CI Atlas · When Hearing Aids Aren't Enough · Module 08

8Cochlear dead regions

Some of the most frustrating hearing-aid failures come from a deceptively simple problem: part of the cochlea is functionally dead. A cochlear dead region is a stretch where the inner hair cells, the neurons, or both no longer work — so the place that should encode those frequencies has no working receiver. The audiogram may still show measurable thresholds there, because the sound is detected off-frequency by spread to a living neighbour, which lures the clinician into amplifying frequencies that can never be properly heard. Doing so does no good, and can actively degrade clarity. Dead regions are common in severe loss and are a likely explanation for the patient whose word recognition stays stubbornly poor despite a well-fitted aid — and a clear pointer toward a cochlear implant.

TA patch with no receiver

A cochlear dead region is an area of the cochlea with no functioning inner hair cells and/or neurons — the place that should transduce a band of frequencies has, in effect, no working receiver. Crucially, the audiogram can still record thresholds in that band, because a tone there is detected off-frequency by spreading along the basilar membrane to a living region nearby.

A dead region is a stretch of cochlea with no functioning inner hair cells or neurons. A tone whose place falls inside it cannot be detected there — at best it is heard off-frequency by spread to a living neighbour, which sounds distorted. Crucially, amplifying that frequency does not help, and may even worsen clarity (Vickers; Moore's TEN test detects this). Dead regions are a hidden reason a patient with usable thresholds still gets little from a hearing aid — and a pointer toward an implant. Schematic.

CWhy amplifying it fails

Because the dead region cannot encode its own frequencies, amplifying them does not restore them — and can make things worse. Vickers and colleagues showed that for listeners with high-frequency dead regions, low-pass filtering (removing the amplified high frequencies that fall in the dead region) improved intelligibility, while amplifying into the dead region did not help. The sound delivered there is either inaudible or perceived in a distorted, off-frequency way.[2001]

CDetecting a dead region

Dead regions are invisible on the standard audiogram but can be revealed by special tests — most practically the TEN (threshold-equalising noise) test, in which a masking noise exposes the off-frequency listening that signals a dead region. The clinical evidence is consistent: patients with dead regions show reduced speech intelligibility and reduced subjective hearing-aid benefit, often despite sensitivity to moderate sound levels.[2005]

CA pointer toward the implant

Dead regions help explain one of the chapter's recurring puzzles: the patient whose word recognition stays poor despite a hearing aid. Many such patients have dead regions to blame — and that makes them candidates for a cochlear implant, whose electrode can stimulate surviving neurons across the cochlea rather than relying on a dead patch to do work it cannot. The dead region is amplification's blind spot and the implant's opportunity.

The standard audiogram cannot see a dead region; the TEN test can. A calibrated background noise is played and the masked threshold measured at each frequency. Over a living region the threshold sits near the noise level (green). Over a dead region it jumps well above it (red) — because the tone is only detected off-frequency, by a distant living place, which the noise masks until the tone is much louder. That elevation, more than ~10 dB above the noise, is the fingerprint of a dead region. Schematic.

Case 10.8 · The patch that won't respond

A patient with measurable high-frequency thresholds gets no benefit from high-frequency amplification, and finds speech clearer when the highs are turned down. A TEN test reveals a high-frequency dead region.

What does this finding mean for the fitting?

Self-assessment — Module 82 questions

Question 1 · Trainee

What is a cochlear dead region?

Question 2 · Clinician

What does amplifying into a dead region achieve, and how is it detected?

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