Chapter 20: Disorders of Hearing and Vestibular Function

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome back to the Deep Dive.

Our mission today, well, it's conquering a massive amount of crucial pathophysiology, all centered on the ear.

It is a lot to cover.

For this deep dive, we've really tried to take the complex mechanisms detailed in Chapter 20 and boil them down into high yield actionable knowledge for you.

Right.

We are talking about the ultimate sensory multitasker.

I mean, the ear handles two completely different critical functions,

capturing and transmitting sound and maintaining our entire sense of balance, our equilibrium.

And that dual function, that's exactly why this stuff can feel, well, overwhelming sometimes.

Our mission here is really precision.

Precision.

Good word.

Yeah.

We need a clear structured guide so you can confidently navigate the key distinctions.

You know, the ones that fundamentally change treatment plans.

Like what specifically?

Well, you absolutely have to be able to tell the difference between acute otitis media and otitis media with effusion, or identify if hearing loss is conductive or sensorineural and figure out if vertigo is coming from a peripheral source, like in the ear itself, or maybe a central lesion in the CNS.

That specificity, that's the core focus of this deep dive.

Okay.

Let's unpack this then.

Starting literally from the outside in with the disorders of the auditory system.

What's the sort of biggest threat to sound conduction right there in the external ear?

Well, probably the simplest and thankfully most reversible issue is usually impacted seramin, earwax.

Yes.

Good old earwax.

Right.

The external acoustic meatus, that's the ear canal.

It's lined with glands producing this protective stuff.

And while it's essential, you know, for filtering debris, if it gets impacted, well, it creates a literal blockage.

A physical block.

Exactly.

And that blockage leads straight to a reversible conductive hearing loss.

Sometimes people feel fullness, pain, or even weird reflex symptoms like vertigo or tinnitus just from the pressure on the eardrum.

Okay.

So if earwax is causing a temporary conductive loss, what's the Yeah.

That would be otitis externa or OE.

This is basically inflammation of the ear canal itself.

Right.

It's typically caused by bacteria like pseudomonas, sometimes fungi that just love it when it stays moist and humid in the canal.

Makes sense.

And the Hallmark clinical sign, the thing that tells you instantly it's OE, is that severe pain when you wiggle the outer ear, the pinna, or press on that little cartilage bump, the tragus, the inflammation's right there.

I got it.

Okay.

Moving inward, past the eardrum, we hit the middle ear,

that air -filled space with those tiny little bones, the ossicle.

Valleys and pieces, stapes.

Yeah.

They amplify the sound.

Right.

And the whole transmission relies on that last one, the stapes, connecting to the oval window to push vibrations into the inner ear fluid.

And connecting the middle ear back to the nasopharynx is the absolutely critical Eustachian tube, the ET.

I think its function is maybe the most underappreciated part of this whole system.

Really?

Why is that?

Well, it has three essential jobs.

First, there's the pressure equalization we all notice on planes, right, when your ears pop?

Yep.

Definitely notice that.

Second, it provides drainage for any secretions produced in the middle ear.

And third, it actually protects the middle ear from stuff coming up from the nasopharynx, like secretions or pathogens.

And dysfunction there in the ET, that's kind of the gateway to middle ear infections, right?

Especially in little kids.

That's a huge factor, yeah.

Because their tubes are shorter, wider, and more horizontal than in adults, it's just, well, much easier for bacteria and secretions to travel up into that middle ear space.

Makes sense anatomically.

And when those tubes fail functionally, we run into immediate issues like barotrauma.

That's pain and damage from rapid pressure changes.

Like on a plane with a cold.

Exactly.

If the ET is blocked, that pressure difference across the eardrum can cause intense pain, sometimes bruising, or even a rupture of the tympanic membrane itself.

Ouch.

Okay, this leads us right into otitis media.

And this distinction, you flagged it earlier, this is where we absolutely must be specific.

Acute otitis media versus otitis media with a fusion.

Why is telling these two apart so critical?

It really comes down to antibiotic stewardship.

It's crucial.

Acute otitis media, or AOM, that's the full -blown active infection.

The nasty one.

Right.

It's defined by abrupt onset, signs like fever, ear pain, otalgia, and usually a history -suggesting infection.

When you look with an otisope, you're looking for a bulging, red, inflamed eardrum.

Significantly decreased mobility, too.

Okay.

Bulging, red, painful.

That's AOM.

But what if we see fluid in a fusion in that middle ear space without all those acute signs?

No fever, no intense pain.

That's OME, otitis media with a fusion.

Precisely.

That fluid is often sterile, actually.

It might hang around for weeks after a cold, or even a resolving AOM.

So the look is different, too.

Yeah.

The membrane in OME might look kind of amber or cloudy.

Maybe you can see air fluid levels behind it.

But it's not acutely inflamed or bulging like in AOM.

And the clinical importance.

Since OME usually resolves on its own, hitting it with antibiotics is unnecessary.

It just drives resistance.

That's why making that distinction is so important day to day.

We should probably also mention the risk factors for AOM, especially in babies, feeding positions.

Absolutely.

Things like supine bottle feeding lying down with a bottle daycare attendance.

And unfortunately, smoking exposure in the home.

Those are all big factors.

Right.

And before we leave the middle ear,

complications.

There are some serious ones, though may be rare.

Definitely need to mention them.

Mastoiditis is a big one.

That's a suppurative infection spreading to the mastoid air cells behind the ear.

Rare, but serious.

And cholesteatoma, that's like an epidermal cyst, kind of pearl -like, that forms from chronic infection or E .T.

dysfunction.

It can erode bone.

OK.

And one more middle ear issue, otosclerosis.

Right.

Otosclerosis.

This is a progressive condition, often hereditary.

Basically, new spongy bone forms around the stapes footplate and the oval window.

So it jams the stapes.

It effectively fixes the stapes in place.

Yeah.

Prevents it from vibrating properly.

And that leading to progressive conductive hearing loss.

Interestingly, and sort of grimly, this process often seems to accelerate during pregnancy.

Hormonal influences, maybe?

Wow.

OK.

Now,

here's where it gets really interesting as we move into the inner ear.

The mechanics of sound transduction.

Yeah.

The deep stuff.

So the inner ear houses the cochlea for hearing, and then the vestibule and semicircular canals for balance.

For hearing, we're zeroing in on the cochlea and that amazing receptive structure,

the organ of corti.

And the cochlea operates in this remarkable fluid environment.

It's all about fluid mechanics.

There are two key fluids.

Paralim, which is high in sodium, baiting the outside of the membranous labyrinth.

OK.

And then endolymph, which is inside the scala media where the organ of corti sits.

And endolymph is unique.

It's very high in potassium.

High potassium.

Why is that so critical?

Well, that high potassium concentration, relative to the surrounding paralymph, creates a really significant electrical potential across the hair cell membrane.

It's called the endolymphatic potential, around positive 80 millivolts.

OK, plus 80 millivolts.

So what does that potential do?

It basically supersensitizes the hair cells in the organ of corti,

makes them incredibly responsive to even the tiniest movement of fluid caused by sound vibrations.

Ah, so it primes them.

Exactly.

So when sound enters, the fluid moves, the basilar membrane vibrates at specific spots, depending on the pitch, that bends the stereocilia on the hair cells, and that mechanical bending opens ion channels, generating the neural signal that travels up the auditory nerve.

It's incredibly elegant.

It really is.

So naturally, any disruption to that complex system, that leads to problems like tinnitus, that ringing or buzzing sound.

Yeah, tinnitus is a big one.

But it's important to remember tinnitus is a symptom, not a disease itself.

Right.

We usually classify it two ways.

Objective tinnitus, which is actually pretty rare.

That's a sound someone else can sometimes detect, often related to vascular things like turbulent blood flow near the ear, pulsatile, usually.

But the vast, vast majority is subjective tinnitus, just perceived by the person.

This usually stems from damage somewhere along the auditory pathway, the sensory receptors, the nerve itself.

Noise exposure is a huge cause, but also impacted seramin, certain drugs, even some foods or stimulants can trigger it in susceptible people.

Okay.

Now, when we talk about actual hearing loss,

we use the decibel scale for severity, right?

Mild starts around 26 DOB loss, up to profound loss over 91 dB.

But clinically, the most important classification is where the problem is.

Absolutely.

That dictates everything.

We basically have three main types.

First is conductive hearing loss.

Like the earwax or the otosclerosis we mentioned.

Exactly.

It's a problem with sound simply getting to the inner ear.

Some mechanical issue in the middle ear is blocking or dampening the sound transmission.

The good news here is it's often medically or surgically fixable.

Okay.

That's conductive.

Then there's the, well, usually more serious type.

Sensory neural hearing loss.

Yeah.

Or sometimes called perceptive loss.

This isn't a transmission problem.

It's damage to the delicate inner ear structures, specifically the hair cells in the cochlea or the auditory nerve itself.

And what causes that kind of damage?

Lots of things.

Intense noise exposure is a big one.

Severe infections like meningitis.

Certain tumors pressing on the nerve.

And crucially, something you must know is ototoxic drugs.

Ototoxic drugs, right.

Which ones should listeners really remember?

Key classes to burn into your memory.

Aminoglycoside antibiotics like gentamisin.

Loop diuretics like furosemide, especially at high doses or IV.

And high dose salicylates like aspirin.

These can cause permanent damage because, well, once those hair cells are gone, they don't typically regenerate in humans.

That irreversibility.

That really hits home with age -related hearing loss, doesn't it?

Presbycusis.

It does.

Presbycusis is that gradual, usually bilateral, high -frequency sensorineural loss we see in older adults.

And their specific complaint is often really telling.

What do they typically say?

They often say, I can hear you.

I just can't understand you.

Especially in noisy environments.

They hear the volume, but the clarity, the speech discrimination is poor.

Why that specific difficulty with understanding speech?

It's because the hair cells that detect the high -frequency sounds are usually the first ones to go.

And those high frequencies carry the information for consonant sounds like S, F, T, K.

Without those sharp consonant sounds, speech becomes muffled, harder to distinguish words.

It's like listening to someone talking with marbles in their mouth, even if the volume is okay.

Low frequencies, vowels, are usually preserved longer.

That makes perfect sense.

So to diagnose these types, differentiate them, we obviously use audiograms now, the gold standard.

But those classic tuning fork tests, the Weber and Wren, they still have a place, right?

Why are they useful?

Oh, absolutely essential, especially for a quick bedside assessment.

They immediately give you clues about the location of the problem.

How does the Weber work again?

Fork on the forehead.

Yep, vibrating tuning fork on the midline of the forehead or top of the head.

You ask the patient where they hear the sound loudest.

If it lateralizes sounds louder in one ear, it tells you something.

If it lateralizes to the ear with the hearing loss, that suggests a conductive loss in that ear.

The blocked ear hears the bone conduction better.

If it lateralizes to the better hearing ear, that suggests a sensorineural loss in the opposite, worse ear.

Okay, clever.

And the Wren test,

comparing air and bone.

You compare how well they hear the fork held next to the ear canal, air conduction AC, versus placed on the mastoid bone behind the ear, Wren conduction BC.

Normally, AC is better than BC.

Air conduction should be louder or longer.

Exactly.

But if they have a conductive loss, the bone conduction BC will actually be heard better or longer than the air conduction AC, because the sound bypasses the middle ear blockage when going through the bone.

So BCAC points right at a conductive problem.

It's quick, low tech, but gives vital information.

Brilliant.

Okay, let's pivot now.

Away from hearing, towards balance,

the vestibular system.

What's its main job?

Its main job is essentially keeping us oriented in space.

It manages postural orientation, perceives motion, and crucially, it integrates inputs from our eyes and our body's proprioceptors to stabilize our gaze and maintain balance.

And the hardware for this?

In the inner ear?

Yep.

The peripheral vestibular apparatus lives right next to the cochlea.

It consists of three semicircular canals and two otolithic organs, the utricle and the saccule.

Okay, canals and otolith organs.

What do they each detect?

The three semicircular canals are oriented in different planes, roughly perpendicular to each other.

They're designed to detect rotational movement or angular acceleration, like when you turn your head, nod, or tilt side to side.

It works by fluid and a limb moving within the canals deflecting hair cells.

Okay, so canals rotation.

What about the otolith organs, utricle and saccule?

Those detect linear movement.

Think acceleration or deceleration in a straight line, like in a car or an elevator, and also the pull of gravity, head tilt relative to gravity.

How do they do that?

They contain specialized areas with hair cells, and overlying these hair cells is a gelatinous membrane containing tiny calcium carbonate crystals.

These are the otoliths, sometimes called ear stones.

Otoliths, right.

When your head accelerates linearly or tilts, gravity pulls on these dense otoliths, causing that membrane to shift, which bends the underlying hair cells.

That bending signals the brain about the linear motion or head position.

Fascinating.

So these balance signals are critical for reflexes, right?

By keeping our eyes steady when we move.

Absolutely.

The key ones are the vestibulo -ocular reflexes, or VORs.

These are incredibly fast reflexes that move your eyes in the opposite direction of your head movement, allowing you to maintain a stable visual field even when you're moving around.

And if that system goes haywire, we can see the nystagmus, those involuntary eye movements.

Exactly.

Nystagmus is that rhythmic jerking movement of the eyes.

There's physiologic nystagmus, which is normal part of the VOR when tracking objects or doing large head movements.

But pathologic nystagmus occurs without appropriate stimulation, indicating a problem in the vestibular system or its central connections.

The pattern can give clues to the location.

And the main symptom someone feels when the system is off is vertigo.

We need to be really clear on this definition.

Yes, please.

Vertigo is specifically an illusion of motion, usually spinning or whirling.

It is not just feeling lightheaded, dizzy, or like you're going to faint.

That's more likely presyncope, often tardiovascular.

Got it.

Illusion of motion.

Can it feel different ways?

Yeah.

We talk about subjective vertigo, where the person feels like they are moving or an objective vertigo, where it feels like the environment is moving around them.

The key is that false sense of motion.

And just like hearing loss, we classify vertigo based on location, right?

Peripheral versus central.

Correct.

Peripheral vertigo originates from problems in the inner ears, vestibular structures, or the vestibular nerve itself.

This is the most common type.

And how does that usually present compared to central?

Peripheral vertigo tends to be much more intense, often severe, and usually comes in episodes.

It's frequently associated with other ear symptoms, like hearing loss or tinnitus, because the auditory and vestibular parts are right next to each other.

Nausea and vomiting are common, too.

Okay.

Intense, episodic ear symptoms often present.

And central vertigo.

Central vertigo stems from problems within the central nervous system, the brain stem, or cerebellum, usually.

Think strokes, tumors, multiple sclerosis.

Symptomatically, it's often milder, maybe more constant or persistent, and less likely to have associated hearing loss.

It might have other neurological signs, though.

Got it.

Okay, let's quickly hit the three most common peripheral vestibular disorders, starting with the absolute most frequent cause of pathologic vertigo.

BPPV.

Benign paroxysmal positional vertigo.

BPPV.

It's almost a beautiful, albeit annoying example of mechanics gone slightly wrong.

Remember those tiny otoliths, the crystals in the utricle and saccule?

Well, sometimes, often due to age or minor head trauma, some of these crystals can get dislodged.

And these loose crystals, now sometimes called penoliths, can drift into one of the semi -circular canals, most commonly the posterior one.

So they're in the wrong place.

Exactly.

And because the canals are designed to detect fluid movement for rotation, when the person changes their head position relative to gravity,

like rolling over in bed, looking up or bending down these loose canalis shift within the canal fluid.

And that triggers the hair cells incorrectly.

Precisely.

It sends a false signal of intense rotation to the brain, causing a brief, usually less than a minute, but often violent burst of vertigo and associated nystagmus.

It only happens with those specific position changes.

But the good news is?

The good news is it's often easily treated.

We can use specific bedside maneuvers, like the Epley maneuver, to physically reposition those canalis out of the canal and back into the utricle where they belong.

Often provides immediate relief.

Amazing.

Okay, next up, Meniere disease.

This one has a classic triad of symptoms.

It does.

The triad for Meniere's is, one, fluctuating, often low -frequency sensorineural hearing loss.

Two, episodic, often severe and prolonged vertigo attacks lasting hours.

And three, tinnitus, usually roaring or low -pitched, often accompanied by a sensation of fullness or pressure in the affected ear.

And what's thought to be going on pathologically?

It's linked to an overaccumulation or distension of the endolymph within the inner ear's membranous labyrinth, a condition called endolymphatic hydrops.

Hydrops.

Too much fluid pressure.

Essentially, yeah.

The exact cause isn't fully understood, but this pressure buildup is thought to periodically rupture the delicate membrane separating endolymph and perilymph, mixing the fluids and causing those sudden debilitating attacks that force the person to lie completely still until it passes.

Sounds awful.

Okay, third common one.

Acute vestibular neuronitis, sometimes called labyrinthitis, if hearing is also affected.

Right.

This is basically inflammation of the vestibular nerve itself, or sometimes the labyrinth.

It's often thought to follow a viral illness.

And the symptoms?

It causes a very acute onset of severe, sustained vertigo -lasting days, not just minutes or hours like BPPV or Meniere's attacks, along with nausea and vomiting.

Usually there's no hearing loss if it's just neuronitis affecting the nerve.

The intense vertigo slowly improves over days to weeks as the inflammation resolves and the brain starts to compensate.

How do we diagnose these vestibular issues generally?

Any specific tests?

Well, the history is absolutely key.

Timing triggers associated symptoms.

The physical exam includes looking for nystagmus and simple tests like the Romburg test.

Romburg.

Standing with eyes closed.

Yeah, you have the person stand feet together first with eyes open, then closed.

If they become significantly unstable only when they close their eyes, it suggests they're overly reliant on vision and might have a vestibular or proprioceptive deficit.

Closing the eyes takes away the visual input, unmasking the balance problem.

Makes sense.

Any higher tech tests?

Sure.

Video nystagmography, or VNG, is common.

It uses infrared goggles to record and measure eye movements, nystagmus, in response to various stimuli positional changes,

visual tracking, caloric testing, where warm or cool water air is introduced into the ear canal to stimulate the vestibular system.

It helps pinpoint the location and severity of the lesion.

And treatment.

Beyond BPPV maneuvers.

Often involves medications to suppress the acute vertigo and nausea like antihistamines, meclizine, or antibiotics.

But long term, vestibular rehabilitation exercises are really important.

These are exercises designed to help the brain adapt and compensate for the damaged vestibular input by strengthening reliance on visual and proprioceptive cues.

Okay, so let's try to wrap this up.

What does this all mean for the listener?

We've journeyed from the outer ear canal right through to the inner ear's balance organs.

Yeah, we've seen how the outer and middle ear mostly act as sound transmitters, which is why conductive loss there is often fixable.

We dove into the inner ear, how that potassiumage endolymph is key for powering the hair cells for hearing.

Right, the sensor neural side.

And how the vestibular system cleverly separates out rotational versus linear motion detection using canals and otoliths.

Clinically, what's the absolute bottom line takeaway?

I think it's that knowing these differences, these distinctions we've hammered on, it fundamentally changes your management path.

Knowing if your patient have AOM versus OME, tells you whether to reach for antibiotics right away or practice watchful waiting.

Crucial distinction.

And knowing if hearing loss is conductive versus sensorineural, that informs the entire prognosis.

Is it potentially reversible surgery or medication, or are we looking at irreversible damage needing hearing aids, cochlear implants, and immediate intervention planning?

The implications are huge.

Absolutely.

And maybe a final thought for people to chew on after this deep dive.

Maybe consider the recovery process after someone loses vestibular input completely on one side, say after vestibular neuronitis or surgery for a tumor.

Despite the initial incredibly violent vertigo and imbalance, the body adapts remarkably quickly.

Within weeks or months, the central nervous system learns to compensate, relying more heavily on the remaining vestibular input from the other ear, plus visual and proprioceptive cues.

People often get back to near normal function, even driving again.

That rapid adaptation just highlights the incredible plasticity of the brain.

Its ability to rewire and compensate for even pretty profound sensory deficits is truly amazing.

It really is a powerful lesson in CNS compensation.

Well, thank you so much for joining us for this deep dive into the sometimes confusing, but always fascinating world of auditory and vestibular pathophysiology.

We really hope you feel better informed and ready to tackle those clinical distinctions out there.