The Physiology of the Outer and Middle Ear

Will Styler - LIGN 113

Today’s Plan

• The Tympanic Membrane

• The Acoustic Reflex

• Conductive vs. Sensorineural Hearing Loss

• Middle Ear Implants

The Physiology of the Tympanic Membrane

Review: The Tympanic Membrane

The Tympanic Membrane is complex

• Around 0.1mm thick and 10mm in Diameter

• Three layers

  • Outer Cutaneous Layer

  • Middle Fibrous Layer

  • Inner mucous membrane

The tympanic membrane can be perforated!

This can happen in many ways

• Infection

  • Acute Otitis Media or Externa

• Physical Trauma

  • Q-Tips, Foreign objects, bad wax removal, head trauma

• Barotrauma

  • Explosions, Scuba diving, air travel, or sudden negative pressure

The tear is usually in the Pars Tensa

This is diagnosed by looking at the TM

• Or by measuring pressure changes when you artificially pull a vaccuum in the OE

• Or by looking at the amount of sound which is absorbed by the ear

  • More on these later

The tympanic membrane usually heals itself

• It usually heals naturally

  • They’ll often heal within a month or so

• Perhaps nature can be helped!

• It can also be fixed surgically

Tympanoplasty

Sometimes you perforate the TM on purpose!

• Myringotomy is the cutting of the pars tensa to relieve pressure

• But sometimes you want it to last!

Tympanostomy

Tympanostomy tubes

• Simple, outpatient procedure

  • Children are usually anesthetized for this

  • Adults power through

• Not particularly painful

• They fall out on their own in 6-9 months

Tympanostomy Downsides

• Insertion is non-fun

• You can’t swim without earplugs

• Liquids can get into the ME

• Your ears don’t pop

  • But they don’t need to!

So, the TM is magical

• And really, really important!

• Be kind to your TM!

The Acoustic Reflex

Anatomy Review

Anatomy Review

The Acoustic Reflex

• Also referred to as the ‘Stapedius Reflex’

• Causes a temporary conductive hearing loss

  • 15-20dB Hearing loss

• There’s debate as to whether the Tensor Tympani is involved in humans

  • Some say it’s only the Stapedius muscle!

How does it work?

What triggers it?

• Sound >60 dB above the hearing threshold

  • Different sources have a range of 60-90 dB

• Speech also triggers it

  • Chewing too!

You must have hearing to have an acoustic reflex!

• People with severe cochlear hearing loss have no acoustic reflex

• Severe conductive loss can reduce/prevent it

• Same thing with serious cochlear nerve damage

• This means that it’s triggered in the pons, not the ear!

How long does it take?

• 10ms to start, roughly

• 100ms+ to reach full tension

  • Compare to 300 microseconds blast from a gunshot

• Tension reduces within a few seconds

  • But there’s some role in long term noise exposure

• This is now in use in cars

Why do we care?

• It’s awesome

• It’s super diagnostic!

• Check out this paper giving more detail!

Speaking of diagnosis

• We want to know where hearing loss is coming from

Conductive vs. Sensorineural Hearing Loss

Sound has a long path with many points of failure

• External obstructions

• Middle Ear Pathologies

• Cochlear Physiology issues

• Different causes of HL get different names!

Conductive Hearing Loss

• Caused by problems between the world and the Oval Window

• External obstructions and infections

• Fluid, OM, Pressure issues

• Tumors in the ME

• TM perforations, ossicular dislocations

Sensorineural (‘Cochlear’) Hearing Loss

• Caused by problems beyond the Oval Window

• Issues inside the Cochlea

• Issues with the hair cells

• Issues with other physiological enablers of hearing

• Issues with the auditory nerve

• Neural issues

These are very different!

• Conductive loss is often more treatable medically

• Conductive loss can generally be restored to a greater extent

• With profound sensorineural loss, nothing else matters

  • Doesn’t matter what the middle ear is doing if nobody’s listening

• … but a complete conductive loss is bad news!

  • The complicated parts are working, but the sound can’t get there!

Complete conductive loss is possible

• Missing ossicles

• Irreparable TM

• Missing or scarred-shut ear canals

What can we do to help people with conductive loss?

• Hearing aids are one approach

  • “Make the incoming signal louder”

• … but if we want to get fancy, we can use…

Middle Ear Implants

How can we replace the middle ear?

Let’s vibrate the cochlear fluid ourselves!

Some devices vibrate the Ossicles or Round Window

• ‘Soundbridge’

• ‘Otologics’

Some devices vibrate the skull

• The ‘Baha’ device

The Envoy Esteem

• “Let’s use the TM as a microphone, and then vibrate the stapes”

Middle Ear Implants

• Offer some increased convenience relative to hearing aids

• Don’t look like hearing aids

• Can work where there are no ossicles or TM or ear canal

• Offer another choice for conductive hearing loss

Wrapping Up

• The Tympanic Membrane is awesome, and you want it intact

• The Acoustic reflex is awesome, but it won’t save you

• Conductive and Sensorineural Hearing Loss are different

• Middle Ear Implants are really cool

Next time

• We’ll think more about the OHCs and the Cochlear Amplifier

Thank you!