What is Cranial Nerve Palsy?

This is one of the most common causes of acute double vision in the older population. It occurs more often in patients with diabetes and high blood pressure. These will get better and essentially always resolve without leaving any double vision. However, compressive masses, infections, inflammation and injury can also cause cranial nerve palsy. The eyes are moved by 6 extra-ocular muscles. Four of these are rectus muscles (superior, inferior, medial, and lateral) that attach to the front part of the eye (just behind the iris, the colored portion of the eye). Two muscles (the superior and inferior oblique) attach to the back of the eye. These 6 muscles receive their signals from 3 cranial nerves (the IIIrd [oculomotor], IVth [trochlear], and VIth [abducens]). These nerves originate in the brain stem (at the base of the brain) and enter the eye socket through a fissure in the bone of the skull behind the eye.

Pressure on or Interruption of the blood supply to one of the cranial nerves causes it not to work. If there is interruption of signal to the VIth nerve (which innervates the lateral rectus muscle) the affected eye will not be able to move to the outside. The patient will be aware of side-to-side double vision that will be worse (further separation) when the patient looks towards the affected sideWhen the IIIrd nerve (which goes to multiple muscles) is involved the eye may be limited in up, down, and gaze toward the nose. The patient is usually aware of combined vertical and side to side double vision although there may be no double vision at all since the lid droops and may block the second image. The nerves are not permanently injured and over a period of 6 to 12 weeks the function should recover.