Benjamin Lin MD, Lauren Chen MD, Kara Cavuoto, MD
The primary actions of the extraocular muscles make sense. Superior rectus moves the eye up. Inferior rectus moves the eye down. Etcetera etcetera. This article will teach you how to logically think through the secondary and tertiary actions and why we check extraocular motility in an “H” pattern.
| Muscle | Primary | Secondary | Tertiary |
| Superior oblique | INtorsion | Depression | ABduction |
| Inferior oblique | EXtorsion | Elevation | ABduction |
| Superior rectus | Elevation | INtorsion | ADduction |
| Inferior rectus | Depression | EXtorsion | ADduction |
| Lateral recutus | ABduction | ||
| Medial rectus | ADduction |
Key terms
- Elevation: eye looks up
- Depression: eye looks down
- ABduction: eye looks away from the nose
- ADduction: eye looks towards the nose
- Intorsion: top of the eye twists in towards the nose (e.g. while looking at R eye, it appears to turn clockwise)
- Extorsion: top of the eye twists away from the nose (e.g. while looking at R eye, it appears to turn counterclockwise)
Pearls to memorize (once you finish the article and understand why these occur)
- Both superior muscles INtort (superior rectus and superior oblique)
- Both inferior muscles EXtort (inferior rectus and inferior oblique)
- Both oblique muscles ABduct (superior oblique and inferior oblique), and the corresponding vertical recti ADduct (superior rectus and inferior rectus)
Fundamental concepts
There are three fundamental concepts about EOM anatomy/function you must understand to master all of the EOMs:
1) Attachment on anterior vs. posterior half of eye: The four recti (medial, lateral, inferior, and superior recti) attach on the anterior half of the eye. In contrast, the superior and inferior oblique attach on the posterior half of the eye.
2) 3 possible axis of rotations: The eye can rotate around three different axes – a horizontal axis (x axis), an anterior/posterior axis (y axis) AND a vertical axis (z axis).
3) Axis of globe vs. axis of orbit: The axis of the globe is NOT aligned with the axis of the orbit: When the eye is in primary position (i.e. looking straight ahead), the axis of the orbit actually runs about 23 degrees offset from the globe.
Since the superior, inferior, medial, and lateral recti originate from the annulus of Zinn at the apex of the orbit, they attach to the eye at an offset angle and don’t run straight anterior/posterior.
This is why the superior and inferior recti don’t ONLY elevate or depress the eye. The force vectors that occur when these recti muscles contract run backwards AND nasally. We’ll explain this more in detail below.
Lateral and medial recti
These are the most straightforward. If you look at the path of these two muscles from the front, you can see that they travel in the same horizontal plane as their origin at the annulus, so there are no superior or inferior force vectors. Therefore, these only cause the eye to rotate around the vertical z axis. This is why these each only have one primary action and no secondary/tertiary actions.
– Lateral rectus: The posterior force vector pulls the lateral portion of the globe back, which rotates around the z axis and causes ABduction.
– Medial rectus: The posterior force vector pulls the medial portion the globe back, which rotates around the z axis and causes ADduction
Superior rectus
The superior rectus runs from the annulus of Zinn to the superior, anterior portion of the globe. As it pulls, the force vector runs not only posteriorly, but also medially and inferiorly. Think of how the force vectors will act on each of the 3 possible axes of rotations independently:
1) X axis: The posterior force vector pulls the top of the eye back. This rotates the front of the eye up by rotating around the x axis, causing elevation (primary function).
2) Y axis: The combined medial and inferior force vectors pull the top of the eye medially and downwards, rotating around the y axis and causing intorsion (secondary function).
3) Z axis: The medial force vector pulls the front of the eye medially, rotating around the z axis, causing ADduction (tertiary function).
Inferior rectus
The inferior rectus runs from the annulus of Zinn to the inferior, anterior portion of the globe. As it pulls, the force vector runs not only posteriorly, but also medially and superiorly. Again, consider the force vectors and their action on each of the 3 possible axes of rotation:
1) X axis: The posterior force vector pulls the bottom of the eye back. This rotates the front of the eye down by rotating around the x axis, causing depression (primary function).
2) Y axis: The combined medial and superior force vectors pull the bottom of the eye medially and upwards, rotating around the y axis and causing extorsion (secondary function).
3) Z axis: Like the superior rectus, the medial force vector pulls the front of the eye medially, rotating around the z axis, causing ADduction (tertiary function).
Superior oblique:
This is where things start to get a little weird. Remember that although the superior oblique begins roughly at the annulus of Zinn (technically it actually begins on sphenoid bone very close to the annulus), it runs through the trochlea and then courses posteriorly prior to attaching to the globe. Additionally, in contrast to the four rectus muscles which attach to the anterior half of the globe, both the superior and inferior obliques attach at the posterior half of the globe.
Therefore, the force vector that is created when the superior oblique contracts runs anteriorly, medially and superiorly.
1) X axis: The anterior/superior force vector pulls the back of the eye up, causing the front of the eye to go down, leading to depression (secondary function).
2) Y axis: The medial force pulls the top of the eye towards the nose, rotating around the Y axis, causing intorsion (primary function).
3) Z axis: The medial force also pulls the back of the eye towards the nose, rotating the front of the eye around the Z axis, causing ABduction (tertiary function).
Inferior oblique:
The inferior oblique is just like the superior oblique, but on the inferior portion of the eye. The force vectors are similar because the inferior oblique’s origin on the medial floor of the orbit is analogous to the superior oblique running through the trochlea. Therefore, the force vector runs anteriorly, medially, and inferiorly
1) X axis: The anterior/inferior force vector pulls the back of the eye down, causing the front of the eye to go up, leading to elevation (secondary function).
2) Y axis: The medial force vector pulls the bottom of the eye towards the nose, rotating around the Y axis, causing extorsion (primary function).
3) Z axis: The medial force vector also pulls the back of the eye towards the nose, rotating the front of the eye around the Z axis, causing ABduction (tertiary function).
Why the H?
Now that you understand how the muscles function, the reason we draw an “H” to test extraocular movements will make sense (note that drawing a square will also illustrate the same thing).
For example, both the superior rectus and inferior oblique will elevate the eye in primary position. If you have the patient look straight up from primary position, you can’t localize a deficit to either muscle because both are active at the same time. Similarly, both the inferior rectus and superior oblique can depress the eye, so looking straight down from primary gaze doesn’t help you localize a deficit either.
However, when the eye is ABducted, the axis of the globe is in line with the axis/force vector of superior rectus. This allows the superior rectus to pull straight back and elevate the eye. However, the force vector of the inferior oblique in this position is approximately perpendicular to this eye position. Therefore, it is unable to elevate the eye. Therefore, ABducting the eye isolates the elevating action of the superior rectus by preventing the inferior oblique from acting as an efficient elevator. In a similar manner, ABducting the eye also isolates the depressing action of the inferior rectus by preventing the superior oblique from acting as an efficient depressor.
When the eye is ADducted, the axis of the globe is better in line with the axis/force vector of the superior oblique. This allows the superior oblique to pull straight forward and depress the eye. Therefore, ADducting the eye isolates the depressing action of the superior oblique. In a similar manner, ADducting the eye also isolates the elevating action of inferior oblique.
Therefore…
Abducting the eye:
1) isolates the elevating action of the superior rectus
2) isolates the depressing action of the inferior rectus
Adducting the eye:
1) isolates the elevating action of the inferior oblique
2) isolates the depressing action of the superior oblique
