0:00

Other major nerve that can be, uh, entrapped upon

0:03

around the shoulder is the axillary nerve.

0:05

So our axillary nerve sits, uh, just inferior

0:07

to the axillary recess.

0:09

Um, surrounded by fat again, is a great way

0:11

to identify the course of the nerve

0:13

and it innervates both our deltoid muscle

0:15

and our terry minor muscle.

0:16

Some of the other, uh, major

0:19

muscular structures about the shoulder.

0:22

So a lot of nerve entrapment syndromes have associated, uh,

0:25

names named kind of space syndromes.

0:28

Uh, you can always call them an axillary nerve entrapment

0:31

as kind of the proper terminology.

0:33

But the syndrome is kind

0:34

of the clinical picture in association

0:36

with the imaging findings.

0:37

Uh, so this is also known

0:39

as the quadrilateral space syndrome, and that's

0:42

because of the, this typical site

0:44

where the nerve is entrapped is considered the

0:46

quadrilateral space, right?

0:47

So, uh, these are the major muscles

0:49

that border the quadrilateral space.

0:51

And again, that paralegal cyst is a common cause

0:53

of axillary nerve entrapment.

0:55

Uh, we can also have some anatomic variation in this region,

0:58

or sometimes some fibrosis, uh,

1:01

or other injuries that can, um,

1:02

impinge upon the course of the nerve.

1:06

So this was a 44-year-old man.

1:08

He had isolated axillary nerve palsy.

1:11

He actually had an injury.

1:12

So again, this is not a true entrapment case.

1:14

It's probably more like a, an injury

1:17

and potentially associated axillary um, nerve.

1:20

But it has a great, uh, imaging example of the pattern

1:23

of denervation, which is why I wanted to show this case.

1:26

So on our fluid sensitive sequences, again,

1:28

we're looking at those muscles.

1:30

Where do we see normal signal

1:31

or where do we see abnormal signal?

1:33

And this one's pretty evident.

1:34

It's pretty stark, uh, difference in the signal

1:36

between our affected deltoid and terries minor muscles

1:40

and our other, uh, rotator cuff musculature

1:42

and other musculature about the shoulder.

1:45

One thing and one pitfall about quadrilateral space

1:48

syndrome, you have to realize that the uh,

1:50

terries minor muscle can have some increased signal in

1:52

asymptomatic patient populations.

1:55

So you really have to make sure

1:56

that you're evaluating this in the

1:58

appropriate clinical setting.

1:59

Um, also really scrutinizing

2:00

that deltoid muscle when seen together,

2:03

you should really scrutinize that axillary nerve

2:05

and evaluate for injury.

2:07

Isolated Terries minor

2:08

should give you a little bit of pause.

2:09

If they're worried about an axillary nerve injury,

2:12

it certainly could be a presentation.

2:13

Just keep in mind that many patients can be asymptomatic

2:16

and have that tes minor muscle signal abnormality.

2:20

So in this particular instance, we talked about

2:22

that injury rate.

2:23

This patient did have a paralegal cyst,

2:25

which we can see on this sagittal fluid sensitive sequence.

2:27

It does get close to the nerve.

2:29

It's hard to believe that this small

2:30

cyst could be causing compression.

2:32

It's more likely that the patient had a labral injury,

2:35

also had an axillary nerve injury.

2:37

But it's a great pattern of denervation that fits

2:39

with a quadrilateral space syndrome.