The human body has 12 pairs of cranial nerves that travel from the brain directly to parts of the head. Three of these nerve pairs control the movement of the eye. The third cranial nerve, which is called the oculomotor nerve, controls movement of the eyelid and eyeball. When this nerve is injured or impaired, the resulting condition is third cranial nerve palsy.

Symptoms

The symptoms of third cranial nerve palsy are typically visible to anyone:

• Drooping eyelid – The condition when the eyelid droops or does not open completely is called ptosis. Patients with third cranial nerve palsy typically display sudden drooping in one or both eyelids.
• Pain – There may be pain in the eye or in the head.
• Eye direction – the affected eye typically points down and out. The eye can move toward the middle only when trying to look inward, and is not able to move up or down. This can cause double vision because the eyes no longer track together or point the same direction. In many cases, the drooping eyelid hangs low enough that the affected eye is completely blocked so double vision is not noticed.
• Pupil – Light enters the eye through the pupil. The pupil closes down or shrinks in bright light and dilates or opens in dim light. A patient is said to have pupillary involvement if the pupil is affected by the palsy. In this case, the pupil is often dilated and does not respond when a bright light shines in the eye. Pupillary non-involvement means the pupil of the affected eye responds normally to changes in light. Partial pupillary involvement means the pupil changes size depending on light levels, but reacts sluggishly rather than at a normal rate.

Causes

Third cranial nerve palsy is caused by something disrupting the function of the oculomotor nerve anywhere along its path from the brain to the eye. Possible causes include these brain disorders:

• Trauma – an injury to the head may damage the nerve
• Aneurysm – a bulge in the wall of a blood vessel supplying blood to the brain may put pressure on the nerve
• Hemorrhage – a rupture in a blood vessel may cause a build-up of blood that can put pressure on the nerve
• Tumor – the growth of a tumor or lesions may block or put pressure on the nerve

Third cranial nerve palsy can also be caused by diabetes or, more rarely, by meningitis.

Treatment

Tests to determine the cause and location of the problem include a CT scan or MRI which allow the doctor to see video images of the inside of the brain. If the pupil is involved, a CT scan is usually done immediately to determine if an aneurysm is the cause. This is a potentially life-threatening condition. If an aneurysm ruptures inside the brain, it can cause coma and death. Get immediate medical help to determine the cause if you have symptoms of third cranial nerve palsy.

Sources:
Merck Manual Online
Handbook of Ocular Disease Management
Gateway Community College: Anatomy & Physiology