Chapter 19: Disorders of Visual Function

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Welcome to the Deep Dive.

We're skipping the summary today and getting straight into the real mechanics of altered health states.

And today, it's a big one.

We're mapping out the path of

the entire visual system, pulling our insights from, well, the foundational text.

A really crucial area.

I mean, globally, the numbers are just staggering.

The World Health Organization says 161 million people are visually impaired, and that's not even counting another 153 million with uncorrected refractive errors.

So our goal here is to really understand the why behind these conditions.

We'll work our way, you know, systematically from the eyes, outer defenses, the lids, and such, right back to the visual cortex in the brain.

Okay, let's do it.

Starting right at the front line, then the accessory structures.

Eyelids, lacrimal system, the body's first defense,

and hydration crew.

So eyelid weakness first.

The terminal here is pisus.

That's P -T -O -S -I -S.

It's that physical droop of the upper eyelid.

It signals weakness in the levator muscle, right?

And that often points towards cranial nerve third damage, the oculomotor.

Exactly, CN3.

Or sometimes it's a sympathetic pathway issue, like in Horner syndrome.

That drooping is a key sign.

And once you've assessed weakness, you look at positioning.

So, entropium.

That's where the lid margin turns inward, correct?

Yes, inward.

And that's bad news because the lashes rub right against the cornea, causes irritation, often from scarring or just aging.

Okay, and the opposite?

That's atropian.

The lower lid turns outward, sort of averts.

You see the conjunctiva, and it leads to excessive tearing, usually CNF at weakness, the facial nerve, or again, aging.

Got it.

Now, inflammation.

The common lumps and bumps.

Blepharitis is the chronic one, isn't it?

Both eyes along the lashes.

Chronic bilateral inflammation, often linked to seborrhea or staph aureus.

But yeah, the ones that often get mixed up are the stye and the chelation.

Right.

The hortiolum, that's the stye.

Yeah, the stye, that's your acute infection.

Usually, staph aureus, again, it's painful, red, swollen, you know, hot to the touch.

Acute, painful infection.

How's that different from a chelation, then?

Ah, the chelation is completely different in mechanism.

It's a chronic inflammation.

It forms this little, usually painless, nodule because a meibomian gland gets blocked.

And here is the clinical pearl, right?

Because it's not an infection, just an obstruction in inflammation.

Antibiotics are useless.

Exactly.

Don't need them.

Big difference in management.

That distinction alone is super helpful.

Okay, moving past the lids,

the lancrimal system.

Piers aren't just water, are they?

Oh, definitely not.

They're mostly water, sure, about 98%.

But packed with salts, protective enzymes like lysozymes and antibodies, IgA, IgG, IgE, really vital for cleaning and defense.

And when that system breaks down, we get dry eye disease.

Right.

And it's multifactorial, as they say.

Generally, you categorize it as either aqueous deficient, not making enough watery tears, or evaporative, where the tears disappear too quickly.

A common cause of that evaporative type.

Meibomian gland dysfunction again.

And the symptoms are that classic gritty feeling.

Burning.

Yeah, gritty, burning, sometimes blurred vision that might even improve with blinking.

Very common complaint.

Okay, let's move inward now.

Past the defenses,

past the tears,

to the structures handling light transmission.

Conjunctiva, cornea, uvealtract.

Starting with conjunctivitis, pink eye.

Probably the most common thing people encounter, the absolute key here for diagnosis before even looking closely, is the level of pain.

Conjunctivitis should only cause mild discomfort, maybe itching.

If someone comes in reporting severe eye pain, alarm bells should ring.

Absolutely.

You have to think cornea, or maybe even acute glaucoma.

That's a different league So assuming it's just conjunctivitis, how do you tell the types apart, viral versus bacterial?

Discharge is usually the giveaway.

Viral often add no virus, maybe linked to a cold, gives you lots of watery tearing minimal gunky stuff.

But bacterial, like Staph aureus.

Then you see that thick mucopurulent or even purulent discharge.

Yellowish, greenish.

You mentioned Mr.

Powell's case earlier, eye stuck together like glue.

That's classic bacterial.

Right, and there's a really serious one too.

Yes, hyperacute conjunctivitis.

Often caused by Neisseria gonorrhoe.

It comes on incredibly fast, very inflamed, tons of pus.

That's a sight -threatening emergency needing immediate treatment.

Okay, let's focus on the cornea itself.

Transparent, no blood vessels, gets nutrients from the aqueous humor and tears.

It's got layers, right?

And membranes.

Exactly, and those membranes are crucial.

You have the Bowman membrane near the surface.

It's a barrier to infection.

But here's the catch, it doesn't regenerate.

So if it gets scarred...

Vision is permanently impaired by that opaque scar.

Contrast that with the deeper decimit membrane.

That one does regenerate well after injury.

Big difference in outcome.

Injury leads to keratitis, inflammation of the cornea, and you mentioned contact lenses.

Huge risk factor.

Contact lens wear is the major culprit for infectious keratitis.

We really have to hammer that point home.

And specific infections.

Herpes seems like a big one.

Herpes simplex keratitis is the leading cause of corneal scarring worldwide.

The virus, you know, just hides out in the trigeminal ganglion, pops out periodically.

Causing repeat attacks.

Nasty.

Yeah.

What else?

Well, Viratella zoster ophthalmicus, that's shingles.

The chicken pox virus reactivating, but specifically in the ophthalmic branch of the trigeminal nerve, C and V.

And then there's a canthamema keratitis.

Rare, thankfully.

Linked to contaminated water, think swimming pools, tap water with contacts.

What's special about that one?

The pain.

Patients report pain that is just way, way out of proportion to what you actually see when you examine the eye.

That mismatch is a big clue.

Okay.

Deeper still.

The uveal tract, that's the middle vascular layer.

Choroid, ciliary body, iris.

That's the one.

It provides nourishment, and importantly, it absorbs scattered light thanks to its pigment.

Inflammation here is uveitis.

What causes uveitis?

It can be infections, autoimmune diseases are a big cause, or sometimes even metastatic cancer cells lodging there.

It's serious stuff.

And speaking of pigment, albinism.

That's a genetic lack of pigment, right?

Exactly.

It's a deficiency in the tyrosinase enzyme needed to make melanin.

So the iris and choroid are unpigmented.

The result.

The eye can't absorb stray light properly.

This causes severe photophobia,

painful sensitivity to light, and really core visual acuity because the light scatters everywhere inside.

Wow.

Shows how vital that internal darkness is.

Which brings us neatly to the pupil and controlling light entry, the pupillary reflex.

Myosis, constriction.

That's parasympathetic control via cranial nerve the third.

Madriasis, dilation.

That's the sympathetic nervous system kicking in.

And we test it with a direct and consensual response.

Shine a light in one eye.

And that pupil constricts, that's the direct response.

But the pupil in the other eye should also constrict, that's the consensual response.

Tests the whole pathway.

And a clinical example.

Horner's syndrome.

Right.

Horner's interrupts the sympathetic supply to the eye.

So you lose the ability to dilate properly on that side.

The pupil stays relatively constricted.

It's a subtle but really important neurological sign.

Okay.

Let's pivot now.

Intraocular pressure.

Glaucoma.

This is a big one.

And the definition has changed, hasn't it?

It's not just high pressure anymore.

Correct.

It's now fundamentally understood as an optic neuropathy.

The damage to optic nerve is the key feature, often related to increased pressure, but not solely defined by it.

So how is that pressure normally controlled?

Talk us through the fluid dynamics.

Okay.

Think of it like plumbing.

Aqueous humor, the fluid is made by the ciliary body in the posterior chamber behind the iris.

It flows through the pupil into the anterior chamber, the space between the iris and cornea.

Then it has to drain out.

Through.

Through the trabecular mesh work, which is like a filter system in the angle where the iris meets the cornea.

From there, it enters the canal of Schlem, a channel, and gets back into the bloodstream.

Normal pressure, IOP, is about 9 to 21 millimeters of mercury.

And in glaucoma, that system fails, pressure rises, and damages the optic nerve.

What do we see when we look inside?

On ophthalmoscopy, you see changes to the optic disc.

Specifically, the optic cup, the depression in the center gets larger and deeper, and the nerve tissue looks pale.

That's optic atrophy setting in.

Let's break down the types.

Primary open angle glaucoma, DOAG.

Most common.

Right.

But the name is confusing.

The angle is open.

Exactly.

The drainage angle itself isn't physically blocked by the iris.

The problem is within the trabecular mesh work.

It becomes abnormal, resistant to outflow.

Think of it like a clogged drain filter.

And it sneaks up on you.

Totally insidious.

POAG is usually chronic, progressive, and completely asymptomatic until significant vision loss has occurred, often starting in the periphery.

Major risk factors are age over 40, being of black race, and long -term steroid use.

How do we treat it, then, if it's so quiet?

Chronic management.

Eyedrops are the mainstay.

First choice is often topical beta blockers to reduce aqueous production.

Prostaglandin analogs are also very common.

They increase outflow through a secondary pathway.

Other options exist, too, like carbonic and hydrase inhibitors.

Okay, so that's the slow, silent type.

What about the opposite, the emergency?

Angle closure, glaucoma.

Ah, this is a dramatic acute event.

A true ophthalmic emergency.

Here, the angle is physically blocked.

How does that happen?

The anterior chamber angle is naturally narrow, and some people more common in those of Asian or Inuit descent.

If the people dilates mydriasis, the iris can bunch up at the periphery and completely block the trabecular mesh work.

Fluid can't get out at all.

And the pressure spikes.

Massively and suddenly.

The symptoms are intense.

Severe eye pain, often with nausea and vomiting, blurred vision, seeing halos or rainbows around lights.

Rainbows.

Why?

That's due to corneal edema.

The sudden high pressure forces fluid into the cornea, making it hazy and causing light to scatter like that.

You'll also see a red eye, and the pupil might be fixed in a mid -dilated position.

Unilateral headache is common, too.

Sounds awful.

Treatment.

Immediate emergency treatment to lower the IOP drastically drops, sometimes IV medication.

Once the pressure is controlled,

the definitive treatment is usually a laser peripheral iridotomy creating a small hole in the iris so fluid can bypass the blocked angle.

Okay.

Emergency averted.

Let's move to the lens now.

Its job is focusing light, right?

Fine -tuning what the cornea starts.

Precisely.

The cornea does most of the heady lifting for refraction -bending light.

The lens adjusts for fine focus, especially for near objects.

So refractive errors.

That's about the eye's shape, mostly.

Largely, yes.

Hyperopia or farsightedness.

The eyeball is a bit too short, so light focuses behind the retina without accommodation.

Corrected with convex lenses plus lenses.

And myopia.

Your sightedness.

Eyeball's too long.

Light focuses in front of the retina.

Needs concave lenses minuses lenses to push the focus back.

And astigmatism.

That's in irregular shape.

Usually the cornea isn't perfectly spherical, more like a rugby ball.

Causes light to focus at multiple points, blurring vision at all distances.

Needs specially shaped lenses.

Okay.

And the lens's focusing ability.

That changes with age, doesn't it?

Presbyopia.

Yes.

Happens to almost everyone, usually starting after age 40.

It's about accommodation in the process where the ciliary muscle contracts, relaxing tension on the lens, allowing it to become more convex, more rounded for near vision.

That's controlled by Sia and the Thur, by the way.

So what goes wrong in presbyopia?

The lens itself changes, the fibers inside get thicker, less elastic over time.

So even when the ciliary muscle contracts, the lens just can't round up as much as it used to.

Near focus gets harder.

Hence, reading glasses.

Exactly.

Now the big lens pathology, cataracts, clouding of the lens.

How common is this?

Incredibly common.

It's the leading cause of age -related vision loss worldwide.

Something like 70 % of people over 75 have some degree of cataract formation.

What causes them?

Just aging.

Aging is the biggest factor of senile cataracts, but other things contribute.

Long -term UV light exposure, smoking, diabetes, prolonged corticosteroid use.

It's multifactorial.

You can also get traumatic cataracts from injury or congenital ones present at birth.

And what is the patient experience?

The main things are progressively blurred vision,

visual distortion,

and significant glare.

That cloudy lens scatters light all over the place, making bright lights really troublesome, especially at night.

Treatment is usually surgery.

Yes.

Cataract surgery is one of the most common and successful surgeries performed.

Usually fake emulsification where ultrasound breaks up the cloudy lens, it's suctioned out.

And then an artificial intraocular lens implant is put in its place.

Amazing technology.

Okay.

Let's journey deeper still to the very back of the eye.

The vitreous and retina.

Right.

The vitreous humor is that gel filling the large posterior cavity.

With age, tends to liquefy, kind of break down the little collagen fibers and cellular debris clump together.

And those are floaters.

Exactly.

Those little specks or cobwebs people see drifting around, usually harmless, but a sudden increase can signal problems.

And the retina itself, that's the light sensing tissue.

Yes.

The neural retina contains the photoreceptors, rods and cones, plus bipolar cells and ganglion cells that process the signal.

It sits on an outer pigmented layer.

Rods and cones.

Rods for night vision.

Right.

Rods handle scotopic vision, black and white, low light conditions.

They use a pigment called rhodopsin, which needs vitamin A.

Hence night blindness in vitamin A deficiency.

And cones.

Cones are for photopic vision, daytime color vision.

There are different types sensitive to different wavelengths.

Color blindness is usually an X -linked genetic thing where someone lacks one or more types of cones, most commonly red or green.

Okay.

Now some retinal pathologies.

Papildema sounds serious.

Edema of the optic disc.

Very serious.

Papildema, or a choked disc, is swelling of the optic nerve head where it enters the eye.

It's caused by persistently high intracranial pressure inside the skull.

That pressure gets transmitted along the optic nerve sheath and compresses the central retinal vein, sometimes the artery too.

It's a major red flag for things like brain tumors or other causes of increased ICP.

A late but critical sign.

Wow.

A window into the brain, really.

And speaking of systemic links, diabetic retinopathy.

The leading cause of blindness in working age adults in developed countries.

It's a microvascular complication of diabetes.

We usually divide it into two stages.

Non -proliferative first.

Right.

Non -proliferative or background retinopathy.

You see changes within the retina itself.

Tiny bulges on vessels called microanarysms, engorged veins, little hemorrhages, cotton wool spots, which are nerve fiber infarcts.

But the main cause of vision loss in this stage is usually macular edema swelling in the central part of the retina responsible for sharp vision.

And then it can progress.

To proliferative retinopathy, this is much more severe.

The proliferation refers to neovascularization, the growth of new abnormal fragile blood vessels.

Why is that bad?

New blood vessels sound helpful.

They're terrible.

They grow in the wrong places, often on the surface of the retina or into the vitreous.

They're leaky, so they bleed into the vitreous, causing floaters or sudden vision loss.

And worse, as they scar and contract, they can pull on the retina, causing tractional retinal detachment.

How do you manage diabetic retinopathy?

Number one is tight control of blood sugar and blood pressure.

For specific retinal problems, laser photocoagulation can be used to seal leaky vessels or destroy ischemic retina that's signaling for new vessel growth.

And anti -VEGF injections into the eye are huge now.

VEGF.

Vascular endothelial growth factor.

Exactly.

It drives neovascularization, so blocking it with drugs like Renobizumab or Bevacizumab can cause those bad new vessels to regress.

It's revolutionized treatment, especially for macular edema and proliferative disease.

Okay.

You mentioned retinal detachment.

Let's touch on that more.

How does it happen?

It's when the neurosensory retina of the layer with rods and cones separates from the underlying retinal pigment epithelium, which nourishes it via the choroid underneath.

The most common type is regmatogenous.

It starts with the vitreous gel shrinking and pulling away from the retina, which is normal with age, but sometimes it pulls hard enough to create a tear.

And fluid gets under.

Liquid vitreous seeps through the tear and gets between the retina and the pigment epithelium, lifting it off like wallpaper peeling from a wall.

What is the patient notice?

It's typically painless.

The classic description starts with flashes of lighter sparks, maybe a sudden shower of floaters from the tear or bleeding.

Then, crucially, a shadow or dark curtain appears in the peripheral vision, and progresses across the visual field as the detachment spreads.

And that's an emergency.

Absolutely urgent.

The photoreceptors are cut off from their blood supply, the choroid.

They start to die pretty quickly.

Needs urgent surgery, maybe laser or cryotherapy to seal the tear.

Sometimes a scleral buckle placed around the eye or injecting a gas bubble.

Pneumatic retinopexy.

Got it.

Another major one, especially in older adults,

age -related macular degeneration, AMD, affects central vision, right?

Yes.

AMD is the leading cause of irreversible central vision loss in older adults in developed nations.

It affects the macula, the part of the retina, for sharp, detailed straight -ahead vision.

Two main types.

Dry and wet.

Correct.

Dry AMD is more common, about 85 -90 % of cases.

It involves atrophy and degeneration of the pigment epithelium and photoreceptors in the macula.

You see characteristic yellowish deposits called drusen under the retina.

It progresses slowly, causing gradual blurring or distortion of central vision.

Unfortunately, there's no really effective treatment to stop dry AMD progression, though certain vitamin formulations might slow it slightly in some people, mostly monitoring.

And wet AMD.

Less common, but more severe and rapid.

This is exudative.

It involves choroidal neovascularization, abnormal blood vessels growing from the choroid layer underneath the retina, breaking through into the subretinal space.

Like in diabetic retinopathy, but from underneath.

Sort of, yes.

These vessels are leaky, they bleed, cause swelling, and ultimately lead to scar tissue formation right in the macula, causes rapid and severe central vision loss.

This one is treatable.

Yes.

This is where those anti -VGF injections have made a huge difference.

Injecting drugs like rinobizumab, bevacizumab, or a flibrocept directly into the vitreous can stop the leakage, regress the vessels, and often stabilize or even improve vision.

Needs repeated injections, though.

Okay.

And just quickly, a childhood condition, retinoblastoma.

The most common primary intraocular cancer in children caused by mutations in the RB1 gene.

The key sign parents or pediatricians might notice is leukocoria.

White pupil.

Exactly.

A white reflex from the pupil instead of the normal red reflex when you shine a light, like in a photograph flash, sometimes called a cat's eye reflex.

Any sign of that needs immediate referral.

Crucial point.

Okay, final section.

Let's trace the signal out of the eye and into the brain.

Neural pathways and eye movement.

Damage along the pathway causes predictable field cuts, right?

Very predictable.

It's all about anatomy.

If you have a lesion right at the optic chiasm, where the nasal fibers from each eye cross over, think of a pituitary tumor pressing up from below.

You lose the outer halves of vision.

Correct.

You lose the temporal visual field from both eyes.

That specific pattern is called bitemporal heminopia.

What if the damage is after the chiasm in the optic tract, or the lateral geniculate nucleus, LGN, in the thalamus, or the visual cortex itself?

Then you lose the same half of the visual field in both eyes.

For example, a lesion in the left optic tract causes loss of the right visual field in both eyes.

That's called homonymous heminopia.

And the most extreme case.

Cortical blindness.

If you have bilateral damage to the primary visual cortex in the occipital lobe, the person loses all conscious visual perception, even though their eyes might be perfectly healthy.

There's also that strange condition.

Yeah, that's fascinating.

Damage is usually in the visual association cortex.

The person can see perfectly well.

They can describe shapes, colors, patterns, but they cannot recognize or name what they're seeing.

They see a familiar object, like keys, but have no idea what it is until they touch it.

Mind -boggling.

Okay, lastly, controlling eye movements.

Trebismus or squint.

Misalignment.

Yes, an abnormality of eye coordination or alignment can lead to diplopia, double vision in adults.

And remember the nerves.

CNV eye, abducens, moves the lateral rectus muscle outward gaze.

CNYB truck layer moves the superior oblique.

And CNV oculomotor does all the rest up, down, inward, plus the eyelid and pupil.

Damage to these causes specific types of paralytic trebismus.

The big concern with trebismus in kids, though, is amblyopia, lazy eye.

Huge concern.

Amblyopia is decreased visual acuity in one eye or sometimes both.

That's not correctable by glasses alone and occurs because the brain didn't learn to see properly with that eye during early childhood.

Why would that happen?

Most commonly because of trebismus, the brain suppresses the image from the misaligned eye to avoid double vision.

Or due to unequal refractive error or something blocking vision like a congenital cataract, visual deprivation.

The crucial window for visual development is roughly the first five, six years of life.

If amblyopia isn't detected and treated during this critical period.

The vision loss can become permanent.

Exactly.

Treatment often involves patching the good eye to force the brain to use and develop the lazy eye.

Early screening and intervention are absolutely key.

Okay, we've covered a massive amount of ground from the eyelids and tear film.

Right through the pressure dynamics of glaucoma, the focusing issues with the lens and cataracts.

The retinal catastrophes like detachment and diabetic damage, AMD.

And finally, how the signals get processed or fail to get processed in the brain and how eye movements are controlled.

It really drives home how interconnected everything is.

The eye isn't isolated.

It reflects systemic health diabetes, hypertension,

even pressure inside the skull.

Absolutely.

And it highlights how vulnerable sight is and why timely intervention, especially catching things like amblyopia or retinoblastoma in childhood, is so incredibly critical for preserving this sense throughout life.

A complex system indeed.

Thank you for joining us for this deep exploration into the concepts of altered visual health states.