Question of the Week: October 1, 1999

Describe 3 clinical findings associated with compression of the right 3rd Cranial Nerve.

Two 3rd cranial nerves (oculomotor nerves) are located at the top of the brainstem - one to the right and one to the left.  Motor messages from the 3rd cranial nerve nucleus controls eye muscles on the same side, or, ipsilateral (medial rectus, superior rectus, inferior rectus and inferior oblique).
See Diagram.

Pupillary Control
The 3rd cranial nerve is a parasympathetic response.  The parasympathetic (return to normal) is responsible for pupillary constriction. 

Eye Movement
The 3rd cranial nerve controls the muscle that makes the eye move.  The 4th cranial nerve controls the muscle that allows the eye to look downward and inward towards the nose, and the 6th cranial nerve controls the muscle that makes the eye look towards the temple on the same side.  The ability to move the eye in all other directions is controlled by the 3rd cranial nerve. 
See diagram of Cranial Nerves responsible for eye movement.

Eyelid Opening
The 3rd cranial nerve also controls the ability to open the eyelid.

Compression of the 3rd cranial nerve prevents the transmission of the impulse to the pupil, eyelid and eye muscles, on the same side as the nerve compressed. 

Compression of the 3rd cranial nerve causes the following problems (on the same side as the compression):

  • inability to constrict the pupil in response to light 
  • inability to rotate the eye and follow objects moved through the full visual field
  • inability to open the eyelid

Clinical findings of 3rd cranial nerve compression include:
  • ipsilateral dilation and loss of reactivity of the pupil (because the 2nd cranial nerve carries the light to the oculomotor nucleus, pupillary constriction requires both the 2nd and 3rd cranial nerve to be functioning)
  • ipsilateral ptosis (drooping of the eyelid - mild ptosis may be evident when the affected eye appears to have a "thick" lid with less visible "white" than the unaffected eye)
  • inability to move the affected eye throughout the full ocular range
  • diplopia (blurred vision - which occurs when both eyes fail to deviate in the same direction) 
  • inability to move the eyes upward - diplopia may worsen when attempts are made to look upward, as only the unaffected eye moves in that direction
  • the affected eye is "down and out" - the eyelid is down, and the eye is rotated downward and outward (due to loss of opposing upward movement).