L13 - Hearing Loss

How is hearing loss measured/ investigated?

Using psychophysics = measuring a patient's behavioral ability to detect tones (pure-tone audiology)

The faintest detectable tone is measured over headphones and compared with normal thresholds.

How does the equipment differ for air-conducted thresholds and bone-conducted thresholds?

Air-conducted = headphones

Bone-conducted = bone vibrator to by-pass the middle ear

Conductive vs Sensorineural Hearing Loss

Conductive = if the patient struggles to hear the tone through the headphones but hears it perfectly well through the bone vibrator, it proves their cochlea is healthy. The problem must be a physical blockage or mechanical failure in the outer or middle ear

e.g. Ear Canal blockages, perforated ear drum, fluid-filled middle ear

Sensorineural = If the threshold is elevated with the headphones, and it remains equally elevated when using the bone vibrator, it means that bypassing the middle ear didn't improve anything. Therefore, the mechanical conducting system (outer and middle ear) is fine, and the impairment lies within the cochlea or the auditory nerve.

4 examples of Sensorineural Hearing Loss (often with old age)

1) Loss/absence of outer hair cells

2) Loss/absence of inner hair cells (dead regions)

3) Loss of endocochlear potential

4) Loss of auditory nerve fibres (synaptopathy, acoustic neuroma)

Most Common form of Sensorineural Hearing Loss (in old people)

Loss of Outer Hair Cells

Study of Outer Hair Cell Function

The Furosemide Study (Ruggero & Rich, 1991)

• administered furosemide, a diuretic that is reversibly toxic to OHCs, to an animal to observe basilar membrane motion.

• Before administration, the basilar membrane showed highly enthusiastic, compressed responses to a 9 kHz tone (its characteristic frequency).

• 14 minutes post-administration, basilar membrane velocity dropped drastically as the OHCs were "paralyzed" and stopped amplifying the physical movement.

Evidence for NON-LINEAR Amplification = OHCs amplify quiet sounds heavily, but their amplification effect decreases as sounds get louder. Therefore, when OHCs are disabled by furosemide, the amplification effect became linear.

Evidence for Frequency Tuning = When a 1 kHz tone was played to the 9 kHz region of the cochlea, the OHCs ignored it. They possess intrinsic resonance and only amplify the exact frequency corresponding to their specific place on the basilar membrane.

Study of Outer Hair Cell Loss in Hearing Impaired vs Intact Listerners

Buus & Florentine (2002) = measured loudness magnitude estimation in normal listeners vs hearing impaired listeners

FINDINGS: Normal listeners = experience rapid loudness growth at very low intensities, which then shifts to a slower,

Impaired Listeners = Lacking OHC amplification, hearing-impaired listeners have an elevated absolute threshold. However, once sound reaches their threshold, their perception of loudness grows exceptionally fast (e.g. ‘Speak up…. there’s no need to shout!’) - find loud sounds as distressingly loud

Study of Loss of Outer Hair Cells and Frequency Selectivity

Glasberg & Moore (1986) = measured Reductions in frequency discrimination using the ‘notch-noise’ technique. Normal listeners have very precise frequency selectivity - can only hear the frequency the OHC is tuned for.

Notch-noise techniqie = play a low-frequency rumble and high-frequency hiss (makes up the background noise). Between two noises is a quiet "notch" in the middle where a target tone is played.

FINDINGS: Hearing impaired require a wider notch (the static noise is further away from the target tone's frequency) in order to hear the tone played = its filters have become much too broad.

What happens when Inner Hair Cell are lost?

creates "dead regions" on the basilar membrane where no transduction occurs.

Why is inner hair cell loss hard to detect?

If the tone is loud enough, the physical vibration will spread down the basilar membrane to functioning IHCs in lower-frequency regions.

The patient will still detect the tone, misleading the audiologist into believing the high-frequency region is alive but impaired, rather than completely dead.

The TEN Test for Inner Hair Cell Loss

Moore et al (2000) developed Threshold Equalising Noise Test to to differentiate between a dead region (IHC loss) and standard OHC loss.

Play static noise (lots of frequencies simultaneously) alongside a ‘test tone’ - this prevents the other inner hair cells from detecting the tone as they are already busy detecting the background noise (static)

This means they can test the specific region of IHCs without other IHCs interfering

If the patient has a dead region (IHC loss), the noise masks the functional, adjacent hair cells that the patient was relying on for off-frequency listening. Consequently, their threshold to detect the target tone spikes significantly higher.

What is Endocochlear Potential?

the fundamental voltage (normally around 100 mV) driving both IHC and OHC transduction, powered by the stria vascularis pumping potassium ions into the scala media

What Evidence is there for Endocochlear Potential loss?

1)The stria vascularis degrades with age.

2)Some individuals with known hearing loss have intact hair cells at post-mortem.

3)The endocochlear potential is known to fall with age in animal models.

Why is EP loss hard to detect?

It is behaviourally identical to outer hair cell loss - the patient loses their biological amplification

How does EP loss vary across frequencies?

Since loss of EP affects the whole cochlea, one would expect all frequencies to be affected.

HOWEVER, in reality, most elderly patients have a "sloping" hearing loss, meaning they struggle to hear high frequencies much more than low frequencies.

This has been explained by the fact that outer hair cells naturally provide a massive amount of amplification at the high-frequency base of the cochlea, Therefore, when EP is lost, the high-frequency regions have much more to lose.

What is Cochlear Synaptopathy?

‘Hidden Hearing Loss’ = the destruction of auditory nerve synapses on the inner hair cells

There are still enough auditory nerve fibres left to detect sound (so appear normal in auditory detection threshold tests)

Animal Study of Cochlear Synaptopathy

Liberman et al (2009) =

Mice were exposed to loud noise just once and for a few hours. Their hearing was tested over the next few days. Initially they displayed a “temporary threshold shift” as many humans do after a noisy concert of nightclub; their tone thresholds increased by 10-20 dB for a few days and then they returned to normal.

HOWEVER, despite normal thresholds, autopsies revealed the mice had suffered permanent, catastrophic destruction of their inner hair cell synapses

Is Synapse Loss the same in animals and humans?

The number of IHC synapses in human cadavers declines with age at death.

HOWEVER, tasks such as interaural phase difference detection and amplitude modulation detection remain intact, despite fewer synapses = this implies we don’t loose enough nerve fibres to make a significant difference