Notes

The neurophysiology of the six cardinal positions of gaze, so when we add in the cranial pathologies, we can understand their extraocular manifestations.

In brief, CN 6 innervates the lateral rectus, CN 4 innervates the superior oblique, and CN 3 innervates the rest.
- Lateral rectus directs the eye laterally (called abduction).
- Medial rectus directs the eye medially (called adduction).
- When the eye is abducted, the superior rectus directs the eye superiorly and the inferior rectus directs the eye inferiorly.
  -When the eye is adducted, the superior oblique directs the eye inferiorly and the inferior oblique directs the eye superiorly.

The lateral rectus fails to activate.
The remaining muscles activate.
The eye is medially rotated (adducted); it fails to rotate out.
The patient notices this as double vision when attempting lateral gaze or when looking at a far image.
Can be a warning of brain herniation, because CN 6 is tethered to the dura.

Superior oblique fails to activate.
The remaining muscles activate.
The eye is elevated (aka hypertropic).
CN 4 is long and thin and is the only cranial nerve to make a decussation; so its innervation originates from the opposite side of the brainstem.
Patients are often unaware of this deficit, because they produce a head tilt to counter it: they tilt their head to the side opposite the affected eye in order to bring their eyes into alignment. To understand why, do the following:

into alignment.

Only the lateral rectus and superior oblique activate.
The remaining muscles fail to activate.
The eye is "Down & Out" and the patient has near-constant diplopia.
In diabetic 3rd nerve palsy, a common cause of isolated CN3 injury, these eye movements often slowly recover over the course of several weeks to months.