Warning: Read when sober and after sufficient ingestion of caffeine! The action of the extraocular muscles often causes considerable confusion, this is because people get confused between the anatomical action (where the action of each muscle is considered independently) and how the muscle is clinically tested (where one often has to take into account more than one muscle moving the eye). Before we delve into this, I’d like you to look at Figure 1 and remind yourself about some terminology regarding eye movements. Supraduction and infraduction can be thought of as elevation and depression respectively.
Remember to appreciate the movement achieved by any muscle, one needs to understand its origin, insertion and the direction in which the muscle shortens. To comprehend eye movements we need to look therefore at the shape of the orbit and how the muscle attachments relate to the shape of the orbit and the eye. The orbits are pyramidal in shape, triangular when seen from above, with the apex at the posterior most point – see Figure 2.
What this means is that when you are looking straight ahead, superior rectus (SR) which is attached to the top of the eye (anterior to the equator of the eye – see Figure 2 & 3) does not go back towards the apex (the muscle actually originates from the Annulus of Zinn – which is located at the apex) of the orbit in a straight line, but rather at an angle of about 23 degrees to an imaginary sagittal plane. Had human orbits been cuboidal (rectangular when seen from above) in shape then the superior recti muscles would head back in a straight line (parallel to the sagittal plane) and contraction of their fibres alone would elevate the eye. Instead, the pyramidal shape of the orbit means when SR contracts it elevates and adducts the eye (it also intorts the eye) – see Figure 3 and 5b.
Figure 3 Adapted from Clinical Anatomy By Regions
So how do you get the eye to look straight up? Well, you need a second muscle which also elevates the eye and abducts (the latter action opposes the effect of adduction from SR), this muscle is inferior oblique (IO). Looking at Figure 4, it’s obvious that IO will elevate and abduct the eye. Note it inserts posterior to the equator (in these diagrams the equator is akin to the transverse axis line) of the eye (a feature shared by both oblique muscles) and originates on the medial aspect of the floor of the orbit – see Figure 4.
Figure 4 Adapted from Clinical Anatomy By Regions
When considering the anatomical movement of SO, one can think of its functional origin as the trochlea (tendinous loop in the upper medial wall of the orbit), and the muscle component proximal to this can be ignored conceptually. The insertion is on the superior aspect of the eye but posterior to the equator of the eye. If you, imagine the muscle contracting, the eye will intort, depress and abduct. Hence moving the eye “down and out” – see Figure 4.
So let’s consider looking straight down, again if human orbits were cuboidal, then inferior rectus (IR) from its origin (Annulus of Zinn) to its insertion on the eye (anterior to the equator of the eye) would be a straight line, contraction would produce depression of the eye. In reality, the orbit is pyramidal, which results in IR being at an angle of 23 degrees to an imaginary straight line. Thus on its own, it depresses but also adducts (and extorts) the eye. To get the eye to look straight down, we require a second muscle which also depresses the eye but abducts (abduction will help oppose the adduction effect from IR) it too. SO achieves this, thus to look straight down requires both IR and SO working togather.
The anatomical movements of medial and lateral recti are relatively straightforward (for the purposes of clinical examination), one should remember that when lateral rectus (LR) contracts, the medial rectus (MR) for the same eye relaxes (Sherrington’s law: Increase in innervation to an agonist muscle must include reciprocal decrease in innervation to the antagonist muscle). When the LR of one eye contracts, the MR of the other eye contracts (Hering’s law: Yoked muscles receive equal innervation). The latter ensures integrated conjugate eye movements.
Lastly, we can look at how the extraocular muscles are assessed in clinical examination. Lateral and medial recti are easily examined by asking the patient to look left and right. Testing SR, SO, IR and IO is a little more complicated, this is because looking straight up and down both require 2 different muscles, as we explained earlier. So to test each muscle independently we need to find positions where only one particular muscle is producing the action we observe, e.g. asking the patient to look to their right, is testing the right LR and the left MR.
So let’s try this with the right SR muscle; first remind yourself about the pyramidal shape of the orbit, now in which position does the right eye have to be to ensure elevation is exclusively due to SR? The eye has to be abducted so that the muscle belly is inline with the sagittal axis (aka optical axis) of the eye – see Figure 5C. So abducting the right eye by 23 degrees, means the right SR will be the main muscle elevating the right eye when you ask them to look to the right and up (the right eye is abducted and looking up). Keeping the right eye abducted, but then asking the patient to look down will test IR, again for the same reason as SR, the IR muscle is acting in a straight line, and thus it’s action will predominate in depressing the eye.
Figure 5 Right Superior Rectus
Now the right eye has been abducted, so where will the left eye be? Assuming no strabismus (look it up), the left eye will be adducted (remember conjugated eye movements). In the adducted eye, if the patient looks up, the muscle whose line of action will predominantly achieve this is IO. Looking down in the adducted eye (left eye in this example) predominantly tests SO – Figure 6A.
Figure 6 Left Superior Oblique
So the anatomical action of SO acting purely on it’s own (with the eye originally looking straight forward) is down and out, this assumes no interference from the other extraocular muscles. However when examining a patient, to test the SO muscle independently one must adduct the eye being tested, in this adducted position, if they look down, then SO is the key responsible muscle, if the adducted eye looks up, then IO is producing the movement.
When considering the individual anatomical movements of each muscle, the following rules apply: All recti move in the direction of their namesake and all adduct except lateral rectus. All obliques move opposite to their namesake and abduct. All superior muscles intort, all inferior muscles extort. However in reality, in most movements there is more than one muscle involved; for the purposes of clinical examination one has to isolate movements. For SO, the eye must be examined in the adducted position, where SO is the main muscle that makes the eye look down (the down and in position).
Clinical Anatomy By Regions, Richard Snell
Last’s Anatomy By Chummy Sinnatamby