Cranial Nerve Palsies and Your Eyes

A nerve palsy or paresis occurs when a nerve no longer sends information correctly. This results in a deficit of any actions or movements that the nerve was responsible for signaling.

Nerves That Impact the Eyes

The eyes move rapidly due to the numerous muscles around the eyeball which allow for movement in all directions of gaze.

These muscles, known collectively as the extra-ocular muscles, receive information from the brain from several nerves.

The main nerves which control the extra-ocular muscles are the oculomotor nerve (Cranial Nerve III), the trochlear nerve (Cranial Nerve IV), and the abducens nerve (Cranial Nerve VI).

When an eye movement is signaled, such as trying to look at a sign on the side of the street, the brain will send a signal along the nerves that control the eye muscles to move the eyes to the desired position.

What Can Cause a Nerve Palsy?

A complete nerve palsy is a relatively rare and often very serious condition.

A nerve palsy can be caused by trauma, like a car accident or punch to the head.

Another cause of a nerve palsy is a space occupying tumor which is often cancerous – such as a pontine glioma or a meningioma.

In some cases, the cause of a nerve palsy is due to an aneurysm or another blood vessel problem which may be caused by a disease like diabetes.

It is possible to be born with a nerve palsy that is congenital and has no known or identifiable cause.

Cranial Nerve III Palsy and the Eyes

Cranial nerve III, the oculomotor nerve, is responsible for moving the eyes up, down, and in towards the nose.

The oculomotor nerve is also responsible for raising the eyelids and keeping the pupils small in bright light.

If there is a complete cranial nerve III palsy on one side of the brain, the eye on the affected side will have a droopy eyelid, large pupil, and the eye will be “down and out” as the muscles responsible for moving the eye up and in are not functioning.

The two most common causes of a cranial nerve III palsy are an aneurysm and diabetic damage to the blood vessels in the nerve.

An aneurysm will result in a complete cranial nerve III palsy and can be a life-threatening condition because if the aneurysm ruptures, the brain can become deprived of blood.

However, if diabetes is the cause of the cranial nerve III palsy, the pupil will not be larger than the other eye. In these cases, there is little risk of further complications, and the function may return to normal.

Cranial Nerve IV Palsy and the Eyes

Cranial nerve IV is primarily responsible for moving the eyes down when looking at the opposite side of the body.

A cranial nerve IV palsy will result in the affected eye being higher than the other eye.

Cranial nerve IV palsies are typically caused by blunt trauma or are congenital.

If there are no other associated problems, the main issue with a cranial nerve IV palsy is double vision when looking straight ahead.

Cranial Nerve VI Palsy and the Eyes

Cranial nerve VI is solely responsible for moving the eyes out or away from the nose.

A cranial nerve VI palsy will present with the person unable to move the eye past the midline when trying to look out to the side. Because of this, the person may have a head turn or head tilt.

In adults, a cranial nerve VI palsy is often due to a cancerous tumor, diabetic damage to blood vessels, or trauma.

In children, a cranial nerve VI palsy is more concerning and likely due to severe trauma or a cancerous tumor.

Many causes of cranial nerve VI palsies are sight or life threatening if not treated.

Importance of Diagnosis and Treatment

In any case of a nerve palsy, it is important for your doctor to make a timely diagnosis and likely refer for additional testing.

If the cause of the nerve palsy can be treated, it is often treated with urgency due to the serious nature of the condition.