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Lea Alhilali, MD

@teachplaygrub

May 16, 2022 • 7 tweets • 4 min read • Read on X

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1/
Why is cranial nerve 6 uniquely affected by⬆️ intracranial pressure? Why is it special? A common question after the CN6 tweetorial.
Here is a maybe #tweetorial, but maybe a🧵about why CN6 is alone affected by ⬆️ pressure. #FOAMed #medtwitter #Medstudenttwitter #neurotwitter

2/
Think of the intracranial CSF space like a balloon, distended by CSF instead of air. Cranial nerves begin inside the balloon, and then they exit as they begin their extradural portion

3/
Most cranial nerves move immediately away from the CSF space after they exit—usually going out through their respective foramina. However, CN6 uniquely runs along the outside of the “balloon” in Dorello canal

4/
Increased intracranial pressure is like expanding the balloon. Most cranial nerves are not affected by the expanded balloon because they move away from the surface of the balloon right after they exit

5/
However, because of the unique course of CN6 along the surface of the “balloon” in Dorello canal, the increased intracranial pressure or expanding “balloon” pushes against the extradural portion of CN6

6/
Unfortunately, CN6 has nowhere to go to escape this increased pressure, as on the other side of it is the clivus. So in the increased pressure pushes it against the clivus in Dorello canal

7/
This makes a “CN6 sandwich”! CN6 gets sandwiched between clivus & dura. It's this compression that uniquely gives you an isolated CN6 palsy w/⬆️pressure!
So when you see an isolated CN6 palsy in intracranial hypertension, think of balloons & sandwiches & you’ll remember why!

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More from @teachplaygrub

Lea Alhilali, MD

@teachplaygrub

Oct 15

1/”That’s a ninja turtle looking at me!” I exclaimed. My fellow rolled his eyes at me, “Why do I feel I’m going to see this a thread on this soon…”

He was right! A thread about one of my favorite imaging findings & pathology behind it

2/Now the ninja turtle isn’t an actual sign—yet!

But I am hoping to make it go viral as one. To understand what this ninja turtle is, you have to know the anatomy.

I have always thought the medulla looks like a 3 leaf clover in this region.

The most medial bump of the clover is the medullary pyramid (motor fibers).

Next to it is the inferior olivary nucleus (ION), & finally, the last largest leaf is the inferior cerebellar peduncle.

Now you can see that the ninja turtle eyes correspond to the ION.

3/But why are IONs large & bright in our ninja turtle?

This is hypertrophic olivary degeneration.

It is how ION degenerates when input to it is disrupted. Input to ION comes from a circuit called the triangle of Guillain & Mollaret—which sounds like a fine French wine label!

Read 9 tweets

Lea Alhilali, MD

@teachplaygrub

Oct 13

1/Time to FESS up! Do you understand functional endoscopic sinus surgery (FESS)?

If you read sinus CTs, you better know what the surgeon is doing or you won’t know what you’re doing!

Here’s a thread to make sure you always make the important findings!

2/The first step is to insert the endoscope into the nasal cavity.

The first two structures encountered are the nasal septum and the inferior turbinate.

3/So on every sinus CT you read, the first question is whether there is enough room to insert the scope.

Will it go in smoothly or will it be a tight fit?

Read 19 tweets

Lea Alhilali, MD

@teachplaygrub

Oct 10

1/I always say you can tell a bad read on a spine MR if it doesn’t talk about lateral recesses.

What will I think when I see your read? Do you rate lateral recess stenosis?

Here’s a thread on lateral recess anatomy & a grading system for lateral recess stenosis

2/First anatomy.

Thecal sac is like a highway, carrying the nerve roots down the lumbar spine.

Lateral recess is part of the lateral lumbar canal, which is essentially the exit for spinal nerve roots to get off the thecal sac highway & head out into the rest of the body

3/Exits have 3 main parts.

First is the deceleration lane, where the car slows down as it starts the process of exiting.

Then there is the off ramp itself, and this leads into the service road which takes the car to the roads that it needs to get to its destination

Read 21 tweets

Lea Alhilali, MD

@teachplaygrub

Oct 8

1/Remembering spinal fracture classifications is back breaking work!

A thread to review the scoring system for thoracic & lumbar fractures—“TLICS” to the cool kids!

2/TLICS scores a fx on (1) morphology & (2) posterior ligamentous complex injury

Let's start w/morphology

TLICS scores severity like the steps to make & eat a pizza:

Mild compression (kneading), strong compression (rolling), rotation (tossing), & distraction (tearing in)

3/At the most mild, w/only mild axial loading, you get the simplest fx, a compression fx—like a simple long bone fx--worth 1 pt.

This is like when you just start to kneading the dough. There's pressure, but not as much as with a rolling pin!

Read 13 tweets

Lea Alhilali, MD

@teachplaygrub

Oct 6

1/Does PTERYGOPALATINE FOSSA anatomy feel as confusing as its spelling?

Does it seem to have as many openings as letters in its name?

Are you pterrified of the pterygopalatine fossa (PPF)?

Let this thread on PPF anatomy help you out.

2/The PPF is a crossroads between the skullbase & the extracranial head and neck

There are 4 main regions that meet here:

(1) Skullbase itself posteriorly, (2) nasal cavity medially, (3) infratemporal fossa laterally, and (4) orbit anteriorly.

3/At its most basic, you can think of the PPF as a room with 4 doors opening to each of these regions: one posteriorly to the skullbase, one medially to the nasal cavity, one laterally to the infratemporal fossa, and one anteriorly to the orbit

Read 18 tweets

Lea Alhilali, MD

@teachplaygrub

Oct 3

1/”Tell me where it hurts.”

How back pain radiates can tell a lot you about where the lesion is—if you know where to look!

Here’s how to remember lumbar radicular pain distributions!

2/Let’s start with L1. L1 radiates to the groin. I remember that b/c the number 1 is, well, um…phallic. So the phallic number 1 radiates to the groin.

3/Let’s skip to L3 for a second. I remember L3 is to the knee—easy, it rhymes!

Read 8 tweets

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