Eye Movement Dysfunction

The rotational movements of intorsion and extorsion are necessary to keep a viewed object stable and positioned on the appropriate part of the retina as the head is tilted from side to side.

To illustrate imagine standing upright and looking at an arrow that is pointing superiorly. 

Now tilt your head 45 degrees to the right, does the arrow move with your head? Now point at a 45 degree angle, hopefully not. Because of intorsion and extortion the arrow should still appear to be pointing superiorly. In this case the right eye has been intowarded and the left eye has been equivalently extorted so that the image falls on the same part of the retina as before the head tilt.

Obviously our eye has a limited range of intorsion and extorsion. They cannot spin completely around within the orbit. And if we were to stand on our head the arrow would appear to change direction and appear to point inferiorly.

Now, lets look at the movements produced by each of the extra oculum muscles.

Medial and lateral rectus muscles are responsible for producing a single straightforward eye movement.

Contraction of medial rectus causes adduction of the eye.

Medial rectus is the primary adductor though other extraocculum muscles can assist in this movement.

Contraction of lateral rectus causes abduction of the eye.

Lateral rectus is the primary abductor. Though again other extra-ocular muscles can assist in this movement.

The remaining four extraocular muscles are a little bit more complex. Since they are capable of producing multiple eye movements and their primary function changes depending upon the relative position of the eye  within the orbit. 

The primary result of contraction of superior rectus is elevation of the eye. However superior erectus is most effective at elevating the abducted eye with increasing adduction of the eye. Superior rectus loses the mechanical advantage necessary for producing elevation such that the fully adducted eye can be elevated only by the action of the inferior oblique muscle.

In addition to its primary function as an elevator of the eye superior rectus also produces limited aduction and intorsion of the eye. 

The primary result of contract of inferior rectus is depression of the eye. However, as is the case with superior rectus  the inferior rectus is most effective at depressing the adducted eye and loses the mechanical advantage necessary for depression as the eye is progressively adducted. The fully adducted eye can be depressed only by the superior oblique muscle.

In addition to its primary function as a depresser of the eye inferior rectus also produces adduction and extortion of the eye. 

The primary result of contraction of the inferior oblique is extorsion of the eye. As mentioned earlier inferior oblique also assists in elevation of the eye and is the only effective elevator of the fully adducted eye.

Inferior oblique also contributes to adduction of the eye.

The primary result of contraction of the superior oblique is intorsion of the eye. Superior oblique also assists with depression of the eye and is the only effective depressor  of the fully adducted eye.

Superior obloque also contributes to the abduction of the eye. 

Superior oblique is often referred to as the down and out muscle because if acting in isolation that is with all or most of the other muscles inactive such as occurs with third nerve palsy or a quote blown third nerve it does put the eye in the down and out position.

However the function of superior oblique is not tested by having the patient look in the down and out direction. But rather in the down and in direction. This is because the co ordinated function of other muscles specifically lateral rectus and inferior rectus can put the eye in the down and out position but only superior oblique is able to depress the fully adducted eye that is to put the eye in the down and in position. This discrepancy between the pure isolated function of the superior oblique and the clinical test of the superior oblique function often causes confusion for students and clinicans alike.