin, & R.F. Dixon (Eds.), Hearing measurement: A book of readings (pp. 130-141). New York, NY: Appleton-Century-Crofts.
Dr. Diana Emanuel has been a faculty member at Towson University for 9 years. She is currently the Audiology Graduate Program Director for the Au.D. Program at Towson University. She can be reached at firstname.lastname@example.org.