Oculomotor Nerve Palsy

What is the Oculomotor Nerve?

The oculomotor nerve is one of 12 pairs of cranial nerves. Many of these nerves form part of the autonomic nervous system, which supplies (innervates) organs such as the eyes.

The oculomotor nerve is the third cranial nerve. It enables movement of the eye muscles, constriction of the pupil, focusing of the eyes and the positioning of the upper eyelid.

Cranial nerve III works together with other cranial nerves to control eye movements and support sensory function.

What is the function of the Oculomotor Nerve?

It controls four of the six muscles that enable eye movement. The 3rd cranial nerve makes it possible to:

• Raise the upper eyelid.
• Focus the eyes.
• Respond to light by making the black centre of the eye (the pupil) smaller.
• Move the eyes inward, outward, up and down, and control torsion.

How does the 3rd Cranial Nerve work?

It coordinates eye movement through motions that include:

• Accommodation, focusing on an object that is moving closer to or further away from you.
• The optokinetic reflex, returning the eyes to their previous position after focusing on an object.
• Saccades, quick movements that shift your gaze from one object to another.
• Smooth pursuit (visual tracking), which enables you to hold your gaze on a moving object.
• The vestibulo-ocular reflex, which adjusts the position of the eyes as your head moves.
• Visual fixation, gazing at an object that is not moving.

What conditions can affect the Oculomotor Nerve?

Oculomotor dysfunction is a group of disorders affecting the 3rd cranial nerve. These disorders are also known as third nerve palsy, and they occur when the 3rd cranial nerve becomes paralysed.

Third nerve palsy can be present at birth. It may also develop later in life as a result of:

• Inadequate blood flow: This deprives the nerve of the oxygen it needs to function properly.
• Nerve compression: Abnormal pressure on the nerve.

Conditions that may cause oculomotor dysfunction include:

• Brain aneurysm.
• Brain tumour.
• Head injuries.
• Demyelinating disease (multiple sclerosis).
• Microvascular disease (diabetes and high blood pressure).
• Infections, including HIV and Lyme disease.
• Migraine.

What can be done to correct a 3rd Cranial Nerve palsy?

Treatment for third nerve palsy depends on its cause. There is no treatment to restore function in a congenital third nerve palsy. An acquired third nerve palsy may resolve on its own, depending on the cause. If a brain tumour or aneurysm is causing the palsy, surgery to relieve the pressure on the third nerve may help it function again.

A neuro-ophthalmologist will wait at least 6 months from the onset of a third nerve palsy to see whether it improves on its own. During this time, an eye patch or prism glasses may help with double vision. Eye muscle surgery (strabismus surgery) may help realign the eyes so that they are straight when looking directly ahead, though this can sometimes take a number of procedures. Even after strabismus surgery, people typically still experience double vision when looking to the sides.

When should I call a neuro-ophthalmologist about concerns with the 3rd cranial nerve?

Contact your neuro-ophthalmologist if you experience symptoms of third nerve palsy. These include:

• An abnormally large pupil.
• A pupil that does not constrict in response to light.
• Double vision (diplopia).
• One or both eyes drifting off to the side (strabismus).
• A drooping eyelid (ptosis), which may cover the pupil.

A note from Athens EyeCare Clinic

The oculomotor nerve is the third cranial nerve and controls four of the six muscles that enable eye movement. Conditions affecting cranial nerve III include third nerve palsy, often a complication of medical problems such as uncontrolled diabetes or a brain aneurysm. Third nerve palsy can affect your vision and the appearance of one or both of your eyes. Many people make a full recovery, although this can take around six months.