Category Archives: Anatomy

Eye Muscles and Testing

I remember in my first year of medical school how much time my roomate and I spent trying to figure out the eye muscles. I honestly dont think I truly understood it until I started studying for the Step 1 exam. This was the case for most other subjects as well.

I want to say at the beginning that the exact details of eye muscle functioning are probably not the highest yield material for Step 1 studying. Also, explaining this in words is definitely not the most effective way of explaining the eye. Animation would be ideal here, alas. With that said lets get into this crazyness.

The first thing I want to point here is a foundational point. The eye muscles and the eye socket itself point outward at about a 20 degree angle away from the midline. Let me be clear: NOT the eyeball but rather the orbit and the majority of the muscles come out of the skull at a 20 degree angle away from the midline. You can see this from the picture. The eyeball itself however is pointed forward as you can see.

THE SUPERIOR RECTUS EXAMPLE

Lets look at the superior rectus (the central muscle in the image here) to get an idea of how this particular non-alignment plays out. If you look carefully at the insertion point of the superior rectus here we can see that if it were to contract not only would the eye turn upwards it would also intort. To visualize intorsion lets first imagine a point on the very top of the eyeball – the exact center of eye in the image (since it is a superior view). Intorsion would mean this point moves medially towards the midline causing the eye to essentially rotate about its own axis. Now we can better imagine the action of the superior rectus: eye elevation and slight intorsion. Intorsion, however, would give us a tilted view of the world and with each eye intorting if we wanted to look up, things would appear very confusing.

The question then arises: How can we look up without causing intorsion of the eye?

To prevent this intorsion nature ingeniously created another muscle (the inferior orbital in this case) to essentially cancel out the intorsion by causing extorsion (outward rotation of the eye about its axis). In addition to extorsion, the inferior orbital elevates the eye as well. So to recap we have two muscles working in concert with each other: one causing elevation and intorsion (superior rectus) and the other causing elevation and extorsion (the inferior orbital). The intorsion and extorsion cancel each other out and the eye can look up without tilting.

Look at the following image to get a better understanding of how the inferior orbital cause elevation and extorsion. To really see if you understand this concept try to work out for yourself using these images how the eye might handle looking down. To get you started, realize that in order to look down the eye would need to use both the inferior rectus as well as the superior orbital. Which one would cause the extorsion? Which one causes the intorsion? Do you see how this cancels out to produce leveled depression of the eye?

TESTING THE SUPERIOR AND INFERIOR ORBITAL MUSCLES

One last point I want to make has to do with how we test the superior and inferior orbital muscles. Lets use the superior orbital muscle as an example. In order to test whether it is working generally we ask the patient to turn the eye medially (adduct) and then look down. The reason we do this is to remove the depressing ability of the inferior rectus. If, after removing the inferior rectus’ depressing ability, the eye can look down then the superior orbital muscle is functioning.

To understand how turning the eye medially “removes” the inferior rectus’ depressing ability we need to revisit the idea that the major eye muscles are coming out of the skull at roughly 20 degrees from the midline (see image above). If, for example, the right eye turns medially then its central axis line would be roughly perpendicular to the line of the inferior rectus. At this point if it were the only muscle to contract it would cause the eye to extort or rotate outwardly about its own axis. In reality the other muscles would prevent this from happening.

Now that the depressing ability of the inferior rectus is removed by turning the eye medially would can test the superior orbital muscle ability to depress the eye and therefore its functioning. The Inferior Orbital muscle is essentially the same idea. We test it by  turning the eye medially and then looking up instead.

As always I am more than open to comments and discussion. Good luck with the studying!

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The Infamous Pope’s Blessing

Truth be told, this is more aptly called “pope’s curse” because of the confusion it causes and heated discussions it generates. I spent the majority of a day  sorting this all out (probably not the most efficient use of my time). From all the forums, books, and websites I’ve read on the subject I am sure I found the source of the confusion for at least the majority of people.

The best way to approach the issue is by first understanding two important concepts: 1) The lumbricals and what they do. 2) Is the patient being asked to extend their fingers or make a fist?

The last point in particular might clear up a lot of confusion. We’ll see why later. First lets start with the Lumbricals. What you need to know is that the Lumbricals are responsible for flexing the MCP and extending the PIP and DIP joints. The Median nerve controls the lumbricals for digits 2 and 3. The ulnar nerve controls the lumbricals for digits 4 and 5. Here’s is how a hand would look with all lumbricals working (flexing the MCP joints and extending the PIP and DIPs):

A nice way to remember this is that L umbricals make an “L” shape. Therefore if the lumbricals don’t work then we have the opposite: extended MCP and flexed PIP and DIP. Here are two images (one my hand, the other I got from google images):

Consider this more of a dramatization since I don’t actually have Klumpke’s palsy.

Note: MCPs extended, PIP/DIPs flexed. Klumpke’s palsy has also been called “total claw hand”, because as a commenter wonderfully pointed out there are injuries to both the median AND ulnar nerves.

Ok, so how does all relate to the Pope’s Blessing sign? It really all comes down to which lumbricals aren’t working. Let’s look at a distal ulnar nerve lesion. In this scenario the ulnar lumbricals don’t work, meaning the 4th and 5th fingers have extended MCP and flexed PIP/DIP (remember non-functioning lumbricals). As a result the 4th and 5th fingers appear partially flexed. Now here’s the key: If you ask the patient to extend their fingers you accentuate the discrepancy between fingers. We end up with extended 1st, 2nd, and 3rd digits while the 4th and 5th digits remain partially flexed giving us the classic pope’s blessing sign.

The confusion, I found out, was when in the past the Pope’s blessing sign was considered a distal median nerve lesion. Let’s consider why this was the case and why it might also appear to be a pope’s blessing sign (and why I call it the crooked Pope’s blessing).

With a distal median nerve lesion we are knocking out the lumbricals of digits 2 and 3. Remember what we said about non-functioning lumbricals? We end up with extended MCP and flexed PIP and DIP. If, as before, we ask the patient to extend their fingers we end up with the classic claw hand of a median nerve injury (see below). But, this looks nothing like a pope’s blessing you might ask. That’s because we asked the patient to extend their fingers. Here’s the key, please read carefully: If we asked the patient to “make a fist” instead, we end up with fully flexed 4th and 5th digits with the dysfunctional partially flexed 2nd and 3rd digits. If you do this with your own hand right now…you can see that the 2nd/3rd digits are elevated above the 4th and 5th and might appear as a pope’s blessing, albeit crooked due to the flexed 2nd and 3rd digits, hence the “crooked” pope’s sign.

A picture of a distal median nerve injury with fingers extended:

Here is a chart I made that breaks all of this down in a clear format. I suggest you play with the different variations to see where the differences lie.

To recap, remember it really is about which lumbricals are working and which aren’t. As for the confusion regarding the pope’s blessing sign I hope that cleared things. The 2010 version of First Aid also has the same descriptions as above. The only problem I found was that it wasn’t clear whether the patient was being asked to flex their fingers or to extend them. Even so, I am still human and if you feel that something above is incorrect or unclear please leave a comment below and I will do my best to bring more clarity to the issue.

I hope this helped!

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A quick word about Embryology

Embryology is a very visual subject. I assure you that words can never do justice to this field. Thank God for the glory of technology. If Embryology is proving difficult for you (It was for me) I strongly urge you to look at the Indiana University Animations. They were beyond helpful for me.

Oh, and don’t opt out of doing the surveys. They are just as beneficial for you as they are to the school. They give you a good sense of where you currently stand with your knowledge and when you retake it after the animation you’ll be surprised at how much you learned in a few minutes.

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Brachial Plexus Breakdown

How can we memorize which nerves innervate which parts of the arm? How can we memorize which nerve roots correspond to which nerves? How can we divide the arm, forearm, and hand in a convenient way to organize all this mentally? The answer has to do with the Brachial Plexus; and here’s the trick to the Brachial Plexus: It’s a tube. This tube as we’ll see maps nicely to the entire arm. Let’s work from that premise. I want you to imagine your arm as a tube. Divide it into four anatomical quadrants: medial, anterior, lateral, and posterior. (Bear with me.) Going clockwise: The medial quadrant of this tube is the ulnar nerve, the anterior quadrant is the median nerve, the lateral quadrant are the musculocutaneous and axillary nerves, and the posterior quadrant is the radial nerve. The quadrants nicely parallel the cords division of the brachial plexus (I suggest breaking out Netter’s to follow along) with the only exceptions being that there is no “anterior” cord  and that technically the axillary nerve comes off of the posterior cords.

So why split it into quadrants? The beauty for me behind this is that each quadrant has a group of nerve roots associated with it that correspond nicely to the area of the arm we are dealing with. Again starting with the medial quadrant which we said was the ulnar nerve (C8-T1) and it runs down the medial side of your arm and hand. The anterior quadrant, the median nerve runs down the anterior part of your arm and hand. Another interesting note: because it runs slightly medial to your biceps it would have more of the “medial” quadrant nerve roots (C6, C7, C8, T1) as opposed to the radial nerve which we’ll see later. The lateral quadrant is the musculocutaneous and axillary nerves run for the most part on the outside or lateral portion of your arm (C5,C6). Finally the posterior quadrant is the radial nerve which runs posteriorly innervating the extensors of the arm and forearm. Since the redial nerve runs slightly postero-laterally it will have more of the “lateral” quadrant nerve roots (C5, C6, C7, C8).

If you have trouble with the above try to remember to visualize your arm as a tube with four quadrants.Each quadrant has a set of nerve roots to it. If you get this down I promise you the brachial plexus, the nerve roots, the nerves and the muscles they innervate will all slide into place.

Another interesting trick is to see how the dermatomes for the hand line up nicely with the nerve roots. See the image I drew alongside Kaplan’s Diagram below:

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Abdominal Aorta Anastomoses

I think the key points here have to do with the anastomoses, since this is where clinically relevant questions may be asked. Since I’m a fan of visual learning I start out first getting a mental picture of the abdominal aorta and where the branches come out from. Going in order from Top Down:

Subclavian* (anastomosis with Ext.Iliac via Int.Mammary and Sup./Inf. Epigastrics)

T12- Celiac (anastomosis with SMA via Pancreaticoduodenal)

L1- SMA (anastomosis with Celiac via Pancreaticoduodenal)(anastomosis with IMA via Marginal a.)

L2- Gonadal

L3- IMA (anastomsis with SMA via Marginal a.)(anastomosis with Int.Iliac)

L4- Bifurcation (think bi”four”cation)

Internal Iliac* (anastomosis with IMA via Superior and Middle Rectal a.)

External Iliac* (anastomosis with Subclavian via epigastrics and Int.Mammary)

It would be more than reasonable for the USMLE to ask a question about collateral circulation for each of these points. To help me visualize this I wrote out the above and drew lines connecting the different branches to their connected branch. Keep in mind that these are the arterial anastomoses not the porto-caval anastomoses, which you should know as well.

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