Conductive Hearing Loss

Pattern

A conductive component delays the acoustic stimulus reaching the cochlea. Every ABR wave is shifted right by approximately the same amount; interpeak intervals stay normal¹. The magnitude of the latency shift correlates with the size of the air–bone gap, roughly 0.03 ms per dB at the lower end of the range — the same slope as the cochlear L–I function, since both reflect intensity reaching the cochlea. Foundation

The bone-conducted ABR counter-test

Air-conducted ABR cannot distinguish conductive from cochlear loss on its own — both can produce parallel rightward shifts. Bone-conducted ABR bypasses the middle ear: in pure conductive loss it normalises completely, while in cochlear loss it remains shifted². The AC–BC difference quantifies the conductive component.

Pre-grommet ABR in children Trainee

Bilateral middle-ear effusion is the textbook paediatric scenario. Pre-operative ABR distinguishes the conductive component from any superimposed sensorineural loss and informs counselling. After grommet insertion, the conductive component resolves and the ABR returns to baseline within days to weeks.

Maximum bone-oscillator output

Bone oscillators reach ~50–55 dB nHL at their maximum, so very large losses cannot be fully characterised on bone conduction. A bone-conducted ABR that fails to detect a response at maximum output does not exclude a large conductive component; it may represent either a profound sensorineural loss or a mixed loss with a sensorineural floor.

What argues against pure conductive loss

If the bone-conducted ABR is also shifted, a sensorineural component is present. If interpeak intervals are prolonged on AC or BC, a retrocochlear or brainstem lesion coexists — the conductive layer is hiding something more.