Lea Alhilali, MD Profile picture
Nov 8, 2024 20 tweets 8 min read Read on X
1/Raise your hand if you’re confused by the BRACHIAL PLEXUS!

I could never seem to remember or understand it—but now I do & I’ll show you how!

A thread so you will never fear brachial plexus anatomy again! Image
2/Everyone has a mnemonic to remember brachial plexus anatomy.

I’m a radiologist, so I remember one about Rad Techs.

But just remembering the names & their order isn’t enough.

That is just the starting point--let’s really understand it Image
3/From the mnemonic, we start with the roots—the cervical nerve roots.

I remember which roots make up the brachial plexus by remembering that it supplies the hand.

You have 5 fingers on your hand so we start with C5 & we take 5 nerve roots (C5-T1). Image
4/Next in the mnemonic are the Trunks.

Bc Trunks starts w/T, I can remember how they are named.

T is Top to bottom. Trunks are named top to bottom: Superior, Middle, and Inferior.

But how to remember which nerve roots combine to give you which trunks? Image
5/Pairing of the nerve roots into the trunks is like pairing off at a dance when there is an odd number

Everyone immediately turns to the person next to them & the person in the middle is left out.

For the roots, C7 is in the middle & has to go it alone as the middle trunk Image
6/Next in the mnemonic are the Divisions.

Divisions do what their name implies—they divide the trunks.

Each trunk is split or DIVIDED into an anterior & posterior division.

Divisions will look like scissors coming off the trunks, helping you to remember they are splitting. Image
7/This division results in a fundamental change in the nerves—anterior divisions will supply flexors & posterior divisions will supply the extensors.

This is an important dividing line.

Like rabid soccer fans, once they have chosen a team, they will never mix w/fans of the other teamImage
8/After the split of the divisions, the nerves come back together as the cords. It is kind of like doing jumping jacks—they open up and then close back up again

I remember that they come back together as the Cords bc Cords and Combine both start with C Image
9/It’s like a toll road.

The road widens to let more cars get to the toll booths. Once they have paid the toll, road narrows again

This is what happens w/the divisions—but instead of paying a toll, they are organizing into flexor & extensor groups & coming back together again. Image
10/But it’s more like going through a worm hole than toll booth.

When you go through a wormhole, you are fundamentally changed when you come out the other side (or so I read on the internet).

Once cords emerge from divisions, they’re either team flexor or extensor & can’t go backImage
11/Divisions combine to form 2 ant. cords & 1 post. cord. Why the inequality?

Well, the fundamental purpose of the arm is to flex (pick up things), unlike the leg (which is to extend/stand up).

So bc it’s more important to flex, remember 2 cords to flexors & 1 to extensors Image
12/All post divisions go to the 1 post cord.

How do you remember which ant divisions go into which cord? Image
13/Remember, divisions come from the superior, middle, & inferior trunks.

Superior or even middle class don’t combine w/inferior things.

So superior & middle combine. Poor inferior is left alone Image
14/Names of the cords are based on their relationship to the axillary artery.

Posterior cord (extensors) is posterior to it.

The flexor cord made of the superior & middle divisions is lateral.

Flexor cord made from the lonely inferior division is medial

But this is hard to remember!Image
15/But unless you have an axillary artery in front of you, this is hard to remember.

So I remember that the flexor cord made from the poor inferior division is looked down upon, so it is given the worst seat—at the arm pit.

In anatomic positioning, closest to the arm pit is medial, so it is the medial cord.Image
16/Now the final division into branches.

Remember posterior cord only supplies extensors & is the only extensor cord.

So when it branches, it needs to innervate extensors all along the arm (elbow, forearm, hand).

So it gives off axillary to the upper arm & radial to the lower armImage
17/Now the branches of the flexor cords

As expected from the names, MEDIAL cord gives a branch for flexors/sensation to MEDIAL forearm/hand (in anatomic position = PINKY side, so ulnar nerve)

LATERAL cord gives a branch for motor/sensation to LATERAL forearm (musculocutaneous) Image
18/Now the brachial plexus goes to the hand & ends w/a handshake!

Medial & lateral cords meet at the end in a handshake in the middle.

And what do you call something in middle: MEDIAN

So medial & lateral cords handshake in the middle to make the median nerve Image
19/Now move beyond mneumonics!

Remember, brachial plexus splits & recombines like jumping jacks w/a very palindromic 5-3-6-3-5 pattern

The names tell you if they are splitting or combining (Trunk=Together, Division=Divide, Cord=Combine, Branch=break) Image
20/Now all you need is to recall 1 fact at each stage!

Trunk: C7 is left out

Div: Ant flexors don’t mix w/post extensors

Cord: Sup/mid class don’t mix w/inferiors

Br: Each cord gives a branch to region its name describes (post, med, lat) & a handshake in the middle! Image

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More from @teachplaygrub

May 6
1/Have disagreements between radiologists on the degree of cervical canal stenosis become a pain in the neck?

Worried about sticking your neck out & calling severe cervical stenosis?

This month’s @theAJNR SCANtastic has the latest about Cspine MRI!

ajnr.org/content/46/4/7…Image
@TheAJNR 2/In the lumbar spine, it is all about the degree of canal narrowing & room for nerve roots.

In the cervical spine, we have another factor to think about—the cord.

Cord integrity is key. No matter the degree of stenosis, if the cord isn’t happy, the patient won’t be either Image
@TheAJNR 3/Cord flattening, even w/o canal stenosis, can cause myelopathy.

No one is quite sure why.

Some say it’s b/c mass effect on static imaging may be much worse dynamically, some say repetitive microtrauma, & some say micro-ischemia from compression of perforators Image
Read 16 tweets
May 2
1/Do radiologists sound like they are speaking a different language when they talk about MRI?

T1 shortening what? T2 prolongation who?

Here’s a translation w/an introductory thread to MRI. Image
2/Let’s start w/T1—it is #1 after all! T1 is for anatomy

Since it’s anatomic, brain structures will reflect the same color as real life

So gray matter is gray on T1 & white matter is white on T1

So if you see an image where gray is gray & white is white—you know it’s a T1 Image
3/T1 is also for contrast

Contrast material helps us to see masses

Contrast can’t get into normal brain & spine bc of the blood brain barrier—but masses don’t have a blood brain barrier, so when you give contrast, masses will take it up & light up, making them easier to see. Image
Read 20 tweets
Apr 28
1/Asking “How old are you?” can be dicey—both in real life & on MRI! Do you know how to tell the age of blood on MRI?

Here’s a thread on how to date blood on MRI so that the next time you see a hemorrhage, your guess on when it happened will always be in the right vein! Image
2/If you ask someone how to date blood on MRI, they’ll spit out a crazy mnemonic about babies that tells you what signal blood should be on T1 & T2 imaging by age.

But mnemonics are crutch—they help you memorize, but not understand. If you understand, you don’t need to memorizeImage
3/If you look at the mnemonic, you will notice one thing—the T1 signal is all you need to tell if blood is acute, subacute or chronic.

T2 signal will tell if it is early or late in each of those time periods—but that type of detail isn’t needed in real life

So let’s look at T1Image
Read 20 tweets
Apr 25
1/Radiologist not answering the phone?

Just want a quick read on that stat head CT?

Here's a little help on how to do it yourself w/a thread on how to read a head CT! Image
2/In bread & butter neuroimaging—CT is the bread—maybe a little bland, not super exciting—but necessary & you can get a lot of nutrition out of it

MRI is like the butter—everyone loves it, it makes everything better, & it packs a lot of calories. Today, we start w/the bread! Image
3/The most important thing to look for on a head CT is blood.

Blood is Bright on a head CT—both start w/B.

Blood is bright bc for all it’s Nobel prizes, all CT is is a density measurement—and blood is denser (thicker) than water & denser things are brighter on CT Image
Read 20 tweets
Apr 23
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! Image
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. Image
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? Image
Read 19 tweets
Apr 21
1/Ready for a throw down?

MMA fights get a lot of attention, but MMA (middle meningeal art) & dural blood supply doesn’t get the attention it deserves.

A thread on dural vascular anatomy! Image
2/Everyone knows about the blood supply to the brain.

Circle of Willis anatomy is king and loved by everyone, while the vascular anatomy of the blood supply to the dura is the poor, wicked step child of vascular anatomy that is often forgotten Image
3/But dural vascular anatomy & supply are important, especially now that MMA embolizations are commonly for chronic recurrent subdurals.

It also important for understanding dural arteriovenous fistulas as well. Image
Read 17 tweets

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