CI Atlas · Audiological Evaluation · Module 04

4Bone conduction & the air–bone gap

Air conduction tells you how much a person hears; bone conduction tells you where the problem is. By driving the skull directly, a bone vibrator skips the outer and middle ear and asks the cochlea what it can do on its own. The difference between the two — the air–bone gap — is one of the most informative numbers in audiology: it separates a conductive blockage, which may be treatable and may make an implant unnecessary, from a true sensorineural loss, and it even hints at the cause, from the Carhart notch of otosclerosis to the deceptive low-frequency gap of a third-window lesion. This module covers how bone conduction is measured and interpreted, and the traps — spurious gaps and vibrotactile responses — that can mislead the unwary.

FWhy test bone conduction

A bone vibrator on the mastoid sets the skull — and the cochlear fluids — vibrating directly, bypassing the outer and middle ear. The bone-conduction threshold therefore estimates cochlear sensitivity, and comparing it with the air-conduction threshold gives the air–bone gap.

TReading the air–bone gap

The pattern classifies the loss. A gap with normal bone is conductive; reduced bone with no gap is sensorineural; reduced bone plus a gap is mixed. Because bone conduction has near-zero interaural attenuation, it almost always needs masking (Module 3) — a result without proper masking can be meaningless.[2020]

CClues to aetiology

The gap and its shape point to a cause. Carhart's notch — a dip in bone conduction around 2 kHz — is the classic mechanical artefact of otosclerosis; a flat tympanogram with a gap suggests effusion; a deep tympanogram with a large gap suggests ossicular discontinuity. Reading air, bone and immittance together — the cross-check again — narrows the differential.

CThe spurious gap & vibrotactile traps

Two traps deserve naming. A purely cochlear condition can produce a spurious low-frequency air–bone gap from altered cochlear mechanics — classically an enlarged vestibular aqueduct (a third-window effect) — mimicking middle-ear disease that is not there. And at high levels in profound loss, a patient may feel rather than hear the vibrator — vibrotactile responses that masquerade as bone-conduction thresholds. In the implant work-up, mistaking either could change the recommendation entirely.

The single most useful inference from the audiogram is the type of loss, and it falls out of comparing two curves. Bone conduction reports the cochlea directly; air conduction reports the whole pathway. When they coincide (no air–bone gap) the loss is sensorineural; when bone is normal but air is worse, the gap localises a conductive problem in the outer or middle ear; when bone is elevated and a gap remains, the loss is mixed. This distinction steers the entire work-up — and a cochlear implant addresses the sensorineural component. Schematic.

Case 10.4 · A low-frequency gap that isn't conductive

A child has a low-frequency air–bone gap but a normal tympanogram, normal otoscopy, and supranormal bone-conduction thresholds. Imaging shows an enlarged vestibular aqueduct.

How is the air–bone gap best explained?

Self-assessment — Module 42 questions

Question 1 · Foundation

What does an air–bone gap with normal bone conduction indicate?

Question 2 · Clinician

What is a spurious low-frequency air–bone gap?

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