Palsies of the CN III, IV, VI

by Ji Keon LOOI

Oculomotor Palsy
Characteristic signs of a complete lesion of CN III:-

• Ptosis (drooping) of the superior eyelid, caused by paralysis of the levator palpebrae superioris.


• Eyeball (pupil) abducted and directed slightly inferiorly (down and out) because of unopposed actions of the lateral rectus and superior oblique.


• No pupillary (light) reflex (constriction of the pupil in response to bright light) in the affected eye


• Dilation of pupil, resulting from the interruption of parasympathetic fibers to the sphincter of the pupillae, leaving the dilator pupillae unopposed.


• No accommodation of the lens (adjustment to increase convexity for near vision) because of paralysis of the ciliary muscle.
  

Injury to the Trochlear Nerve
CN IV is rarely paralyzed alone. The nerve may be torn in severe head injuries because of its long intracranial course. Damage to CN IV nerve or its nucleus impair the ability to turn the affected eyeball inferomedially. The characteristic sign of trochlear nerve injury is diplopia (double vision) when looking down (e.g., when going down stairs). Diplopia occurs because the superior oblique normally assists the inferior rectus in depressing the pupil (directing the gaze downward) and is the only muscle to do so when the pupil is adducted. In addition, because the superior oblique is the primary muscle producing intorsion of the eyeball, the primary muscle producing extorsion (the inferior oblique) is unopposed when the superior oblique is paralyzed. Thus the direction of gaze and rotation of the eyeball about its anteroposterior axis is different for the two eyes, especially when looking downward and medially.

Injury to the Abducent Nerve
Because CN VI has a long intracranial course, it is often stretched when intracranial pressure rises, partly because of the sharp bend it makes over the crest of the petrous part of the temporal bone after entering the dura. A space-occupying lesion such as a brain tumor may compress CN VI, causing paralysis of the lateral rectus muscle. Complete paralysis of CN VI causes medial deviation of the affected eye—that is, it is fully adducted owing to the unopposed action of the medial rectus, leaving the person unable to abduct the eye. Diplopia is present in all ranges of movement of the eyeball, except on gazing to the side opposite the lesion.

Source: Moore and Agur; Essential Clinical Anatomy, Lippincott and Williams