Chapter 20: Central Nervous System Pathology

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Okay, let's unpack this.

Welcome back to the deep dive.

Today, we are not just skimming the surface.

We are, uh, we're going all the way to the bottom of the pool.

We really are.

We're taking on what I think is arguably the most complex, the most intricate, and just the most unforgiving system in the entire human body.

We certainly are.

We are tackling the central nervous system.

Specifically, we are doing a comprehensive page -by -page walkthrough of Chapter 20, Central Nervous System Pathology.

From the USMLE Lecture Notes.

The 2017 Pathology Edition, that's the one.

The brain,

the spinal cord, I mean this is the hardware that runs the software of us.

I have the source material right here in front of me and I have to say looking at this stack,

it is dense.

It's a lot.

We're talking about pages of high -yield text, complex tables, MRI scans, and you know these gross pathology photos of brains that have definitely seen better days.

It's a lot to take in.

It is a beast of a chapter, you're not wrong.

But here's the thing about the

It's incredibly intolerant of injury, so it has these very predictable patterns of responding to damage.

Our mission today is to, well, to translate all those dense bullet points and static images into a kind of mental movie for you.

I like that.

We want to help you visualize exactly what's happening when a vessel bursts or, you know, when a protein misfolds, so you don't actually need the book open in front of you to get it.

That is the goal.

A shortcut to being well informed on this.

We are going to act as your tour guides through the chapter's exactological flow.

We're going to start with infections, so meningitis and encephalitis.

Right.

Then we'll move into cerebrovascular disease strokes, aneurysms,

the plumbing problems.

Or plumbing, exactly.

We'll tackle trauma and the terrifying concept of herniation.

Then we'll get into developmental issues, the demyelinating disorders like MS, the heavy hitters of degenerative disease like Alzheimer's.

And finally, we will wrap up with CNS tumors.

It's a very logical progression, if you think about it.

We go from the acute and inflammatory stuff like infections to the structural and vascular.

And then, you know, finally to the chronic degenerative and neoplastic changes.

So for this deep dive, I'm going to play the role of the student.

I've read the chapter, but I need you to help me connect the dots.

I'm looking for those high yield associations, the stuff that examiners just love, but I'm counting on you to provide the clinical reasoning and the why behind it all.

I am ready.

Let's get into the neurons.

All right.

Section

one, infections of the CNS.

We're kicking things off with meningitis.

I feel like everyone knows the word, but anatomically, what are we actually talking about here?

We do need to be precise.

Meningitis is inflammation of the leptomeninges.

Now you have to remember your anatomy.

Okay, lay it on me.

You've got the dura mater on the outside, which is tough as mother.

Then you have the arachnoid and then the pia mater, which basically shrink wraps the brain surface.

Meningitis specifically affects those two inner layers, the pia and the arachnoid.

The dura, the tough outer layer, is usually spared in standard meningitis.

Okay, and the text immediately makes this massive distinction right off the bat.

It separates meningitis into two main buckets,

acute aseptic meningitis and acute purulent meningitis.

Let's break an aseptic first.

I mean, to me, aseptic sounds clean,

sterile.

It sounds that way, doesn't it?

But in this context, aseptic usually just means bacterial cultures are negative.

So you take the spinal fluid, you try to grow bacteria in a dish and nothing grows.

And that's because this is almost always viral.

It's leptomeningel inflammation caused by viruses.

And is there a specific culprit we need to know, the main one?

The absolute most common cause, the one to tattoo on your brain, is enterovirus.

Enterovirus, got it.

Now, if we could shrink down and look at this under a microscope, what does the battlefield look like in viral meningitis?

What kind of cells are we seeing?

You are going to see a lymphocytic infiltration.

That is the hallmark.

Lymphocytes are the foot soldiers of the viral response.

So not neutrophils?

Not neutrophils.

These lymphocytes infiltrate the leptomanages and the superficial cortex.

Now, clinically, these patients are they have fever, they have signs of meningeal irritation, maybe they're a bit confused or lethargic.

But the key takeaway is that the mortality is actually quite low.

Yeah, it's much better to have the viral kind.

Much, much better prognosis than the bacterial kind, absolutely.

There is a specific clinical context mentioned in the notes regarding HIV here that I thought was really interesting.

Yes, this is a very high -yield clinical pearl.

You really need to know this one.

Acute viral meningitis is often the most common neurologic symptom associated with primary HIV infection.

So right when they get it.

Exactly.

Picture a patient who is seroconverting.

They have just contracted HIV.

The virus is replicating like crazy.

Right around that time, they present with an acute, confusional state.

And what's the outcome of that?

It usually resolves on its own after about a month with just supportive care, but that episode, that's a massive red flag for a new HIV infection.

Okay, that covers the viral side.

Now let's talk about the scary one.

Acute purulent meningitis.

Purulent essentially means pus, right?

Exactly.

This is bacterial warfare.

On gross examination, meaning if you were looking at the brain during an autopsy, the leptomeninges look cloudy, opaque, because they're congested with pus.

And microscopically, the whole army changes.

The army completely changes.

It's not lymphocytes anymore.

It is neutrophils.

A sea of neutrophils.

And why is this so much more dangerous than the viral form?

What's the mechanism of death here?

Because that massive neutrophilic inflammation triggers a cascade.

You get the systemic signs of infection, sure.

But locally, you get diffuse cerebral edema swelling of the brain.

And the skull is a rigid box.

It's a rigid box.

That swelling has absolutely nowhere to go.

And that carries a very real risk of fatal herniation, which is often how these patients die.

We'll get to herniation later, but it's the end game for a lot of these pathologies.

We have to hit the classic triad of bacterial meningitis.

This is something every medical student drills into their head.

Yes.

If you see these three things, you have to act immediately.

No questions asked.

Fever, neutral rigidity, which is just doctor speak for a stiff neck because moving the head stretches those inflamed meninges and its agony and altered mental status.

So fever, stiff neck, confusion.

That triad is the hallmark.

Don't miss it.

Now, here's where the notes get very specific, and this is classic matching game material for exams.

The specific bacteria causing the meningitis depends heavily on the patient's age.

It does.

The bug changes with the host.

So let's run through the demographics, starting with neonates newborns, first month of life.

In neonates, the most frequent cause is group B streptococci.

Okay.

This comes from the birth canal during delivery.

However, and this is a nuance the nose highlight, that's greekia coli.

Coli actually causes a greater number of fatalities in this group, even if it's slightly less common than strep B.

So another high yield distinction.

Group B strep is most common, but E.

coli is deadlier.

Precisely.

Okay.

Moving up to infants, children, and adults.

So the general population, who is the main villain here?

The heavyweight champion in this group, and really overall, is streptococcus pneumonia.

It's the most common cause of meningitis across the board once you're out of that newborn phase.

And what about

the classic college dorm scenario or military recruits?

That's a specific one.

That is Neisseria meningititis.

This one is terrifying because it moves so fast.

A person can be healthy in the morning and dead by night.

And is there a specific clinical clue for Neisseria?

The smoking gun you look for is a rash, a patechial or maculopapular rash.

If you see meningitis symptoms plus a rash on a teenager, you don't wait for labs.

You treat for Neisseria immediately.

And finally, what about the elderly folks over 50 or anyone who's immunocompromised?

The bug to think about there is Listeria monocytogenes.

Listeria.

Listeria is tricky.

It tends to infect people with poor cell -mediated immunity.

This is why we tell pregnant women and immunocompromised people to avoid things like soft cheeses and unpasteurized dairy.

Because Listeria can live there.

It can survive there and cause a really nasty infection in the right person.

Before we leave infections, we have to touch on mycobacterial meningoencephalitis.

This is TB.

Right.

Mycobacterium tuberculosis.

Or in AIDS patients, you might see atypical mycobacteria like MAI,

mycobacterium, avium and tricellular.

And how does this happen?

This usually happens in patients with a reactivation of a latent infection.

So they had TB in their lungs years ago.

It went dormant.

And now that their immune system is down, it wakes up and seeds the brain.

Does TB meningitis look different on a scan compared to the others we've discussed?

It really does.

The imaging choice is an MRI with contrast and it shows something very specific.

Basal meningeal enhancement.

Basal meaning the base of the brain.

Exactly.

TB loves the base of the brain.

You might also see hydrocephalus because that inflammation at the base can block the drainage of cerebrospinal fluid and you can even see tuberculoma's little granulomas forming within the brain parenchyma itself.

Okay this is a perfect transition.

Let's visualize table 20 to 1 from the notes.

This is the CSF parameters table.

This is clinically how you tell these different types of meningitis apart.

You stick a needle in the spine.

You pull out the cerebrospinal fluid.

The lumbar puncture.

Exactly.

So I'm going to give you the parameter.

You tell me how it shifts.

Let's start with bacterial.

Acute purulent.

In bacterial meningitis everything is extreme.

The pressure, the opening pressure is markedly elevated.

It practically shoots out of the needle.

The cells.

Neutrophils.

Wall -to -wall neutrophils.

You can see up to 90 ,000 cells per microliter.

Group.

Increased.

Way up because of all the bacteria and cellular debris and inflammation.

But the dead giveaway,

the thing that clinches the diagnosis,

is the glucose.

What happens to the sugar?

It drops like a stone.

Glucose is decreased.

Usually less than 45 milligrams per deciliter.

And why is that?

Because the bacteria are metabolic machines.

They are literally eating the glucose in the CSF for fuel.

They're just fermenting it.

Okay.

Now contrast all of that with viral or aseptic meningitis.

Total opposite on the key findings.

In viral the cells are lymphocytes, not neutrophils.

Right.

The protein might be normal or just slightly increased, but the glucose, the gluteose is normal.

That is your key differentiator.

Why normal?

Viruses don't consume glucose the way bacteria do.

So if the sugar is normal it is very very likely viral.

And finally, what about the tricky middle ground?

Granulomatous.

So fungal or TB?

This one is a mix, which is why it's tricky.

You have lymphocytes, like a viral infection, but you have decreased glucose, like a bacterial infection.

And protein.

Protein is usually increased as well.

So if you see that specific combination of low sugar plus lymphocytes, you have to think TB or a fungus like cryptococcus.

That's a great heuristic.

Okay, let's move from the coverings of the brain to the brain itself.

Encephalitis.

How is this different from meningitis?

It's simple distinction.

Meningitis is inflammation of the covering, the meninges.

Encephalitis is inflammation of the brain parenchyma itself, the actual neural tissue, the neurons and the glia.

The notes mention a general pathology here involving perivascular cuffs.

Can you paint a picture of what it is?

Yeah, this is a classic histologic finding.

Imagine a blood vessel diving from the surface into the brain tissue.

In encephalitis, immune cells, mostly lymphocytes and plasma cells, accumulate in the space surrounding that vessel, the vertigo -robin space.

So it looks like a cuff on a sleeve.

Exactly.

Under a microscope, it looks like a thick cuff or a sleeve of blue inflammatory cells wrapped around the vessel.

Along with that, you see microglial nodules and something called neuronophagia.

Neuronophagia, that sounds ominous.

It's literally neuron eating.

It's exactly what it sounds like.

The immune cells, the microglia, surround a dying infected neuron and just devour it.

It is a grim microscopic scene, but it's the brain's way of cleaning up the infected cells.

The notes list several very specific, very high yield associations for encephalitis.

I'll throw them at you.

Arthropod -borne.

Those are your mosquito -borne viruses.

Things like St.

Louis, encephalitis, eastern and western equine, Venezuelan equine.

Geography matters here, as does the season.

Think summer.

Think mosquitoes.

Okay.

What about HSV1, herpes simplex virus type 1?

This is critical.

You have to know this one.

HSV1 has a very specific predilection for the temporal lobes.

The memory centers.

The memory and emotional centers.

Yes.

If you look at figure 20 to 1 in the text, there's a CT scan showing this intense edema, this swelling,

specifically in the bilateral temporal lobes.

That is classic HSV1.

It causes a hemorrhagic necrosis there.

And under the microscope.

Any clues?

You look for Cowdrey type A bodies.

These are these big, glassy, pink, intranuclear viral inclusions inside the neurons.

Rabies.

Rabies gives you negri bodies.

These are eosinophilic, so pink, inclusions found in the cytoplasm, not the nucleus.

And where do you find them?

Specifically in the hippocampal neurons and the purkinje cells of the cerebellum.

That location explains the symptoms.

The hippocampus involvement relates to the agitation and aggression, and the brainstem involvement relates to the hydrophobia and pharyngeal spasms.

What about HIV?

HIV encephalopathy is more of a subacute process.

You see microglial nodules, and very specifically you see multi -nucleated giant cells which are formed from fused macrophages.

And this causes what, clinically?

It creates a dementia -like picture, an AIDS dementia complex.

It can also affect the spinal cord, causing something called vacuolar myelopathy, which looks a lot like a vitamin B12 deficiency, clinically.

One last viral one that's mentioned.

PML.

Progressive multifocal

leukoencephalopathy.

A mouthful.

This is caused by the JC polyomavirus.

And who gets this?

It really only strikes when the immune system is completely wiped out, like in late stage AIDS or on certain immunosuppressive drugs.

It attacks oligodendrocytes, the cells that make myelin in the CNS.

So it's a demyelinating disease?

Right.

You get these patchy areas of demyelination.

The histological clue is

enlarged oligodendrocytes with these glassy viral -filled nuclei.

Let's briefly hit fungal and parasitic infections before we move on.

Okay.

For fungal, remember aspergillus and mucor.

These molds are angioinvasive.

Angioinvasive.

They have a tropism for blood vessels.

They love to invade the vessel wall, which causes vasculitis, thrombosis, and massive hemorrhage.

They are devastating.

Miscryptococcus.

Cryptococcus is another big one, especially in AIDS patients.

It has this thick gelatinous capsule.

It invades the vertorabin spaces, those paravascular spaces we talked about, and expands them until they look like soap bubbles on imaging.

We literally call them soap bubble lesions.

And parasites.

What's the main one to know?

In the U .S., for an AIDS patient, it's toxoplasmosis, caused by the parasite toxoplasma condii.

This is the most common cause of brain abscesses in that population.

And what does that look like on an MRI?

You see characteristic ring -enhancing lesions, usually multiple of them.

Speaking of abscesses, the notes highlight a few risk factors.

How does bacteria get deep into the brain to form an abscess in the first place?

There are a few ways.

It can come from local direct spread, like a bad sinus infection or a middle -year infection mastoiditis that just eats through the bone and gets into the brain.

Or it travels through the blood, hematogenous spread.

A classic systemic risk factor is acute bacterial endocarditis -infected heart valves throwing off little septic emboli, little clots of bacteria.

And there's another one mentioned here, right -to -left shunts.

Yes, cyanotic heart disease with a right -to -left shunt.

Can you explain that connection?

Why does a hole in the heart put the brain at risk for an abscess?

Normally, the lungs act as a fantastic filter for any bacteria that might get into the venous blood.

The capillaries there are so fine they track them.

If you have a heart defect where deoxygenated blood skips the lungs and goes right from the right side of the heart to the left, a right -to -left shunt, then any bacteria in that blood completely bypass the lung filter.

And go straight to the brain.

And can lodge straight in the capillary beds of the brain, setting up shop and forming an abscess.

Scary stuff.

Okay, before we close out this whole section on infections, we have to talk about prion diseases.

This is the stuff of, well, nightmares.

Koitzfeldt -Jakob disease, CJD.

This is a fundamentally different kind of disease.

It's not a virus.

It's not a bacteria.

It's a disease of protein folding.

So what's the protein?

We all have a normal prion protein called PRPC.

The C is for cellular.

It's shaped like an alpha helix.

It sits on our cell membranes.

And it seems to mind its own business.

We're still not entirely sure what it does.

And in this disease?

In this disease, that protein spontaneously, or due to a mutation, flips its shape.

It refolds into PRPC, a beta pleated sheet.

The CC is for screpey, the sheep version of the disease.

And why is that one shape change so catastrophic?

Two reasons.

One, the beta pleated form is incredibly stable and resistant to digestion.

The cell's proteasomes can't break it down, so it accumulates.

Two, and this is the really scary part, it acts as a template.

It touches other normal PRPC proteins and forces them to flip their shape into the bad beta pleated form.

Two, it's a chain reaction.

It's infectious at a protein level.

And what does this build up of bad protein do to the brain tissue?

These misfolded proteins accumulate and cause what we call spongiform change -fine vacuolization in the gray matter.

The brain literally turns into a sponge full of tiny holes.

And the symptoms, I imagine, reflect that widespread destruction.

Absolutely.

Rapidly progressive dementia.

We are talking about a decline over weeks to months, not years, and a very characteristic feature called startle myoclonus, which is involuntary jerking, especially in response to a loud noise.

Is this genetic?

Most cases, about 85 % are sporadic.

They just happen.

About 15 % are familial due to a mutation.

And then there's the very rare variant CJD, which is associated with eating beef from cows with mad cow disease, bovine spongiform encephalopathy, and that tends to affect younger patients.

All right, let's shift gears.

A very different mechanism of injury.

Section two.

Cerebrovascular disease.

The notes say this is the third most frequent cause of death in industrialized countries.

It is a massive public health issue.

And broadly, it breaks down into two main categories.

Two big problems you can have with the brain's plumbing.

Which are?

Ischemia, which is a lack of blood flow, and hemorrhage, which is bleeding.

Okay, let's start with global cerebral ischemia.

This isn't just one clogged pipe.

This is the whole system going down, right?

Right, this is usually due to systemic hypotension.

So shock, cardiac arrest, severe blood loss.

The entire brain is starved of oxygenated blood all at once.

But does the whole brain die at once?

No.

And this is a key concept.

Not all neurons are created equal in their sensitivity to hypoxia.

We call this selective vulnerability.

So some areas are more sensitive than others.

Who are the canaries in the coal mine here?

The Purkinje cells in the cerebellum are exquisitely sensitive.

They die very quickly.

The hippocampus, specifically the pyramidal neurons in the CA1 area, also known as the summer sector.

Some memory problems after a cardiac arrest.

Exactly.

If you have a patient who survives cardiac arrest but has severe memory deficits or balance issues, this is why.

Those specific cells died first.

You also get infarcts in the watershed areas.

Watershed.

What does that mean?

Think of it like irrigating a field.

The areas at the very end of the line at the border zones between two different arterial supplies are the most vulnerable.

They're the first to run dry when the overall pressure drops.

Okay, that makes sense.

Moving to the more common stroke.

The text says 85 percent of strokes are infarction and 15 percent are hemorrhage.

Under infarction, we have three subtypes thrombotic, embolic, and small vessel.

This feels like a plumbing exam.

How do we distinguish them?

It is plumbing.

Let's start with thrombotic strokes.

These are usually due to atherosclerosis plaque buildup right there in the brain's arteries, like the cerebral artery or in the carotids in the neck.

And what do these look like?

These create what we call anemic or white infarcts.

Why white?

Because the flow is cut off gradually and completely by the thrombus.

The tissue downstream dies, but there's no blood getting back into it, so it stays pale.

Contrast that with embolic strokes.

Embolic strokes are different.

The clot doesn't form there.

It travels there.

The usual source is the heart like a clot from atrial fibrillation or from a valve.

The clot flies up, slams into a vessel, and blocks it.

So why is that a red infarct?

Because that embolus can break up or move.

It's called reperfusion.

If the clot dissolves, blood suddenly rushes back into that dead damaged tissue.

But the vessel walls there are now leaky and necrotic, so the blood just extravasates out.

It causes bleeding into the infarct.

So embolic strokes are often hemorrhagic or red infarcts.

Got it.

And the third type, small vessel disease.

This is the result of chronic, long -standing hypertension.

High blood pressure damages the walls of tiny penetrating arteries deep in the brain.

How does it damage them?

It causes a process called hyaline arterial sclerosis.

The walls get thick and glassy.

Eventually the lumen just closes off, causing these tiny little infarcts we call lacanar infarcts.

Lacanar, like a lake?

A little lake or hole, yes.

They're tiny, usually less than 15 millimeters, and they happen in the deep structures like the basal, ganglia, thalamus, and pons.

Okay.

Now for the 15 % that are hemorrhages.

Bleeds.

Right.

You have intracerebral hemorrhage, which is bleeding into the brain parenctoma.

The number one cause, by far, is hypertension rupturing those small vessels we just talked about.

And then you have subarachnoid hemorrhage.

The subarachnoid hemorrhage, this is the one famous for the patient describing it as the worst headache of my life.

An absolute thunderclap headache.

And the culprit there is almost always the berry aneurysm.

Correct.

Or saccular aneurysm.

These are congenital defects.

The media, the muscular layer of the vessel wall, is missing at the branch points of the arteries.

So it's a weak spot.

A congenital weak spot.

Over years of blood pressure, the wall pushes out like a balloon or a berry.

They usually sit at the branch points of the anterior circle of Willis.

Are there any genetic conditions associated with these?

Yes.

If a patient has Marfan syndrome, Ehlers -Ganlos syndrome, or adult polycystic kidney disease, they are at much higher risk for these aneurysms because their connective tissue is fundamentally weaker.

Figure 20 -2 shows a beautiful example on angiography.

It looks just like a little sack hanging off the artery, waiting to pop.

Now, for the students listening, table 20 -3 is pure gold.

It's the evolution of a stroke.

This is a timeline of death and cleanup in the brain.

Let's walk through it.

Imagine a patient has an ischemic stroke.

What do we see under the microscope 12 to 24 hours later?

This is the era of the red neuron.

This is the first visible sign of irreversible hypoxic injury.

The neurons are dying.

Their cytoplasm becomes intensely isosynophilic, or pink, and the nucleus shrinks, a process called pinosis, and eventually fragments.

Okay, red neurons, what about 24 to 48 hours out?

The immune system arrives.

Neutrophils invade the tissue to start the inflammatory response.

This is when the tissue starts to get soft, what we call liquefactive necrosis.

Days 2 to 10.

The heavy lifters, the cleanup crew arrives,

the neutrophils die off, and macrophages, or histiocytes, as we call them in the brain, come in to eat the dead neurons and all the myelin breakdown products.

And what does the tissue feel like?

It becomes very soft, friable, and it literally starts to liquefy.

You could scoop it out with a spoon.

And three weeks and beyond, what's left?

The cleanup is done.

You are left with a fluid -filled cystic cavity.

Now, this is a key point.

In the rest of the body, scars are made of collagen by cells called fibroblasts, but the brain is different.

No fibroblasts in the brain.

Right.

The brain scars with astrocytes.

So you get a gliotic scar made of reactive astrocytes lining the cystic cavity.

It doesn't fill in.

It just forms a glial wall around a hole.

Fascinating.

All right.

Moving on to section three, CNS trauma and herniation.

Let's talk about getting hit in the head.

First, the basics.

Concussion versus contusion.

A concussion is a transient functional injury.

You get your bell rung.

You might lose consciousness briefly.

You wake up confused.

Maybe you have a headache.

You don't remember the hit.

But usually, if you're CT -scanned that person, the brain paranchuma looks normal.

So it's a software problem, not a hardware problem.

In a sense, yes.

A contusion is a hardware problem.

It's a bruise.

It's actual physical damage, hemorrhage, and necrosis of the brain tissue, usually in a wedge shape with the base on the surface of the brain.

This is where we get the very famous terms coup and contrecoup.

Physics again.

Yeah.

You have to imagine the brain is floating in a bowl of fluid, the CSF, inside the rigid skull.

Right.

If your head is moving and suddenly stops, say you hit the dashboard in a car crash, the brain keeps moving and slams into the front of the skull.

That's the coup injury, the injury at the site of impact.

The contrecoup.

Then the brain bounces off the front and ricochets back, hitting the opposite side of the skull.

That's the contrecoup injury.

So you can have bleeding on the side directly opposite the point of impact.

And if we were to look at a brain years later, what would that old contusion look like?

That old contusion heals into a depressed, yellow, gliotic scarred area.

We call it plac jaune, which is just French for yellow plaque.

Now there's another type of injury here, diffuse axonal injury.

This sounds particularly bad.

It is devastating.

This isn't from a direct impact.

It's from shearing forces.

It happens with rotational forces or rapid acceleration, deceleration like getting T -boned in a car or shaken baby syndrome.

So the brain twists.

The brain twists inside the skull and the long axons that connect everything get stretched and sheared, especially at the nodes of Ranvier.

The patient goes into a coma instantly, but the initial CT scan might look deceptively clean because you don't always see a big bleed.

So how do you diagnose it?

Microscopically.

You see these axonal swellings or spheroids.

It's where the transport system inside the axon, the axoplasmic flow, has been blocked by the shearing and material just piles up.

Let's distinguish the hematomas.

Epidural versus subdural.

This is a classic high yield comparison.

It is.

Let's start with epidural hematoma.

This is an arterial bleed.

It's usually a rupture of the middle meningeal artery.

And what causes that?

It's almost always associated with the skull fracture, typically of the temporal bone, which is very thin.

The artery runs right under it.

The blood pumps out under high pressure and it dissects the dura away from the inner surface of the skull.

And on a CT scan?

It looks like a lens.

It's biconfex.

It bulges inward.

A key feature is that it does not cross suture lines because the dura is tacked down very tightly at the sutures of the skull.

And what about subdural?

Subdural is totally different.

It's a venous bleed.

It's the rupture of the bridging veins.

Bridging veins.

These are veins that cross the subdural space from the surface of the brain to the big dural sinuses where the blood drains.

This is very common in the elderly.

Why the elderly?

Because as we age, the brain atrophies.

It shrinks.

That stretches those bridging veins, making them tight and taut like a guitar string.

So even minor trauma, a simple fall can snap them.

In the CT appearance.

This bleed is crescent -shaped.

It spreads out over the surface of the brain and it can cross suture lines because it's under the dura.

Okay, that brings us to herniation syndromes.

This is the pressure cooker scenario we talked about earlier.

When the brain swells or bleeds, it has nowhere to go but out.

What are the three main types we need to know?

First, you have subfulcine herniation.

The brain has that big dural divider down the middle called the falc cerebre.

Right.

If one cerebral hemisphere swells, the cingulate gyrus on that side gets pushed under the falx to the other side.

This can compress the anterior cerebral artery, causing a secondary stroke in that territory.

Okay.

What's the second type?

Trans -stentorial or uncle herniation.

This is when the innermost part of the temporal lobe gets pushed over the edge of the tentorium cerebelli.

And what's right there?

Cranial nerve the third, the oculomotor nerve is right there.

So compression of that nerve is the first sign.

What does that look like clinically?

A blown pupil.

The pupil on that side becomes dilated and fixed.

It doesn't react to light.

As it progresses, it can also compress the posterior cerebral artery and cause duret hemorrhages in the brain stem from tearing small vessels.

And the third and most deadly.

That would be tonsillar herniation.

This is when the cerebellar tonsils get pushed down through the foramen magnum, the big hole at the bottom of the skull where the spinal cord comes out.

And what lives right there at that opening?

The medulla, which contains the respiratory and cardiac centers.

Compression here leads to respiratory arrest and death.

It's the final common pathway for many fatal brain injuries.

Let's take a breath while we still can and move to section four,

developmental abnormalities and perinatal injury.

Right.

So here we're looking at things that go wrong in utero during development.

The section kicks off with neural tube defects.

And the big association here, the one everyone knows is folate deficiency.

Absolutely.

The neural tube, which becomes the brain and spinal cord, zips up very early in pregnancy.

If it fails to close properly, you get these defects.

The most severe is?

An encephaly.

Abscess of the cranial vault and most of the brain.

The brain barely forms.

It is unfortunately incompatible with life.

And then you have the spinal defects.

Can you break those down?

Spina bifida culta versus meningotrally versus meningomyelosal?

Sure.

Spina bifida culta is the mildest form.

It's just a bony defect in the vertebra.

The vertebral arch doesn't fuse.

Often it's totally asymptomatic.

Maybe just a little dimple or a tuft of hair on the lower back.

Meningotrally.

That's when the meninges, the coverings, pouch out through that bony defect, forming a fluid -filled sac.

But the spinal cord itself is still in the right place.

And meningomyelosal is the worst one.

Right.

In the meningomyosal, the meninges and the spinal cord itself pouch out into that sac.

That causes severe neurologic deficits in the legs and bladder and bowel dysfunction.

What about syringomyelia?

That's a strange sounding word.

It refers to a syrinx, which is a fluid -filled cyst or tube that forms inside the central canal of the spinal cord.

It's very strongly associated with the Arnold -Chiari malformation type 2.

And as that cyst expands, what does it damage first?

It expands from the inside out.

So it destroys the center of the cord first.

And that's exactly where the crossing fibers of the spinoflamic tract are located.

Those are the fibers for pain and temperature sensation.

So what's the classic symptom?

A loss of pain and temperature sensation in a cape -like distribution over the shoulders and down the arms.

But they keep their sense of touch and vibration because the dorsal columns in the back of the cord are spared, at least initially.

It's a very specific sensory dissociation.

You mentioned Arnold -Chiari.

Let's clarify the cerebellar malformations.

Dandy -Walker versus the Chiari malformations.

Okay.

Dandy -Walker is a triad of findings, a very large posterior fossa, a huge cyst that's continuous with the fourth ventricle,

and hypoplasia or underdevelopment of the cerebellar vermis.

And the Chiari malformations.

Arnold -Chiari type 1 is usually found incidentally in adults.

The cerebellar tonsils just hang down a bit low, below the form and magnum, often asymptomatic.

But type 2 is a big deal.

Arnold -Chiari type 2 is the severe one seen in infants.

You have a small posterior fossa and the cerebellar vermis and the medulla are physically displaced downward through the form and magnum.

This causes obstructive hydrocephalus and is almost always associated with a lumbar meningo -myelosal.

They go hand in hand.

And briefly, perinatal injury.

What's the single biggest risk factor for brain injury in a newborn?

Prematurity.

A preemie's brain is just not ready for the outside world.

The blood vessels, specifically in an area called the germinal matrix near the ventricles, are incredibly fragile.

So they can bleed.

They're at very high risk for germinal matrix hemorrhage.

They can also get paraventricular leukomalacia, which is necrosis or death of the white matter around the ventricles due to ischemia.

Okay, let's move into section 5.

Demyelinating and degenerative disorders.

This covers a lot of ground, a lot of big name diseases.

Let's start with multiple sclerosis, MS.

MS is the classic autoimmune demyelinating disorder of the CNS.

The body's own immune system, T -cells specifically, attacks the myelin sheath that insulates the axons.

Is there a genetic link?

Yes.

There's a strong association with the HLA -DR15 genotype.

And there are environmental factors, too, like vitamin D deficiency and, famously, your distance from the equator as a child.

So what are the actual lesions in the brain?

You see plaques.

These are areas of demyelination.

Acute plaques have active inflammation in myelin breakdown products being eaten by macrophages.

Chronic plaques are just quiet gliotic scars where the myelin is gone for good.

And where are they located?

They are typically periventricular located right around the ventricles.

That's a classic location on an MRI.

And the clinical symptoms?

I learned of the mnemonic SI.

That's Charcot's triad, SIN, scanning speech, which is a staccato broken up speech pattern,

intention tremor, so shaking when you're trying to reach for something, and nystagmus, the darting eye movements.

And another classic one is optic neuritis.

Optic neuritis, painful vision loss, is a very common presenting symptom.

To help diagnose it, you look at the CSF oligoclonal bands, which are evidence of antibody production happening inside the brain.

What about central pontine myelinolysis?

That sounds very specific.

It is.

And it's a terrifying iatrogenic condition.

Iatrogenic meaning doctor caused.

Yes.

It is a focal demyelination of the base's pontus, the base of the pons.

And it happens if you correct hyponitremia low sodium too rapidly.

There's a mnemonic for that too, right?

From low to high, your pons will die.

If you rush the saline infusion and bring the sodium up too fast, you cause a massive osmotic shift that strips the myelin off the axons in the pons.

And the result?

It can lead to locked -in syndrome, where the patient is fully conscious and aware, but completely paralyzed from the neck down, only able to move their eyes.

A devastating outcome.

Now let's talk about the big degenerative diseases, the dementias and movement disorders.

Let's start with Parkinson's disease.

Parkinson's is a hypokinetic movement disorder.

Hypo, meaning too little movement.

It's caused by the loss of dopaminergic neurons in a part of the midbrain called the substantia nigra.

And what does that look like?

Grossly, the substantia nigra, which is usually dark because of neuromelanin, looks pale.

Histologically, the key finding is the Lewy body.

What is a Lewy body?

It's a round eosinophilic pink inclusion inside the cytoplasm of the remaining neurons.

It's composed of a misfolded protein called alpha -synuclein.

And the classic clinical symptoms?

We use the mnemonic TRAP.

T for tremor, a classic pill -rolling tremor at rest.

R for rigidity, a cogwheel -type stiffness.

A for akinesia or bradykinesia, which is slowness of movement.

And P for postural instability, which leads to falls.

Okay, now let's contrast that with Huntington disease.

Huntington's is the opposite.

It's a hyperkinetic movement disorder.

Too much movement.

And the genetics are key here.

Absolutely.

It's autosomal dominant on chromosome 4.

It involves CAG trinucleotide repeats.

The more repeats a person inherits, the earlier the disease starts.

A phenomenon called anticipation.

And pathologically, what's happening in the brain?

You see profound atrophy of the caudate nucleus.

You lose the gaborergic inhibitory neurons there.

Think of them as the brain's brakes.

So without the brakes?

Without the brakes, you get chorea, these involuntary writhing dance -like movements, and eventually a progressive dementia.

Then there's the big one, Alzheimer disease.

The most common cause of dementia, by far.

Genetics are very important here.

Early onset, before age 65, is strong light associated with mutations in the APP gene on chromosome 21 and the present Allen 1 and 2 genes.

Wait, chromosome 21?

Is that why patients with Down syndrome, trisomy 21, get Alzheimer's so early?

It's exactly why.

They have an extra copy of the APP gene, so they produce more of the precursor protein that leads to amyloid.

For the more common late onset form, the biggest genetic risk factor is carrying the APOE E4 allele.

And what's actually happening in the brain tissue in Alzheimer's?

What are the pathologic hallmarks?

There are two main protein problems.

First, you have extracellular neuritic plaques, which are composed of a beta amyloid.

These are like sticky garbage between the neurons.

And the second?

Second, you have intracellular neurofibrillary tangles.

These are composed of a hyperphosphory -related protein called tau.

Tau normally stabilizes the neuron's internal skeleton, but here it clumps up and kills the cell from the inside.

And the result of all this is?

Massive diffuse cortical atrophy.

The brain literally shrinks.

The gyra get thin.

The cells get wide.

The ventricles enlarge to fill the space.

We call that hydrocephalus ex vacuo.

It's not a pressure problem.

It's a volume loss problem.

Let's hit a few others quickly before we get to tumors.

ALS.

Amyotrophic lateral sclerosis.

Lou Gehrig's disease.

This is unique because it affects both upper motor neurons in the brain and lower motor neurons in the spinal cord.

So you get a mix of signs.

Exactly.

You get spasticity and hyperreflexia, which are UMN signs.

Mixed with weakness, muscle atrophy, and fasciculations, which are LMN signs.

But notably, sensation is spared.

There is no sensory loss.

They feel everything.

Free drichotaxia.

This one is autosomal recessive.

It's another trinucleotide repeat disease, GAA repeats.

It affects the fritaxin gene, which is involved in mitochondrial iron regulation.

So it's a mitochondrial disease.

Essentially.

Without fritaxin, iron builds up and creates damaging free radicals.

Clinically, you see the ataxia, the gait problems, but you also see systemic issues like diabetes and hypertrophic cardiomyopathy.

Heart failure is often the cause of death.

OK.

Final stretch.

Section 6.

CNS tumors.

Let's do it.

The first rule of real estate applies here.

Location, location, location.

Meeting.

In adults, primary brain tumors are usually supertentorial, above the tentorium, in the cerebral hemispheres.

In children, they are usually infra -tentorial in the posterior fossa,

so the cerebellum and brainstem.

And what about metastasis to the brain?

That's actually very common.

About 25 % to 50 % of all brain tumors are metastases from elsewhere, usually from the lungs, breasts, or melanoma.

They typically show up as multiple lesions right at the gray -white junction, where the blood vessel is narrow and trap the tumor cells.

Let's break down the primary tumors.

Gliomas first.

And under that, astrocytomas.

Right.

So in children, you have the paleocytic astrocytomas, usually found in the cerebellum.

The classic look on an MRI is a cyst with a mural nodule.

And the histology.

Histology shows these corkscrew -shaped eosinophilic fibers called Rosenthal fibers.

It's a benign, grayed -eye tumor with a great prognosis if you can resect it.

But in adults, it's a different story.

In adults, the worst -case scenario is the glioblastoma, which is a grade -fourth astrocytoma.

It's the most common and most malignant primary brain tumor in adults.

What are its hallmarks?

It has central necrosis and vascular proliferation.

And under the microscope, you see this classic feature called pseudopalicating, where the tumor cells line up around the areas of necrosis.

It has a terrible prognosis.

It also tends to cross the corpus callosum, forming what's called a buttersly glioma.

Figures 20 to 6 and 27 in the book show this.

It's just a massive destructive infiltrating lesion.

OK.

What about oligodendroglioma?

These have a very characteristic fried egg appearance on histology.

A clear halo around a central nucleus.

It's actually a processing artifact, but it's what we look for.

They often have calcifications you can see on the CT scan.

There's a key genetic pearl here.

Yes.

If the tumor has a co -deletion of chromosome arms 1p and 19q, they respond much better to chemotherapy and have a better prognosis.

Ependymoma.

These arise from the ependymal cells that line the ventricles.

In children, they usually hit the fourth ventricle, blocking CSF flow and causing hydrocephalus.

The buzzword for histology is perivascular pseudorescence.

OK.

What about the primitive tumors?

Medulloblastoma.

This is the most common malignant brain tumor in children.

It's always in the cerebellum.

Histology just shows sheets of small round blue cells.

It's highly malignant and can spread down the spinal fluid, what we call dropments.

But the good news is it is very radio sensitive.

And finally, let's cover the non -glial tumors.

Meningioma.

These are dural -based tumors arising from the arachnoid cap cells.

They are usually benign and slow growing.

Who gets them?

They are much more common in adult women because they often have estrogen receptors.

Histologically, you see cells in a world pattern and somoma bodies, which are these laminated calcifications.

Figure 20 to 8 shows a classic one.

It's a big dural -based mass that pushes into the cortex but doesn't actually invade it.

It compresses rather than destroys.

These usually arise on a cranial nerve, most commonly cranial nerve 8th, the vestibulocochlear nerve.

We call it an acoustic neuroma.

It sits right in the cerebellopontine angle and causes hearing loss and tinnitus.

And the histology.

Histology shows two patterns.

A dense antony A pattern and a loose mixoid antony B pattern.

If you ever see bilateral schwannomas, you must think of the genetic condition neurofibromatosis type 2 or NF2.

Last one, craniopharyngeoma.

These occur in the cellar region right near the pituitary gland and usually present in children.

They are derived from remnants of Rathke's pouch, which is the embryologic structure that forms the anterior pituitary.

And what's the connection there?

Rathke's pouch is derived from oral ectoderm.

So it's like odontogenic or tooth -forming epithelium.

That's why these tumors can have calcifications and even keratin.

They're often cystic and filled with this thick cholesterol -rich fluid that looks like machine oil.

Wow.

We just sprinted through the entire CDS pathology chapter.

That was intense.

We did.

It's a lot of information, no question.

But I think seeing the patterns, the why behind the locations and the ages and the histology helps it stick.

So synthesizing all this, what's the big takeaway for you?

What's the unifying theme of brain pathology?

If we connect this to the bigger picture, it really highlights the brain's unique set of vulnerabilities.

First, it's trapped in a rigid box.

So any extra volume, whether it's pus from meningitis, blood from a hematoma or tumor, is catastrophic.

It causes pressure and herniation.

Second, it has an extremely high nonstop metabolic demand with no energy reserves.

So ischemia, even for a few minutes, kills neurons.

And finally, for reasons we're still figuring out, it's incredibly susceptible to these protein misfolding diseases, whether it's prions, amyloid in Alzheimer's or alpha -synuclein in Parkinson's.

It's just a delicate, high -performance machine.

It really emphasizes how fragile that system is.

Here's a final thought for you to chew on.

We often separate vascular disease and neurodegeneration in our minds.

We think of stroke as one thing and Alzheimer's as another.

But notice how many overlaps there are.

Vascular dementia is a thing.

There's amyloid angiopathy in Alzheimer's.

The risk factors for stroke are also risk factors for dementia.

So it's all connected.

I think maintaining vascular health -controlling blood pressure, cholesterol, not smoking,

might be the single strongest lever we have for preserving cognitive function late in life.

It's all one connected system.

A powerful thought to end on.

Thank you so much for joining us on this very deep dive.

My pleasure.

It was fun.

And to you, the listener, we hope this helps you visualize those pages and connect the dots.

A warm thank you from the Last Minute Lecture team.

See you next time.