What the Carhart notch is
On a hearing test, the Carhart notch is a dip in the bone-conduction line, usually at 2000 Hz (2 kHz), in an ear whose sound transmission is impaired, most typically because the stapes is fixed by otosclerosis. The dip is typically 10–15 dB, sometimes a little more. On either side of it, bone conduction is often close to normal.
Bone conduction is meant to test the inner ear directly, bypassing the ear canal, eardrum and ossicles, so a dip in the bone line looks at first sight like inner-ear damage. In this case it mostly is not.
It is named after Raymond Carhart, the American audiologist who described the pattern in otosclerosis in 1950, and it has been taught as a classic sign of stapes fixation ever since. The evidence for that reputation is weaker than the textbooks suggest.
Why it happens: mechanics, not damage
Bone conduction is not one pathway but several. When the vibrator behind the ear shakes the skull, the cochlea is stimulated partly because its bony shell is moved and squeezed directly. Part of the stimulus comes indirectly: the ossicles and inner-ear fluids lag slightly behind the moving skull because of their own inertia, and that lag rocks the stapes in the oval window and adds to the signal.
This ossicular share matters most in the middle frequencies, around the natural resonance of the middle ear. When the stapes footplate is locked in bone, that share is lost. The cochlea is unchanged, but it receives slightly less stimulation from the vibrator, so the measured threshold rises and a notch appears.
Models of human bone conduction reproduce this mid-frequency dip from stapes fixation alone, without any change in the cochlea. The notch is therefore sometimes called a pseudo-sensorineural loss: it behaves like an inner-ear loss on the graph, but it is a mechanical artefact of the middle ear.
What it looks like on the audiogram
On a typical audiogram in otosclerosis, the air-conduction symbols lie well below normal, often most in the low frequencies, while the bone-conduction symbols (brackets or arrows) lie near the top of the chart. The difference between them is the air-bone gap. In an ear with a Carhart notch, the bone line drops at 2 kHz and comes back up towards normal at 4 kHz, forming a shallow V.
The notch is not always at 2 kHz, and not always present. In one series of people having laser stapedotomy, four in five had a notch, split roughly evenly between 0.5, 1 and 2 kHz. In another, three-quarters had one, most often at 2 kHz. With a strict definition (at least 10 dB at 2 kHz below the neighbouring frequencies), it was found in only about a third of fixed stapes.
Definitions and mechanism
There is no agreed definition. Kashio and colleagues took the 2 kHz bone threshold minus the mean of the 1 and 4 kHz thresholds, with a notch at 10 dB or more (mean depth about 17 dB in qualifying ears); other studies accept 7.5 dB, or a dip at any frequency from 0.5 to 4 kHz. Prevalence figures cannot be compared without checking the criterion. In Stenfelt's lumped-element model, stapes fixation depresses the fluid-inertia component of bone conduction, leaving inner-ear compression dominant above about 400 Hz; the model predicts a Carhart-type dip as well as the supranormal bone conduction of a canal dehiscence.
After surgery: the notch often fills in
When stapedotomy restores movement of the chain, the ossicular share of bone conduction returns. Bone conduction at the notch frequency often improves: by about 8 dB on average at 2 kHz in published series, more in some ears and not at all in others. Averaged across the usual test frequencies the gain is small, under 2 dB in one series of more than 200 patients.
This produces what is called overclosure: after surgery, air conduction can end up better than the bone conduction measured before the operation. In one series, four in five patients showed overclosure at one or more frequencies, most at 1 and 2 kHz. It is good news, because the inner ear was better than it looked. It is also a trap when results are reported: comparing post-operative air conduction with pre-operative bone conduction makes the air-bone gap look smaller than it is. The correct comparison is post-operative air with post-operative bone, as explained on bone-conduction preservation.
A notch that does not fill in is not, by itself, a sign of failure. Part of the dip may have been genuine inner-ear loss, or the change may be too small to show in 5 dB steps. The other direction matters more: a fall in bone conduction after surgery, particularly at 4 kHz, is a sign of inner-ear strain, and an average across frequencies can hide it behind an improvement at 2 kHz.
How much the notch tells you
The notch has long been called a hallmark of otosclerosis. The evidence for its diagnostic value is modest.
- A 2013 systematic review found only three eligible studies, all at moderate or high risk of bias, and concluded that the notch is a useful hint towards otosclerosis but cannot confirm it.
- In ears with a normal eardrum that were explored surgically for a conductive loss, a 2 kHz bone dip was found about as often with a disconnected chain (26%) or a fixed malleus or incus (30%) as with a fixed stapes (31%).
- The notch also appears in glue ear. In one series of children with otitis media with effusion, about a quarter had a notch, nearly always at 2 kHz, and in most of them bone conduction improved after myringotomy.
So the notch says “something is loading or stiffening the middle ear”, not “this is otosclerosis”, and its absence does not exclude otosclerosis. The diagnosis rests on the whole picture, set out on interpreting hearing tests: a normal eardrum, an air-bone gap, absent stapedial reflexes, the history and, in selected cases, a CT scan. Other ossicular problems and third-window lesions such as superior canal dehiscence can mimic otosclerosis; the reflexes and imaging, not the notch, tell them apart.
Pitfalls: masking and ordinary variation
Masking. Bone-conducted sound reaches both cochleas almost equally, so the other ear has to be occupied with masking noise to be sure which ear is answering. In otosclerosis both ears often have an air-bone gap, and then masking is genuinely difficult: the noise needed to occupy the other ear can itself cross the skull and mask the ear being tested. Too little or too much masking can create a notch or hide one.
Ordinary variation. Thresholds are measured in 5 dB steps, and a repeat test commonly differs by 5 dB. A 10 dB dip at one frequency on one occasion can be no more than two measurements drifting apart. A notch is more convincing when it appears on repeated audiograms and fits the rest of the picture.
Other dips. A dip around 3–6 kHz in both air and bone conduction is the usual pattern of noise exposure, a genuine inner-ear loss, not a Carhart notch.
Test conditions. Vibrator placement and whether the test ear is covered change bone thresholds, so the test should be done the same way before and after surgery.
What it means for the inner-ear reserve
The bone-conduction line is the inner-ear reserve: the ceiling for air conduction after a perfect operation. A notch means that at 2 kHz the reserve is probably a little better than measured. In practice:
- The pre-operative bone line is a slightly pessimistic estimate at the notch frequency. An improvement of several decibels is common, but not guaranteed, and should not be the reason to operate.
- When bone conduction is reduced across many frequencies, not just in one dip, that part is genuine inner-ear loss. Closing the gap then helps less, and a hearing aid may still be needed afterwards.
- Word recognition is a useful cross-check: excellent understanding of amplified speech supports a healthy cochlea.
- After surgery, post-operative bone conduction shows whether the inner ear was protected. Overclosure at 2 kHz is welcome but is not proof of a gain elsewhere.
The decision to operate, to use a hearing aid or to wait is the same with or without a notch. It rests on the air-bone gap, the other ear and what matters to the person.
Reporting results when there is a notch
Calculate the post-operative air-bone gap from post-operative air and bone conduction measured at the same session. Report bone-conduction change frequency by frequency, including 4 kHz, because a 2 kHz recovery and a 4 kHz loss cancel out in a 0.5–4 kHz average. Report overclosure as such rather than counting it as gap closure. Whether a pre-operative notch is prognostic is unsettled: in a single-centre series of 140 patients, Wiatr and colleagues found a less favourable stapedotomy result in ears with a notch, and the opposite after stapedectomy.
Sources
- Carhart R. Clinical application of bone conduction audiometry. Arch Otolaryngol. 1950;51(6):798-808. PubMed
- Stenfelt S. Inner ear contribution to bone conduction hearing in the human. Hear Res. 2015;329:41-51. PubMed
- Wegner I, Bittermann AJ, Hentschel MA, et al. Pure-tone audiometry in otosclerosis: insufficient evidence for the diagnostic value of the Carhart notch. Otolaryngol Head Neck Surg. 2013;149(4):528-532. PubMed
- Kashio A, Ito K, Kakigi A, et al. Carhart notch 2-kHz bone conduction threshold dip: a nondefinitive predictor of stapes fixation in conductive hearing loss with normal tympanic membrane. Arch Otolaryngol Head Neck Surg. 2011;137(3):236-240. PubMed
- Ahmad I, Pahor AL. Carhart’s notch: a finding in otitis media with effusion. Int J Pediatr Otorhinolaryngol. 2002;64(2):165-170. PubMed
- Perez R, de Almeida J, Nedzelski JM, et al. Variations in the “Carhart notch” and overclosure after laser-assisted stapedotomy in otosclerosis. Otol Neurotol. 2009;30(8):1033-1036. PubMed
- Sabbe A, Verhaert N, Joossen I, et al. Otosclerosis: shift in bone conduction after stapedotomy. B-ENT. 2015;11(3):183-189. PubMed
- Wiatr A, Składzień J, Stręk P, et al. Carhart notch: a prognostic factor in surgery for otosclerosis. Ear Nose Throat J. 2021;100(4):NP193-NP197. PubMed
Common questions
What is a Carhart notch on an audiogram?
It is a dip in the bone-conduction line, usually at 2000 Hz and typically 10 to 15 dB deep, in an ear with a conductive hearing loss. It is named after the audiologist Raymond Carhart, who described it in otosclerosis in 1950. Although bone conduction is meant to test the inner ear, this dip is mostly caused by the stiff middle ear, not by damage to the cochlea.
Does a Carhart notch mean I have otosclerosis?
Not on its own. The notch is common in otosclerosis, but it also appears when the ossicular chain is disconnected or another ossicle is fixed, and in children with glue ear. Many people with otosclerosis have no notch at all. The diagnosis rests on the whole picture: a normal eardrum, an air-bone gap, absent stapedial reflexes, the history and sometimes a CT scan.
Does the Carhart notch go away after surgery?
Often, at least partly. When stapedotomy restores movement of the chain, bone conduction at the notch frequency improves in many ears, on average by about 8 dB in published series. In some ears it fills in completely; in others it hardly changes. A notch that persists does not mean the operation failed, but your surgeon will compare bone conduction before and after to check that the inner ear was preserved.
Is a Carhart notch a sign of nerve or inner-ear damage?
Usually not. The notch appears because a fixed stapes removes one of the routes by which skull vibration normally stimulates the cochlea, so the inner ear seems a little less sensitive than it really is. That is why it is sometimes called a pseudo-sensorineural loss. Bone conduction that is reduced across many frequencies, rather than in one dip, is more likely to reflect a genuine inner-ear loss.
Can glue ear cause a Carhart notch?
Yes. Fluid in the middle ear also loads and stiffens the ossicular chain, and a similar dip in bone conduction, nearly always at 2 kHz, has been found in about a quarter of children with glue ear in one series. In most of those ears bone conduction improved after a small opening was made in the eardrum to drain the glue, which again points to a mechanical cause.
This page provides general educational information. It cannot replace an individual assessment, which depends on a full history, examination, audiometry and, where relevant, imaging. If you have sudden hearing loss, severe pain, facial weakness, severe dizziness or discharge with fever, seek medical care promptly.